1958 , Volume v.10 n.3-4 , Issue Nov/Dec-1958

1958 , Volume v.10 n.3-4 , Issue Nov/Dec-1958
- h p -
Vol. 10 No. 3-4
5 x 10'VWeek Time Base Accuracy
in the 10 MC Frequency Counter
REFINEMENT of techniques in several
fields is currently generating a need for
making routine frequency measurements to in
creased degrees of accuracy. A good example
of this need lies in the single-sideband field
where it has become desirable to be able to
make fast, convenient meas
urements of VHP frequencies
to within a matter of cycles in
checking transmitter carriers, receiver oscilla
tors, filter response, etc.
For such work the -hp- 10-megacycle fre
quency counter is now available with a new
time base crystal oscillator of very high per
formance. The new time base is rated as having
a long-term stability of within ±5 parts in 108
per week. This tolerance is directly transfer
able to frequency measurements and means,
for example, that a measurement of a 100megacycle frequency (using the counter with
its heterodyne converter) will be accurate with
in ± 5 cycles in the week following calibration
or ±10 cycles in the second week, ± short-term
stability in each case.
Actually, the performance of the oscillators
has been well above ratings, and the ratings are
considered to be subject to later improvement.
Short-term stability, for example, is rated as
within ±3 parts in 108, but typical short-term
performance is considerably better, as shown in
Fig. 2. Note that one major vertical division in
Fig. 2 is equal to 1 part in 108 so that the sta
bility can typically be described in parts in 109.
It is of interest to recall that this performance,
while within about one order of magnitude of
that of most high-quality commercial crystal
L. 1 5
- t
Fig. 2. Typical short-term stability of Model 524D time
base. Stability of this order permits very close adjustments
to be made in work such as with single-sideband equip
ment where increased precision is currently involved.
Fig. 1 (left). New Model 524D 0-10 me Frequency
Counter incorporates new high-stability time base for ap
plications requiring advanced precision or where unit is
used as frequency standard. Decade form is used for all
columns of readout. Counter is shown with panel plug-in
unit for measuring frequencies from 100 to 220 mega
cycles. Basic counter will also be available with in-line
readout as pictured on page 4.
U . S . A .
© Copr. 1949-1998 Hewlett-Packard Co.
1 9 5 8
H E W L E T T - P A C K A R D
C O .
#lj#2 #3 #4 #5 #6 #7 #8 #9
—15 MIN.
standards, is supplied within the
economic limitations necessary for
one circuit of a complete frequencymeasuring instrument whose total
selling price is well below that of
such commercial standards alone.
Space limitations make it imprac
tical to present extensive recorded
data on long-term stability, but in
typical cases the crystals show aging
rates of a few parts in 109 per week
after the initial stabilization period
that crystals normally exhibit has
passed. Fig. 3 shows sections spaced
a week apart from a long-term sta
bility record to illustrate typical sta
bility over a one-week period. It can
be seen that the aging rate is less
than 1 part in 108 for the week. At
the -hp- plant, in fact, the equip
ments are not released until the crys
tals have shown an aging rate of not
more than 2.5 parts in 10s per week
for a 2-week period. This rate nor
mally continues to improve, as in
dicated in Fig. 3.
Fig. 4. Record of frequency
change in Model 524D time
base oscillator resulting from
8 different oscillator tube re
placements. Large spikes are
where recorder went offscale as tubes u'ere removed
and new tube warmed.
A question that often arises in re
gard to stable oscillators concerns
the amount of frequency shift that
occurs when a tube is changed. This
is a factor that is more under the
control of the designer than is often
realized, and in this oscillator it is
one of the factors that has been care
fully controlled. Fig. 4 shows typi
cal performance in this respect.
Eight replacement tubes were in
stalled and allowed to operate for
several minutes each. Overall effect
among all nine tubes was less than 1
part in 10s. Tube-to-tube changes,
in fact, were less than 3 parts in 109
in all but one case.
