Circuit Description - Kenwood TK-782 Service Manual

Frequency Configuration
The TX-RX unit (A/2) incorporates a VCO, based on a frac-
tional N type PLL synthesizer system, that allows a channel
step of 12.5kHz to be selected. The incoming signal from
the antenna is mixed with a first local oscillation frequency
to produce a first intermediate frequency of 44.85MHz.
The signal is then mixed with a second local oscillation
frequency of 44.395MHz to produce a second intermediate
frequency of 455kHz. This is called a double-conversion sys-
tem. The TX-RX unit (A/2) contains a wide/narrow MCF and
CFs. The transmit signal is produced by the PLL circuit for
direction oscillation and division. The signal output from the
VCO is amplified by a straight amplifier and transmitted.
220~235MHz
1st
44.85MHz
MIX
ANT RX
–
SW AMP +
175.15~
190.15MHz
POWER TX
AMP
AMP
220~235MHz
Fig. 1 Frequency configuration
Receiver System
" Outline
The incoming signal from the antenna passes through a
low-pass filter and a transmission/reception selection diode
ANT
LPF

CIRCUIT DESCRIPTION /

CF 455kHz
(Narrow)
CF 455kHz
(Wide)
MCF
MIX/IF/DET AFO
44.395MHz
PLL/VCO MD
VCXO MB
16.8MHz
Q202 L202~204
D211,212 HPF AMP BPF
ANT
SW
Fig. 2 Receiver system /
switch (D211,D212) and goes to the front end of the receiver.
The front-end filter is a variable BPF consisting of three coils
and three varicap diodes (D206, D207, D208) to eliminate
unwanted out-of-band signal components. The low-noise
amplifier (LNA) (Q202) uses a bipolar transistor to achieve
wide-band and low-distortion amplification.
The signal passes through the BPF and is down-converted
with the first local signal by IC202 to produce the first IF sig-
nal of 44.85 MHz. The first local signal passes through an
LPF and an attenuator to eliminate unwanted harmonics com-
ponents and implement the optimum input level to the mixer,
then enters IC202. A DBM is used as a mixer to achieve a
high potential.
The signal output from the mixer passes through two
MCFs (XF1). The signal is amplified by an intermediate fre-
quency amplifier and input to the FM IF IC (IC11).
The first intermediate frequency signal is mixed with the
second local signal of 44.395MHz to produce the second IF
signal of 455kHz.
The unwanted near-by signal components are then elimi-
nated by a wide ceramic filter (CF1) or a narrow ceramic filter
(CF2) and the resulting signal goes back to the FM IF IC. The
signal is quadrature-detected in the IC to produce an audio
signal, which is amplified by a DET amplifier (IC2) and output
to the TX-RX unit (B/2).
"
IC202 XF1 Q15
MIX BPF AMP
IC11
–
+ MIX,IF ,DET
X2
1st local 2nd
OSC (HT) local OSC
TK-782
IC2
AMP
DEO
CF1/
CF2
35