Whistle On Air, Perhaps Only In Stereo Reception; Interference From Stereo Into Sca - Orban OPTIMOD-FM 8500S Operating Manual

5-4
TROUBLESHOOTING

Whistle on Air, Perhaps Only in Stereo Reception

Interference from stereo into SCA

The most likely cause is oscillation in the analog input or output circuitry. If the oscil-
lation is in the output circuitry and is between 23 and 53 kHz, it will be detected in a
receiver's stereo decoder and translated down into the audible range.
If you encounter this problem, check the analog or digital outputs with a spectrum
analyzer to see if the spurious tone can be detected here. If it appears at both out-
puts, it is probably an input problem. If it only appears at the analog output, then it
is likely a problem with the left/right D/A converter or other analog circuitry. If it
appears only when you use the composite output, then it is likely a problem in the
composite D/A converter or output amplifiers.
A whistle could also be caused by power supply oscillation, STL problems, or exciter
problems.
A properly operating 8500S generates an immaculately clean baseband, with pro-
gram-correlated noise below –80 dB above 57 kHz even when the composite limiter
is used aggressively. If the 8500S and the rest of the transmission system are operat-
ing 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 8500S's composite output to verify that
57.088 kHz
0
dBVpk
10
dB/div
-100
0 Hz
dBVpk
FFT 1 Log Mag BMH
Figure 5-1: Typical 8500S baseband spectrum with
heavy processing, 0-100 kHz.
To prevent truncation of the higher-order Bessel sidebands of the FM modulation,
the RF system following the exciter must be wideband (better than ±500 kHz) and
must have symmetrical group delay around the carrier frequency. An incorrectly
tuned transmitter can exhibit an asymmetrical passband that will greatly increase
crosstalk into subcarriers.
-72.881 dBVpk
51.2 kHz 102.4 kHz
PkhAvg 20000
program correlated noise is
less than –80 dB below
SRS
100% modulation from 57
to 100 kHz. Any inadvertent
composite clipping will dra-
matically degrade this pro-
tection. Make sure that the
link between the 8500S's
composite output and the
transmitter has
headroom.
If the exciter is nonlinear,
this can cause crosstalk. In
general, a properly operat-
ing exciter should have less
than 0.1% THD at high fre-
quencies to achieve correct
operation with subcarriers.
ORBAN MODEL
sufficient