Keying Inputs; Compressor (Automatic Level Control (Alc)); Outputs To Modulators - Tait T880 II Series Adjustment Manual

M880-00
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,
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.
Copyright TEL
T881 Circuit Operation
relative to each
C2.9
10/07/00