Evolution in power measurement – intelligent sensor technology

Evolution in power measurement – intelligent sensor technology
GENERAL PURPOSE
Power meters
Microwave power meters are only
as good as their sensors, which is
why they were the focus of devel43 877/7
opment for the new Power Meter
FIG 1
A powerful team: R&S NRP with 18 GHz Power Sensor R&S NRP-Z21
Series R&S NRP. The sensors offer
a dynamic range of up to 90 dB
for modulated signals of any RF
Power Meter R&S NRP
bandwidth, plus time gating, high
Evolution in power measurement –
intelligent sensor technology
measurement speed and low
measurement uncertainty. Whether
Intelligent sensors herald a new
generation
used for digital mobile radio, wireless LANs, or classic applications:
Digital mobile radio triggered a flood
of developments in RF test and measurement engineering, which have also
affected power meters. At first, it was
the time structure of the test signals that
presented new challenges. Today it is
these sensors set new standards in
terms of versatility and accuracy.
News from Rohde&Schwarz
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Number 174 (2002/II)
the broadband modulation techniques of
third-generation mobile radio. And this is
only the beginning. Wireless LANs with
RF bandwidths of more than 100 MHz
are already being discussed.
The problems cannot be solved by
conventional sensor designs, especially if you want to keep the great-
Power
est advantage of power meters, their
high measurement accuracy. For that
reason, Rohde & Schwarz again takes
an extremely innovative approach with
the new generation, comparable to the
introduction of intelligent sensors for
the URV5 and NRV models in the early
1980s. All the signal processing is relocated into the sensor, which is the key
to exploiting the potential of multiplepath sensor technology. The link to the
basic unit or any controller is established
via the standard USB interface (universal serial bus). The new family of power
meters launches with the universal Sensors NRP-Z11 (-Z21) from 10 MHz to
8 (18) GHz and a basic unit of futureoriented design (FIG 1).
P1
P2
P3
P4
P5
P6
P7
P8
Time
Exclusion period
FIG 2 Multislot measurement: for the usual time division methods (e.g. GSM / EDGE, DECT),
average power can be measured in all timeslots at the same time.
able –67 dBm. And this does not change
much if the power is to be measured
within a single GSM timeslot (FIG 3).
Even for signal-triggered measurement
of the mean power of single bursts or
the generation of a power/time template,
a wider dynamic range is available than
with conventional designs.
90 dB dynamic range
If it is true that the popularity of a power
sensor grows with its dynamic range,
then the NRP-Z11 and NRP-Z21 stand
a very good chance of becoming real
favourites. For the first time, a range
of 90 dB for broadband modulated signals has been achieved, while the lower
measurement limit (defined by noise
and zero offset) remains a very respect-
Signal-synchronized
measurements
The NRP-Z11 and NRP-Z21 sensors can
measure the average power not only
in the classic manner, i.e. continuously
without temporal reference to the signal
content, but also synchronized with the
signal over definable periods of time.
Up to 128 intervals (26 when controlled
by the basic unit) can be acquired and
measured at one go (FIG 2). This allows
entire frames of GSM / EDGE signals to
be analyzed. Unwanted portions in the
transition from one timeslot to the next
can be blanked by freely definable exclusion periods at the beginning and end.
To measure the power/time template of
recurring or non-recurring waveforms
(FIG 4), the number of test intervals
Dynamic range for measuring average power
Bandwidth of test signal 100 MHz/5 MHz/0 (CW)
Technology ↓
Mode
→
Continuous
Timeslot
1 out of 8
(external trigger)
Burst
duty cycle 1:8
(internal trigger)
50 / 50 / 50 dB
–
–
Sensor in square-law region
43 / 43 / 50 dB
–
–
CW sensor
43 / 43 / 90 dB
–
–
Peak sensor
33 / 50 / 80 dB
– / 50 / 57 dB
– / 33 / 37 dB
Multiple-path sensor
80 / 80 / 80 dB
–
–
R&S smart sensor technology
90 / 90 / 90 dB
85 / 85 / 85 dB
60 / 60 / 60 dB
Thermoelectric sensor
Diode
FIG 3 Dynamic range of various sensor technologies as a function of the RF bandwidth of the test
signal (peak-to-average ratio always 7 dB).
News from Rohde&Schwarz
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Number 174 (2002/II)
Power vs. time
256 points
(external trigger)
– / 50 / 57 dB
70 / 70 / 70 dB
GENERAL PURPOSE
Power meters
(points) can be increased to 1024; signal
details down to durations of about 10 µs
can thus be resolved. Extensive trigger functions, derived from an external
source or the test signal, ensure stable
conditions.
