Optimizing and Troubleshooting the Measurement
R&S
®
FSW-K7
176
User Manual 1173.9240.02 ─ 23
10 Optimizing and Troubleshooting the Mea-
surement
If the results do not meet your expectations, consider the following notes and tips to
optimize the measurement.
Determining the demodulation bandwidth
A frequent cause for measurement errors and false results is an
incorrectly defined
demodulation bandwidth
(DBW).
If the DBW is too large, the actual signal takes up only a small part of the demodulated
range. That means that any noise or additional signal parts may be included in the
measured results, which are then false.
On the other hand, if the DBW is too small, part of the signal is cut off and thus not
included in the calculation of the results.
An easy way to determine the required DBW is to display the RF spectrum of the input
signal. If the entire signal is displayed there and takes up most of the diagram width,
the DBW should be appropriate.
This procedure is demonstrated in the measurement example described in
"Measurement Example: Demodulating an FM Signal"
For further recommendations on finding the correct demodulation bandwidth see
ter 4.2, "Demodulation Bandwidth"
Adjusting the displayed span
Be aware that the span of the RF Spectrum display is not automatically increased for a
wider DBW, since it may be useful to display only a small range from the demodulated
bandwidth. However, this means the RF spectrum may not show the entire demodula-
ted bandwidth. In this case you must increase the span manually to show the entire
signal.
Determining the SINAD and THD
The signal-to-noise-and-distortion ratio (SINAD) and the total harmonic distortion
(THD) of the demodulated signal are a good indicator of the signal quality sent by the
DUT. Both values are calculated inside the AF spectrum span and thus only if an AF
spectrum window is displayed. If either value deviates strongly from the expected
result, make sure the demodulation bandwidth is defined correctly (see
