Instrument Functions
R&S
®
NRP2
73User Manual 1173.9157.02 ─ 03
signal. Samples at equal time intervals over each acquisition interval are taken. From the
sum of the samples partial measurement results are formed. By forming the difference
from adjacent acquisitions, an improved result is achieved, that is, the DC offset voltage
and 1/f noise of the sensor amplifiers are significantly reduced. Depending on the
selected averaging number, a multiple of these results is averaged to further reduce
noise, see chapter 4.2.3, "Filter / averaging", on page 79.
The length of the acquisition interval depends on the set measurement mode:
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 signal
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 applications
With low-frequency modulation, the aperture time should match the modulation period to
optimally suppress modulation-induced variations of the measurement result, see chap-
ter 4.2.1.2, "Continuous average-power measurements", on page 66.
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-specific 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 averaging -
Data acquisition and parameters
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