General Description
R&S
®
CMW-KM645
13
User Manual 1173.9740.02 ─ 04
3.5.2 RF Modulation Results
The "RF Modulation Results" characterize the properties of the audio signal which
modulates the RF carrier (baseband signal).
Fig. 3-5: FM Stereo Radio: RF Modulation Results
The table contains the following results:
●
Pilot Deviation, RMS * SQRT2:
Frequency deviation of the RF signal due to the
19 kHz pilot tone in the baseband signal. The pilot tone causes a harmonic fre-
quency deviation; therefore the RMS average times the square root of 2 provides a
very accurate estimate of the peak deviation.
●
RDS Deviation:
Frequency deviation of the RF signal due to the signal in the
Radio Data System (RDS) band around 57 kHz.
●
Audio Left Deviation,
±
Peak / 2:
Frequency deviation due to the left audio chan-
nel, half of the peak-to-peak value. See also
●
Audio Right Deviation,
±
Peak / 2:
Frequency deviation due to the right audio
channel, half of the peak-to-peak value.
●
Pilot Frequency Error:
Frequency error of the pilot signal relative to the rated fre-
quency of 19 kHz.
FM baseband spectrum
The FM stereo baseband signal is built from two audio channels L (left) and R (right)
and a pilot tone with constant frequency.
●
The "mono" audio signal is the sum of the left and right channels: M = (L + R) / 2. It
occupies the frequency range between 30 Hz and 15 kHz. In a stereo transmitter
this signal is directly fed to the FM modulator.
●
The "stereo" difference signal S = (L – R) / 2 modulates a 38 kHz subcarrier using
the Double Sideband Suppressed Carrier (DSSC) AM modulation scheme. The
subcarrier is suppressed in the transmitted FM signal.
●
A 19 kHz "pilot tone" provides the frequency and phase reference for the 38 kHz
subcarrier in the transmitter. It is also used to recover the suppressed subcarrier in
the FM stereo decoder.
Measurement Results
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