ABELL A-82 UHF Service Manual
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CIRCUIT DESCRIPTION
1
．
Frequency configuration
The receiver utilizes double conversion. The first IF is 38.85MHz and the second IF is
450 KHz. The first local oscillator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary frequencies. The Chart 4 display all these
frequency.
2
．
Receiver
The receiver is double conversion super heterodyne, designed to operate in the frequency range of
400-440MHz, 430-470MHz.
1
）
Front-end RF amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q301) after passing through a
transmit/receive switch circuit (D301 and D302 are off) and a band pass LC filter. After the signal is
amplified, the signal is filtered through a band pass filter to eliminate unwanted signals before it is
passed to the first mixer
（
Q300
）
.
Because of the LC band-pass filter’s insufficient bandwidth, control the capacitance value of variable
capacitance diode(D303-D308) by adjusting the voltage of AFC to reach the bandwidth need, the
voltage of AFC is controlled by CPU. (See the Chart 5)
ANT SW
RF
AMP
ANT
MCF
38.85MHz
CF
450KHz
IF SYSTEM
AF
AMP
SP
TCXO
12.8MHz
MIC
MIC
AMP
PLL
VCO
RX
TX
TX
AMP
PA
AMP
Chart 4.
Circuit Structure
X3
multiply
38.4MHz
