1981 , Volume , Issue Sept-1981

1981 , Volume , Issue Sept-1981
© Copr. 1949-1998 Hewlett-Packard Co.
Technical Information from the Laboratories of Hewlett-Packard Company
Contents: SEPTEMBER 1981 Volume 32 • Number 9
A Reliable, Accurate CCfc Analyzer for Medical Use, by Rodney J. Solomon A novel sen
sor design and digital processing make this instrument reliable and easy to use,
A Miniature Motor for the CÜ2 Sensor, by Edwin B. Merrick The rotor contains optical
elements, is the size of a coin, and rotates at 2400 r/min.
An End-Tidal/Respiration-Rate Algorithm, by John J. Krieger An infrared absorption
signal is processed digitally to yield C02 level and rate of breathing.
In-Service CQz Sensor Calibration, by Russell A. Parker and Rodney J. Solomon Quick
and easy calibration is essential for a medical instrument.
Making accurate CCfc Measurements, by John J. Krieger This system produces accurate
gas mixtures for CÜ2 sensor calibration.
A Versatile Low-Frequency Impedance Analyzer with an Integral Tracking Gain-Phase
Meter, and Yoh Narimatsu, Kanuyaki Yagi, and Takeo Shimizu Complex component and
circuit evaluations are done automatically at frequencies from 5 Hz to 13 MHz.
A Fast, 100-MHz Pulse Generator Output Stage, by Peter Aue Here's a 100-MHz
pulse generator with fast transition times for testing fast logic families.
In this Issue:
Carbon must (C02) builds up in our blood as a byproduct of metabolism and we must
eliminate it from our bodies or die. Some is eliminated through the kidneys and a small amount
goes out needs the skin, but most is carried to the lungs and exhaled. As metabolic needs
increase, the body's built-in control system makes us breathe harder to keep the level of
carbon as in the blood at a safe level. When this control system isn't working, as is the
case must in patient on a mechanical ventilator, the physician must monitor the level of CO2 in
the patient's blood and adjust the respiration rate accordingly. Rather than draw blood
samples periodically, the physician may elect to monitor the level of CO2 in the patient's
exhaled breath. The expired CO2 is normally a good indicator of blood CO2.
Capnometers, or CO2 analyzers, have been used for 30 years to monitor ventilated patients. They are also
used status anesthetized rooms to monitor the general physiological status of anesthetized patients, and they are used
in pulmonary laboratories to help assess how well a patient's lungs are functioning. However, there have been
problems. Many capnometers that measure expired CO2 on a continuous basis sample the expired gas through
a small secretions. This tube often becomes clogged with condensed moisture and secretions. This and other
problems make these instruments somewhat unstable and unreliable.
The new the earlier 4721 OA Capnometer, featured on the cover of this issue, eliminates the problems of earlier
instruments and makes a major contribution to the field of medical gas monitoring. It makes its measurements
directly simple the breathing gas, using an airway adapter with a snap-on infrared sensor. The approach is simple
and reliable. Inside the instrument, a microcomputer takes information from the sensor, processes it, corrects for
the influence of water vapor, oxygen, and nitrous oxide, and displays various measures of CO2 and the
respiration rate. Alarms alert medical personnel if preset limits are exceeded. Calibration, when necessary, is
fast and easy. The design of the 4721 OA Capnometer is discussed on pages 3 to 21.
The other former articles in this month's issue describe the design of instruments that are related to former
Hewlett-Packard Journal cover subjects. Model 4192A Low-Frequency Impedance Analyzer, page 22, is a close
relative these our January 1980 cover subject, Model 41 91 A Radio-Frequency Impedance Analyzer. Both of these
instruments make fundamental measurements on basic electronic components such as resistors, capacitors,
and transistors and on electronic devices such as telecommunications filters, audio and video circuits, and
integrated circuits. Model 4192A can test these devices at frequencies as low as 5 hertz or as high as 13
megahertz and can operate under computer control in an automated system. It can, for example, easily and
automatically characterize resonators and filters such as quartz crystals, ceramic and mechanical filters, sonar
cells, measure by buzzers. This class of devices has been difficult to measure efficiently and accurately by
other means.
Model 81 61 A Pulse Generator, page 29, is a faster relative of our May 1979 cover subject, Model 8160A.
Today's as fast digital integrated circuits are capable of switching between voltage levels in as little as a few
thousandths of a millionth of a second. Model 8161 A generates fast, accurate voltage pulses or staircases for
testing these circuits. Its output voltage level, pulse rate, and switching speed are completely programmable. It'll
be used circuits use production, and incoming inspection of fast integrated circuits and products that use them.
-R. P. Dol an
Editor, Richard F. Doian • Associate Editor. Kenneth A. Shaw • Art Director. Photographer, Arviu A. Danielson
Illustrator, Nancy S. Vanderbloom • Administrative Services, Typography, Anne S. LoPresti • European Production Manager, Dick Leeksma
H E W L E T T - P A C K A R D
1 9 8 1
H e w l e t t - P a c k a r d
© Copr. 1949-1998 Hewlett-Packard Co.
C o m p a n y
1 9 8 1
P r i n t e d
i n
U . S . A .
A Reliable, Accurate CO2 Analyzer
for Medical Use
Measuring the amount of carbon dioxide in a patient's
breath is an important medical diagnostic tool. This
instrument makes the measurement quickly and easily
without cumbersome calibration requirements.
by Rodney J. Solomon
CARBON DIOXIDE (CO2) is one byproduct of human
metabolism. Accumulation of this gas by the body
leads to a shift in intracellular acidity which is
incompatible with life. Elimination of COa occurs
mainly through blood transport to the lungs, although
some CÜ2 is eliminated through the kidney and a small
amount is passed through the skin. Being able to measure
the partial pressure of carbon dioxide (PCOa) in the
patient's inhaled and exhaled breath is important because
it allows the clinician to predict the arterial blood CÜ2
concentration and thus adequacy of ventilation (breathing).
The HP Model 47210A Capnometer shown in Fig. 1 is
designed to make these measurements easily and accu
rately without causing discomfort to the patient. The pa
tient's breath is inhaled and exhaled through an airway that
has an infrared source mounted on one side and an infrared
detector mounted on the opposite side. The instrument
measures the amount of infrared radiation absorbed by the
patient's breath to determine the partial pressure of CÜ2
present. This direct measurement is done quickly and
The new capnometer is very useful for monitoring pa
tients whose breathing is being done for them by a mechan
ical ventilator. Normally, someone who is breathing spon
taneously has a natural feedback system for controlling
blood PCO2 level. As the person's metabolic needs increase,
so does the respiration rate to maintain the blood PCO2 at a
constant value (usually around 40 mmHg for a normal per
son). For a patient on a mechanical ventilator, the physician
who adjusts the ventilation controls the level of CÜ2 in
the patient's blood. The physician may use the patient's
expired CÜ2 level instead of blood gas analysis for control
ling ventilation. The expired CÜ2 reading is immediate,
while with blood gas analysis, there may be a delay of a
dozen minutes or so.
Arterial blood concentration of CO2 is the primary
Fig. 1. The HP Model 4721 OA
Capnometer noninvasively mea
sures breath CO* content to pro
vide accurate monitoring of a pa
tient's breathing in operating, re
covery, and respiratory care
areas. Its accuracy, ease of use,
and simple calibration a/so make it
useful for pulmonary laboratory
© Copr. 1949-1998 Hewlett-Packard Co.
stimulus for respiratory control. That is, we are driven to
eliminate carbon dioxide, not to acquire oxygen. Monitor
ing the CÜ2 concentration in the exhaled breath and com
paring that value to the CCb concentration of a drawn arte
rial blood gas sample can be used to make a determination
of the ability of the lung to eliminate CÜ2. Additional diag
nostic information can be gleaned from an examination of
the CU2 partial-pressure waveform as function of time.1
Carbon-dioxide analyzers and capnography, the study of
the PCCh waveform, have been used since the early 1950s to
assess patient condition. These analyzers use a nondispersive infrared technique whereby all radiation from an emit
ter is passed through a gas sample and the energy absorbed
at the wavelength of interest is measured. In the infrared
radiation region a number of gases have absorption bands
(see box at right). Carbon dioxide has a strong absorption
band centered around a wavelength of 4.26 /am. The energy
absorption is primarily a function of COz concentration and
total radiation path length. There are other influences on
the total energy absorbed such as total gas pressure, tem
perature and presence of other gases.
Some Problems
CÜ2 analyzers have had a number of problems which can
make routine monitoring difficult. Most CCb analyzers op
erate on a sampling technique. A gas sample is drawn via a
capillary tube to the analyzer from the plumbing used to
connect the patient to a ventilator. This capillary tube is
prone to clogging from sputum and water vapor. In addi
tion, the instrument requires an air pump and flow reg
ulators, and contamination has to be removed from the gas
before it enters the sample chamber. The small passages and
traps in such a system can make it difficult to maintain
adequate cleanness. Instrument calibration, usually
needed quite frequently, is done by using gas from premixed cylinders. This is cumbersome, time-consuming and
subject to operator error.
+ 15V
14360A CO^Sensoj;
Processor Box
Digital Section
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 2. Block diagram of the
4721 OA Capnometer. The output
from the COz sensor is controlled,
amplified, and converted to digital
data by the analog section in the
processor box. This data is opti
cally coupled to the rest of the pro
cessor box to be analyzed and
Sapphire Windows
Fig. 3. Crass section of the airway adapter and sensor as
sembly. The sensor body contains a/I of the infrared compo
nents required to measure the infrared absorption of the pa
tient's breath passing through the airway adapter.
Design Goals
The fundamental approach of placing all the infrared
components at the patient end of the cable offers many
advantages over the sampling tube approach. The "on air
way" concept is not, however, without its share of prob
lems. The sensor must be small, rugged, lightweight, and
easily cleaned, and perhaps most important, must help
isolate the processor box from any high voltages caused by
the use of defibrillation equipment. It also should not be a
shock hazard for the patient. If a patient makes contact with
a piece of equipment such as a motorized bed that has a
defective ac line connection, a current can flow through the
patient if the patient is grounded. The goal is to assure that
the CÜ2 analyzer does not provide either the ac source or the
grounded path for the patient.
The airway adapter must be rugged and lightweight. It
must be sterilizable and the infrared path length must be
stable and consistent from unit to unit to minimize the total
system error. Mating the airway adapter to standard ventila
tion plumbing must be simple and reliable.
The variability of the infrared components in the sensor
must be compensated by the processor box. If the goal of a
simple calibration scheme is to be realized, the processor
box must be more than just a power supply for the sensor
and a simple analog-to-digital (A-to-D) converter. The ef
fects of interfering gases and total pressure variation (al
titude) on the CÜ2 measurement mean additional process-
The 47210A CQz analyzer was developed to alleviate the
problems with presently available instruments. In particular:
• The small sampling tube is a trouble spot and is avoided.
• The method of instrument calibration eliminates the
need for unwieldy gas cylinders.
• The stability of measurement is adequate to allow confi
dent use for extended periods of time.
• The instrument is easy to use in a monitoring application
because the complex compensation routines are per
formed by the instrument rather than by the operator.
The 47210A Capnometer provides the above features by
making some significant changes in the basic measurement
approach. A simplified block diagram of the 47210A is
shown in Fig. 2.
The 47210A consists of an airway adapter, a sensor, and a
processor box. The airway adapter, a hollow aluminum
casting with sapphire windows, is inserted in series with
the ventilator plumbing. The sensor is snapped over the
airway adapter windows (Fig. 3), and the measurement is
made directly on the artificial airway through which the
patient is breathing (Fig. 4). This sensor contains all the
optical components necessary to make the infrared mea
surement and is connected to the processor box by a cable
2.44 metres long. The processor box powers the sensor,
processes the return signal, and presents the data via LED
(light-emitting diode) displays. A simple, self-contained
calibration system attached to the processor box substitutes
a foolproof method fora previously difficult and potentially
error-inducing calibration procedure using premixed
gases. The microprocessor in the processor box performs
numerous tasks that greatly simplify the use of the system.
Compensations for interfering gases and total pressure are
performed by the processor rather than by the operator.
Fig. 4. The47210A Capnometer is connected to the gas to be
sampled for CÜ2 content by the 14361 A Airway Adapter. The
adapter is inserted in series with the ventilation plumbing as
© Copr. 1949-1998 Hewlett-Packard Co.
Cable Connector
Sensor Case Half
with Cable and
Wiring Connectors
Airway Adapter
ing is necessary if the instrument is to be beneficial in a
patient monitoring situation where ease of use is given a
high priority.
Sensor Design
Starting with the lead-selenide photoresistor in the detec
tor assembly, each part of the design interacts with the other
parts. No one parameter can be changed without affecting
others in the system. The design begins with the sensor.
The detector assembly, located on the opposite side of the
airway adapter from the source, has a rotating filter wheel, a
thin-film infrared bandpass filter and a lead-selenide
photoresistive detector arranged as shown in Fig. 5. The gas
sample is always in the infrared path. Modulation for drift
rejection is accomplished by the rotating filter wheel. The
filter wheel consists of two hermetically sealed cells with
sapphire windows, one open chamber with sapphire win
dows, and four permanent magnets. The magnets form the
rotor for a brushless dc motor (see box on page 8). Each cell
is rotated into the infrared energy beam 40 times per sec
ond. The output of the detector is shown in Fig. 6. The
output waveform is generated as the wheel successively
rotates into the infrared energy beam first one sealed cell,
then the open chamber, the second sealed cell, and finally
an opaque region for zero output. The conversion of this
waveform to a CCh value is discussed in the box on page 12.
