Orban OPTIMOD 5750 Operating Manual page 111

5-4
Troubleshooting
Figure 5-1: Typical 5750 baseband spectrum with heavy processing, 0-100 kHz.
Interference from the stereo into a subcarrier is best diagnosed with a spectrum analyzer. First examine the
spectrum of the 5750'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 between the 5750'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 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 ("synchronous AM") can cause program-
related crosstalk into subcarriers. Synchronous AM should be better than 35 dB below 100% modulation as
measured on a synchronous AM detector with standard FM de-emphasis (50µs or 75µs).
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.
Shrill, Harsh Sound
If you are using the Five-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 excessively 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).
If you are driving an external stereo encoder with built-in pre-emphasis, you must set the 5750's output to Flat in
the System Setup > Output screen to prevent double pre-emphasis, which will cause very shrill sound (and very poor
peak modulation control).