Circuit Description - Kenwood TK-890/(B) Service Manual

2. Receiver Circuit
Incoming signals from the antenna pass through a low-
pass filter in the final unit of the transmitter system, and are
switched to the front-end of the receiver system via a re-
ceive/transmit switch (switching diode D3, D4 and D5 : TK-
890/(B), switching relay K1 : TK-890H(B)).
The signal is then passed through an antenna matching
coil. After passing through the 2-pole BPF (L101), the signal
is amplified by the RF amplifier (Q101). The 3-pole BPF
(L104) is then passed, and it enters the 1st mixer.
The 1st mixer use a double balanced mixer (A101). The
DBM is an excellent IM (intermodulation distortion) and im-
age. The 1st mixer mixes the signal with the 1st local oscil-
lator frequency and converts it to the 1st IF (73.05MHz).
The signal is amplified by the 1st IF amplifier 1 (Q102).
The signal then passes through two monolithic crystal fil-
ters (XF101 : Wide, XF102 : Narrow) to remove unnecessary
nearby frequency components. The signal from the XF101
or XF102 is used as the 1st IF signal.
The 1st IF signal is amplified by 1st IF amplifier 2 (Q107)
and fed into IC101 in the FM IF IC. The IF signal is then
mixed with the 2nd local oscillator frequency of 73.505MHz
to generate the 2nd IF of 455kHz. The 455kHz signal is then
passed through a ceramic filter (CF101, CF103 : Wide,
CF102, CF104 : Narrow) and fed back into IC101 for addi-
tional amplification.
The AF signal output from IC101 then passes through
the control unit's HPF and D/A converter IC. After entering
the AF power amplifier (IC522), the signal is output from the
speaker.
FINAL UNIT : TK-890H(B)
ANT
LPF
L7,6,5
LO GND GND
1 2
6 5
GND
IF RF
Fig. 6 Double balanced mixer

CIRCUIT DESCRIPTION

FINAL UNIT : TK-890/(B)
ANT
LPF
D3,4,5
L7,5,4
RA
ANT
K1
SW
RSSI
DET
D107
SQ
NOISE NOISE
DET
IC518
IC504 (2/2)
3
AMP HPF
27
ASQ
26
RSSI
4
TK-890/
Item
Nominal center frequency
Pass bandwidth
Attenuation bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 1 Crystal filter (L71-0514-05) : Wide
Item
Nominal center frequency
Pass bandwidth
Attenuation bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 2 Crystal filter (L71-0515-05) : Narrow
TX-RX UNIT
BPF 1st MIX
RF AMP BPF
L101
Q101 L104 A101
73.05MHz
IC101
RSSI
FM SYSTEM IC
DET
Q109
WIDE
AMP
CF103 CF101
Ceramic
filter
X101
NARROW
73.505
CF104 CF102
MHz
Ceramic
filter
IC524 IC524
(1/2) (2/2) IC515
BEF EXP
DE-EMP
IC516
CPU
Fig. 5 Receiver circuit
( B
Rating
73.05MHz
± 7.5kHz or more at 3dB
± 30kHz or less at 40dB
1.0dB or less
3.0dB or less
85dB or more at fo+400~+1000kHz
fo–200~–1000kHz
2.0kΩ / –0.5pF
(TX-RX unit XF101)
Rating
73.05MHz
± 3.75kHz or more at 3dB
± 20kHz or less at 40dB
1.0dB or less
4.0dB or less
85dB or more at fo+400~+1000kHz
fo–200~–1000kHz
2.0kΩ / –0.5pF
(TX-RX unit XF102)
f –73.05MHz
RX
RF AMP RF AMP
LPF Q104 Q103 D308
T/R
SW
1st IF AMP2 MCF 1st IF AMP1
XF101
Q107 Q102
WIDE
NARROW
455kHz
MCF
XF102
CONTROL UNIT DISPLAY UNIT
IC512
VOL
VOL
IC518
BUFF
(1/2)
IC522
AF PA
SP
) /H ( B )
VCO
RX VCO
27