To obtain adequate signal-to-noise ratio so that the
specified output noise of 0.5 mmHg rms can be achieved, a
3-mm-square photoresistor is used. Dark-current noise is
excessive if detectors smaller than this are used. This
photoresistor size defines the minimum aperture of the
rotating wheel cells that can be used and still provide an
adequate dwell period. This aperture controls the wheel
diameter and thus the overall detector assembly size. The
result is a roughly 20-mm-square by 10-mm-thick
beryllium-copper investment-cast housing for the detector
assembly. In addition to the filter wheel and its motor drive
coils, the housing also supports two thermistors: one for
temperature sensing and one for heating.
The infrared source is a heated broadband black-body
radiator. The heating element is a thin-film cermet resistor
Sensor Case Half
with Sapphire Windows
Fig. 5. Exploded view of the as
sembly for the 14360 A Sensor.
deposited on a 0.064-mm-thick sapphire substrate. Two
conductors are deposited over the resistor material such
that a square heater is defined in the center of the element
(Fig. 7a). The heating element is mounted in a twin-lead
TO-5 transistor package with a sapphire window in the top.
The element is supported on the two lead posts and faces
a collimating mirror mounted to the package's header
(Fig. 7b).
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 6. (a) Typical output waveform from the infrared detector
assembly, (b) Viewing the waveform from left to right, the
first peak VR is the output detected through the sealed
COz reference cell, the second peak VH is through the open
cell, and the last peak Vs is through the sealed nitrogen ceii.
Heating Element
Sapphire Window
its very lengthy curing cycle which can continue after the
sensor is in service. This makes this adhesive unusable
inside the sealed sensor because the detector assembly can
not differentiate CCfe in the sample chamber from any high
concentrations of CCh in the infrared path that may be
trapped inside the sensor.
Finding an adhesive that would adequately bond the
sapphire windows to the case material involved much trial
and error. The bond has to be strong and survive exposure to
sterilizing materials. A heat-cured epoxy preform is used.
Precision tooling and carefully controlled technique are
required because any voids in the adhesive will com
promise the high-voltage breakdown resistance so neces
sary in an instrument used near a patient who may be
subject to defibrillation.
The result is a sensor that survives drops from a height of
one metre onto concrete floors without damage. The sensor
has been dropped from as high as 2.4 metres. Damage was
confined to the outer case structure. This severely abused
sensor still measured CCh with no perceptible shift in
Airway Adapter
TO-5 Transistor
Fig. 7. (a) Cermet resistor heating element used for the in
frared source, (b) Infrared source assembly. The heating ele
ment is mounted face downward so that the mirror can focus
the infrared radiation.
The detector assembly and the source define the basic
dimensions and weight of the sensor. What about ruggedness, cleanability and voltage isolation? The design of the
outer case assembly, from the cable entrance down to the
sapphire windows, is an exercise in materials selection
and testing.
The case assembly consists of two halves (Fig. 5 and Fig.
8). The upper half of the assembly contains the cable and
interconnects, and the lower half aligns the infrared com
ponents. An O ring makes a watertight seal between the two
halves. The thermoplastic bumpers bonded to the corners of
the outer case provide necessary cushioning.
The case material is a glass-filled thermoplastic polyes
ter. This material has the solvent resistance and dielectric
strength required for the application. However, its solvent
resistance makes it difficult to find a suitable adhesive for
potting the cable, attaching the bumpers and sealing the
sapphire windows. Each application presents a different set
of requirements.
For example, the bumper-to-case bond has to be flexible
and still survive multiple exposures to liquid sterilizing
agents. A two-part polyurethane adhesive is used. But this
adhesive generates small amounts of carbon dioxide during
The sensor performs its job by passing infrared energy
through the airway adapter and measuring the amount ab
sorbed by any CCh in the airway. The airway adapter also
has a number of critical requirements. Sterilizability, a sta
ble infrared path length, and ruggedness dictate a series of
materials requirements. Measurement accuracy is related
directly to the infrared path length through the sample. Any
variation from the nominal 3-mm gap results in an error
proportional to the difference in the gap from the nominal
value. To achieve the required stability in view of the other
requirements, the airway adapter is made of aluminum. The
choice of an aluminum investment casting is dictated by the
detail necessary in the part. Grooves for the ball detents
used to secure the sensor assembly to the airway adapter,
the complex gas passageway with its mushroom-shaped
cross-section, and the mating ends of the airway adapter
precluded fabrication processes other than casting. Invest
ment casting is used because the number of units and the
(continued on page 9)
Fig. 8. Disassembled 14360A Sensor showing case halves
(right and left), infrared source (top center), and filter wheel
assembly (center).
© Copr. 1949-1998 Hewlett-Packard Co.
A Miniature Motor for the CO2 Sensor
(with Thanks to Kettering)
by Edwin B. Merrick
Into the midst of the tightly packed infrared components of the
sensor for the 4721 OA Capnometer it is necessary to insert a
highly reliable and efficient motor whose only purpose is to keep a
little cell-carrying wheel turning at a constant rate. Among the
rather stringent requirements are that the motor must not signifi
cantly increase the sensor size and cost. A sensor the size of a
brick of will not do. It is also desirable to minimize the number of
wires in the sensor cable, thereby keeping the cable small and
light. be patient's airway tubing is already sufficiently rigid to be
an encumbrance. The cable to the COi sensor should also not
add to the leverage applied to hose connections. Since the sensor
Optical Sync Track
is temperature controlled at as low a temperature as possible it is
necessary that the motor dissipate the least possible power.
Finally, whatever motor is developed must be agreeable to accu
rate speed control.
Various alternative motor and drive configurations were con
sidered and discarded. A dc brush motor? Brush noise, life and
the added commutator and winding space ruled it out, although
only requiring two leads was attractive. An ac induction motor?
The addition of a squirrel cage or suitable conductor to the simple
plastic filter wheel seemed to add size, weight or complexity when
the requirements of the infrared paths were considered. Effi
ciency was also doubtful since the usual approach is to apply high
magnetic field intensity to keep the rotor from slipping and provide
speed control. A step motor approach? Now that had some prom
ise since small permanent magnets could be added to the
perimeter of the filter wheel and interspersed with the four infrared
openings. This would have negligible effect on the size of the
wheel. Two drive coils could be positioned away from the infrared
detector on the same side of the wheel as the detector without
adding to the thickness of the assembly since the detector and
infrared filter already required a reasonable space (see Fig. 5 on
page 6). But how should it be driven?
An optical sync track applied to the edge of the wheel as shown
in Fig. 1 a was tried first. This was the only wheel surface that was
not otherwise occupied and could still be viewed. This track was
used to drive a solid-state commutator in the processor box which
then pulsed the drive coils at appropriate times in proper polarity.
Initially the drive coils always received the same high pulse
amplitude. This was inefficient. Thus, proportional control was
applied to the pulse amplitude, applying only the drive level
necessary to maintain the proper 2400 rpm. Although this motor
ran, in required several wires for the optical sync signal, it in
creased the size of the sensor somewhat, and introduced a re
quirement for a second axis of optical alignment to the wheel. Also
it seemed possible that efficiency might improve if just the right
waveform were applied rather than an arbitrary square pulse.
(continued on next page)
Three Wires to
Processor Box
Sense Coils
Fig. 1. (a) An optical sync track along the edge of the filterwheel rotor as shown was initially used to control motor speed,
(b) The sync track was eliminated in the final design by using
sense coils to detect motor speed. The physical arrangement
of the drive and sense coils is shown in Fig. 11 a on page 1 1 .
Edwin B. Merrick
"i Ed Merrick is a native of Andover,
^t^f *v Massachusetts and received the
flr ^k BSEE degree from the University of
New Hampshire in 1963. He came to
Mm -jf^ * ' HP that same year and is currently
an engineering manager at HP's
Hospital Supplies Operations in
Chelmsford, Massachusetts. Ed is
co-author of an earlier HP Journal
article on the 47201 A Oximeter and
a co-inventor on a patent for a blood
perfusión measurement. He is a
member of the American Associa
tion for Medical Instrumentation. Ed
is married, has three children, and
supports a menagerie consisting of a dog, three cats, four pigs,
and a rabbit. He lives in Stow, Massachusetts where he is restor
ing a 200-year-old house and barn and has built a hydraulic log
splitter to supply firewood for wood heating. During the summer
Ed enjoys sailing his 32-foot sloop along the New England coast.
© Copr. 1949-1998 Hewlett-Packard Co.
Enter Kettering.
In describing his life's work as an inventor Kettering referred to
his work on the development of a piston for diesel locomotive
engines. He said the problem was not so much knowing when you
had a bad piston because the engine would tell you that soon
enough. The trick was to simply let the engine tell you which
pistons it liked best. In this case we let the motor show us the
proper drive waveform. Simply adding a pair of sense coils in the
proper location and a simple integrator in the processor box
produces a properly phased and shaped waveform with which to
power the drive coils (see Fig. ib). Speed control is achieved
by comparing the period of the sense-coil waveform with a oneshot time reference. The speed is increased by increasing the
gain gain. the drive amplifier and decreased by decreasing the gain.
The result is a motor that requires only three wires, uses inex
pensive components, is easily speed-controlled, does not in
crease the size or complexity of the sensor, and runs on less than
50 mW.
detail required make die casting undesirable.
The airway adapter has two sapphire windows that are
epoxy bonded to each side of the gas passage. The gap
between the windows forms the precise path length for the
gas sample. A tolerance of ±13 /urn was deemed necessary
and possible. This small variation alone can add approxi
mately ±0.4 mmHg error to a 100-mmHg PCCh measure
ment. The gap is set to the desired value during assembly by
placing a shim of the correct thickness between the two
windows (Fig. 9). By firmly clamping the windows to the
shim while the epoxy bond cures, the gap is formed.
Processor Box
The diversity of disciplines and requirements for the
sensor's mechanical design is matched by the diversity of
design challenges presented by the processor box, which
performs the following functions:
• Sensor motor control
• Sensor temperature control
Spring Plunger
of Airway Adapter
Aluminum Casting
Epoxy Preform
Fig. 9. To insure the proper gap between the two sapphire
windows in the airway adapter during the curing of the epoxy
adhesive they are held against a shim whose thickness is
equal to the desired gap. The shim is then removed.
m Sensor infrared source supply
• Sensor infrared detector amplifier
• Calibration of sensor
• PCCh computation
• Altitude correction
• Interfering gas correction
• Sensor internal PCCh correction
• End-tidal (peak) detection
• Respiration rate computation, and
• Various display and system interface tasks.
The first four functions are performed in the electrically
isolated portion of the processor box. The sensor interface is
isolated from chassis ground as a second line of defense
against microshock hazard to the patient (the plastic sensor
case being the first). A dangerous situation can occur if the
sensor cable shield or internal conductors become exposed
(continued on page 1 1 )
Fig. 10. Simplified schematic of
the filter-wheel motor speed con
trol circuit.
© Copr. 1949-1998 Hewlett-Packard Co.
Fabrication of the Sensor Requires Special Care
Because of the small size of the sensor, normal assembly
techniques are difficult to use. Assembling the bearing used to
support the filter wheel is an example. The constraints are to
produce a low-friction, low-noise bearing that will survive the
rigors of being mounted in a sensor that will be subjected to
severe handling. It is extremely important to maintain the location
of the the axially and radially. If there is too much variation in the
position of the wheel as sensor orientation is changed, the output
will the affected. To maintain the orientation-induced error of the
sensor within acceptable levels, the radial play in the two bear
ings, one on each side of the wheel, is held to less than 10 /xm.
Axial play is held to less than 50 yu.m.
The normal approach for building an assembly such as this is to
take the two raw castings, assemble them, and pin the two cast
ings so that they can be disassembled and reassembled in
exactly the same relative location. The bearing bores are then
machined together. The castings are disassembled, the bearings
inserted, the filter wheel is positioned in place and the castings
are reassembled. This procedure has a number of drawbacks.
First, there is insufficient room for pins. Second, because of the
inherent lack of flatness and uniformity of the castings, reassem
bling the castings so that the bores will still be in line cannot be
assured. Also, for the above technique to work, the bearing's
outside and inside diameters must be held to tolerances tight
enough that the bores in the bearings will line up when the casting
holes do. This is not possible when sintered bronze bushings are
used. Sintered bronze bushings were selected instead of ball
bearings for the sensor because they are more rugged and quiet
The sotution is to mount the unoiled bushings in the castings
with to modified acrylic anaerobic adhesive. No pins are used to
align the two castings. The assembly process requires two sepa
rate fixtures that are illustrated in Fig. 1. The first takes the two
casting pieces, with the wheel and shaft captive in the bearing
bore, and aligns the bearing bores. The bearings are not yet in
place. The four screws holding the castings together are then
tightened. The second fixture is used to install the two bearings in
the castings on either side of the filter wheel. This fixture holds the
wheel, bearings, and casting so that the correct axial play is
maintained and the bearings are aligned, but the radial clearance
between the shaft and the bearing is not reduced.
The problem that must be addressed is that the shaft may be
bowed slightly; thus the filter wheel may not turn freely throughout
its full 360 degrees of rotation. By allowing the bearings in the
castings to be self aligning with respect to the shaft and biasing
the wheel and shaft to one side with a specified side load during
the adhesive curing cycle, the radial clearance is established and
alignment between the shaft and bearings is assured. The shaft
and bearings are supported by spring-loaded pins. By pressing
the bronze bearings firmly against the two jeweled ringstone
thrust and on the filter wheel and elastically deforming and
holding the casting during the cure cycle the proper end play is
established when the casting returns to its unstressed state upon
removal from the fixture.
Other portions of the sensor are designed to use fixturing as an
integral part of the manufacturing process. Fixtures are used to
locate the infrared detector and the four motor coils on the printed
circuit board during assembly. The upper and lower case assem
bly technique also uses fixtures to align components such as the
sapphire windows during the bonding phase.
