Interference From Stereo Into Sca; Shrill, Harsh Sound - Orban Optimod-FM 8300 Operating Manual

5-4
TROUBLESHOOTING

Interference from stereo into SCA

Shrill, Harsh Sound

A properly operating 8300 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 8300 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 8300's composite output to verify that
57.088 kHz
0
dBVpk
10
dB/div
-100
0 Hz
dBVpk
FFT 1 Log Mag BMH
Fig. 5-1: Typical 8300 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.
Amplitude modulation of the carrier that is synchronous with the program ("syn-
chronous AM") can cause program-related crosstalk into subcarriers. Synchronous
AM should be better than 35 dB below 100% modulation as measured on a syn-
chronous AM detector with standard FM de-emphasis (50s or 75s).
The subcarrier receiver itself must receive a multipath-free signal, and must have a
wide and symmetrical IF passband and a linear, low-distortion FM demodulator to
prevent program-related crosstalk into subcarriers.
If you are using the Multi-Band structure, this problem can be caused by excessive
HF boost in the HF Equalizer and HF Enhancer. It could also be caused by an exces-
sively high setting of the BAND 4 THRESH control, or by excessively high settings of
the BAND 4 MIX and BAND 5 MIX controls (located in Intermediate and Advanced
Modify).
-72.881 dBVpk
51.2 kHz 102.4 kHz
PkhAvg 20000
ORBAN MODEL 8300
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 8300's
sufficient