Kenwood TK-272G Service Manual Download Page 16

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TK-272G

Fig. 3

Wide/Narrow changeover circuit

CIRCUIT DESCRIPTION

1. Frequency configuration

The receiver utilizes double conversion. The first IF is 49.95

MHz and the second IF is 450 kHz. The first local oscillator

signal is supplied form the PLL circuit.

The PLL circuit in the transmitter generates the necessary

frequencies. Fig. 1 shows the frequencies.

Fig. 1

Frequency configuration

1) Front end (RF AMP)

The signal coming form the antenna passes through the
transmit/receive switching diode circuit, (D3,D7) passes
through a BPF(L307), and is amplified by the RF amplifier
(Q301).
The resulting signal passes through a BPF (L305 and L306)
and goes to the mixer. These BPFs are adjusted by variable
capacitors (D302, 303,305). The input voltage to the
variable capacitor is regulated by voltage output from the
microprocessor (IC13)

2) First mixer

The signal from the front end is mixed with the first local
oscillator signal generated in the PLL circuit by Q19 to
produce a first IF frequency of 49.95 MHz.
The resulting signal passes through the XF1 MCF to cut
the adjacent spurious and provide the opitimun
characteristics, such as adjacent frequency selectivity.

Fig. 2

Receiver section

2. Receiver

The frequency configuration of the receiver is shown in Fig. 2.

Q14

Q17

D14

R78

R75

C263

C265

R80

D13

Q23

R98

R108

C133

CD1

R79

W/N

"H" : Wide
"L" : Narrow

MXO

IFI

MXI

IFO

AFO

IC4
FM IF SYSTEM

Q22

W/N

C107

R74

C108

CF1

CF2

R81

QAD

R105

3) IF Amplifier circuit

The first IF signal is passed through a four-pole monolithic
crystal filter (XF1) to remove the adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q22) and then applied to the lF system IC (IC4). The IF
system IC provides a second mixer, second local oscillator,
limiting amplifier, quadrature detector and RSSI (Received
Signal Strength Indicator). The second mixer mixes the
first IF signal with the 50.4MHz of the second local oscillator
output (TCXO X3) and produces the second IF signal of
450kHz.
The second IF signal is passed through the ceramic filter
(CF1; Wide (without E), CF2 ; Narrow and E type) to remove
the adjacent channel signal. The filtered second IF signal
is amplified by the limiting amplifier and demodulated by
the quadrature detector with the ceramic discriminator
(CD1). The demodulated signal is routed to the audio
circuit.

4) Wide/Narrow changeover circuit

Narrow and Wide settings can be made for each channel
by switching the ceramic filters CF1 (Wide) and CF2
(Narrow).
The WIDE (high level) and NARROW (low level) data is
output from IC5 (OUTPUT EXPANDER), pin 4.
When a WIDE (high level) data is received, Q14 turn off
and Q17 turn on. When a NARROW (low level) data is
received, Q14 turn on and Q17 turn off. D14 and D13 are
switched to ceramic filters when a high/low level data is
received.
Q23 turns on/off with the Wide/Narrow data and the IC4
detector output level is changed to maintain a constant
output level during wide or narrow signals.

ANT

TX/RX : 150~174MHz (K)
TX/RX : 136~150MHz (K2)

ANT

AMP

MCF

49.95MHz

50.4MHz

450kHz

SYSTEM

AMP

RX : 199.95~223.95MHz (K)
RX : 185.95~199.95MHz (K2)

RX : 399.9~447.9MHz (K)
RX : 371.9~399.9MHz (K2)