Orban OPTIMOD-FM 5518, Operating Manual

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5518-FM DIGITAL TROUBLESHOOTING
5-3
You may be overdriving the 5518’s left/right protection limiter and/or composite lim-
iter. It is unwise to do more than 3 dB of gain reduction in the protection limiter or
to set the
C
OMPOSITE
L
IMIT
D
RIVE
control higher than
1.0
DB
. Review the level match-
ing and setup instructions in section 2 of this manual starting on page 2-11.
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 5518 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 5518 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 5518’s composite output to verify that
program correlated noise is
less than –80 dB below 100%
modulation from 57 to 100
kHz. Any inadvertent compos-
ite clipping will dramatically
degrade this protection. Make
sure that the link between the
5518’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 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
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 5518 baseband spectrum with
heavy processing, 0-100 kHz.