OPTIMOD-FM TROUBLESHOOTING
5-5
The 8400 uses independent composite DACs for the Composite 1 and Com-
posite 2 outputs. Therefore, a problem of this nature would likely show up at
only one composite output.
A whistle could also be caused by power supply oscillation, STL problems, or exciter
problems.
Interference From Stereo Into SCA
A properly operating 8400 generates an immaculately clean baseband, with program-
correlated noise below –80 dB above 57 kHz even when the composite limiter is used
aggressively. If the 8400 and the rest of the transmission system are operating correctly,
subcarriers should experience no interference.
Interference from the stereo into a subcarrier is best diagnosed with a spectrum analyzer.
First examine the spectrum of the 8400’s composite output to verify that program corre-
lated noise is less than –80 dB below 100% modulation from 57 to 100 kHz. Any inad-
vertent composite clipping will
dramatically degrade this protec-
tion. Make sure that the link be-
tween the 8400’s composite output
and the transmitter has sufficient
headroom.
If the exciter is nonlinear, this can
cause crosstalk. In general, a prop-
erly operating exciter should have
less than 0.1% THD at high fre-
quencies to achieve correct opera-
tion with subcarriers.
To prevent truncation of the higher-
order Bessel sidebands of the FM
modulation, the RF system follow-
ing the exciter must be wideband
(better than ±500 kHz) and must
have symmetrical group delay around the carrier frequency. An incorrectly tuned trans-
mitter can exhibit an asymmetrical passband that will greatly increase crosstalk into sub-
carriers.
Amplitude modulation of the carrier that is synchronous with the program (“synchronous
AM”) can cause program-related crosstalk into subcarriers. Synchronous AM should be
better than 35 dB below 100% modulation as measured on a synchronous AM detector
with standard FM de-emphasis (50μs or 75μs).
The subcarrier receiver itself must receive a multipath-free signal, and must have a wide
and symmetrical IF passband and a linear, low-distortion FM demodulator to prevent
program-related crosstalk into subcarriers.
SRS
57.088 kHz -72.881 dBVpk
0 Hz
FFT 1 Log Mag BMH PkhAvg 20000
51.2 kHz 102.4 kHz
-100
dBVpk
0
dBVpk
10
dB/div
Fig. 5-1: Typical 8400 baseband spectrum with
heavy processing, 0-100 kHz.
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