JETStream JT220M Service Manual page 8


Encode
The CTCSS and DCS signal that is output from CPU(U2) of P31 pass through two-way digital switches
and buffer amplifier(U8A) to amplify. The DTMF signal that is output from DTMF encoder passes through
buffer amplifier (U8A) to amplify, and amplified by the other U504 (B/4) to perform IDC operation. The
signal then passes through a low-pass filter (splatter filter) U504 (C/4and D/4) and cuts 3kHz and higher
frequencies. and the resulting signal goes to the D/A converter (U505).
The D/A converter (U505) adjusts the balance between the MOD and CTCSS/DCS levels. Signal of
CTCSS/DCS port is summed with MOD and the resulting signal goes to the VCO. This signal is applied to
a varicap diode in the VCO for direct FM modulation. (See Fig. 13.)
U2 DCS/CTCSS
CPU
U4
DTMF
DTMF
Encode

Decode
CTCSS/DCS/ DTMF
The signal from (AFWN) entering into AF signal and higher audio frequencies output by pin 1 of U501: A
are cut by low-pass filter U503 and amplified. then led to pin 42 of CPU. The input signal is compared with
the programmed tone frequency code in the CPU. The squelch will open when they match.
The signal (DTMF-DET) goes to P27 (pin 43) of CPU (U2).
goes to compared U8B. The compared
CPU (U2). (See Fig. 14.)
CIRCUIT DESCRIPTION
CS
WR
U2
CPU
DI
P12
Fig. 12 Display circuit
U8A
U7
2-CHANNEL BUFFER
DIGITAL SW
AMP
Fig. 13 Encode circuit
signal goes to the CPU for processing
U501B/U502
DTMF-DET
LPF/HPF
FILTER
U501D
AFWN
BUFFER AMP
Fig. 14
6
U3
COM 0~COM 3
LCD
LCD
SEG0~SEG27
driver
Q6,Q7
8C
SW
D5
BLUE
TCXO
MOD
X200
TCXO
U505
D/A
VCO MOD
The DTMF input signal from the
U8B
COMPARATOR
U2
CPU
U503
LPF
FILTER
Decode circuit
JT220M
U205
PLL
VCO
DTMF-DET
and be decoded within the