The sensor requires two stable gas mixtures, each hermetically
sealed in a transparent cell for use in the filter wheel. One cell has
to contain an accurately determined, stable reference concentra
tion of COz, the other has to contain only nitrogen. The fabrication
problem is two-fold. The first is how to construct the cell such that
acceptable hermeticity can be maintained, and the second is how
to fill and then seal the cell with an accurately determined con
centration of C02 inside. The approach chosen bonds a sapphire
window to each end of a metal cylinder. After a number of trials,
the window-to-ring bonding method selected is a glass
frit seal to a Kovarâ„¢ metal ring. Other methods such as epoxy
adhesives or brazing operations were either too permeable, thus
allowing the gas concentration to change, or too expensive.
(continued on next page)
Bearing Bores
Fixture Frame
Load to Hold Bearings
Against Wheel
Load to Deflect
Casting 0.051 mm
Side Load on Wheel to
Locate Bearings Radially
with Respect to Shaft
Fig. 1. (a) This fixture design aligns the bearing bores in the
detector assembly before the four screws holding the assem
bly together are tightened, (b) This fixture design aligns and
loads the filter wheel shaft and bearings in the detector case
assembly while the adhesive securing the bearings cures.
© Copr. 1949-1998 Hewlett-Packard Co.
Filling the cell and then sealing it is the next challenge. Using a
filling tube brazed to the metal ring and closed by crimping was
cumbersome and expensive. A radial hole located directly in the
wall of the ring is the approach used. The filling and sealing
process is done by inserting a solder preform loosely in the hole
and then placing many filter cells in a sealing vessel. The sealing
vessel is first evacuated and baked to remove residual gas con
tamination. Next, the vessel is backfilled with the appropriate COz
concentration and sealed. The temperature is then elevated past
the solder melting point. The solder flows, sealing the hole. The
vessel is then allowed to cool slowly. The cells are removed from
the vessel and measured to assure that the CQz concentration
contained in each cell is within acceptable limits before they are
mounted in the filter wheel.
and the patient contacts the exposed wires. If at the same
time the patient contacts a current source such as a defec
tive, of operated hospital bed, this second line of
defense prevents more than 100 microamperes of current
from flowing through the patient to earth ground via the
Accurate speed control and good dynamic response are
necessary for the control of a somewhat unusual motor (see
box on page 8). Because the infrared beam is modulated by
the rotating filter wheel, it is necessary to sample the in
frared signal at just the right time.
The periods when the infrared signal rises and falls are
contaminated with light rays that have experienced multi
ple reflections from the sidewalls and edges of the many
infrared components. This situation results in an error of up
to one mmHg of PCOa for each 50 /us of sampling time
change. In addition, 50 and 60-Hz power-line fields can add
to the signal noise if the sampling rate of the infrared signals
is not controlled. The importance of the dynamic response
of the speed control will be appreciated if it is remembered
that the patient's respiratory efforts inpart motion to the
ventilator plumbing and thus the sensor. This motion can
momentarily rotate the sensor housing slightly with respect
to the filter wheel, causing an apparent change in its rota
tional velocity.
The circuit diagram in Fig. 10 shows the motor speed
control. Amplifier Al and capacitor Cl form an integrator
whose output is proportional to the flux change in the sense
coils produced by the four moving magnets on the motor's
rotor (the filter wheel). Note that the output amplitude is not
a function of rotational velocity. Resistors Rl and R2 form
an attenuator which provides a low-level signal to Ql, a
FET used as a controlled-resistor element. Amplifier A2
supplies the motor drive coils. Amplifier A3 and capacitor
C2, also connected as an integrator, are responsible for
keeping the dc output of A2 at zero volts. Four times each
revolution a zero crossing of magnetic flux is detected by
comparator A4, which then triggers Tl. The average value
of Tl's output is compared to a reference voltage set by R8.
Any difference between these two voltages is integrated by
A5 and, through a sample-and-hold circuit Si, the result is
applied to the gate of Ql. Thus, if Tl is triggered too often
(indicating a high speed) the gate of Ql will be adjusted in a
negative direction, thereby reducing the drive applied to
the motor. Network C4-R7 compensates for the inertia of the
motor's rotor.
The temperature of the detector assembly is another criti
cal parameter that must be controlled accurately. Of all the
infrared elements, the interference filter is the most sensi
tive. Small changes in temperature cause the filter's center
wavelength to change sufficiently to alter the CCh mea
surement. A straightforward thermistor-amplifier-heater
circuit, heavily compensated for thermal lag, is used.
One feature of the temperature control is the placement of
the temperature sensor (a thermistor) and the heater relative
to the infrared filter (see Fig. 11). In a typical analog heater
control, variable gradients occur between the heated area
and the thermistor in response to thermal load. As heat is
removed from the system, the thermistor will cool below a
desired temperature and the heater will be turned on. Be
cause the system has finite gain, the area near the heater will
get much warmer due to the increased heat input while the
thermistor is still cooling. In this particular case, with the
filter between the thermistor and the heater, a system gain is
chosen so that temperature changes at the filter are mini
mal, providing the filter with optimal temperature control
and therefore minimizing the temperature sensitivity of the
(continued on page 13)
Infrared Bandpass Filter
(Sensitive Component)
Fig. 11. (a) Photograph of the infrared detector assembly
showing the filter wheel and infrared filter on the left and the
printed circuit board with the motor coils and infrared detector
on the right, (b) Infrared filter temperature variations are
minimized by locating the thermistor and heater for the filter's
temperature control on opposite sides of the filter as shown.
© Copr. 1949-1998 Hewlett-Packard Co.
An End-Tidal/Respiration-Rate Algorithm
by John J. Krieger
One of the features of the 4721 OA Capnometer, when used with
an airway adapter, is the generation of derived parameters: respi
ration rate, end-tidal PCO2, and inspiratory minimum PCOa. The
main purpose of the capnometer is to measure a carbon-dioxide
partial-pressure waveform accurately and precisely. The
waveform itself is useful for diagnostic purposes, but most cap
nometer users are interested in the parameters derived from the
PCOs waveform. In the 472 1 0A the microprocessor controlling the
capnometer also analyzes the PCCte waveform and derives these
desired parameters.
Simply speaking, an airway PCÜ2 waveform looks like a dis
torted square wave (see Fig. 1). The waveform increases to a
peak value at the end of each breath cycle; this is called the
end-tidal (ETCOa) value. In patients that do not have chronic
obstructive lung disease, the ETCOa value very closely approxi
mates the PCÜ2 level in the arterial blood.
Some CÜ2 analyzers on the market attempt to derive an ETCÜ2
value from a PCÜ2 waveform by using analog circuitry and per
forming a simple peak-finding function. This may work fine for
normal waveforms, but most real PCÜ2 waveforms contain noise
and waveform artifacts caused by the patient coughing or moving
(or crying in the case of children) or by ventilator waveform distor
tion. For exam pie, amplitude variations can be caused by intermit
tent mandatory ventilation (IMV). IMV is used where patients usu
ally can breathe on their own, but occasionally need help by
forced ventilator breathing. Under these conditions a simple,
analog peak-finder does not perform very well in deriving accu
rate ETCÜ2 values.
To develop the end-tidal/respiration-rate algorithm for the
47210A Capnometer, many field trials were performed at several
hospitals internationally. At these trials, PCÜ2 waveforms were
recorded on magnetic tape from many patients with a variety of
diseases and a variety of ventilators — including operating room
ventilators. The dozens of hours of recordings were digitized and
stored on a computer disc file for easy random access. An al
gorithm development system was programmed on an HP Model
9845B Desktop Computer with CRT graphics. A proposed endtidal algorithm was entered into the 9845B. Then, a variety of real
patient PCO2 waveforms was retrieved from the disc file to test the
proposed algorithm. The waveform and the derived parameters
were displayed on the CRT screen. The algorithm was then
evaluated for effectiveness and modified to achieve best perfor
mance. Once the optimum algorithm was achieved, it was coded
into microprocessor assembly language for use in the 47210A
The end-tidal/respiration-rate algorithm has several require
ments. Not only does it have to find the minimum and maximum
PCÜ2 and the period between breaths, but it also must recover
from all possible error conditions. The error conditions may be
caused by a variety of sources: initial instrument warmup tran
sients, pseudo-CO2 waveform glitches caused by electromagnetic
interference (EMI) from electrosurgery devices, changing the
CÜ2 sensor from one patient to another, patient breathing artifacts
(coughing, crying, sighing, long periods between breaths, or
resisting the ventilator), and ventilator artifacts caused by poppet
valves, tubing elasticity, trapped water, pulsation, et cetera. Nor
mal people have ETCO2 values of about 40 mmHg. Patients with
chronic obstructive lung disease may have ETCÜ2 values in ex
cess of 90 mmHg!
The problem of extracting the ETCO2 from a patient on a ven
tilator under intermittent mandatory ventilation proved to be dif
ficult. In that situation, the question becomes "which waveform
peak is the ETC02?" An IMV PCO2 waveform ensemble contains
several ETCOs peaks of varying amplitude. The forced ventilation
breath usually produces the largest ETCO2 value and the several
intermediate and weaker voluntary breaths produce ETCOa val
ues of decreasing amplitude. This is probably because the forced
breath is effective and the shallow, voluntary breath is less effec
tive tubing represents the rebreathing of a larger fraction of tubing
dead-space. In the transition stages when a patient on IMV is
getting stronger, the patient's voluntary breath ETC02 values are
about the same as the forced breath ETCO2 values.
Sometimes the PCO2 waveform is meaningless and no reason
able made. or respiration-rate derivation can or should be made.
In this case, an error message is displayed on the 4721 OA front
panel data. indicate something is wrong with the quality of the data.
This can alert the nurse or physician that a patient or ventilator
problem exists.
The final algorithm has some resemblance to its analog pre
decessors in that it is the digital equivalent of a low-pass filter. A
low-pass-filtered waveform is used as an adaptive threshold for
determining when a maximum (end-tidal) or minimum (inspiratory
minimum) PCO2 value has occurred. The digital algorithm then
makes waveform feature decisions based on time window
criteria. The time windows are also adaptive and are updated
based filtering the patient's past breathing history. This type of filtering
in both the time and frequency domains is very difficult to. do with
pure analog circuitry.
Actually, two low-pass-filtered threshold values are computed.
The mean PCO2 value is the low- pass-filtered result of the instan
taneous PC02 waveform and has a time constant of about 60
seconds. The peak-to-peak index is a measure of the deviations
of the instantaneous PCOa waveform from the mean PC02 value.
This second value is analogous to the amount of ripple in the
output of a full-wave rectifier circuit and also has a time constant of
about 60 seconds. The peak-to-peak index is scaled down and
MediumEnd-Tidal PCO2
Instantaneous PCO2
Low-Pass *•Filter—.
Fig. 1. (a) Partial-pressure CÜ2 waveform. Each pu/se cor
responds to one breath by the patient. The capnometer
software algorithm derives the medium-to-high (b) and lowto-medium (c) waveforms to determine respiration rate and
eliminate artifacts.
© Copr. 1949-1998 Hewlett-Packard Co.
used to create a hysteresis band about the mean PCÜ2 value. Two
threshold levels are derived as follows:
Threshold (HI) = mean + (peak-to-peak index)/4
Threshold (MED) = mean - (peak-to-peak index) 4
The instantaneous waveform is divided into three sections: tow,
medium, and high (see Fig. 1). By counting the transitions from
one region to another, the basic breath cycles can be determined .
The time interval required to complete a cycle (low-mediumhigh-medium-low-medium) determines the respiration interval.
Using the three-section approach helps reduce false end-tidal
values ad by artifacts of a patient resisting a ventilator ad
justed for intermittent positive pressure ventilation (IPPV), IMV, or
other causes. A window of minimum time in the high and low
regions is computed from the respiration rate. Transitions that
occur in less than the minimum window time are defined as un
desirable artifacts in the PCOz waveform. Detection of such a
result is displayed by an error message on the front panel. This
error condition is cleared as soon as a good, artifact-free wave
form is detected.
Once the PCOz waveform is judged to be in the high region, the
end-tidal value is selected to be the highest quick-average value
within that region before the PCOa value drops to the medium or
low regions. This quick-averaging width is proportional to the time
window, which is computed from the respiration-rate information.
The determination of the inspiratory minimum value is identical
to that of determining the end-tidal value except that the point of
interest, of course, is the minimum value in the low region just
Since the relative magnitudes of the alternating infrared
signals are compared by the digital processing that follows,
it is not important to maintain a constant infrared source
output. A simple dropping-resistor scheme adequately con
trols the power delivered to the source element. With the
dropping resistor equal to the nominal infrared source re
sistance, changes of 2 to 1 in source resistance caused by
component variation and aging produce only an 11%
change in source power. This is more than adequate to
maintain sufficient signal-to-noise ratio and source
To A-to-D
Current Source
Fig. 12. Biasing and amplification circuit for the infrared
before the PCOi value changes to the medium region.
Some averaging of breath intervals is done to obtain a less
erratic respiration-rate display. Moving averaging is done on the
last six breath-to-breath intervals. All six breaths are equally
weighted. After an apnea (that is, no breath cycles for more than
30 seconds), a break in breathing is detected and an alarm is
initiated. When breathing resumes, the algorithm accumulates
good breaths as they come in and averages those accumulated
until there are six new values and then the algorithm resumes
normal averaging.
John J. Krieger
John Krieger is a native of Santa
Monica, California and attended the
University of California at Los
Angeles (UCLA) where he received
the BSEE and MSEE degrees in
electronics and biomedical en
gineering in 1974. John then joined
HP and was a development en
gineer for the 4721 OA Capnometer.
