JETStream JT220M, Service Manual

Page: 3 / 42

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
MIC
MIC
AMP
RX
TX
1st
MIX
RF
AMP
1/2
X2 MULTIPLY
38.4MHz
Q200
RX: 216.000-280.000
TX: 222.000-225.000
ANT SW
IF system
(PLL)
CF201(wide)
ANT
1st Local OSC
BUFFER AMP
U501(4/4)
U200
IF AMP
Q204
MCF
XF200, XF201
MIXER
Q205
BPF
RF AMP
Q206
BPF
D210,D211
U505
D/A
TV
CPU
CF200(Narrow)
D/A CONVERTER
U505
W/NO
D207, D208
L206, L207
D205,D206
L205, L206
X2 MULTIPLY
Q211
TCXO
X200