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Line output
A separate audio output is taken from the DAC (OUT 2) to the line audio output level control U65B, which is
of the same type as used for the loud speaker volume control. This is part of a feed back circuit around
amplifier U6B. The output from U6B, (LINE AUD OUT) goes to the clock filter and output balancing amplifier
combination U39B and U39A. The line output emerges at the DB25 rear connector. The line level is set by
the microprocessor.
Tones
There are two sources of the various tones used in the transceiver. All of the sine wave tones for
transmission are produced in the DSP. These tones include Transmitted SELCALL tones, Automatic Link
Establishment (ALE) tones, CW tone and alarm tone transmissions.
The microprocessor produces PWM tones (U1, pin 85) for such things as key presses, SELCALL received,
and GPS detected tones. The PWM carrier is chosen such that its frequency is above human hearing and
the resultant signal is passed through an anti-aliasing filter U4B to remove the carrier products. DSP
generated tones, are also cleaned with the same anti-aliasing filter (U4B) which also resistively mixes the
two tone sources, bypassing the volume control, to the input of the speaker amplifier U19.
Transmit
Microphone input. Remote head / Front Panel
The front panel and the remote head are exactly the same unit and the two terms are used interchangeably
in this manual.
Audio from the hand held electret microphone arrives via the Front Panel interface board at connector P1
pins 11 and 12, and is amplified and converted to balanced audio in U2A and U2B on front panel PCB. The
now balanced and amplified signal leaves the front panel on connector P1, pins 5 and 6. This connector is
plugged into the small Remote Head Interface Board. From here the audio signal passes to the Front panel
Interface Board via an eight way cable with RJ45 connector at each end. This cable is a long one where the
head is used remotely or a very short cable when it is used as a front panel.
From the Front Panel Interface board the microphone audio is connected to the Microprocessor/ RF board.
Line input
Balanced line audio is fed into the 25 pin rear D connector, P1 pins 11 (BAL AUD IN 1) and 24 (BAL AUD
IN 2) on the microprocessor board. The balanced audio is terminated in a 680-ohm resistor R105 and then
it is converted to an unbalanced signal with U5A. The signal is then amplified by U5B. This stage has a line
level setting digitally controlled potentiometer U23A. The gain is set by the microprocessor.
Microphone / Line input selection
Either the microphone audio or the line audio is selected with an analogue bus switch U11. The DC outputs
of amplifiers U2A and U5B (line and microphone amplifier outputs) supply the necessary bias for the switch.
The TX SELECT line when low selects microphone audio and when high, selects line audio for
transmission.
Compression
The selected audio gets compressed with the circuit around U3 and Q1. This works in the following way.
Resistors R77, R79 and MOSFET Q1 act together as a variable Tee attenuator. When no signal is present,
the voltage gain of U3A is about 5. This amounts to 20dB. U3B is an amplifier which feeds a rectifier D2.
As the input signal level increases, U3B output causes the rectifier to generate a positive going DC voltage.
This voltage, when high enough causes the FET Q1 to start conduction. This in turn causes the signal into
the amplifier to reduce, because part of the signal is shunted to ground by Q1, and thus the output level
does not increase as rapidly as before. The net effect is that an increase in audio level of 60dB will cause
