Demodulation Bandwidth - R&S FSW-K7 User Manual

®
R&S FSW-K7
The AM DC, FM DC and PM DC raw data of the demodulators is fed into the "Trace
Arithmetic" block that combines consecutive data sets. Possible trace modes are:
Clear Write, Max Hold, Min Hold and Average. The output data of the "Trace Arith-
metic" block can be read via remote control ([SENS:]ADEM:<evaluation>:RES?,
see
[SENSe:]ADEMod<n>:AM[:ABSolute][:TDOMain]:RESult?
The collected measured values are evaluated by the selected detector. The result is
displayed on the screen and can be read out via remote control.
In addition, important parameters are calculated:
●
A counter determines the modulation frequency for AM, FM, and PM.
●
average power = carrier power (RF power)
●
average frequency = carrier frequency offset (FM)
●
The modulation depth or the frequency or phase deviation; the deviations are
determined from the trace data
AC coupling is possible with FM and PM display.

4.2 Demodulation Bandwidth

The demodulation bandwidth determines the span of the signal that is demodulated. It
is not the 3 dB bandwidth of the filter but the useful bandwidth which is distortion-free
with regard to phase and amplitude.
Therefore the following formulas apply:
●
AM: demodulation bandwidth ≥ 2 x modulation frequency
●
FM: demodulation bandwidth ≥ 2 x (frequency deviation + modulation frequency)
●
PM: demodulation bandwidth ≥ 2 x modulation frequency x (1 + phase deviation)
If the center frequency of the analyzer is not set exactly to the signal frequency, the
demodulation bandwidth must be increased by the carrier offset, in addition to the
requirement described above. This also applies if FM or PM AC coupling has been
selected.
In general, the demodulation bandwidth should be as narrow as possible to improve
the S/N ratio. The residual FM caused by noise floor and phase noise increases dra-
matically with the bandwidth, especially with FM.
For help on determining the adequate demodulation bandwidth see

demodulation bandwidth"

A practical example is described in
an FM
User Manual 1173.9240.02 ─ 23
on page 176.
Signal", on page 170.
Chapter 9, "Measurement Example: Demodulating
Measurement Basics
Demodulation Bandwidth
on page 307.
"Determining the
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