5-4
TROUBLESHOOTING
transmission system. An uncompressed digital STL will perform much better than any
analog STL. (See
Studio-Transmitter Link,
starting on page 1-15.)
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 DACs or other analog circuitry. If it appears on-
ly when you use the composite output, then it is likely a problem in the composite
DACs 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 8700i 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 8700i 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 an-
alyzer. First examine the spectrum of the 8700i’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 dramatically degrade this
protection. Make sure that the link be-
tween the 8700i’s composite output
and the transmitter has sufficient
headroom.
If the exciter is nonlinear, this can
cause crosstalk. In general, a properly
operating exciter should have less than
0.1% THD at high frequencies to
achieve correct operation with subcar-
riers.
To prevent truncation of the higher-
order Bessel sidebands of the FM mod-
ulation, 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.
Figure 5-1: Typical 8700i baseband spectrum
with heavy processing, 0-100 kHz.
Содержание OPTIMOD-FM 8700
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