The NRP sensors mark a great step
towards resolving these problems. The
expression smart sensor technology (see
opposite page) stands for a whole series
of measures intended to give the sensors the behaviour characteristic of thermal sensors. This includes very accurate measurement of average power,
regardless of modulation (FIG 5), and
high immunity to incorrect weighting
of harmonics, spurious and other interference signals. The maximum speed
of up to 1500 triggered measurements
per second nevertheless equals that of
diode sensors (in buffered mode, measurement interval 2 x 100 µs).
High system accuracy
The small measurement uncertainty of
broadband power meters will continue
to be the decisive argument for their use.
In practice, the data sheet specifications
of about 2 % (0.09 dB) for unmodulated,
spectrally pure signals of well-matched
sources can seldom be achieved. This is
due to those error sources that relate to
the test signal or external circuitry: harmonics and nonharmonics, modulation,
mismatch of the source, and the influence of attenuators and directional couplers connected ahead of the sensor for
level matching.
The effect of mismatched sources is
reduced to the extent technically feasible by the sensors’ small SWR (max.
1.13 between 30 MHz and 2.4 GHz),
which is largely independent of the
power to be measured. Despite this, the
value given still results in an uncertainty
0.2
Error / dB
0.15
0.1
0.05
0
–0.05
–0.1
–0.15
–0.2
– 60
–50
– 40
–30
–20
–10
0
10
20
Power level / dBm
FIG 5 Modulation-related errors of an NRP-Z11 or NRP-Z21 power sensor for a 3GPP test signal
(test model 1–64) compared to a CW signal of the same magnitude.
Red: default setting; yellow: transition area between measurement paths shifted by –6 dB;
light blue: uncertainty caused by noise (modulation effect below –30 dBm negligible).
News from Rohde&Schwarz
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Number 174 (2002/II)
FIG 4 Power/time template of a nonrecurring RF burst for an application in medical electronics, measured with the NRP-Z11 (LabView
application without basic unit; readings in W
and ms, no averaging).
of ±4 % (0.17 dB) on a source with an
SWR of 2. This value, which dominates
all other errors, can now be reduced
to almost zero with the NRP sensors, if
the complex reflection coefficient of the
source is transmitted to the sensors via
the USB data interface. The sensors
then correct the matching error, taking
into consideration their own impedance
mismatch.
There is a similar problem if the sensor
cannot be connected directly to the
source, and a level-matching attenuator or connecting cable is necessary. In
this case, the interactions between three
components must be taken into account
– a non-trivial bit of mathematics involving complex numbers. Here, too, the
user is offered a straightforward, standardizable solution. With the help of
a small software tool that runs on any
PC, the complete S-parameter data set
of the twoport connected ahead can be
loaded into the sensor’s memory in the
common s2p (Touchstone) format, which
every vector network analyzer can provide, and is then taken into account in
the measurement. After the source’s
complex reflection coefficient has been
transmitted, a perfectly corrected reading of maximum accuracy is obtained.
GENERAL PURPOSE
Power meters
Costs per measurement halved
Universal basic unit
And evolution continues
The price of a power meter that meets
the requirements of modern communications technology is substantial, and
is a sizeable part of the total cost of an
RF measurement system. Users consequently often save in the wrong place,
and shift the job of power measurement to other, less accurate instruments, or keep the number of test points
low. There is no need for such compromises with the NRP sensors, since these
can be operated directly on a controller, which is usually available anyway,
thus saving the cost of the basic unit.
One of the two USB adapters (NRP-Z3
or NRP-Z4) and the software tool kit
included as a standard accessory are
required for controlling by a PC. The
software tool kit comes with both a
DLL (dynamic link library) for individualized use of the entire sensor functionality under Windows™, and the Power
Viewer, a virtual power meter with basic
measurement functions for the PC workstation (FIG 7).
For those applications requiring a basic
unit, the R&S NRP offers everything you
expect from a modern power meter –
and much more. No other instrument is
as small, lightweight and rugged, and
the optional battery pack ensures several hours of operation without line
power. The NRP has a Windows-style
menu interface, a high-resolution graphical display, and is operative in seconds,
making it a pleasure to use. Depending
on requirements, it can be fitted with
one, two or four measurement inputs,
an IEC/IEEE bus port being provided as
standard. The shortest measurement
time, from triggering to readout of the
result, is 4 ms; only one modulation
period is needed to measure very lowfrequency modulated signals.
The new family will be expanded continuously, starting with the extension of the
frequency range. Sensors with upstream
power attenuators as well as DC-coupled thermal sensors will be available.
Since the influence of the basic unit is
nonexistent, the latter will be the most
accurate power references commercially
available. Display of power versus time
and remote control via the USB interface
or Ethernet (optional) will round off the
functionality of the basic unit.
Thomas Reichel
FIG 7
More information and data sheet at
www.rohde-schwarz.com
(search term: NRP)
The Power Viewer turns any PC (under Windows 98 / 2000 / ME / XP) into a power meter.
News from Rohde&Schwarz
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Number 174 (2002/II)
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