Technical Description

communicated to the Receiver and Exciter modules by way of bussed data and clock
lines on SKC/D-18 and SKC/D-15 (Exciter/Receiver) and an individual module strobe on
SKC/D-17. A lock detect signal from each module on SKC/D-16 is read by the micro
controller.
In addition to the synthesizer programming bus an I2C bus goes to each of the modules.
This bus is currently unused.
IC25 is a non predictive, full duplex CTCSS encoder/decoder which is under the control
of the microprocessor.
4.4.2. TX Signal Processing
TX audio may be sourced from a number of different paths. These include VF from line,
Talk through audio, the microphone, DC FM/Wideband input, Tone generator from
microcontroller, Test TX VF inject from the Monitor port, CTCSS generator and the
internal RF modem.
The TX VF path is readily user configurable with most major functional blocks being
possible to either select or bypass by means of links ( refer to 2.2.2.1 for details of link
settings). The VF from line enters the board on SKH-14 and RV4 (TX VF gain) provides
gain adjustment to accommodate different line levels. This is followed by a selectable
compressor (IC34A) with 30rib dynamic range. The output of the compressor apart from
being fed to the main TX audio path is also connected to the input side of the 'Line' FFSK
modem and via a gain control pot the speaker amp (IC60). Following the main TX audio
path, IC36B serves to switch the VF further on the TX audio path or, under the control of
the external input on SKH- 13, to loop it back to the line output via RV7 which provides
level adjustment for the looped back signal. From this switch the VF passes though a
second switch IC32B under the control of the Micro Controller and the external TX talk
line on SKH-10. This switch is used to disable the line TX VF path. From here the VF
passes through a pre-emphasis stage C66, R67 and a 300Hz high pass filter (IC30B and
IC29A) each of which is selectable through links. The output of this filter is fed to IC30A
a summer/limiter amp.
The summing point of this amp also serves to combine the audio from the talkthrough
path which is level adjusted by RV3, the microphone which is processed through a
similar compression, pre-emphasis and filtering chain as the line VF, the tone ( Morse
code) signal from the microprocessor, the DC FM input which enters the board on SKH-6
and the Test TX VF injection from SKE-2. The latter three inputs are all at fixed levels.
RV2 on IC30A provides overall gain adjustment. Following the limiter amp is the TX VF
low pass filter. The standard Bessel filter has a 3.4kHz cut off frequency but may
optionally be ordered as a 3kHz Chebychev filter. A second summing amplifier iC24B
follows which combines the CTCSS, optional modem (IC52 and IC54) and Wideband
audio inputs with the TX VF. Level adjustment of the CTCSS signal is in three 0.85dB
steps either above or below the nominal level ( 10% of max dev ). These level steps are