Analog Demodulation (Option K7) R&S FSL
1300.2519.12 4.142 E-11
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 is displayed.
• AC coupling is possible with FM and PM display. The deviations are determined from the trace
data. +Peak, –Peak, ½ Peak–Peak and RMS are displayed.
Demodulation bandwidth
The demodulation bandwidth is not the 3 dB bandwidth 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)
Note: If the center frequency of the analyzer is not set exactly to the signal frequency, the demodulation
bandwidth must be selected larger 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 dramatically with the bandwidth,
especially with FM.
AF trigger
The analog demodulation option allows triggering to the demodulated signal. The display is stable if a
minimum of five modulation periods are within the recording time.
Triggering is always DC–coupled. Therefore triggering is possible directly to the point where a specific
carrier level, phase or frequency is exceeded or not attained.
Stability of measurement results
Despite amplitude and frequency modulation, the display of carrier power and carrier frequency offset is
stable.
This is achieved by a digital filter which sufficiently suppresses the modulation, provided, however, that
the measurement time is
3 x 1 / modulation frequency, i.e. that at least three periods of the AF signal
are recorded.
The mean carrier power for calculating the AM is also calculated with a digital filter that returns stable
results after a measurement time of
3 x 1 / modulation frequency, i.e. at least three cycles of the AF
signal must be recorded before a stable AM can be shown.
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