Technical Description

set up using the Configure screen of MXTOOLS. The output of this final stage is fed to
two microprocessor controlled digipots which serve to adjust the modulating signal level
to the VCO and the reference oscillator. A third digipot is used to provide an offset null
for the reference oscillator center frequency. Adjustment of these levels is also by way of
MXTOOLS using the Channel Edit screen.
4.4.3. RX Signal Processing.
In a similar fashion to the TX audio path the major functional elements of the RX audio
path are capable of being selected or bypassed by means of links.
Discriminator audio enters the board on SKD-3 and is fed to a selectable 300Hz highpass
filter comprising IC37 and IC38B. This output or an unfiltered version of the
discriminator audio is fed via a switch IC32A to the TTR path. The switch is used to
disable talk through audio under the control of the microprocessor and the external TX
talk line on SKH-10. Following the HPF a selectable 3.4kHz cutoff low pass filter IC38A
and IC39B connects to the de-emphasis circuit IC39A. From here the audio passes
through the mute switch IC32C which is under control of the microprocessor and on to
the RX talk switch ICIC36C which is controlled externally from SKH-3. This second
switch is used to disable RX audio to line. A final switch IC36A selects between RX
audio and TX looped back audio for output to line via amplifiers IC40A and IC40B.
Secondary inputs to these amplifiers are the TX audio from the line modem and
microphone audio to line. The output on SKH-2 forms an unbalanced 600• VF output to
line and, alternatively, outputs SKH-2 and SKH-11 form a differential output.
Discriminator audio is also fed via a low pass filter IC31 to the CTCSS decoder as well
as to a level detector D9 the output of which is connected to ADC 1 on the
microprocessor.
Discriminator audio is also fed to the mute detection circuit. This comprises a high pass
filter IC41 followed by RV4 which sets the mute threshold. Following amplification by
IC42A and rectification by IC42B and IC43A a comparator, IC43B, determines when the
detected signal passes a fixed threshold. The output of the comparator is fed to the
microprocessor and via a selectable inverter to SKH-4.
Discriminator audio is also fed to RX port of the RF modem IC54.
4.4.4. RF Power Control
Forward power is controlled by the microprocessor through two mechanisms. Based on
preprogrammed per channel adjustments the microprocessor sets the digipot IC 10 to a
reference setting. IC23A serves as a comparator and, with the non-inverting input