Audio and Control Circuit 2-5
3.5.3 Dual VCO
The dual VCO module contains an RX VCO and a TX VCO. They are configured as Colpits
oscillators and connected to power through cascade buffers. Only one VCO is selected at a time.
The power source to each block is switched by transistors Q305 and Q111, and is dependent on the
states of TX_EN1 and RX_EN. The varicap diodes CR701 and CR702 produce a change in
frequency with a change in voltage. L703 and L706 are resonant tuning coils, which adjust the
operating frequency with specified DC tuning voltage applied. The audio signals from low pass filter
(U105-C) is applied to CR703 varicap diode in TX VCO to be modulated TX RF signal.
4.0 Audio and Control Circuit
4.1 RX Audio
The RX audio signal comes through the de-emphasis pass six order elliptic 300 Hz HPF and 3 order
300Hz HPF. This 300Hz High Pass Filter (U203) removes all signals under 300Hz to keep any
undesired signal from being heard from the speaker. After passing through this filter, audio signal is
de-emphasized by R209,C212 to get 6db/oct response. The de-emphasized audio signal passes
through volume control (SW/VOL1) and is amplified to a sufficient level to drive a loud speaker by
U601 Audio AMP. U601 has mute/un-mute function by the audio-mute control in CPU. If U601 pin 2
is low, the audio amplifier goes into active mode. If pin 2 is high, the IC goes into mute mode.
TONE-FIL
LPF
AF-AMP
HPF
MIC MUTE
MIC
N/S SW
U104-A
U104-C
Q109
TONE-FIL U202
AUDIO-
PATH
Q103
AF PA
U601
AUDIO-
MUTE
Q101
to SPK.
TONE-GEN.
U104-D,
U105-C
U105
MUTE-
CON
AUDIO-MUTE
CON
MOD. ADJ
R159
6 Order
Elliptic HPF
8 Order LPF
to CPU
RX LOGIC
3 Order HPF
6 Order LPF
3 Order LPF
to MOD.
TX LOGIC
+ 3 Order HPF
260Hz
From
IF IC
300Hz
300Hz 3KHz
SW/VOL1
Q108
De-EMP
AUDIO-FIL U203
300Hz
R209,C212
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