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M850-00

T856/857 Circuit Operation

C2.9

2.5.3 Keying Inputs

There are four ways to key the exciter:

• pulling the Tx-Key line low (pin 13 on D-range 1 [PL100]) 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 D-range 1 [PL100]) when electrical

isolation is required. This features a constant current sink (Q270) to ensure reli-

able activation of the opto-coupler (IC250) at low keying voltages.

2.5.4 Compressor (Automatic Level Control (ALC))

The input signal is fed via a current controlled attenuator (Q230, Q220) to a high gain

stage (IC230) from which the output signal is taken. This signal is passed to a compara-

tor (IC230) which toggles whenever the audio signal exceeds a DC threshold deter-

mined by RV220. 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 (C233) which controls the attenuator (Q230, Q220), thus complet-

ing the feedback loop.

The compression level is set by adjustment of the comparator threshold (RV220).

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 there-

fore be avoided where possible.

2.5.5 Outputs To Modulators

The output signal from the limiter (IC210, IC230) is summed with a CTCSS tone at a

summing amplifier (IC260). The signal is then low pass filtered (IC260) and split to sup-

ply the two modulators.

Since the VCO modulator is a true frequency modulator, its audio is simply buffered

(IC260). The reference modulator, however, is a phase modulator and its audio must

first be integrated (IC210).

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. Once

set, adjustments to absolute deviation may be made only via the deviation digital pot.

(IC220 - adjustable via PGM800Win).

Tait T850 II Series User Manual

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