User manual

Analog Modulation
R&S
®
Spectrum Rider FPH
223User Manual 1321.1011.02 ─ 05.00
10.1 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. See Table 10-2.
Therefore the following formulas apply:
●
AM: demodulation bandwidth ≥ 2 x modulating frequency
●
FM (CARSON's rule): demodulation bandwidth ≥ 2 x (frequency deviation + highest
modulating frequency)
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.
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.
10.2 Sample Rate, Measurement Time and Audio Lowpass
Filter
Depending on the sample rate, the maximum demodulation bandwidths listed in the
Table 10-2 can be obtained during the measurement. The permissible value range of
the measurement time depends on the selected demodulation bandwidth and audio
lowpass filter.
Example: AM modulation with a modulating frequency of 1kHz
1. Calculate the demodulation bandwidth you require.
Demodulation Bandwidth = 2 x 1 kHz
2. Select the minimum demodulation bandwidth from table Table 10-2.
As a result, the demodulation bandwidth of "3000" is selected from the table. This
shows that a minimum measurement time of 1.31 seconds is required to capture
3125 of sample rate
3. Select the appropriate audio lowpass filter from Table 10-3.
4. Depending on the sample rate and modulating frequency range, "DBW/1" is
selected as the audio lowpass filter.
5. If a faster measurement time is desired, e.g. 6 ms, the audio lowpass, "DBW/10"
should be selected to fulfill the requirement on the modulating frequency.
Sample Rate, Measurement Time and Audio Lowpass Filter