Somewhat related data are shown
in Fig. 5. This curve demonstrates
the effect of discrete changes in
capacitance at three points in the
oscillator circuit. Information of
this nature is used to establish ap
proximate coefficients of frequency
change to evaluate design factors.
Fig. 5. Record of frequency change re
sulting from placing lumped capacitance
of 10 mmf at grid, plate and output of
oscillator to investigate effect on oscil
lator of very small changes in circuit
The first upward step in the curve
shows the frequency shift caused by
placing a lumped capacity of 10
mmf directly across the grid of the
oscillator. A frequency change of
about 6 parts in 10s or a coefficient
of 0.6 parts in 108 per mmf resulted.
This information is both a guide to
tube replacement changes as well as
a measure of the effect of possible
slight changes in circuit capacity
owing to slight shifts in wiring, etc.,
that might occur in transporting the
instrument. Such shifts probably
cannot exceed a few tenths of a
micro-microfarad so that resulting
Fig. record 524D taken a week apart from long-term stability record of typical Model 524D time base. Low time-base
oscillator aging rate minimizes need for calibration checks.
© Copr. 1949-1998 Hewlett-Packard Co.
— 115VFig. time-base stability. effect of large line-voltage variations on time-base oscillator stability.
— 15 MINr-
Fig. time previous over-night standby, a feu'-minute warmup returns time base to its previous frequency and stability.
shifts in frequency should be less
than 2 parts in 109.
The next two smaller steps in Fig.
5 show the effect of a 10 mmf in
crease in capacitance at the oscilla
tor plate and at the oscillator out
put. Additional buffering is used be
tween the oscillator output and the
point where the signal is extracted,
and a 10 mmf increase at this point
causes no detectable change in fre
The frequency counter is rated for
operation from power line voltages
within the range from 103.5 to 126.5
volts. Within this range line volt
age level has very little effect on
time base stability. This is demon
strated in Fig. 6 which shows the
effect of very severe steps in line
voltage— actually somewhat beyond
ratings. The only noticeable fre
quency change occurred when line
voltage was changed from 102 volts
to 128 volts, i.e., a positive step of
Two other situations about which
performance data are of value to the
user concern standby operation and
initial turn-on characteristics. The
instrument is provided with a stand
by power switching arrangement
such that crystal oven temperature
is maintained as long as the instru
ment remains connected to a power
source. Fig. 7 shows turn-on charac
teristics from standby operation
after the instrument proper has been
turned off for an overnight interval.
A few minutes' warmup in this con
-15 MIN.—
dition is adequate for accurate
Standby operation is also essen
tially free of the permanent or semi
permanent shifts sometimes ex
hibited by crystal oscillators. This
is demonstrated in Fig. 8, which
shows the stability during three
switchings to standby and back.
When the instrument has been
completely disconnected from pow
er for a significant time such that
the crystal has cooled to room tem
perature, the cooling and subse
quent re-heating of the crystal often
causes it, in common with any
quartz crystal, to undergo a new but
accelerated aging cycle which, in
effect, may cause a small, permanent
shift in the crystal frequency. It is
not practical to place ratings on
: !/2 HOUR'
10 MM
Fig. sometimes when oper standby feature is free of shifts in frequency sometimes encountered when crystal oscillators are thus oper
ated, as indicated by three switchings to standby and back in above record.
© Copr. 1949-1998 Hewlett-Packard Co.
Fig. base will complete cool-off of instrument and crystal, time base oscillator will
typically follow a warm-up characteristic as indicated above.
— 1 HOUR —
these effects, but they are usually
relatively small. In any case, where
highest precision is sought, it is de
sirable to make use of the standby
feature and to minimize complete
turn-offs. Short-time turn-offs, of
course, such as might be caused by
short power failures or moving the
instrument a limited distance, do
not appreciably alter the crystal
temperature and thus cause no
noticeable effect.
When the instrument has been
cooled to room temperature, it can
be expected to follow an initial pat
tern similar to that shown in Fig. 9.
