Measurements On Fm Signals - Rohde & Schwarz R&S FSP Series Operating Manual

R&S FSP
3. Activate the marker function for AM depth measurement
2.5.2

Measurements on FM Signals

Since Spectrum Analyzers only display the magnitude of signals by means of the
envelope detector, the modulation of FM signals cannot be directly measured as is
the case with AM signals. With FM signals, the voltage at the output of the envelope
detector is constant as long as the frequency deviation of the signal is within the flat
part of the passband characteristic of the resolution filter which has been selected.
Amplitude variations can only occur if the current frequency lies on the falling edge
of the filter characteristic. This effect can be used to demodulate FM signals. The
center frequency of the R&S FSP is set in a way that the nominal frequency of the
test signal is on the filter edge (below or above the center frequency). The resolution
bandwidth and the frequency offset are selected in a way that the current frequency
is on the linear part of the filter slope. The frequency variation of the FM signal is
then transformed into an amplitude variation which can be displayed in the time
domain.
The R&S FSP's analog 4
have a good filter-slope linearity, if the frequency of the R&S FSP is set to 1.2 times
the filter bandwidth below or above the frequency of the transmit signal. The useful
range for FM demodulation is then almost equal to the resolution bandwidth.
2.26
Press the SPAN key and enter 5 kHz.
Press the MEAS key.
Press the MODULATION DEPTH softkey.
The R&S FSP automatically positions a marker on the carrier signal in the
middle of the graph and one delta marker on each of the lower and upper AM
sidebands. The R&S FSP calculates the AM modulation depth from the ratios
of the delta marker levels to the main marker level and outputs the numerical
value in the marker info field
Fig. 2-3 Measurement of AM modulation depth. The modulation depth is indicated
by MDEPTH = 49.346 %. The frequency of the AF signal is indicated by the
delta markers
th order filters with frequencies from 300 kHz to 3 MHz
Getting Started
Measurements on Modulated Signals
Operating Manual 1164.4556.12 - 06