Circuit Description - Kenwood TK-885 Service Manual

Frequency Configuration
The TX-RX unit incorporates a VCO, based on a fractional
N type PLL synthesizer system, that allows a channel step
of 6.25 and 12.5kHz to be selected. The incoming signal
from the antenna is mixed with a first local oscillation fre-
quency 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
system. The TX-RX unit contains a wide/narrow 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.
440~470MHz
1st
MIX 44.85MHz
ANT RX
+
SW AMP –
395.15~
425.15MHz
POWER TX
AMP
AMP
440~470MHz
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
switch (D209) and goes to the front end of the receiver. The
front-end filter is a variable BPF consisting of two two-pole
helical resonators and eight varicap diodes (D203, D204,
D205, D206, D212, D213, D214, D215) to eliminate un-
wanted out-of-band signal components. The low-noise am-
plifier (LNA) (Q201) uses a bipolar transistor to achieve wide-
band and low-distortion amplification.
ANT
LPF D209,210

CIRCUIT DESCRIPTION

CF2 455kHz
CF1 455kHz
MCF
MIX/IF/DET AFO
44.395MHz
PLL/VCO MD
VCXO MB
16.8MHz
D211
L203 Q201 L207
BPF AMP BPF
ANT
SW
1st local
OSC (HT)
Fig. 2 Receiver system
The signal passes through a BPF and is down-converted
with the first local signal by IC200, then converted to the
first IF signal of 44.85MHz. The first local signal passes
through an LPF and an attenuator to eliminate unwanted
harmonics components and implement the optimum input
level to the mixer, then enters IC200. A DBM is used as a
mixer to achieve a high potential.
The signal output from the mixer is amplified by an inter-
mediate frequency amplifier and input to two MCFs (XF1).
The signal is amplified by another intermediate amplifier and
goes to the FM IF IC (IC11). The first intermediate fre-
quency 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 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 control unit.
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
AUDIO
DEO
PROCE.
FIg. 3 AF signal system
IC200 Q203 XF1 Q15
MIX AMP BPF AMP
+
–
X2
2nd
local OSC
TK-885
IC502 IC5 IC10
AFO
SUM D/A
AF PA
AMP CONV.
DTMF BPO
IC2
AMP
IC11
MIX,IF,DET
DEO
CF1/
CF2
SP
13