Tait T830 Series, Руководство по обслуживанию

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31/01/96 Copyright TEL
2.4.3 Keying Inputs
There are four ways to key the exciter:
• pulling the Tx-key line low (pin 13 on the D-range connector at the rear of the
set);
• pushing the "Carrier" button on the front panel - this will inhibit all audio;
• using the PTT button on the local microphone, disabling audio from the line;
• via the opto-key inputs (pins 11 and 12 on the D-range connector) where elec-
trical isolation is required. This features a constant current source (Q106) to
ensure reliable activation of the opto-coupler (IC100) at low keying voltages.
2.4.4 Compressor
The input signal is fed via a current controlled attenuator (Q103, Q104) to a high gain
stage (IC102a) from which the output signal is taken. This signal is passed to a compa-
rator (IC102b) which toggles whenever the audio signal exceeds a DC threshold deter-
mined by RV104. Thus, the comparator produces a square wave whose mark-space
ratio is determined by the amplitude of the audio signal. This square wave pumps up
the reservoir capacitor (C129) which controls the attenuator (Q103, Q104), thus complet-
ing the feedback loop.
The compression level is set by adjustment of the comparator threshold (RV104).
Note: Although the high dynamic range of the compressor allows the use of very
low audio signal levels, such conditions will be accompanied by a degrada-
tion of the signal to noise ratio. Very low audio input levels should therefore
be avoided where possible.
2.4.5 Outputs To Modulators
The output signal from the limiter (IC102d, IC103a) is added to any incoming CTCSS
tone at a summing amplifier (IC103b). The signal is then low pass filtered (IC103c) and
split to supply the two modulators.
Since the VCO modulator is a true frequency modulator, its audio is simply buffered
(IC104b). The reference modulator, however, is a phase modulator and its audio must
first be integrated (IC103d).
It is vital that the audio levels to the modulators are accurately set,
relative to each
other
. Hence the inclusion of level adjustment in the reference modulator path (RV105).
Once set, adjustments to absolute deviation may be made only via the deviation pot
(RV106).