Orban OPTIMOD-FM 8500S, Руководство по эксплуатации

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5-4
TROUBLESHOOTING ORBAN MODEL
Whistle on Air, Perhaps Only in Stereo Reception
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.
Interference from stereo into SCA
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
program correlated noise is
less than –80 dB below
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 sufficient
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.
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.
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
Figure 5-1: Typical 8500S baseband spectrum with
heavy processing, 0-100 kHz.