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Telemetry Radio Frequency Problem Solving
Frequent Squelch
Appearance of the 15-Hz squelch triangle waveform during
monitoring indicates a data loss for at least two sample periods.
Because of the nature of RF communications, occasional short
bursts of squelch on any given channel are to be expected. These
are attributed to conditions such as a signal null or RF noise.
In a telemetry system, occurrences of squelch should be very
infrequent; however, if squelch occurs frequently (on a per channel
basis) or if the duration of squelch is often longer than two seconds,
it indicates a possible system problem from external interference,
poor antenna system coverage, a defective component in the
antenna system, a defective transmitter, a weak battery in the
transmitter, or a defective receiver.
To isolate a squelch problem
If squelch frequently occurs on more than one channel, suspect a
problem with the common equipment: antenna system, the preamp-
splitter PCBA in the module housing, etc.
Observe LED4 (refer to
receivers. If many are OOS (out of sync) often or for long periods,
suspect a problem with the antenna system gain or coverage area.
Alternatively, the preamp-splitter PCBA in the 90479-A receiver
housing may be defective. The 90479-B/C contains no preamp.
Observe LED3 on several channels. If the diversity antenna system is
functioning correctly, LED3 should be switching between ON and
OFF regularly. About half the time should be spent ON and half OFF
(if the transmitters are stationary, LED3 will not switch as often). If
this is not happening on several channels, suspect a problem with
one of the diversity antenna sides or with the preamp-splitter PCBA
in the 90479-A receiver housing.
90479-A Receiver Housing
The preamp-splitter PCBA in the receiver housing contains a
preamp followed by an 8-way splitter for each antenna input. The
gain of this combination is approximately -4 dB. To assess the
functionality of the PCBA, monitor the RF signal amplitude at the
output of the antenna 1 cable at the rear panel of the receiver
housing. Reconnect this cable and monitor the antenna 1 output on
one of the front panel RF cables. If these levels are approximately
the same, the preamp-splitter PCBA is functioning correctly. Repeat
these measurements for ANT 2. It may be necessary to check all 16
front panel wall outputs to verify that a cable is not bad.
