Circuit Description - Kenwood TK-290 Service Manual

TK-290
1. Overview
The KENWOOD model TK-290 is a VHF/FM hand-held
transceiver designed to operate in the frequency range of
136 to 174MHz, the unit consists of a receiver, a transmit-
ter, a phase-locked loop (PLL) frequency synthesizer, power
supply circuits, a control unit.
2. Circuit Configuration by Frequency
The receiver is a double-conversion superheterodyne
with a first intermediate frequency (IF) of 44.85MHz and a
second IF of 455kHz. Incoming a signals from the antenna
are mixed with the local signal from the PLL to produce the
first IF of 44.85MHz.
This is then mixed with the 45.305MHz second local os-
cillator output to produce the 455kHz second IF. This is de-
tected to give the demodulated signal.
The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the microphone. It
is then amplified and sent to the antenna.
ANT
MCF : 44.85MHz
TX/RX
: 136~174MHz XF300 : Wide
1st MIX
ANT RF
SW AMP
XF301 : Narrow
180.85~218.85MHz
PA
TX
AMP
AMP 136~174MHz
Fig. 1 Frequency configuration
3. Receiver System
3-1. Front-end RF amplifier
The signal are then passed through an antenna matching
coil, where the high-frequency components are amplified by
a GaAs FET (Q200). The signals are then fed into band-pass
filter that uses varactor diode tuning to reject unwanted sig-
nal components, and is fed to the 1st mixer.
3-2. First mixer
The 1st mixer uses the GaAs IC (IC200). The 1st mixer
mixes the signal with the 1st local oscillator frequency from
the VCO, and converts it to the 1st IF (44.85MHz).
The signal then passes through monolithic crystal filter
(XF300 ; Wide, XF301 ; Narrow) to remove unnecessary
nearby frequency components. The signal from the MCF is
used as the 1st IF signal.
16

CIRCUIT DESCRIPTION

CF
455kHz
CF300,301
: Wide
CF302,303
: Narrow
FM IF AF
SP
SYSTEM AMP
45.305MHz
PLL
MIC
MIC
VCO
AMP
3-3. IF amplifier
The 1st IF signal is amplified (Q302) and fed into IC300 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 ce-
ramic filter (CF300, CF301 ; Wide, CF302, CF303 ; Narrow)
and fed back into IC300 for additional amplification.
Q200
BPF RF AMP BPF BPF
Q302
MCF
IF AMP
XF300
X300
45.305
MHz
MCF
XF301
Fig. 2 Receiver section
Item
Nominal center frequency
Pass band width
Attenuation band width
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 1 Crystal filter XF300 (L71-0523-05) : Wide
Item
Nominal center frequency
Pass band width
Attenuation band width
Ripple
Insertion loss
Guaranteed attenuation
Terminating impedance
Table 2 Crystal filter XF301 (L71-0502-05) : Narrow
IC200
1st MIX D7
BPF
T/R
SW
IC300
FM IF SYSTEM
CF300,301
: Wide
CF302,303
: Narrow
Rating
44.85MHz
±5~7kHz or more at 3dB
±25kHz or less at 3dB
1.0dB or less
4dB or less
80dB or more at fo±910kHz
40dB or more within fo±1MHz
350 / 4.5pF
Rating
44.85MHz
±3.75kHz or more at 3dB
±12.5kHz or less at 25dB
1.0dB or less
4dB or less
80dB or more at fo±910kHz
40dB or more within fo±1MHz
350 / 4.5pF