It is rather usual that the frequency
will return to within a few parts in
10s of its previous value within a
few hours, but the previous stability
may not be reached for several days.
- 2 0 Â ° C /
+ 50°C
+ 25°C
Fig. frequency Record of time-base oscillator stability as complete frequency counter was
temperature-cycled in chamber which reached —20° and +50°C.
The-hp- ÃO-megocyc/e coun
ter with the advanced time
base described in the accom
panying article has the model
number 524D. In addition to
its new time base this instru
ment also has a full 8-decade
readout as shown on fronf
page. In other respects, the
instrument is the same as the
welt-known Model S24B.
The new time-base counter
will a/so be available in the
late spring with an in-line
readout using glow-indicator
tubes (left) for applications
where this type of readout is
preferred. The in-line readout
instrument has the model
number 524C.
-hp- instruments are rated for
operation over normal room tem
peratures, but -hp- policy has al
ways been to design for consider
ably wider temperature ranges as a
safety factor, and instruments are
normally "type-tested" in this and
other respects over wider than rated
ranges. Fig. 10 is a record of time
base frequency stability during a
temperature test in which the am
bient temperature was varied to
-20°C and to +50°C as a check on
the adequacy of the oven design as
well as a check on the remainder of
the frequency counter. During this
test the time base changed less than
2 parts in 10s, while the complete in
strument operated properly.
The -hp- 10-megacycle frequency
counter using the new time base dedescribed above can be seen to pro
vide a very high level of accuracy,
making the instrument suited to use
in laboratory measurements as well
as exacting production measure
ments. The instrument can also
serve in the dual role of a laboratory
standard frequency generator, since
it provides an output (at 100 kc)
(Concluded on page 6)
© Copr. 1949-1998 Hewlett-Packard Co.
T T IS OFTEN of special interest to
engineers visiting the -hp- plant
to observe the procedures used to
ascertain the accuracy and stability
of precision time base oscillator cir
cuits—particularly those that achieve
stabilities of within a few parts in
108 such as the Model 524D time
base discussed previously in this is
sue. By way of illustration, it can be
noted that an accuracy of 5 parts in
108 (the rating of the Model 524D)
corresponds to knowing the distance
between a point in New York City
and a point in London to an overall
accuracy of within about 1 foot. Ob
viously, precision of this order re
quires extensive and very careful
procedures on the part of an instru
ment manufacturer.
Since the stability of the time base
in the Model 524D is rated in terms
of a week, it is necessary in proving
these time bases to conduct sufficient
tests to assure that ratings will be
met over a week's time when the
instrument is in service. Because the
heart of the time base is a quartz
plate and because newly-cut-andprocessed plates usually have not as
sumed their final character, such as
surance means that tests must be
conducted over a several-to-many
week period on each plate and that
test results must be significantly bet
ter than ratings, as dictated by good
engineering practice.
As supplied by a crystal processor,
quality quartz plates are usually
rated as to their equivalent electrical
units— inductance, Q (activity), reso
nating capacitance, etc. They may
also be rated as to aging, based on
the methods and care that the proces
sor uses. The operation of a plate in
a circuit, however, as well as the
quality of performance the plate ex
hibits during and after its break-in
period must be determined.
At -hp- the testing of high-preci
sion quartz plates is carried out in
three main steps. Newly-received
Daily measurements of crystal frequency are made and recorded while plate
is continuously operated in test stands for several-week period. Before aging,
crystal temperature characteristic is plotted with automatic set-up (fore
ground) and proper oven temperature determined.
plates are connected into special
test instruments that first measure
their Q and electrical values as a
preliminary test of general accept
ability. The operating temperature
of a plate is then continuously in
creased from room temperature to
insure that the plate Q does not ex
hibit any irregularities or any se
vere decreases up to the temperature
at which it will be operated. This
procedure insures that the plate
characteristics do not change in such
a way as to cause the oscillator to
stop oscillating during some portion
of a future warm-up.
