JT220M
1
CIRCUIT DESCRIPTION
Frequency configuration
The receiver utilizes double conversion. The first IF is 38.850 MHz and the second IF is 450kHz. The
first local oscillator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary frequencies. Fig. 1 shows the frequencies.
Fig. 1 Frequency configuration
Receiver
The receiver is double conversion super heterodyne, designed to operate in the frequency range of 216
to 280MHz.
The frequency configuration is shown in Fig. 1.
Front - end RF amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q206) after passing through a
transmit/receive switch circuit (D210 are off) and a band pass filter (L206, L207 and varactor diodes: D207,
D208). After the signal is amplified (Q206), the signal is filtered through a band pass filter (L205, L206 and
varactor diodes: D205, D206) to eliminate unwanted signals before it is passed to the first mixer.
The voltage of these diodes are controlled by tracking the D/A (U505) center frequency of the band
pass filter. (See Fig. 2.)
Fig. 2 Receiver section configuration
ANT SW
MCF
38.850MHz
IF SYSTEM
450KHz
CF
ANT
AF
AMP
SP
TCXO
19.200MHz
POWER
AMP
RF
AMP
PLL
VCO
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