Configuring the RF Signal
R&S
®
SMBV100B
396
User Manual 1178.4460.02 ─ 03
About the measuring principle, averaging filter, filter length, and achieving stable
results
A sensor measures the average or peak RF power of the source continuously. The
measurement results are displayed in the "NRP Power Viewer" dialog.
The power viewer function uses
averaging filters
for getting a stable readout.
Measurement results could be interfered for instance by to much noise in your setup,
by a bad supression of harmonics or non harmonics or when you reach the sensitivity
level of your power sensor.
Measurements are continuously repeated in a predefined time window. The measure-
ment result is obtained by averaging the measured values for the last 2N time win-
dows. This approach is referred as a
two-step averaging process
.
The factor of 2 in the formula arises because the output signals from the microwave
detector are chopped at the same rate as the time windows to suppress low-frequency
noise. An independent measured value can only be obtained from two consecutive val-
ues.
The variable N in the formula indicates the
filter length
. The filter length then directly
influences the measurement time. The filter length can be selected automatically or it
can be manually set to a fixed value.
Depending on the R&S NRP power sensor type, the manual setting of the filter length
varies in resolution:
●
Resolution = 1 for the R&S NRPxx power sensor family
●
Resolution = 2
n
for R&S NRP-Zxx power sensors, with n = 1 to 16
Follow the following general recommendation to find out the
optimum filter length
:
●
Always start a measurement in auto mode ("Filter > Auto").
Check if the measurement results are sufficient.
●
If the power is not constant, select the filter length manually ("Filter > User").
Trigger the "Auto Once" function to search for the optimum filter length for the cur-
rent measurement conditions.
The estimated value is indicated as filter length.
●
If the target measurement accuracy value is known, select "Filter > Fixed Noise".
The averaging factor is selected automatically and so that the sensor's intrinsic
noise (two standard deviations) does not exceed the specified noise content.
●
Different sensor types achieve the same filtering result with different filter and time
window lengths.
The time window length depends on the sensor type:
–
For most sensors, it is fixed to 20 ms.
–
For the R&S NRP-Z81 sensor, it is 10
μ
s.
The R&S NRP-Z81 uses filter length that is 1000 times larger than the filter
length for other sensors.
About zeroing
Activates the auto zero function.
Improving Level Performance
