Table of Contents
Advertisement
ANT
LPF
D209,210
AF Signal System
The detection signal (DEO) from the TX-RX unit goes to
the audio processor (IC504) of the control unit. The signal
passes through a filter in the audio processor to adjust the
gain, and is output to IC502. IC502 sums the AF signal and
the DTMF signal and returns the resulting signal to the TX-
RX unit. The signal (AFO) sent to the TX-RX unit is input to
the D/A converter (IC5). The AFO output level is adjusted by
the D/A converter. The signal output from the D/A converter
is added with the BEEP signal (BPO) and the resulting signal
is input to the audio power amplifier (IC10). The AF signal
from IC10 switches between the internal speaker and
speaker jack (J1) output.
CONTORL UNIT
IC504
IC502
AUDIO
SUM
DEO
PROCE.
AMP
DTMF
FIg. 3 AF signal system
Squelch Circuit
The detection output from the FM IF IC (IC11) is ampli-
fied by IC2 and the signal (DEO) is sent to the control unit.
The signal passes through a high-pass filter and a noise am-
plifier (Q503) in the control unit to detect noise. A voltage is
applied to the CPU (IC511). The CPU controls squelch ac-
cording to the voltage (ASQ) level. The signal from the RSSI
pin of IC11 is monitored. The electric field strength of the
receive signal can be known before the ASQ voltage is input
to the CPU, and the scan stop speed is improved.
MIC
IC711
IC504
AF AMP,
AF AMP
IDC, LPF
TA75S01F
TC35453F
IC511
CPU
MIC KEY
30620M8
INPUT
-394GP
CIRCUIT DESCRIPTION
D211
L203
Q201
L207
BPF
AMP
BPF
ANT
SW
1st local
OSC (HT)
Fig. 2 Receiver system (M type)
IC5
IC10
SP
AFO
D/A
AF PA
CONV.
BPO
IC3
Q103
VCO
SUM AMP
2SK508NV
TA75W558FU
(K53)
X1
IC300
VCXO
PLL
16.8MHz
SA7025DK
Fig. 5 Transmitter system (K,E type)
IC200
Q203
XF1
Q15
MIX
AMP
BPF
AMP
+
–
X2
2nd
local OSC
CONTROL UNIT
IC2
IC11
AMP
IF
SYSTEM
Fig. 4 Squelch circuit
Transmitter System
Outline
The transmitter circuit produces and amplifies the de-
sired frequency directly. It FM-modulates the carrier signal
by means of a varicap diode.
VCO/PLL Circuit
The TK-785 has a VCO for the transmitter and a VCO for
the receiver in a sub-unit (A1). They are housed in a solid
shielded case and connected to the TX-RX unit through
CN101. One of the VCOs is selected with an ST signal. A
filtered low-noise power supply is used for the VCOs and
varicap diodes.
The VCO for the transmitter is described below. It is de-
signed so that Q103 turns on with a prescribed frequency
when a reverse bias is applied to D102 and D107 by using
the control voltage (CV) through CN101. The control voltage
is changed by turning the trimmer capacitor (TC102). The
output from Q103 is applied to the buffer amplifier (Q106) to
generate a VCO output signal. This signal is used as a drive
input signal or a local signal of the first mixer.
Q106
Q7
Q203
RF AMP
BUFFER
RF AMP
2SC4215
2SC4215
2SC3357
(Y)
(Y)
Q300
BUFFER
2SC4215
(Y)
TK-785
IC2
AMP
IC11
MIX,IF,DET
DEO
CF2
IC503
Q503
AMP
NOISE AMP D509
DEO
HPF
DET
RSSI
Q204
IC400
POWER AMP
RF AMP
M68729
2SC2954
IC511
CPU
ANT
15
Hide quick links:
Advertisement
Table of Contents
