Whistle On Air, Perhaps Only In Stereo Reception; Interference From Stereo Into Sca - Orban OPTIMOD-FM 5500 Operation Manual

5-4
TROUBLESHOOTING

Whistle on Air, Perhaps Only in Stereo Reception

Interference from stereo into SCA

In this case, it is wise to use the Orban 8200ST Studio AGC to perform the AGC func-
tion prior to the STL transmitter and to control the STL's peak modulation. This will
optimize the signal-to-noise ratio of the entire transmission system. An uncom-
pressed digital STL will perform much better than any analog STL. (See Studio-
Transmitter Link, starting on page 1-15.)
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 5500 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 5500 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 5500'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 5500 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
-72.881 dBVpk
51.2 kHz 102.4 kHz
PkhAvg 20000
ORBAN MODEL 5500
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
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.
the 5500's
sufficient