Analog Demodulation
R&S
®
FPC
152
User Manual 1178.4130.02 ─ 07
Selecting the demodulation bandwidth
The demodulation bandwidth (DBW) determines the frequency range around the cen-
ter frequency that is demodulated. It is not the 3 dB bandwidth of the filter but the use-
ful bandwidth which is distortion-free regarding phase and amplitude.
Basically, 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 dramati-
cally with the bandwidth, especially with FM.
For AM, the DBW should be
≥
2 x modulation frequency
For FM, the DBW should be
≥
2 x (frequency dev modulation frequency)
1. Press the "BW" key to open the bandwidth menu.
2. Select the "Manual DBW" menu item.
3. Enter the demodulation bandwidth you need.
If you enter a value that is not supported, the R&S
FPC rounds to the next suppor-
ted bandwidth.
Note that the selected bandwidth also defines the output sample rate and mea-
surement time (see
).
Selecting the audio lowpass filter
The audio lowpass filter is a decimating filter that improves the measurement of low
modulation frequencies.
contain information about the different audio frequency ranges
as a function of the demodulation bandwidth and the audio filter.
In most cases, the audio filter with a "DBW/1" characteristic is sufficient. If you need to
measure low modulation frequencies that are below the DBW/1 range, consider to
select another audio filter that fits your requirements. When you are using the "DBW/
10", "DBW /30" or "DBW/100" audio filter, both, the lower and the upper audio corner
frequencies decrease by a factor of /10, /30, or /100, respectively.
1. Press the "Meas" key to open the measurement menu.
2. Select the "Audio Lowpass" menu item.
3. Select the filter you want to apply ("DBW/1", "DBW/10", "DBW/30" or "DBW/100)").
Example:
Assume that the modulating frequency of an AM signal is 1 kHz.
For proper demodulation, the demodulation bandwidth must be at least 2
kHz (2 x
modulation frequency).
As a 1
kHz DBW is not available, select the next higher one (10
kHz).
You can use the default "DBW/1" audio filter, because that filter supports a frequency
range from 30
Hz to 5
kHz (see
If you have to measure lower modulation frequencies, use a different audio filter. With
the "DBW/10" filter, for example, the audio frequency measurement range is reduced
by a factor of 10 to a range of 3 Hz up to 500 Hz (see
Bandwidth Configuration
