Circuit Description - Kenwood TK-885 Service Manual

TK-885
Frequency Configuration
The TX-RX unit incorporates a VCO, based on a fractional
N type PLL synthesizer system, that allows a channel step of
5, 6, and 25kHz 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 contains a wide/narrow MCF, a wide CF,
and a narrow CF . The transmit signal is produced by the PLL
circuit for direction oscillation and division. The signal out-
put from the VCO is amplified by a straight amplifier and trans-
mitted.
350~390MHz
ANT RX
+
SW AMP
305.15~
345.15MHz
POWER TX
AMP AMP
350~390MHz
Fig. 1 Frequency configuration
20

CIRCUIT DESCRIPTION /

CF 455kHz
(Narrow)
CF 455kHz
(Wide)
1st MCF
MIX 44.85MHz
MIX/IF/DET
–
44.395MHz
PLL/VCO
VCXO
16.8MHz
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 BPF which is adjusted by a variable ca-
pacitor (D902, D903). The signal is amplified by Q201 and
passed through the BPF and BEF to eliminate unwanted out-
of-band signal components. The low-noise amplifier (LNA)
(Q201) uses a bipolar transistor to achieve wide-band and
low-distortion amplification.
The signal passes through a BPF and BEF, and is then
down-converted with the first local signal by IC200, then con-
verted to the first IF signal of 44.85MHz. The first local signal
passes through an LPF and an attenuator to eliminate un-
wanted 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 one pair of MCFs
(XF1). The signal is amplified by another intermediate ampli-
fier and goes 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 control unit.
AFO
MD
MB