Technical Description

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 oflC23A. 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 PWMI 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.
4.4.5. User Interface
All user interface to the MX800 except the RF connections is made by way of the Micro
Controller board.
§ Output latch IC18 drives the six LED indicators on the front panel. The function is
described in section 1.1.1.
§ Input latch IC14 serves to accept the programmed status of the 8 bit binary
channel select DIP switch SW1. Refer section 6.3 for programming instructions.
§ Input latch IC 13 serves to accept the programmed status of the 6 bit binary mode
select DIP switch SW2. Refer section 2.2.2.2 for mode selection instructions.
§ Input latches IC48 and IC47 provide two eight bit +5V CMOS compatible digital
input ports (ports A and B respectively). The inputs may be either internally
pulled high and active low or internally pulled low and active high. Port B inputs
are configured as external channel select inputs. The inputs are buffered by 10k.Q
§ Output latch IC46 provides 8 uncommitted +5V CMOS compatible digital outputs.
Each output is buffered by a 1kohm resistor.
§ IC60 is a 1 watt speaker amplifier, which drives the internal speaker. This IC
includes the volume control function connection to the front panel toggle switch.
§ Microphone audio is fed via amplifier IC61A to a compressor (IC34B). This VF is
pre-emphasized and filtered before being fed to the main TX voice path.
§ IC22 provides CMOS to RS-232 conversion for the serial port.