DRAFT
DRAFT
11
throughTR6 which is turned on only when the radio is in the receive mode and
not squelched.
Volume control is provided by a d.c. level, generated by IC6 and IC11, supplied
to the volume control pin on IC2. IC6 is a shift register which is connected to
the microcontroller and the appropriate volume level is set from information
generated by the raise and lower keys on the front panel when Volume is
selected. This provides a 32 level control of audio.
When any key is pressed a beep is created by the microcontroller generating a
tone, IC10 pin 12, which is filtered by R94, C106, C105 and R93 and fed to the
audio amplifier IC2. TR13 provides momentary muting of the received audio to
increase the clarity of key beeps. If a key beep is required when the radio is
squelched TR16 is turned on to mute the audio whilst the audio amplifier is
turned on via TR6. This is to prevent any clicking from the loudspeaker. As
soon as the key beep has finished, power to the audio amplifier is turned off.
Transmit Audio. DC bias to the microphone is provided by R63 and R64.
When the microphone is not required muting is enabled by the microcontroller
turning on transistor TR10. IC7 and associated components provide
amplification and audio frequency filtering. The circuit also acts as a limiter
using feedback provided by TR8 and TR9. The output from the final amplifier
stage, IC7a is fed via a resistive divider R41, VR3 and R42 to the modulator
diode of the VCO located on the RF PCB.
Communications. To enable programming of the radio a simple serial bi-
directional data link is provided via connection to the antenna socket; isolated
by a network on the RF PCB. This signal is buffered by IC4a and IC4b, for
receive and transmit respectively, and connected to the RxD/TxD lines of the
UART on the microcontroller. When communications are not required the
circuitry is isolated by TR7.
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