MX800 Transceiver Technical Manual
© SPECTRA ENGINEERING 2006 Revision 4.2.3
72
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 cut-off low pass filter IC38A and IC39B
connects to the de-emphasis circuit IC39A. From here the audio passes
through the mute switch IC32C that is under control of the microprocessor
and on to the RX talk switch IC36C, 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 600Ω 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
ADC1 on the microprocessor.
Discriminator audio is also fed to the mute detection circuit. This comprises a
high pass filter IC41 followed by RV6/6B, 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 pre-programmed per channel adjustments the microprocessor sets
the digipot IC10 to a reference setting. IC23A serves as a comparator and,
with the non-inverting input connected to the wiper of the digipot, is set up
with a reference voltage. The detected actual PA forward power is fed to the
inverting input of IC23A. The error voltage at the output of IC23A is fed to
the exciter output power control circuit via SKC-4 and the action of the
control loop is to set the RF power such that the actual detected volts equals
the reference volts. The digipot setting is static for each channel unless the
required forward power is changed.
The voltage to the top of the digipot is set up by the microprocessor through
the Pulse Width Modulator output PWM1. On PTT the ON duty cycle of the
PWM1 output is progressively increased and the filtered result of this forms a
ramp to the top of the power control digipot. Once 100% duty cycle is
reached full power is produced. This results in a fast but controlled RF power
rise characteristic.
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