He recently left HP and now lives in
Goleta, California with his wife and
daughter. John is a member of the
Society of Motion Picture and Tele
vision Engineers and enjoys scuba
diving, skiing, Chinese cooking, and
The infrared detector used in the sensor is a lead-selenide
photoconductor with a nominal dark resistance of 400 kii.
This resistance varies about ±50% from device to device.
However, the infrared energy reaching this detector causes
only about 0. 1 5% change in its resistance. Since this change
in resistance must be resolved to at least 12-bit precision, it
is apparent that quiet, accurate bias and amplifier circuits
are required. Fig. 12 shows a circuit diagram of how these
requirements are met.
Resistor Rl supplies 100 /u.A to the summing junction of
amplifier Al. Under dark conditions (no incident infrared
radiation), the bias voltage applied to one end of the detec
tor is adjusted to remove the 100 fj.A supplied by Rl. Any
change in the detector current flows through R2, provided
of course that Al has enough gain to keep its summing
junction at zero volts. Since infrared energy causes a de
crease of the detector's resistance, Al's output is positivegoing for increasing infrared intensity. Once each filter
wheel revolution, an autozero control circuit causes FET
switch Ql to close for 600 /xs. Any output voltage from Al
causes a current in R3 which, via feedback around A2, is
forced to flow in capacitor Cl. Therefore, after each revolu
tion the charge on Cl is updated to cancel the error seen in
Al's output. A2's output is applied to the control input of a
current generator I which is adjusted each wheel revolution
to make the detector current approach the current in Rl.
Capacitor C2 has two important functions. First, it filters
out any noise produced by the active current source I. Sec
ond, C2 appears to the detector to be a voltage source during
the short time required for one wheel revolution. Thus all of
the small-signal detection-current changes flow into Al's
summing node.
With the five functions of motor control, temperature
© Copr. 1949-1998 Hewlett-Packard Co.
From Isolated
Analog Section
780 System
Interrupt Line
Strobe Lines
Fig. the 3. Block diagram of the digital electronics section in the
processor box of the 47210A.
control, source power control, detector bias and amplifica
tion accomplished, the sensor output is suitable for conver
sion to digital form for further processing. The sensor's
analog output waveform is prescaled by one of four possible
gains in a preamplifier. The gain switching is done via
control from the microprocessor and allows the analog-todigital (A-to-D) converter to function as a fixed-point 14-bit
A-to-D converter, thus minimizing unused bits. This pre
scaled waveform is converted to four sampled values cor
responding to the three filter-wheel-cell readings plus one
dark reading. The triple-slope A-to-D converter is triggered
to sample the appropriate 248-/us-wide pulse of the infrared
output waveform by the motor sense-coil signal. The four
14-bit words are transmitted serially to the grounded elec
tronics portion of the processor box by an optical coupler
(Fig 2).
various system components to perform several tasks neces
sary to convert the digital data into PCOz and other derived
parameters. A main program (see Fig. 14) performs most of
the major tasks.
When electrical power is first turned on, each of the MPU
system components is tested for functionality. A checksum
test is performed on each of the three program ROMs. If a
ROM fails this test, a message indicating its printed circuit
board location is displayed on the capnometer's front panel.
The volatile RAMs are tested by a checkerboard RAM test
pattern. The EAROM, the programmable interface adapter
(PIA), and the keyboard/display scanner devices are also
tested by appropriate algorithms. Again, if any device fails
its test, its printed circuit board location is shown on the
display. If nothing is defective, the main program continues
by initializing all the RAM data structures and I/O devices.
The main program then awaits the data-available signal flag
before continuing.
This wait loop can be interrupted by hardware when a
high-priority, real-time event (such as when an A-to-D con
version is completed) needs attention. When an I/O device
(such as an A-to-D converter) requests service, the main
program is stopped and its state is stored so that it can be
restarted later when the cause of the interrupt has been
serviced. The MPU then vectors to an interrupt service
routine that identifies the source of the interrupt and per
forms the appropriate action. The interrupt may be due to
one of several causes (see Fig. 15).
The non-maskable interrupt (NMI) is reserved for instru
ment servicing. In this case, a test loop generates a stable,
synchronous digital pattern needed for digital signature
analysis testing. This makes it possible to test field failures
down to the component level.
The maskable interrupt (IRQ) can be activated by the
A-to-D converter, the 100-Hz real-time clock, or the
keyboard/display scanner chip. When the IRQ is activated,
its service routine polls the I/O devices to determine the
cause of the interrupt and then the appropriate I/O device is
Main Program
Power On
Digital Processing
The 472 10 A Capnometer's processor box contains mi
croprocessor electronics to perform all the magic needed to
convert the stream of digital data from the sensor to mean
ingful physiological data. The digital electronics (Fig. 13)
consists of a microprocessor, volatile random-access mem
ory (RAM), 12K bytes of program read-only-memory
(ROM), a nonvolatile electrically alterable ROM (EAROM),
input logic, and logic to output information to the user. The
output information consists of front-panel numeric dis
plays for CÜ2 and respiration rate parameters, front-panel
light-emitting-diode (LED) annunicators such as alarm
lights, rear-panel analog outputs driven by a digital-toanalog (D-to-A) converter, and other rear-panel output sig
nals which are used when the capnometer is connected to
an arrangement of other bedside monitors.
The microprocessor (MPU) receives command sequence
information from the program ROM. The MPU activates its
© Copr. 1949-1998 Hewlett-Packard Co.
Output To
Output to
LEOs and
End-Tidal and
Fig. 14. Flow chart of the main control program for the cap
Interrupt Service Routine
Altitude Correction
The total pressure effect on the measurement is handled by
the instrument and does not burden the operator during
use. The instrument is programmed to compensate for the
line-broadening effect on the measurement caused by dif
ferences in total gas pressure. At elevations other than sea
level, the instrument will remain calibrated when properly
set via internal rotary switches.
It is desirable to enter altitude instead of average total
pressure for instrument calibration. This makes the initial
instrument setup easier. The equation relating total pres
sure to elevation is
Read A-to-D
Set Data
box on page 16). Algorithms that perform the com pensation
routines for these errors are incorporated into the processor
box firmware.
(-3.394X \
Fig. 15. Flow chart of the subroutine used to determine the
source of an interrupt.
serviced. If the A-to-D converter is the cause, a digital value
and status bits are collected. The data-available flag is set by
the A-to-D service routine only when all the elements of the
A-to-D message have been collected from four revolutions
of the transducer's filter wheel. The real-time clock inter
rupt is used for time-related computations, such as convert
ing a CCh waveform breath-to-breath interval to respiration
rate (breaths per minute). The keyboard/display scanner
chip triggers an interrupt whenever a front-panel switch is
pressed. This is important because the user may wish to
change the operational state of the capnometer and a switch
interrupt avoids the need to poll all switches constantly.
When the main program receives the data-available sig
nal, it processes the packet of digital data. This data, along
with transducer calibration information previously stored
in the EAROM, is transformed into partial pressure of
CCh. This is made possible by a series of mathematical
functions that include multiply, divide, square root, and
The basic CCh calculation described in the box on page 1 2
is only the next step leading to the PCCh and respiration rate
display. After the PCCh is calculated, corrections must be
applied to compensate for the various interfering parameters.
As was mentioned earlier, the sensor is influenced by
effects other than just the partial pressure of CCh in the
sample. Total pressure (altitude) as well as the presence of
gases such as oxygen, water vapor, and nitrous oxide also
affect the amount of infrared absorption. The effect of al
titude was determined empirically on the static station de
scribed in the box on page 19. This was done by measuring
the sensor output for different total pressures while main
taining a constant partial pressure of CCh. The static station
was also used to measure the effect of interfering gases (see
where BAR PRESS is the average barometric pressure in
mmHg and x is altitude in hectometres. A lookup table is
used to store fourteen constants relating sensor error to the
PCCh measured at each of the 50 possible altitude settings (0
thru 49 hectometres). When an altitude setting is entered,
the 14 corresponding constants are used to generate a
piecewise-linear approximation of the appropriate correc
tion curve. A linear interpolation is done between adjacent
points every 100 ms (at each PCCh calculation) and a correc
tion constant (ALTK) is determined. The actual PCCh is
calculated from the following relationship:
PCCh measured
Interfering Gas Compensation
The final correction applied to the PCCh computation
compensates for the errors caused by interfering gases (see
box on page 16). Since the instrument is calibrated on a
binary carbon dioxide and nitrogen mixture, the question is
how to allow the operator to reduce the effect of the interfer
ence to acceptable limits and not compromise ease of use. It
seems appropriate to allow the microprocessor to perform
the correction computation, so the problem becomes one of
simplifying the data entry. Three assumptions were made:
1. The expired gas is saturated with water vapor at 33°C.
2. The expired oxygen partial pressure (PCh) is related to
the inspired pressure by the following:
expired ~
and the minimum inspired PCh is 21%.
3. When nitrous oxide is used, it is administered in a binary
mixture with oxygen in concentrations from 0% to 65%.
Three pushbutton switches on the front panel are used to
implement the nitrous oxide and oxygen corrections. The
first is for nitrous oxide compensation, the second is for
oxygen concentrations between 21% and 50%, and the third
pushbutton is for oxygen concentrations greater than 50%
(continued on page 18)
© Copr. 1949-1998 Hewlett-Packard Co.
In-Service CO2 Sensor Calibration
by Russell A. Parker and Rodney J. Solomon
The success of the 4721 OA Capnometer is very much depen
dent on the ease of use of the 1 4360A Sensor. The ideal situation
is achieved with a completely interchangeable sensor. However,
because of differences in interference filters, zero and reference
cell each filling, infrared alignment and some other variables, each
sensor, when used with a different 4721 OA, must be calibrated
against a gas standard to achieve the desired measurement
accuracy. This standard is the gas-cell calibration stick. Its valid
ity is based on a major assumption that, within limits, all sensors
will look alike to the processing algorithm in the 4721 OA after
calibration on the zero and 55 mmHg gas cells. The detector
signal must therefore be processed in such a way as to generate a
standard response curve, the values of which relate in some
known way to the partial pressure of carbon dioxide (PCOz).
The infrared system requires that the signal always pass
through the sample. To avoid errors caused by source and detec
tor output changes, signal path blockage, etc., the detector's
output (VR) while a reference gas cell filled with approximately
1 60 mmHg of COa is in series with the sample is compared to the
detector's output (VS) when a zero cell containing no CO? is in
series with the sample. These cells are part of the filter wheel
assembly in the sensor. The ratio VR:VS is called Q. As CC>2 gas
fills the sample volume, VS decreases more rapidly than VR. Q,
therefore, increases (Fig. 1).
The capnometer sensor uses an infrared bandpass filter in
series with the infrared beam. The passband of this filter is of
sufficient width to allow the energy not absorbed by many of the
discrete lines of the infrared absorption band to pass through to
the detector (see box on page 4). At the concentrations of interest
and given the sample path length, the relationship between Q and
PCÜ2 will not be a simple exponential as is described by Beer's
Law. If Beer's Law held, Q would be constant regardless of the
sample concentration, and the series path scheme used in the
capnometer would not work.
The task then is to find an expression for Q versus PCÜ2 which,
when response at only two points, will still represent the response
for any given sensor within allowable tolerances. To accumulate
the needed data, special sensors were designed and built to
allow rapid changing of each infrared component. A matrix of
experiments was performed on the static station (see box on page
19) with infrared components at tolerance extremes. After
a number of trials, the form of the equation used to relate Q to
PCO2 is:
1 0 0
P C 0 2
Fig. 1 . The relative amplitudes of the VR and VS portions of
the infrared detector output waveform (Fig. 6 on page 6) vary
with PCÜ2 level as shown. Q = VR/VS.
differences between sensor responses by compensating for the
actual QO and Q55 values.
A general multiparameter least-squares curve-fitting routine1
was used to relate S to Q. The goal was not to linearize S. but
rather to minimize the differences between various sensors for S
versus PCÜ2 (see Fig. 2). The routine was allowed to determine
the coefficients A, B and C for optimum similarity of S at each value
of E used. Weighting factors were employed to force curve S to
Nominal Sensor
where PCOa is a function of S. This function is a second-degree
polynomial where Q is the Q at a given PCOa from the data set, E is
a variable incremented by 0.5 from 0.5 to 4.5, Q0 and Q55 are the
sensor Q values at 0 and 55 mmHg CO?, and D is found from the
following equation:
D = AQ0 +
+ C
where A, B, and C are variables derived by a curve-fitting
routine. D is a modifier of Q that attempts to remove some of the
* Beer's Law states that the absorption of light by a solution changes exponentially with
the concentration of the solution if no other factors change at the same time.
5 5
1 0 0
P C O 2 ( m m H g )
Rg. 2. Plots of S versus PCÜ2 level for various sensors.
© Copr. 1949-1998 Hewlett-Packard Co.
Measured PCO2 (mmHg)
Rg. 3. P/ois oÃ- the error in the uncompensated PCOz value
caused by the presence of various nitrous oxide and oxygen
concentrations. PO2 is the inspired Oz pressure in mmHg and the
balance of the gas mixture is NzO for each curve.
remain in a region where, essentially, the data has the greatest
slope between points. One unacceptable solution to coefficients
A, B.Cand E forces S to be zero at all points. Simi lar for all sensors
yes, but this value of S can hardly be configured by a seconddegree polynominal to yield PCOa. A further constraint is that COz
can be generated inside the sensor for a variety of reasons.
Therefore, the data for all sensors was shifted to simulate 5, 10,
and 15 mmHg of internal CÜ2.
The result is that interference filter variations can be effectively
eliminated by the "Q-D" equation. The internal COa problem is
minimized by selecting the proper value of E, its most sensitive
variable, and finding A, B and C for optimum similarity.