The second step is a detailed ex
amination of the temperature char
acteristic of the plate. While operat
ing in a test circuit, the plate is
heated to a high but safe tempera
ture and allowed to cool at a slow
rate which previous experience has
shown to be satisfactory. During the
cooling period the frequency change
of the crystal is automatically plot
ted against temperature in order to
determine its temperature coeffici
© Copr. 1949-1998 Hewlett-Packard Co.
ent characteristic as well as its zerocoefficient temperature at which the
plate will eventually be operated.
At the same time the operation of
the plate is carefully observed to in
sure that no coupling to unwanted
vibrational modes exists and that no
mechanical instability exists in its
At this point a full set of data ex
ists on the plate as to its characteris
tics, the change of these character
istics with temperature, and the
mechanical suitability of the mount
ed plate. It remains to determine
that the plate is stable over long in
tervals. To accomplish this, the
plate is installed in its final oven
which has been pre-adjusted to oper
ate at the zero-coefficient tempera
ture determined from the earlier
tests. The oven is then inserted in
special test stands wherein the plate
is operated at its rated frequency (1
megacycle in the case of the Model
524D). The test stand continuously
operates the plate and includes
(Concluded on page 6)
Typical crystal temperature characteristic
automatically plotted with equipment
shown on page 5.
(Continued from page 4)
from its time base. The aging rate
of the new time base is sufficiently
low that calibration need be done
only infrequently, probably less
than once a month, for the instru
ment to retain very high accuracy.
-hp- MODEL 524D
(Basic Unit for Frequency Measure
ments from 0 cps to 10.1 me)
Range: 10 cps to 10.1 me.
Gafe Time: 0.001, 0.01, 0.1, 1, 10 seconds or
manual control.
Accuracy: ±1 count ± 0.000 005%.
Reads In: Kilocycles; decimal point automati
cally positioned.
PERIOD MEASUREMENT (without plug-in units)
Range: 0 cps to 10 ke.
Gafe Time: 1 or 10 cycles of unknown.
Accuracy: ±0.3% (measurement one period).
±0.03% (ten period average).
Standard Frequency Counted: 10 cps, 1 kc, 100
kc, or 10 me; or externally applied frequency.
Reads in: Seconds, milliseconds, or microsec
onds; decimal point automatically positioned.
Registration: 8 places (99,999,999 maximum
high -precision frequency -measuring
equipment so that the day-to-day changes
in the vibrating frequency can be deter
mined. Typically, for "good" plates of the
type used in the Model 524D, a daily
change in frequency of about 1 part in
108 will occur in the earlier part of the
run-in period. In plates of this quality
these day-to-day changes are usually at
tributed to changes in the lattice structure
and will normally occur in the same di
rection and thus be additive. As the plate
continues to operate, the day-to-day
changes become smaller and usually taper
off into changes that are only a few parts
in lO9. When a plate has stabilized such
that it operates within 2.5 parts in 108 per
week for a two-week period, it is consid
ered to have aged satisfactorily and is
released for production instruments, pro
vided its history during the stabilization
period is entirely free of any erratic per
formance. Some plates, for example, may
show a sudden, discrete shift in frequency
and these are not released, at least until a
much longer testing period has shown
them to be satisfactory.
When the crystal in its oven is now in
stalled in a production counter, it is al
At the same time the new time base
is provided in an instrument that is
the industry's first and most proved
high-speed frequency counter. Com
bined with its heterodyne convert
ers, it can be used to measure fre
quencies up to 220 megacycles. It
can also be used with the -hp- trans
Stability: Short term: ±3 parts in 10"; long
term: -±5 parts in lO1* per week; may be
standardized against WWV or used with ex
ternal 100 kc or 1 me primary standard.
Display Time: Variable 0.1 to 10 seconds but
not less than gate time. Display can be held
Output Frequencies: Secondary standard fre
quencies available at front panel: 10 cps, 1
kc rectangular; 100 kc positive pulse; 10 me
sine wave. (Stability as above.)
