Instrument Functions
NRP2
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User Manual 1173.9140.02 ─ 07
Table 4-2: Acquisition interval of the respective measurement modes
Mode
Acquisition interval identical to ...
Cont Av
aperture time (default or user-specific)
T'slot Av
nominal width x number of slots
T'gate Av
gate length
Burst Av
burst length
After one acquisition has been completed, the output polarity of the detector output sig-
nal is inverted. The next acquisition does not start until a sensor-specific settling time
has been passed. With the "T'slot Av", "T'gate Av" and "Burst Av" modes, where the
start of an acquisition must additionally be synchronized with the signal, the trigger
period or a multiple of it mainly determines the acquisition rate.
In "ContAv" mode, the acquisition interval can arbitrarily be set by defining the aperture
time. In most cases, the sensor-specific default value is sufficient for optimum perform-
ance. It is defined by the manufacturer as to effectively suppress 1/f noise. This
means, that for a given integration time (= averaging number x 2 x aperture time), the
noise content of the measurement result is minimal with the default setting.
Depending on the sensor, the default aperture time widely varies between 20 ms (for
R&S
NRP-Z11/21/22/23/24/31/32/28/98 multipath power sensors) and 10 µs (for
R&S
NRP-Z81/85/86 wideband power sensors). Thermal power sensor normally use a
5 ms default aperture time (20 ms in the past). Vastly differing averaging numbers
when using different sensor types are thus quite normal.
There are two cases where the default aperture time may not be the best selection:
●
if measuring signals with low-frequency modulation
●
if very short measurement times are needed, e.g. with remote-controlled applica-
tions
With low-frequency modulation, the aperture time should match the modulation period
to optimally suppress modulation-induced variations of the measurement result, see
chapter 4.2.1.2, "Continuous average-power measurements"
Since this may lead to acquisitions that vastly exceed the 10 µs default aperture time of
wideband power sensors, a noticeable increase of fluctuations in the measurement
result due to noise must be considered with these sensors. Eventually the sensor-spe-
cific lower measurement limit cannot be reached. The same applies for "T'slot Av",
"T'gate Av" and "Burst Av" power measurements when the acquisition rate drops down
below 10 kHz for these sensors. For details refer to the data sheet.
With R&S
NRP-Z11/21/22/23/24/31/32/28/98 multipath power sensors, a reduction of
the default aperture time may be essential when fast measurements - without averag-
ing - are needed. Taking a 100 µs idle time of these sensors into consideration, a
reduction of the aperture time even below 500 µs will merely speed up measurements
but significantly increase noise.
Smoothing modulated signals
Continuous average-power measurements on signals with low-frequency modulation
or on pulsed signals with low repetition rates may suffer from fluctuations in the mea-
Data acquisition and parameters
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