2 0
4 0
6 0
8 0
1 0 0
Measured PCO2 (mmHg)
Fig. 4. Plots of the error in the uncompensated PCOz value
caused by interference from various oxygen concentrations.
80 100 120 140 160
Measured PCO2(mmHg)
Rg. 5. Plots of the error in the uncompensated PCO2 value
caused by the presence of various water vapor pressures.
Finding a value for E that minimizes the effect of internal CO2 is
only not the solution. This internal CC>2 concentration is not stable,
because it varies with time and sensor temperature. To eliminate
the effect of varying COa in the sensor, a third hole is used in the
filter wheel. This third hole is an open chamber that is used as an
alternate VS cell to measure the detector's output VH not only
as a function of COa in the sample chamber but also as a func
tion of CO? inside the sensor. A new Q value, labeled U, is cal
culated from the VR:VH ratio. The normal Q, using the sealed
cell, and U, using the open chamber, are compared to yield
information about the concentration of CO2 inside the sensor.
During calibration a U0 value and a U55 value as well as the Q0 and
Q55 values are stored in an electrically alterable read-only
memory (EAROM) in the 4721 OA.
The sensor housing PC02 index PH is the difference between
the PCO2 calculated from the Q channel and that calculated from
the U channel. It is assumed, due to the integrity of the sensor
sealing, that the change in internal COa will be slow. Therefore,
while Q and U are calculated by the 4721 OA every 100ms, PH is
heavily filtered to provide a stable internal COs reading. If PH is
greater than 0.5 mmHg C02, the CO2 values from both the Q and
the U readings are used to solve a quadratic equation for the
actual sample PCOa. If PH is greater than 2 mmHg, an error code
is displayed by the 4721 OA indicating recalibration is necessary
to rezero the Q and the U readings. This entire scheme compen
sates if changes in either direction. Namely, the system works if
the sensor is calibrated with COa inside and drifts low afterwards
or if the sensor is calibrated and then CO2 accumulates inside.
Two coefficient sets (A, B, C) are used for Q, one for PCOa less
than cali calibration point and one for PCO2 greater than the cali
bration point. They are substantially different, and optimize the perfor
mance throughout the O-to-100-mmHg range. Two sets of coeffi
cients for U are also used. The remaining major variable, refer
ence toler filling, need be controlled only within moderate toler
ances during manufacture.
© Copr. 1949-1998 Hewlett-Packard Co.
The final task is to characterize the sensor response to interfer
ing gases. Gases such as water vapor, oxygen, and the anesthet
ic nitrous oxide, can be present in the gas sample to be mea
sured. The sensor is calibrated by using a binary CCVnitrogen
mixture to avoid any possible chemical reactions in the calibration
cells. This means that when a gas other than nitrogen is in the
sample, and causes a change in the measured CÜ2, it will contri
bute to the overall system error. The task then is to characterize
the sensor response so that the processor box can compensate
for that added error. This error should be consistent from sensor to
sensor. Verification of this was necessary. An empirical approach
was taken. The special sensors discussed earlier were used on
the static station. The sensor was first calibrated using binary
mixtures of COa in nitrogen. Next a mixture was introduced into the
static station with the same CÜ2 partial pressure but with some
amount of interfering gas. Data was accumulated in this manner
for a number of transducers with varying amounts of interfering
gas and COa.
Figs. 3, 4, and 5 are plots of this data and show the PCOz
measurement errors that would occur if some form of compensa
tion was not used. The processor box in the 47210A uses a
compensation routine derived from this data to reduce these
errors. The user simply specifies the presence of interfering gases
by pressing the appropriate correction factor pushbuttons on the
front panel of the 4721 OA.
Russell A. Parker
Russ Parker received the BS degree
in chemistry from Trinity College,
Hartford, Connecticut in 1967. He
then attended Purdue University,
West Lafayette, Indiana where he
was awarded a Master's degree and
a PhD degree in analytical chemis
try in 1 972. After working for a while
as an analytical chemist and an
electronics designer. Russ joined
HP in 1975 and has worked on ac[ItJIj^ I cessories for the 4721 OA Oximeter,
the 4721 OA Capnometer, expand^^ ing an electroplating area, and qual.JHIPt«J& 1ÉKMB ity control. He is co-author of
three and about an on-line computer for chemical analysis and
is a co-inventor for one pending patent on a timer/controller. Russ
was born in Hartford, Connecticut and now lives in Holliston,
Massachusetts. He has a 10-year-old son and enjoys photo
graphy, playing tennis and volleyball, skiing, spelunking, listening
to blues music, and not jogging. His friends call him "the crazy
1. "A General Multiparameter Least Squares Curve Fitting Computer Programme and
some of its Applications," Talanta, Vol. 19, 1976, pp. 1131-1139
of the inspired gas. These switches select any one of five
possible compensation states. With no buttons engaged, the
instrument assumes standard conditions and a correction
for 21% inspired oxygen and saturated water vapor is im
plemented. This compensation is a simple factor applied to
the measured value.
The compensation for Ch concentrations greater than
21% and 50% is also a simple factor. The value chosen
minimizes the error in each band. Thus, at one specific Ch
inspired concentration in each range, the added error due to
interfering gases is nearly zero. A similar approach is taken
for nitrous-oxide-plus-oxygen interference, the difference
being a instead of a straight-line error compensation, a
second-order polynomial compensation is used to fit the
error characteristics of the sensor response to nitrous oxide,
oxygen and water vapor more closely. These correction
factors are only implemented during the operational
modes of the instrument. When the calibration stick
supplied with the instrument is extended, the processor box
assumes a calibration check is being performed on the CCh
and nitrogen-filled calibration cells. Thus no interfering
gas or altitude compensation is performed.
The goal of providing an instrument that is easy to use
and calibrate rests partly on the software and partly on the
hardware. The two-point calibration scheme requires two
stable, known reference concentrations to be introduced
into the sample chamber one at a time. The microprocessor
must also be informed when each reference is present. The
scheme uses two calibration cells similar to those used in
the sensor's filter wheel, one filled with nitrogen and one
filled with a CCh-in-nitrogen mixture. This CCh-in-Nz mix
ture is used as a stable reference concentration. The calibra
tion cells are considerably larger and have thicker windows
but otherwise they are similar to the cells in the sensor's
filter wheel. The thicker windows help the calibration cells
survive the rigors of handling during calibration. The two
cells are secured in a glass-filled polycarbonate carrier (the
calibration stick) attached to the front of the processor box.
In addition, there are two momentary pushbutton switches,
one associated with each cell, to allow the operator to inform
the processor box when the sensor is in place over that
calibration cell. The system is calibrated when the sensor is
changed or if the system has drifted outside the prescribed
bounds. To calibrate the instrument, the calibration stick is
extended (Fig. 16) and the sensor is first placed on the
calibration stick over the zero cell, which contains only
nitrogen. The associated button is pressed and the display
shows LO CAL. After an internal delay of approximately i%
minutes to establish thermal equilibrium, the PCCh display
will read 0.0. This shows that the first calibration point has
been entered into the EAROM in the instrument. Next the
sensor is placed over the nitrogen-and-CCh-filled cell and
its pushbutton is pressed. The display shows HI CAL for
three minutes after which it shows the same value as is
stamped on the calibration stick (55.2 in Fig. 16). This
indicates that the second calibration point has been stored
in the EAROM. The system is now calibrated and ready for
use. This simple, foolproof scheme allows the operator to
check the instrument accuracy or calibrate the instrument
without using any other equipment such as gas bottles and
their associated plumbing.
The corrected waveform can be processed to get useful
respiration parameters: respiration rate, end-tidal PCCh,
and inspiratory minimum PCCh (see box on page 12).
These parameters can be tested against alarm limits to sig
nal a doctor or nurse of an undesirable change in the patient's
(continued on page 20)
© Copr. 1949-1998 Hewlett-Packard Co.
Making Accurate CO2 Measurements
by John J. Krieger
One of the requirements of developing any fundamental mea
surement, such as the partial pressure of carbon dioxide, is an
absolute standard. That is, what is truth in the context of COz
exhaled by a patient? Given a sensor that can measure COz level
changes, it is necessary to calibrate its response against "truth."
The problem is complicated by several variables that affect the
CÜ2 measurement.
• Total barometric pressure (or altitude). Measurement interfer
ence can come from other gases such as oxygen, water vapor,
nitrous oxide, and other anesthetic gases commonly used in a
medical environment.
• Charles' and Boyles' Gas Laws (PV=nRT) govern effects
caused by changes in environment, gas temperature and
sample gas flow rate.
• Optical and mechanical tolerances can accumulate when in
terchanging system components.
• Long-term drift (aging) of system components.
One way to calibrate and verify performance of the 4721 OA
instrument is to measure many CO? samples with all of the possi
ble variations and combinations listed above by both a 4721 OA
system under test and a known perfect CO? sensor. An algorithm
could be developed relating the 472 10A system and the perfect
sensor. However, the perfect standard CÜ2 sensor does not exist.
Even a to good one doesn't exist. The development goal is to
make the 4721 OA more accurate and stable than any other medi
cal sensor. This requires the CÜ2 standard to be at least ten times
better than the 4721 OA accuracy and stability goals. Even
analyses of gas bottles supplied by the U.S. National Bureau of
Standards (NBS) are less accurate than what is needed.
Instead, we use an indirect, but more precise technique that
uses different partial pressures at constant volume and tempera
ture. Although the NBS cannot certify gas samples accurately
enough, they can certify pressure and temperature very accu
rately. Given a fixed volume and PV=nRT, very accurately known
CO? mixtures can be made. A custom, automated static gas
station was built and is used to test, calibrate, verify, and manufac
ture the 4721 OA Capnometer.
The static station (Fig. 1 ) consists of : (1 ) a stainless-steel mixing
chamber, (2) an isothermal circulating-water jacket, (3) an array of
solenoid-controlled valves, including a precision, electrically op
erated, analog mixing valve and a pneumatically operated purg
ing valve, (4) a custom interface box to control the valves, (5) an
ultra-accurate MKS Inc. manometer which is read by an HP3455A
Digital Voltmeter, (6) one or more 4721 OA processor boxes to
function as analog-to-digital (A-to-D) converters for the one or
more 14360A Sensors, and (7) an HP 9825A Desktop Computer
as the static station's system controller. A series of HPL programs
was written to run the static station.
The static station system was run over a period of several years
to mix and record thousands of COa sample calibration points.
The results are stored on magnetic tape or disc media for analysis
and algorithm development.
The construction of the static station system was an engineer
ing project in its own right involving several engineering disci
plines: mechanical, electrical, and software. The heart of the static
station is the stainless-steel mixing chamber. The mixing chamber
Fig. calibrate the diagram of the static station developed by HP to calibrate the 4721 OA
© Copr. 1949-1998 Hewlett-Packard Co.
has several openings to external devices: the MKS Inc. manome
ter, mix 5-cm-diameter high-vacuum purge valve, the analog mix
ing valve, and one or more 14361A Airway Adapter chambers.
The 14360 Sensors to be calibrated are placed on the airway
adapters and connected to the 472 10A processor box which is
used is an A-to-D converter. A special HP-IB interface board is
attached to the 4721 OA to allow direct reading of its A-to-D con
verter by the 9825A Computer.
The mixing chamber is filled with various gas mixtures by the
analog mixing valve M which receives one gas at a time from a
manifold. The manifold is filled with either 100% carbon dioxide,
nitrogen, oxygen, nitrous oxide or other commonly used
operating-room anesthestic gases. The desired fraction of gas
partial pressure is let into the mixing chamber. The mixing valve is
then closed and the manifold is evacuated through the purging
valve P. Then a different gas can be let into the evacuated man
ifold without fear of gas cross-contamination.
The mixing chamber is constructed of stainless steel to avoid
possible chemical reactions with the gas mixtures (at one time we
mixed in water vapor as one of the gases and we didn't want the
mixing box to rust). Stainless steel, however, is a poor thermal
conductor. For reasons that will be discussed later, the mixing
chamber has to be kept at a constant temperature. This is done by
completely surrounding the mixing chamber with an aluminum
water jacket. A precision temperature controller and a circulating
pump is coupled to the water jacket. Constant temperature is
more important than accurate temperature in this application.
The airway adapters used in the static station are constructed
specially to eliminate variability and assure that the accumulated
data reflects nominal conditions.
A desired gas mixture sample point is orchestrated by the
9825A Computer by the proper sequencing and timing of the
various valves. As an example, suppose we want to make a
sample point of 55 mmHg COs, 50% nitrous oxide, 21% oxygen,
and the balance nitrogen to make a total pressure of 760 mmHg
(sea level). The control sequence is as follows:
1. Vent mixing chamber for three minutes (valve V).
2. Purge manifold for one minute (valve P).
3. Let COa into evacuated manifold.
4. Fill mixing chamber until it has a total pressure of 55 mmHg.
5. Purge manifold for one minute (valve P).
6. Let nitrous oxide into evacuated manifold.
7. Fill mixing chamber until it has a total pressure of
55 + 50% x 760 = 435 mmHg.
8. Purge manifold for one minute (valve P).
9. Let oxygen into evacuated manifold.
10. Fill mixing chamber until it has a total pressure of
435 + 21% x 760 = 594.6 mmHg.
11. Purge manifold for one minute (valve P).
12. Let nitrogen into evacuated manifold.
13. Fill mixing chamber until it has its final total pressure
594.6 + 165.4 = 760.0 mmHg.
The static station controller then waits until the various layers of
gas form a homogeneous mixture by Brownian motion. The mixing
chamber has a mixing time constant of three or four minutes, so all
the nooks and crannies are equilibrated in about 10 to 15 minutes.
