Circuit Description - Kenwood TK-390 Service Manual

TK-390
1. Overview
The KENWOOD model TK-390 is a UHF/FM hand-held
transceiver designed to operate in the frequency range of
450 to 490MHz, 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 44.395MHz 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
: 450~490MHz XF300 : Wide
1st MIX
RF
ANT
SW AMP
XF301 : Narrow
405.15~455.15MHz
PA
TX
AMP AMP
450~490MHz
Fig. 1 Frequency configuration
3. Receiver System
3-1. Front-end RF amplifier
The signal coming from the antenna passes through the
transmit/receive switching diode circuit, is amplified by the
RF amplifier (Q200), and passes through a BPF (L205). The
resulting signal is further amplified by the RF amplifier
(Q201), passes through a BPF (L210) and goes to the mixer.
The band-pass filters (L205 and L210) are two-pole helical
resonators that uses varactor diode tuning to reject un-
wanted signal components.
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
44.395MHz
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 44.395MHz 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
L205 Q201
RF AMP BPF RF AMP
ANT
SW
Q302
MCF
IF AMP
XF300
X300
44.395
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
L210
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
350Ω / 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
350Ω / 4.5pF