Kenwood TK-250 Service Manual page 22

TK-250
CIRCUIT DESCRIPTION
7. CONTROL UNIT
The control unit consists of microprocessor IC221 and its
peripheral circuits. It controls the TX-RX unit and transfers
data to and from the display section. The CPU (IC221) mainly
performs the following :
1) Switching between transmission and reception by PTT
signal input.
2) Reading channel, frequency, and program data from
IC212, 220.
3) Sending frequency program data to the PLL.
4) Controling squelch ON/OFF by the pulse signal input
from the squelch circuit.
5) Controling the audio mute circuit by decode data in-
put.
6) Transmiting encode data.(OT, DOT)
7) Sending serial data to output expander (IC213) to con-
trol various function in the unit.
7.1 OT, DOT encode
The OT, DOT encoder tone is set by the data from the
microprocessor(lC221). The OT, DOT tones are applied to
the VCO and VCXO (X1) after passing through the
DI
A con-
vertor (IC206) for tone deviation adjustment and the sum-
ming amplifier (IC201).
7.2 OT ,DOT decode
A part of the recovered audio signal obtained at the am-
plifier IC208
(1/2)
are the OT and DOT tones and are low
pass filtered by IC214
(1/2)
and passed to the microproces-
sor for decoding.
L. S. D(QT. DQT)
from CPU(IC221)
IC201 ~
(2/2) ~
5
~ R219
6i18
5MT
BIAS2
R2S' Rl66 A2G8
N
SM
IQ <D
<Ot
T~ ~
IC214
~ I C208 ~ ( 1/2)
a:
(1/2)
3
3 "'-
~l ;;;
2 4
'"
C260
a:::
LSD IN
to CPU( IC22
Decode circuit
A216 RZ12
C23B R230
R232
IC205
DET
to
TX VCO
R220
BIAS2
N
N
a:
V02,3 R224
Encode circuit
IC206
~/A v03110
(AOJ)
19
,81
v
13
VREF
to
VCXO(Xl)
IBUFF!
AMP
IC210
( 1/2)
1 •.•.• 3
C259
R255
CXJ
"'
,.
N
U
..,. "'
N
a:
Fig. 8 OT, DOT en code, decode
7.3 CPU clock shift
When the CPU (IC221) 12MHz clock (X202) high frequency
(actually the integral double high frequency of 6MHz because
it is halved) is multiplexed with the recept ion frequency, it
becomes an internal beat signal, supressing the signal sen-
sitivity. To prevent this, by turning 0209 ON, the clock
frquency is shifted (about 4kHz) .
(Shift ON/OFF can be set through programming.)
..,
.•.
..,
a:
C331
~
Q209
~Xl
251 X2
IC221
[§]
VDD ,58
Fig. 9 CPU clock frequency shift
22
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