Typically mixing and measuring each gas data point takes about
25 to to minutes. This computerized system makes it possible to
do the and calibration of a sensor and airway overnight and
reduces the drudgery of what would otherwise have to be done
The partial-pressure filling subroutine mentioned above is a
complicated procedure involving feedback from the MKS Inc.
manometer. The computer calculates the target pressure that the
mixing chamber is to be filled with and fills it with successively
smaller pressure pulses. An attempt was made to approxi
mate an isothermal expansion. This can only be done per
fectly by an infinitesimal flow rate. A quick expansion of a gas
causes it to cool down (e.g., the use of Freonâ„¢ in a refrigeration
system). An empirically derived algorithm was developed for the
analog mixing valve that is used. At first long gas pulses at high
flow rates are used. Then, as the target pressure is approached,
short, low-flow-rate pulses of short duty cycle are used to creep
slowly up to the target pressure. In this way, an isothermal
expansion is achieved in a minimum time. The constant-tempera
ture mixing-chamber water jacket is very important for the mix
ing task.
respiration status.
The main program then displays the fully processed
parameters on the capnometer's front panel and outputs the
instantaneous PCCh waveform to a rear-panel analog output
Rodney J. Solomon
Rod Solomon joined HP in 1972 with
several years of experience working
- with small computer peripherals, mis
sile guidance, and photoreconnais* sanee. At HP he has worked on the
mechanical design for an ECG monitor,
led a strip-chart recorder project and
served as project manager for the
472 10A Capnometer. A native of
Syosset, New York, Rod attended Pratt
Institute in Brooklyn, New York, earning
the BME degree in 1967. He is a co-in
ventor for a patent on an automobile
anti-theft device. Rod is married, has
two children, and lives in Needham,
Massachusetts. He enjoys flying, tinkering with automobiles, and fix
ing up his 60-year-old home.
-' YÃ
Fig. 16. The calibration stick supplied with the 47210A con
tains two cells that have reference concentrations of gas
sealed in them. The sensor is placed on one cell at a time to
check and calibrate the instrument.
© Copr. 1949-1998 Hewlett-Packard Co.
for use with chart recorders or slow-trace oscilloscopes.
\Vhen the processing is completed, the data-available flag is
cleared and the main program awaits the A-to-D converter's
service routine to signal the arrival of the next digital data
Many people at Hewlett-Packard Laboratories in Palo
Alto, California laid the groundwork for the 47210A Capnometer, notably Charlie Hill and John Bridgham. At HP's
Ualtham Division the intricate mechanical design of the
sensor assembly was performed by Ed Parnagian with assis
tance by Ross Frushour and John Allen. Al Bond contrib
uted to the processor box design and the sensor manufac
turing techniques. A truly superb job was done on the elec
HP Model 4721 OA Capnometer
tronics by a trio of engineers: John Krieger. Gerry Kager and
Ray Stelting. Russ Parker, our resident chemist, performed
a remarkable job of characterizing the sensor. Russ' \vork
was the key to success with the simple-to-perform calibra
tion scheme. Tom Hayes, our gas monitoring product
line manager, provided valuable guidance regarding the
needs of the medical user. A final thanks goes to our section
manager. Ed Merrick, who demonstrated his abilities as an
exceptional engineer many times in the course of develop
ing the 47210A. His help was greatly appreciated.
1. B. Smallhout and Z. Kalenda, "An Atlas of Capnography,"
Kerckebosch. Zeist, The Netherlands, 1975.
Range: 0 to 150 mmHg (0 to 7.5V).
•Accuracy: ±2 mmHg from 0 to 40 mmHg.
±5% of reading from 40 to 100 mmHg.
Response Time: Delay. 150±25 milliseconds.
Rise Time (90% rise to a PCO2 step change), 200 milliseconds.
Noise: ±0.5 mmHg (rms), or 1.5% of reading, whichever is higher.
Range: 0 to 150 mmHg.
•Accuracy: ±2.5 mmHg from 0 to 40 mmHg.
±5.5% of reading from 41 to 100 mmHg.
Response Time: Delay, 150 ±25 milliseconds.
Rise Time (90% rise to a PCO2 step change), 200 milliseconds.
RANGE: 0 to 150 mmHg (0 to 3.0V).
'Accuracy: ±2 mmHg from 0 to 40 mmHg.
±5% of reading from 40 to 100 mmHg.
Range: Oto 150 mmHg
"Accuracy: ±2.5 mmHg from 0 to 40 mmHg.
±5.5% of reading from 41 to 100 mmHg.
Range: 0 to 75 BPM (0 to 3.0V).
Accuracy: ±2 BPM (4 to 75 BPM).
Range: 0 to 75 BPM.
Accuracy: ±2 BPM (4 to 75 BPM).
1. Accuracy of End-Tidal readings applies to measurements greater than 10 mmHg.
2. Standard conditions:
Gas Mixture: CO2, N2, O2 (inspired) at 21%, and water vapor.
Temperature: 33°C (airway adapter).
Pressure: 760 mmHg (altitude setting - 0)
Water Vapor Pressure: 38 mmHg
"REPEATABILITY: ±0.8 mmHg from 0 to 40 mmHg
±2% of reading from 41 to 100 mmHg
STABILITY: ±1 mmHg for 7 days at 55±5 mmHg, after 30-minute warmup
"Conditions: The repeatability specification applies if the same sensor, airway adapter, and
capnometer are used immediately after calibration. Each gas sample measured must be at
the same temperature and pressure.
COMPENSATION: Front-panel pushbuttons provide mean corrections for gas compositions
when the error oxygen is other than 21%. The following tables indicate the added error
at the reduced gas compositions and their extremes. These added errors can be reduced
to nearly zero by treating them as proportional to the difference from the mean composition.
In the tables, all gas percentages are for inspired gas and standard conditions (Measure
ments, in 2). In all cases, the actual PCO2 is the displayed PCO2p/us the error in the table.
For altitude settings other than zero {sea level), add ±0.3 mmHg.
Connector pin assignments are compatible with standard signal lines in the HewlettPackard Patient Monitoring Series.
Capnometer: 0°C to 55 C
14360A Sensor: 17°C to 38°C
Capnometer: 5% to 95% relative humidity at 40"C
14360A Sensor: 5% to 95% relative humidity at 38"C
LINE VOLTAGE: 100. 120. 220, 240Vac.
+5%- 10%, 50 to 60 Hz.
CHASSIS LEAKAGE CURRENT TO GROUND: Less than 50 microamperes at
127Vac. 60 Hz.
measured from 14360A sensor case to power-cord third-wire ground at 40°C
and 95% relative humidity.
INPUT PROTECTION: Protected against defibrillator potentials. Free from electrocautery interference under most circumstances.
DIMENSIONS: Capnometer (HWD): 19.1 x 21.3 x 38.1 cm.
AIRWAY ADAPTER LENGTH (with tubing couplers): 9.5 cm.
TUBING COUPLERS (ANSI Standard Z79): 15-mm diameter.
WEIGHTS: Capnometer: 7.7 kg.
SENSOR: 56 g without cable
195 g with cable
AIRWAY ADAPTER: 18 g with tubing couplers.
STERILIZATION: Capnometer may be wiped with cold chemical disinfectant. Sensor
may be sterilized with buffered gluteraldehyde. Airway adapter less disposable
tubing couplers may be autoclaved, gas sterilized (ethylene oxide), or cold-chemical
DEAD SPACE: Airway Adapter (with tubing couplers): 15cc.
PRICE IN U.S.A.: $6.400.
175 Wyman Street
Waltham, Massachusetts 02154 U.S.A.
© Copr. 1949-1998 Hewlett-Packard Co.
A Versatile Low-Frequency Impedance
Analyzer with an Integral Tracking
Gain-Phase Meter
This instrument measures impedance parameters, gain,
phase, and group delay of individual components, circuit
sections, and complete circuits. The measurements are
automatic, wideband, and made under variable frequency
and/or dc bias voltage conditions.
by Yoh Narimatsu, Kazuyuki Yagi, and Takeo Shimizu
THE DEVELOPMENT of more complex electronic
systems and components requires improved in
strumentation to evaluate impedance and trans
mission characteristics of individual components and cir
cuits. Ideally, this evaluation should be done under the
conditions of frequency, bias, and signal level at which the
device or circuit is to operate.
Measuring both impedance and transmission parameters
usually requires at least two instruments: an analog vector
impedance meter or a digital LCR meter to measure imped
ance parameters, and a gain-phase meter or network
analyzer to measure transmission characteristics.
Hewlett-Packard's Model 4192A LF Impedance Analyzer
(Fig. 1) introduces a new concept in the measurement of
these parameters. To our knowledge, it is the industry's
first fully automatic, wideband, variable-frequency, multiparameter impedance meter that is equipped with a track
ing gain-phase meter. It can also measure group delay and
be operated via the HP-IB. The 4192A is designed to
simplify and improve the testing of discrete complex de
vices and in-circuit components, and the evaluation of cir
cuits, materials, and semiconductor products. Some of its
features are:
• All measurements and test conditions are specified with
pushbutton ease. There are no knobs or dials to adjust. An
internal microprocessor can automatically select the
measurement range and circuit mode (equivalent series
or parallel) appropriate for the specified conditions and
measured parameter value.
• Eleven impedance parameters ( Z , | Y| , 6, R, X, G, B,
L, C, D, and Q) can be measured. Equivalent series or
parallel mode is selectable manually or automatically.
• Test signal frequency can be automatically or manually
swept in either direction within a range of 5 Hz to 13
MHz. A fixed test signal frequency can be specified any* Hewlett-Packard's implementation of IEEE Standard 488 (1978).
Fig. 1. The 4192 A LF Impedance
Analyzer makes accurate mea
surements of impedance and
gain-phase response for compo
nents, materials, and two-port de
vices in the frequency range of 5
Hz to 73 MHz. Test signal levels
are programmable. The analyzer
has frequency-sweep capability,
4 Vz-digit resolution, built-in dc bias
that can be swept, andX-Y record
er outputs.
© Copr. 1949-1998 Hewlett-Packard Co.
where within that range with a resolution better than
• Test signal amplitude is selectable from 0.005 to 1.100
Vrms in steps as small as 1 mY. The actual test signal
amplitude across the device under test (DUT) can be
monitored by the 4192A.
• An internal ±35Vdc bias source can be automatically or
manually swept in 10-mY steps.
• Five gain-phase parameters (|B— A|, 6, group delay,
|A|, and |B|) can be measured with a maximum gain
resolution of 0.001 dB and a phase resolution of 0.01°.
The dynamic range for the input level is 100 dB and the
level can be measured in dBm or dBY.
• X-Y recorder outputs between zero and ±1 V. These out
puts are driven by digital-to-analog converters (DACs)
controlled by the instrument's microprocessor. The con
sistent output ramp simplifies recorder setup.
• An internal backup memory, consisting of five non
volatile storage registers, stores all of the settings for up
to five independent measurement setups.
• All front-panel keys and test parameter settings can be
controlled by means of the instrument's HP-IB capability.
The 4192A LF Impedance Analyzer is part of HP's
variable-frequency impedance analyzer family. It covers
the frequency range below that of the 4191 A RF Impedance
Analyzer1 which does impedance measurements in the HF
to UHF frequency range. Together the 4191A and 4192A
can make impedance measurements over the very wide
frequency range of 5 Hz to 1 GHz.
The main applications for the 4192A are testing of com
plex discrete components, either separately or in-circuit.
and evaluating circuits, materials, and semiconductor de
vices. An example is measuring quartz crystal parameters.
Manufacturers and users of quartz crystals require an accu
rate way to measure a crystal's series or parallel resonant
frequency, equivalent series resistance, shunt capacitance,
and Q. The 41 92 A can perform all these measurements
quickly. If an HP-IB controller is available, they can be made
Fig. in Gain-phase characteristics of a 3. 2-MHz crystal filter in
its passband are measured easily by the 4192A.
automatically and plotted graphically (Fig 2). If an X-Y
recorder is connected to the 4192A's recorder outputs, mea
surement results can be recorded on standard logarithmic
or linear graph paper.
When used with an HP-IB controller, the 4192A can au
tomatically measure the frequency characteristics of cored
inductors while keeping the test current through the DUT
constant. This is possible because the output level of the
signal source can be remotely controlled while monitoring
the test signal level.
Unlike most impedance measurement instruments, the
4192A can measure the input or output impedance of de
vices and circuits grounded on one side, such as filters and
The combination of impedance and gain-phase mea
surement capabilities provides many benefits, especially in
designing video, communication, and hybrid 1C circuits.
The greatest benefit is that the designer can evaluate both
HP 4192R
STHRT FRO. -3999. 2 KHZ
STOP FRO. -4000.6 KHZ
Conductance (mS)
FREO. f kHz)
Fig. measured 4192A Impedance characteristics of a 4-MHz quartz crystal, measured with the 4192A LF
Impedance Analyzer, (b) Admittance vector locus of the 4-MHz quartz crystal near the seriesresonance frequency.
© Copr. 1949-1998 Hewlett-Packard Co.
the characteristics of an entire circuit and the impedance
characteristics of the components that make up the circuit.
For example, Fig. 3 shows measurements made on a crystal
filter, and Fig. 2 shows measurements on a crystal. There
fore, the designer can precisely analyze circuit performance
down to the component level. The 4192A also makes it
possible to troubleshoot and upgrade the circuit easily. This
not only improves design efficiency, but also contributes to
reliable design.
LF Bridge
The major sections of the low-frequency impedance
analyzer are the LF bridge, the vector ratio detector, the
signal source, and the digital data and control section.
The bridge section (Fig. 4) in the 4192A uses an approach
different from that used in similar instruments like HP's
4274A and 4275A Multifrequency LCR Meters.2 For
impedance measurements this section provides a complex
voltage across the device under test (DUT) and another
complex voltage proportional to the complex current
through the DUT at bridge balance.
