Rohde & Schwarz FSQ series, Quick Start Manual

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R&S FSQ Basic Measurement Examples
Measurements in the Time Domain
5.30 Quick Start Guide 1313.9652.62 - 02
5.5.5 Measurement of FM-Modulated Signals
Since analyzers can display only the magnitude of the measurement signal by using the
envelope detector, the modulation of FM-modulated signals cannot be measured directly as
in the case of AM-modulated signals. The voltage at the output of the envelope detector
remains constant for FM-modulated signals as long as the frequency deviation of the signal is
located within the flat part of the passband characteristic of the resolution filter that is used.
Amplitude variation occurs only if the instantaneous frequency extends into a falling edge of
the filter curve. This behavior can be used to demodulate FM-modulated signals. The center
frequency of the R&S FSQ is set in such a manner that the nominal frequency of the
measurement signal is located on a filter edge (below or above the center frequency). The
resolution bandwidth and the frequency offset must be selected in such a manner that the
instantaneous frequency is located in the linear part of the filter edge. As a result, the
frequency variation of the FM-modulated signal is transformed into an amplitude variation that
can be displayed on screen in the time domain.
In the case of the four-circuit filters from 200 kHz to 3 MHz implemented as analog filters, you
can obtain good linearity of the filter edge if the frequency of the R&S FSQ 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 approx. equal to the resolution bandwidth.
5.5.5.1 Example - Display of the AF of an FM-Modulated Carrier
Test setup:
Settings on the signal generator (e.g. R&S SMIQ):
Measurement with the R&S FSQ:
1. Reset the instrument.
➢
Press the PRESET key.
The R&S FSQ is now in the default state.
2. Set the center frequency to 99.64 MHz and the span to 300 kHz.
➢
Press the FREQ key and enter 99.64 MHz .
➢
Press the SPAN key and enter 300 kHz.
Frequency: 100 MHz
Level: -30 dBm
Modulation: FM 0 kHz deviation (i.e. FM modulation is deactivated), 1 kHz AF
Signal
generator
R&S FSQ