Circuit Description - Kenwood TK-880 Service Manual

TK-880/H
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 5, 6.25kHz 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
system. 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.
450~490MHz : K,HK
485~512MHz : K2,HK2
400~430MHz : K3,HK3
MIX
ANT RX
+
SW AMP
405.15~
445.15MHz : K,HK
440.15~
467.15MHz : K2,HK2
355.15~
385.15MHz : K3,HK3
POWER TX
AMP AMP
450~490MHz : K,HK
485~512MHz : K2,HK2
400~430MHz : K3,HK3
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 (TK-880 : D209, D210, D211, TK-880H : D402, D210,
D211) 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 (K3,HK3 : D203~D206,
D212~D215, K,K2,HK,HK2 : D203~D206) 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.
The signal passes through a BPF and is down-converted
ANT
LPF
28

CIRCUIT DESCRIPTION

CF 455kHz
(Narrow)
CF 455kHz
(Wide)
1st MCF
44.85MHz
MIX/IF/DET
–
44.395MHz
PLL/VCO
VCXO
16.8MHz
D402,211 L203 Q201
D209,210
BPF AMP
ANT
SW
Fig. 2 Receiver system
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 wide ceramic filter (CF1) or a narrow ceramic fil-
ter (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).
Wide/Narrow Changeover Circuit
The W/N port (pin 11) of the shift register (IC7) is used to
switch between ceramic filters. When the W/N port is high,
AFO
Q24 turns on and the ceramic filter SW diode (D22, D23)
CF1 turns on to receive a Wide signal. At the same time,
Q16 turns on and one of the filters is selected so that the
wide and narrow audio output levels are equal.
When the W/N port is low, Q23 turns on and the ceramic
filter SW diode (D22, D23) CF2 turns on to receive a Narrow
MD
signal.
MB IF system
C107 Q32 R121
DET
+
OUT
+
DM
Q16
Fig. 3 Wide/Narrow changeover circuit
L207 IC200 Q203 XF1
BPF MIX AMP BPF AMP
+
–
1st local
OSC (HT)
IC11
IFI
C118
MXO
CF1
(Wide)
AFO
CF2
(Narrow)
D22
W/N
IC7 11pin
Wide : H
Narrow : L
Wide : L
Narrow : H
Q15 IC2
AMP
IC11
MIX,IF,DET
DEO
X2
CF1/
CF2
2nd
local OSC
5C
Q23
Q24
D23