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M8SL2-00-002-812 T854 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 res-

ervoir capacitor (C233) which controls the attenuator (Q230, Q220), thus completing 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 therefore

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 by IC220, a 256-step 10k elec-

tronic potentiometer (EPOT), which is controlled via PGM800Win. The EPOT is made

up of 256 resistive sections (representing approximately 39Ω each) which can be indi-

vidually addressed by the microcontroller. Each section can be switched in or out of cir-

cuit to achieve the required total resistance, thus adjusting the absolute deviation level.

Tait T800 SL2 25W User Manual

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