A heterodyne method is used to balance the bridge over
the frequency range of 5 Hz to 1 3 MHz. The bridge uses two
mixers. One is placed right after the I-V converter and the
other at the front of the low current amplifier. Therefore,
the IF amplifier and the null detector can operate at the
intermediate frequency (78.125 kHz) regardless of the mea
surement frequency and the vector generator can operate at
40 MHz. Thus, it is easy to obtain two 90° phase shifters to
drive the null detector and the vector generator. This made
it possible to design a simplified digital phase tracking
The I-V converter, filter, low current amplifier, and
measurement cables cause a phase shift large enough to
prevent the bridge from balancing at high measurement
frequencies. This phase shift has to be compensated ap
propriately so that the bridge can be balanced over the full
frequency range. A phase shift between the reference signal
of the null detector and the IF signal causes a phase shift
between the bridge input (LPOT) and output (LCUR)
in Fig. 4, provided that the phases of the local, 40-MHz,
and VCO signals are kept constant. A digital phase tracking
circuit varies the phase shift of the reference of the null
detector. It consists of a preset binary counter and latches. If
one pulse of the 16IF signal is removed from the input
pulses during each period of the IF signal, the reference
phase of the null detector can be delayed 22.5° from the IF
signal. The phase tracking data is programmed according to
the measurement frequency and cable length, and is stored
in a ROM for use by the instrument's microprocessor.
The bridge of the 4192A uses a four-terminal-pair config
uration3 to avoid measurement errors caused by mutual
inductance between measurement cables. Errors can be
avoided because the magnetic fields generated by the cur
rents flowing in the inner and outer conductors of the mea
surement cable cancel each other when the bridge is bal
anced. To maintain this condition over the full test fre
quency range, the bridge section uses floating power
supplies for the low current amplifier, the I-V converter,
and the coaxial baluns. The floating power supplies pro-
High Current
F Circuit Common of
High Current Amplifier
0 40 MHz
Circuit Common of
I-V Converter
Phase Tracking
Circuit Common of
Low Current Amplifier
Fig. and sections section of the 4192A. The high current, low current and I-V converter sections
each have an independent floating power supply to make measurement of one-side-grounded
devices possible.
© Copr. 1949-1998 Hewlett-Packard Co.
V, (High Potential)
T o
D i g i t a l
Integrator Section
Fig. 5. conversion. ratio detector. This circuit uses only one frequency conversion.
vide excellent isolation between the low current amplifier
and the I-V converter and chassis ground. Each power sup
ply is a dc-to-dc converter with a 1-MHz switching fre
quency and low-pass filters. Common-mode noise is ap
proximately 10 (¿V rms. These floating supplies provide the
grounded-on-one-side impedance measurement capability.
If the bridge is unbalanced, an error current ij flows into
the I-V converter and is converted into an IF error signal by
the first mixer. The amplified IF' error signal is added to the
null detector to control the 40-MHz vector generator. The
signal converted from the 40-MHz error signal is fed back to
the other side of the reference resistor until the system
becomes balanced, that is, until \¿ = 0 and V¿ = 0.
â € ”
= 1
â € ”
â € ”
enough to allow detection of weak signals, especially for
gain-phase measurements. Maximum input level is approx
imately 2V rms.
To satisfy these requirements, field-effect transistor
switches and operational amplifiers are used to provide a
double-balanced mixer having a wide dynamic range (Fig.
6). Compared with conventional diode mixers, this mixer
exhibits much better linearity for high input levels and
lower power requirements for the local port. The equivalent
input noise is about the same as that of the operational
amplifiers. To keep the signal level fortheanalog-to-digital
converter (ADC) constant in the impedance measurement
mode, the signal conversion gain of the mixer is controlled
according to the test signal level.
Besides the desired feedthrough component, several unInput
Therefore Zx = - Rr Vr
As seen from the above equation, all that is needed to
calculate the complex impedance of the DUT are the values
of Rr and the vector ratio between Vx and Vr.
Vector Ratio Detector
Fig. 5 shows a simplified block diagram of the vector ratio
detector section. This section detects the precise complex
voltage ratio (both magnitude and phase difference ratios)
between two signals.
To obtain good tracking characteristics and linearity for
various test signal levels, the buffered signals are multi
plexed in a timesharing manner to share one signal path.
The output from the multiplexer is converted into an IF
signal by a mixer to cover the measurement frequency range
from 5 Hz to 13 MHz. Typical tracking accuracy is 0.03% for
midrange frequencies and 0.3% for the high and low fre
quencies. The equivalent input noise of the mixer is low
Local Port
Fig. 6. Double-balanced mixer provides good linearity over
the entire operating range.
© Copr. 1949-1998 Hewlett-Packard Co.
wanted signals are also applied to the IF stage from the IF
port of the mixer. Examples are the sum of the signal and the
local of feedthrough from the local port to the IF port of
the mixer, and sideband noise introduced around the IF
signal by various noise sources. Conventional methods to
eliminate these unwanted components use a high-Q
bandpass filter in the IF stage. If this approach is used, the
frequency components as near as 10 Hz from the IF must be
attenuated by more than 80 dB. Such high-Q filters are very
costly and introduce a significantly slower response to
input level changes. Therefore, a different approach was
chosen in the 4192A for filtering.
The integration time of the ADC is used as a filter func
tion. It is well known that an integrator whose integration
time is TI has a frequency response Ff as follows:
Ff =
where f is the input frequency.
Since the phase detector located in front of the integrator
is regarded as a mixer whose input component is mixed
with the IF signal, the unwanted components described
above (except sideband noise) are converted to components
whose frequencies are multiples of the measurement fre
quency. Therefore, if Tj is set to be a multiple of the recip
rocal of the measurement frequency, the term sin(7rfTj) be
comes zero and unwanted components are eliminated. The
higher the measurement frequency becomes, the further the
unwanted components appear from the IF signal. The IF
filter (following the mixer in Fig. 5) becomes effective only
for measurement frequencies above 10 kHz. Consequently,
rejection by using an integration time T¡ is especially nec
essary for the lower measurement frequencies.
T¡ is varied by changing the integration time counter via
the microprocessor. Since the integration time must vary
from approximately 2.5 ms to 600 ms, an ordinary dualslope integrator cannot be used because of its limited
dynamic range. An integrator similar to the one used in the
HP 3455A Digital Voltmeter4 is used, because its dynamic
range is practically infinite.
The noise bandwidth of filters of this type is equal to the
reciprocal of the integration time. For example, an integra
tion time of 20 ms gives a noise bandwidth of 50 Hz. This is
approximately equal to the noise bandwidth of a filter with
a -3-dB bandwidth of 32 Hz if the filter is approximated by
a single-tuned bandpass filter having a Q of 2400.
Signal Source
Fig. 7 shows a simplified block diagram of the 4192A's
signal source section. In impedance measurements or
gain-phase measurements, a good quality test signal is es
sential for obtaining high-resolution measurements with
good repeatability. The basic concept used in the 4192A is
to phase-lock the 40-to-53-MHz signal from VCO #1 to a
100-kHz signal that is divided down from a 40-MHz signal
by using fractional-N frequency synthesis.5 The VCO #1
output is then converted to the 5-Hz-to-13-MHz test signal
by mixing.
A local frequency that is always higher than the test
signal frequency by the IF (usually 78.125 kHz) is obtained
by subtracting the 40-MHz-IF signal from the VCO #1 output.
The 40-MHz-IF signal is generated by VCO #2 which is
phase-locked to the IF frequency divided from the 40-MHz
signal. Leveling of the output signal is achieved by feeding
back an ALC (automatic level control) signal to the PIN
modulator inserted in the path of the 40-MHz signal. Con
trol of output level is achieved by changing the reference
level of the ALC loop, which is controlled by an 8-bit
digital-to-analog converter (DAC).
Digital Circuitry
All data and analog controls are managed by an MC68BOO
microprocessor. A battery-supported backup memory re
tains five independent instrument setups when the instru
ment is turned off or line power is removed. Troubleshoot
ing is done easily with the instrument's built-in test capa
bility and a specially designed troubleshooting kit that is
PIN Modulator
IF Control
fiF=78.125 kHz/69.44 kHz
Fig. 7. The signal source section
of the 4192A generates the test
and local signals by using a
fractiona/-N synthesizer.
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 8. The 16095A probe fixture
is useful for probing circuits and
devices grounded on one side.
Fig. 9. The 16096A test fixture
is useful for measurements of
two-port devices and circuits.
supplemented by signature analysis.
Test Fixtures
Two different types of test fixtures and an accessory kit
are available for circuit probing or measurement of two-port
devices. Also, most of the test fixtures usable with HP's
4274A and 4275A Multifrequency LCR Meters can be used
with the 4192A.
The 16095A probe test fixture (Fig. 8) is convenient for
measurement of devices grounded on one side, circuits, and
in-circuit components. Before using the probe, zero offset
compensation should be performed to cancel the effects of
any residual inductance and stray capacitance inherent in
the probe fixture.
The 16096A test fixture (Fig. 9) is used to measure the
input/output impedance and gain-phase response of twoport devices and circuits.
© Copr. 1949-1998 Hewlett-Packard Co.
HP Model 41 92 A LF Impedance Analyzer
IMPEDANCE PARAMETERS: Z|-9, | Y|-9, R-X, G-B, L-R, G, D, Q, C-R, G, D, Q rela
tive gain {B-A), phase, group delay, absolute gain (A, B).
GAIN-PHASE PARAMETERS: (B-A)-6, (B-A)-group delay, A, B.
DEVIATION MEASUREMENT: Displays measured value as deviation (A) from stored
reference or as percent deviation (A%) for all parameters.
TEST SIGNAL (internal synthesizer):
FREQUENCY RANGE: 5.000 Hz to 13.000000 MHz.
FREQUENCY STEPS: 1 mHz (5 Hz to 10kHz), 10mHz(10tO 100kHz). 100 mHz(100 kHz
to 1 MHz), 1 Hz (1 to 13 MHz).
SIGNAL gain- L (open-circuit for impedance measurement or terminated with 50Ã1 for gainphase measurement): 5 mV to 1.1Vrms.
SPOT voltage. Measurements at specific frequency or bias voltage.
SWEEP MEASUREMENT: Linear or logarithmic sweep measurements.
INPUT IMPEDANCE OF CHANNEL A AND B: 1 Mil ±2% in parallel with 25 pF±5 pF.
Basic Accuracy"
Impedance |Z|
Admittance JY[
Relative Gain
Group Delay
0.1 mflto 1.2999 Mil
1 nSto 12.99 S
-180.00° to +180.00°
0.001 dB to 100 dB
0.02 dB
0.1 ns to 10 s
Absolute Gain
A, B
O.SdBVto -100dBV
Calculated by
accuracy of phase
0.4 dBm
0.4 dBV
1. T. Ichino, H. Ohkawara and N. Sugihara, "Vector Impedance
Analysis to 1000 MHz," Hewlett-Packard Journal, January 1980.
2. K. Maeda and Y. Narimatsu, "Multi-Frequency LCR Meters Test
Components Under Realistic Conditions," Hewlett-Packard Jour
nal, February 1979.
3. K. Maeda , "An Automatic Precision 1-MHz Digital LCR Meter,"
Hewlett-Packard Journal, March 1974.
4. A. that "A Fast-Reading, High-Resolution Voltmeter that
Calibrates Itself Automatically," Hewlett-Packard Journal, Feb
ruary 1977.
5. D. with and S. Froseth, "A Synthesized Signal Source with
Function Generator Capabilities," Hewlett-Packard Journal,
January 1979.
Y o h N ar i m at su
Yoh Narimatsu joined YokogawaHewlett-Packard in 1971. After several
years as a development engineer, he
transferred to HP's Santa Clara Division
and was involved in the development of
the 5342A Counter. He worked on the
4275A LCR Meter and designed the
vector ratio detector section of the
4192A Impedance Analyzer. Yoh holds
a BSEE degree from Kyoto University
and an MSEE from Stanford University.
He enjoys playing volleyball and tennis,
1. is married, and has a two-year-old son.
'Varies depending on measurement frequency and test signal level.
"Basic basic accuracy. At frequencies below 400 Hz and above 1 MHz, the basic
mainframe accuracy begins to roll off.
CIRCUIT MODE: Series, parallel, and automatic.
MEASURING TERMINALS: 4-terminal pair configuration.
DISPLAYS: 4V2-digit display in average and normal mode. 31/2-digit display in high-speed
INTERNAL DC BIAS: -35V to +35V, 10-mV steps.
RECORDER OUTPUTS: - 1 V to + 1 V for display A and B. 0 to 1 V for frequency/bias, 1 -mV
IMPEDANCE MEASUREMENT: 140 to 170 ms (60 to 90 ms in high-speed mode).
GAIN-PHASE MEASUREMENT: 180 to 220 ms (90 to 130 ms in high-speed mode).
OPERATING TEMPERATURE: 0 to 55°C, 95% relative humidity at 40°C.
POWER: 100, 120, 220V ±10%, 240V +5%, -10%, 48 to 66 Hz.
DIMENSIONS APPROXIMATE (HWD): 247 mm x 425 mm x 547 mm.
WEIGHT: Approximately 19 kg.
ACCESSORIES FURNISHED: 16047A Test Fixture, two 11048C 50Ã! feedthroughs,
11652-60009 son Power Splitter, 1250-0216 BNC Adapter, and two 11170A Cable
ACCESSORIES AVAILABLE: 16095A Probe Fixture, 16096A Test Fixture, 16097A
Accessory Kit, 1 6047B/C Test Fixtures, 1 6048A/B/C Test Leads and 1 6034B Test Fixture.
PRICE IN U.S.A.: $11, 550.
