TM-V708A
4
CIRCUIT DESCRIPTION
Fig. 2 Main VHF Receiver circuit
Fig. 3 Main UHF Receiver circuit
2. Receiver system
2-1. Main VHF Receiver
The incoming signal from the antenna passes through a
filter circuit and goes to the RF amplifier (Q205). The
amplified signal enters the helical resonator (L209, L207,
L204). The helical resonator is tuned to the desired frequency
according to the BPF voltage output from the CPU with the
varicap (D206, D205, D202). After the signal passes through
a filter, it is amplified by another RF amplifier (Q202) and goes
to the single balanced mixer (Q200, Q201). The frequency of
the signal is converted by the first local oscillator (Upper
heterodyne) to generate a 38.85MHz first IF signal. The signal
passes through the MCF (XF100), is amplified by the IF
amplifier (Q112) and enters the IF IC (IC100). Then the signal
is converted to the 450kHz second IF signal by the second
local oscillator, which is tripled 12.8MHz (Lower heterodyne).
Then it is detected to generate an audio signal.
2-2. Main UHF Receiver
The incoming signal from the antenna passes through a
filter circuit and enters the RF amplifier (Q508). The amplified
signal passes through the SAW filter (L517), is amplified by
another RF amplifier (Q505). Then it passes through the LC
filter (L508) and enters the single balanced mixer (Q501,
Q502). The frequency of the signal is converted by the first
local oscillator (Lower heterodyne) to generate a 45.05MHz
first IF signal. The signal passes through the MCF (XF400), is
amplified by the IF amplifier (Q406) and enters the IF IC
(IC400). The signal is converted to the 455kHz second IF
signal by the second local oscillator (Upper heterodyne). Then
it is detected to generate an audio signal.
IF IC
IC100
IF AMP
X3
LO
AF
MCF
XF100
CF
12.8MHz
SINGLE BALANCED
MIXER
RF AMP
BPF
RF AMP
CPU
LPF
L209,L207,L204
TUNE BY VOLTAGE
ANT
Q112
2SC5108
(Y)
Q102
2SC4649
(N.P)
Q200,201
2SK302
(GR)
Q202
3SK239A
Q205
3SK239A
D206
D205
D202
38.4MHz
IF AMP
LO
MCF
XF400
LC BPF
L508
AF
45.505MHz
IF IC
IC400
CF
SINGLE BALANCED
MIXER
RF AMP
SAW FILTER
RF AMP
LPF
L517
HPF
ANT
Q406
2SC5108
(Y)
Q501,502
2SK302
(GR)
Q505
2SK1875
(V)
Q508
3SK239A
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