Receiver Circuit; Double Balanced Mixer; Circuit Description - Kenwood TK-790 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 and D4 : TK-790/
(B), switching relay K1 : TK-790H(B)).
The signal is then passed through an antenna matching
coil. After passing through the BPF (L.C filter), it is amplified
by the RF amplifier (Q101). The tuned varactor BPF
(L110~112) 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 (44.85MHz).
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 45.305MHz
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-790H(B)
ANT
LPF
L9,8,7
LO GND
1 2
6 5
GND
IF
Fig. 6 Double balanced mixer

CIRCUIT DESCRIPTION

FINAL UNIT : TK-790/(B)
ANT
LPF
D3,4
L7,5,4
ANT
K1
SW
RSSI
DET
D103
SQ
NOISE
DET
GND
IC504
3
AMP
4
RF
TK-790/
Item
Nominal center frequency
Pass bandwidth
Attenuation bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 1 Crystal filter (L71-0512-05) : Wide
(TX-RX unit XF101)
Item
Nominal center frequency
Pass bandwidth
Attenuation bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 2 Crystal filter (L71-0513-05) : Narrow
(TX-RX unit XF102)
TX-RX UNIT
RA
RF AMP BPF
BPF Q101 L110~112
TV
IC101
RSSI
FM SYSTEM IC
DET
Q109
NOISE
WIDE
AMP
CF103 CF101
Ceramic
filter
X101
NARROW
45.305
CF104 CF102
MHz
Ceramic
filter
IC518 IC524 IC524
(2/2) (1/2) (2/2) IC515
HPF BEF EXP
DE-EMP
IC516
CPU
27
ASQ
26
RSSI
Fig. 5 Receiver circuit
( B ) /H
Rating
44.85MHz
± 5.5~7.5kHz or more at 3dB
± 25kHz or less at 40dB
1.0dB or less
3.0dB or less
85dB or more at fo+400~+1000kHz
fo–200~–1000kHz
3.2kΩ / –1.0pF
Rating
44.85MHz
± 3.5kHz or more at 3dB
± 15kHz or less at 40dB
1.0dB or less
4.0dB or less
85dB or more at fo+400~+1000kHz
fo–200~–1000kHz
3.0kΩ / –0.5pF
f +44.85MHz
RX
1st MIX
RF AMP
A101 LPF Q104 D306
T/R
SW
44.85MHz
1st IF AMP2 MCF 1st IF AMP1
XF101
Q107
WIDE
NARROW
455kHz
MCF
XF102
CONTROL UNIT DISPLAY UNIT
IC512
VOL
VOL
IC518
BUFF
(1/2)
IC522
AF PA
( B )
VCO
RX VCO
Q102
SP
27