Self Check: Panel control provides automatic
count of internal standard 100 kc and 10 me
frequencies to assure accuracy of gate and
proper operation of counter.
Input Voltage: 1 volt rms minimum/1.5 volt
peak. Rise time .2 sec (max.).
Input Impedance: Approximately 1 megohm,
40 ¿¿/¿f shunt capacitance.
External Standard: 100 kc or 1 me signal from
external primary standard can be applied to
unit for highest accuracy. 2 volts rms re
quired. Nominal input impedance: 1 meg
ohm, 40 /¿Mà shunt.
Connectors: BNC Type.
Power Supply: 115/230 volts ±10%, 50-60
cps, approximately 600 watts.
Dimensions: Cabinet Mount: 20" wide, 21 W
high, 23'/2" deep overall.
Rack Mount: 19" wide, 191/4" high, 22V2"
deep behind panel.
Weight: Cabinet Mount: Net 118 Ibs., Shipping
168 Ibs.
Rack Mount: Net 108 Ibs., Shipping 172 Ibs.
© Copr. 1949-1998 Hewlett-Packard Co.
lowed to fully reach its oven temperature
and then adjusted to be within 1 part in
10* of its f, value in the new circuit. The
complete frequency counter is then con
nected to a power-cycling device which
applies power to the instrument for
alternate half-hour periods for 4 days.
The instrument is thus turned on some 96
times and turned off a like number of
times. This test is designed as an overall
check on the complete instrument as well
as the crystal oscillator. At the end of this
period the oscillator frequency is again
measured and adjusted to be within a few
parts in 109.
From this time on during the remain
der of testing that the instrument under
goes the oscillator must remain within 2
parts in 108 in order to be released at
final test.
It can be seen from the foregoing that
-hp- time bases are put through extremely
rigorous tests indeed to insure that ratings
can be depended on in field use. Further,
since quartz plates that are free of imper
fections and properly operated circuitwise have a broad history of improving
with time, their performance can, in gen
eral, be expected to improve with time up
to the limits set by the oven itself.
fer oscillator to measure up to 12
kilomegacycles. At the other ex
treme the counter can be used with
its period multiplier unit to meas
ure the average period of low fre
quencies to high accuracy.
—LaThare N. Bodily and
Leonard S. Cutler
Accessories Furnisfiwf: -hp- AC-16K Cable As
sembly, 48" RG-58/U cable, terminated each
end with UG-88/U BNC male Connectors.
Plun-in Units: -hp- 525A Frequency Converter,
10 cps to 100 me. $250.00.
-hp- 525B Frequency Converter, 100 me to
220 me. $250.00.
-hp- 526A Video Amplifier, 10 cps to 10.1 me.
-hp- 5268 Time Interval Unit, 1 fisec. to 107
sec. $175.00.
-hp- 526C Period Multiplier, 0 cps to 10 kc.
Complementary Equipment: -hp- 540A Transfer
Oscillator, Cab:net Mount, $615.00.
Rack Mount, $600.00.
-hp- Model 440A Detector Mount, $85.00.
-hp- 560A Digital Recorder, Cabinet Mount,
Rack Mount, $1,250.00.
Accessories Available: Test Cable Set to inter
connect -hp- 524 and 525/526 units during
test and adjustment of auxiliary units, Soecify -hp- 524B-16P, 16 conductor cable $20.00,
and -ho- 52¿B- 16Q, 8 conductor cable $15.00.
-hp- 524D-95A, Model 560A Digital Recorder
Adopter Kit for field installation, $35.00.
Price: -hp- 524D Electronic Counter, Cabinet
Mount, $2,150.00.
Rack Mount, $2,125.00.
For Model 560A Digital Rororder operation,
add $75.00, specify C03-524D.
Prices f.o.b. Palo Alto, California.
Data subject to change without notice.
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