9-1, Takakura-cho
Hachioji-shi, Tokyo, Japan
The 4192A design team members who deserve special rec
ognition are Seiichi Kikuta and Tomio Wakasugi, signal
source section, Kiyoshi Suzuki, power supply, Masahiro
Yokokawa and Eiichi Nakamura, digital section and the
software, Tetsuya Shiraishi, mechanical design, Hiroshi
Shiratori, test fixtures, and Tsuneji Nakayasu, industrial
Special thanks are also due Masahide Nishida for his
general management and encouragement, and to Shiro Kito
who gave much useful advice and encouragement to the
© Copr. 1949-1998 Hewlett-Packard Co.
* * 7
Kazuyuki Yagi
Kazuyuki Yagi joined YokogawaHewlett-Packard in 1973 just after re
ceiving his BSEE from Tokyo Institute of
Technology. After a few years as a de
velopment engineer, he transferred to
HP's Stanford Park Division in 1 976 and
was involved in the development of the
8656A Synthesized Signal Generator.
During his stay in California, he re
ceived his MSEE from Stanford Univer
sity. He designed the bridge section of
the 4192A Impedance Analyzer. He is
married and has a two-year-old daugh
ter and a one-year-old son. He enjoys
playing Shogi, Igo, and ping-pong in his
spare time.
Takeo Shimizu
Takeo Shimizu received his BSEE from
Fukui University, Japan in 1961 and
joined Yokogawa Electric Works as an
R&D engineer. He joined YokogawaHewlett-Packard in 1964. He worked on
the 4270A Automatic Capacitance
Bridge and served as a project leader
for several other projects before be
coming project leader of the 41 92A. He
is married, has two sons, and enjoys
playing golf.
A Fast, Programmable Pulse Generator
Output Stage
A new pulse generator supplies fast-transition pulses for
testing 100k ECL, advanced Schottky TTL, and other fast
logic families.
by Peter Aue
WITH THE INTRODUCTION of fast new integrated
circuit logic families like 100k ECL (emittercoupled-logic) and the ongoing development
of current technologies such as Schottky TTL and advanced
Schottky TTL, the volume of fast integrated circuits is in
creasing rapidly. Characterization of the parameters of such
ICs in R&D, production, incoming inspection, or produc
tion testing of modules designed with these circuits re
quires a fast, accurate pulse generator with variable transi
tion times to match those of the logic family under test. A
new HP-IB programmable pulse generator, Model 8161A
(Fig. 1), allows these measurements to be performed under
remote control. Increased throughput, decreased develop
ment time, and easier long-term reliability tests are among
the benefits of this 100-MHz pulse generator with 1.3-ns
transition times (Fig. 2).
Design Approach
Because major portions of the proven 8160A Programma
ble Pulse Generator1 were perfectly suitable for the new
instrument, the basic mechanical design, power supply and
timing circuitry of the 8160A were adopted. Only minor
changes were necessary to meet the needs of the 816lA's
100-MHz repetition rate and the different internal supply
currents. The microprocessor hardware was redesigned to
implement signature analysis and the latest-design ROMs.
The major challenge was the design of the programmable
transition time generator and output amplifier.
Transition Time Generator
The main parts of the transition time generator are the
current-switching differential amplifier, the clamp voltage
circuitry, two buffer amplifiers, and seven high-precision
current sources (see Fig. 3). To achieve the fast transition
times, push-pull differential circuitry was chosen.
To describe thebasic principles of operation, let's assume
the four switching current sources are off and Ql has been
off and Q2 on for a long period of time. In this case Isum =
Fig. 1. Model 8T6TA Programma
ble Pulse Generator produces
pulses with amplitudes up to 5V
and transition times of 1.3 ns to
900 iJS. at rates up to 100 MHz.
Programming is done manually
using the front-panel keyboard or
remotely via the HP-IB (IEEE 488).
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 2. Model 81 61 A's 1 .3-ns transitions (upper trace) are
faster and cleaner than most fast logic families, such as 10k
ECL (lower trace).
!LEE + ITRE f lows through Q2. Voltage VC1 =V1 =VCLl +0.7V
and voltage VC2 = V2 = VCL2 - 0.7V. When Ql is switched
on and Q2 off, Isum flows through Ql and Cl is discharged
with ILEE - Isum = - ITRE- On the other hand, C2 is charged
with ITRE, since Q2 is off. When Ql is switched off and Q2
on again, Cl is charged with ILEE an<l C2 is discharged
= ~ ILEE- The voltage swing between
~ Is
and V2 is limited by VCL1 - VCL2 + 1.4V, which is approx
imately 2V. The slew rate is proportional to ILEE or ITRETherefore, with increasing current values, the transition
times decrease.
Cl and C2 consist of intrinsic and stray capacitances
only. The ramp signal is buffered and amplified using buf
fer amplifier 1. For transitions between 5 ns and 99.9 ns
fixed capacitances are switched in by turning the four
switched current sources on. In this case part of the signal
goes through buffer amplifier 2. For transitions longer than
99.9 ns, additional capacitances are switched in.
The ILEE and ITRE current sources are controlled by two
10-bit multiplying digital-to-analog converters (DACs) for
transitions ^5 ns. For shorter transition times, a separate
custom-designed DAC is used because the control doesn't
follow a simple function and has to be approximated. ILEE
and ITRE are made equal to achieve a cleaner waveform.
Output Stage
A dual cascode differential amplifier (Fig. 4) drives the
complementary outputs. (In the Option 020 dual-channel
Fig. 3. The transition time
generator provides programma
ble transition times from 7.3 ns to
900 /us.
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. 4. Output amplifier and at
tenuator. A dual cascode differen
tial amplifier drives the com
plementary outputs.
instrument, one output is taken from each channel).
The electronic attenuator consists of a pair of differential
amplifiers and a vernier current source. The input signals
control the share of current through Ql and Q2 and there
fore the current through each differential amplifier.
Similarly, Vv controls the current in attenuator transis
tors Q3, Q5 or Q4, Q6. Assume that Vv is set to an attenua
tion factor of 2 , 1 = 1 00 m A, and the input is such that both I:
and I2 = 50 mA. Then I3 = %\ = 25 mA and I4 = y2I2 = 25
m A, which adds to a vernier current of 50 mA. The sum I3
+ I4 remains constant for all input signal conditions with It
and I2 split appropriately so that the remaining currents I5
and I6 cause the desired output amplitude at the load resis
tors. This is true for all attenuation ratios. The active vernier
operates over a dynamic range of 10 dB. 10-dB and 20-dB
step attenuators can be switched into the signal path for a
total amplitude variation of 40 dB, giving an amplitude
range of 50 mV to 5V. Without addition of an offset current
source, the output pulses would always be negative. A
bipolar current source is added to supply any current value
between -200 mA and +200 mA to allow an offset voltage
of +5V maximum. For ease of use, calculation of the
amplitude, offset and step attenuator setting is done by the
microprocessor. An additional feature in the Option 020
instrument, besides the availability of a second output
channel, is the selectable built-in 50Ã1 passive adder. In the
A-add-B mode, the sum of the output signals is available at
the channel A connector and the channel B outlet is dis
abled. This feature can be used to generate waveforms like
fast simu (Fig. 5) and multilevel signals, or for the simu
lation of glitches, spikes, and overshoot on signals with
transition times as low as 1.5 ns (Fig. 6).
Fig. 5. The 8161 A Option 020 can generate fast staircases for
testing A-to-D converters, multilevel logic, or pulse-height dis
Fig. 6. The A-add-B mode makes it possible to simulate
glitches, spikes, and overshoot with transistion times as low as
1.5 ns.
© Copr. 1949-1998 Hewlett-Packard Co.
Uwe Neumann contributed the software and the redesign
of the microprocessor board. Hartwig Bartl modified the
mechanical design to meet RFI requirements. A lot of help
ful ideas were provided by lab section manager Werner
Huttemann and product manager Jürgen Brettel.
Peter Aue
A Diplom Ingenieur graduate of the
University of Stuttgart, Peter Aue joined
HP's Boblingen Instruments Division in
1976. Before he became project leader
of the 8161 A, he contributed to the de
sign of the 8 160A timing circuitry. Peter
is married, has three daughters, and
lives in Boblingen. He spends his spare
time working on his house, doing
woodwork and building model trains.
1. W. Hüttemann, L. Kristen, and P. Aue, "A Precision, Program
mable Pulse Generator," Hewlett-Packard Journal, May 1979.
HP Model 8161A Pulse Generator
Pulse Parameters (50fl Load)
RANGE: 10.0 ns to 980 ms.
RESOLUTION: 3 digits (best case 100 ps)
ACCURACY: ±3% of programmed value ±0.5 ns (period <100 ns)
±2% of programmed value (period 5*100 ns)
MAXIMUM JITTER: 0.1% of programmed value +50 ps.
DELAY, times, PULSE, WIDTH: (Specifications apply for minimum transition times,
measured at 50% of amplitude. Delay is measured from trigger to main output).
DELAY (DEL) RANGE: 0.0 ns to 990 ms.
DOUBLE PULSE (DBL) RANGE: 8.0 ns to 990 ms.
WIDTH (WID) RANGE: 4.0 ns to 990 ms.
RESOLUTION: 3 digits (best case 100 ps).
ACCURACY: ±1% of programmed value ± 1 ns.
MAXIMUM JITTER: 0.1% •*• 50 ps («999 ns)
0.05% (999 ns to 9.99 /is)
0.005% (>9.99 /is)
DELAY: for DEL s 50 ns, DEI^g,, <0.94 PER - 30 ns.
for DEL < 50 ns, DELmax independent of period.
WIDTH: for WID s 50 ns, WIDmax <0.94 PER - 30 ns.
for WID < 50 ns, WIDmax <0.94 PER - 3 ns.
HIGH LEVEL (MIL) RANGE: -4.95V to 5.00V.
LOW LEVEL (LOL) RANGE: -5.00V to 4.95V.
RESOLUTION: 3 digits (10 mV).
AMPLITUDE: 0.06V minimum, 5.00V maximum.
LEVEL ACCURACY: ±1% of programmed value ±3% of amplitude ±25 mV.
SETTLING TIME: 20 ns plus transition time to achieve specified accuracy.
In A add B Mode (Opt. 020 only):
HIGH LEVEL (HIL) RANGE: -1.75V to 1.80V.
LOW LEVEL (LOL) RANGE: -1.80V to 1.75V.
TRANSITION TIMES (10-90% Amplitude):
LEADING EDGE (LEE): 1.3 ns' to 900 /is.
TRAILING EDGE (TRE): 1.3 ns' to 900 us.
*<1 ns (20-80% amplitude)
*1.5 ns in A add B mode (Opt. 020 only).
LINEARITY: ±5% for transition times >30 ns.
PRESHOOT, OVERSHOOT, RINGING: ±5% of amplitude ±10 mV for transition times
^2.5 ns, may increase to ±10% of amplitude ±10 mV for transition times <2.5 ns.
A ADD B: Adds Channel A and B outputs (Opt. 020).
OUTPUT FORMAT: 8161A: Simultaneous normal and complement output.
8161A selectable. 020: Channel A and B, normal/complement independently selectable.
Operating Modes
NORM: Continuous pulse stream.
GATE: External signal enables rate generator. First output puise sync with leading
edge. Last pulse always complete.
TRIG: Each input cycle generates a single output pulse.
BURST: Each input cycle generates a programmable number (0 to 9999) of pulses.
Minimum time between bursts is 1 period. Minimum period setting in burst mode is 1 5.0 ns.
MAN: Simulates external signal when EXT INPUT switched OFF.
SINGLE PULSE: Provides a single pulse independent of input and period settings.
WARM-UP TIME: 30 minutes to meet all specifications.
REPEATABILITY: Factor of 2 better than specified accuracy.
Specifications apply from 20°C to 40°C.
Accuracy derating for temperatures from 20°C to 0°C and from 40°C to 50°C with factor
(1 + 0.05 xA°C), where A°C is the temperature deviation outside the 20°C-40°C
HUMIDITY RANGE: 95% R.H., 0°C to 40°C.
POWER-OFF STORAGE: After eight hours of operation, batteries maintain all stored data
up to 2 weeks with instrument switched off. Hardwired addressable location contains a
fixed set). state for confidence check (standard parameter set).
POWER: 115/230V rms + 10%, -22%; 48-66 Hz; 675VA maximum.
WEIGHT: Net 20.8 kg (46 Ibs), Shipping 25 kg (55 Ibs).
DIMENSIONS: 178 mm high, 426 mm wide, 500 mm deep (7 x 16. B x 19.7 in).
PRICES Second U.S.A.: 8161A Programmable Pulse Generator. $14.940. Option 020 Second
Channel. Includes delay, width, double pulse, transition times, and output amplifier,
Hewlett-Packard GmbH
Herrenberger Strasse 110
D-7030 Boblingen
Federal Republic of Germany
Bulk Rate
U.S. Postage
Hewlett-Packard Company, 1501 Page
Road, Palo Alto, California 94304
SEPTEMBER 1981 Volume 32 . Number 9
Technical Information from the Laboratories of
Hewlett-Packard Company
Hewlett-Packard Company, 1501 Page Mill Road
Palo Alto. California 94304 U.S.A.
Hewlett-Packard Central Mailing Department
Van Heuven Goedhartlaan 121
1181 KK Amstelveen. The Netherlands
Yokogawa-Hewlett-Packard Ltd.. Suginami-Ku
Tokyo 168 Japan
040009403 5&?I& HANS £QMOO
CODE i LOG N 244-7
b change your address or delete your name from our mailing list please send us your old address label. Send
hanges 94304 Hewlett-Packard Journal, 1501 Page Mill Road, Palo Alto, California 94304 U.S.A. Allow 60 days.
© Copr. 1949-1998 Hewlett-Packard Co.
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