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Circuit description, Overview, Circuit configuration by frequency – Kenwood TK-480 User Manual

Page 15: Receiver system, Fig. 1 frequency configuration, 1. rf unit, 2. if unit, Fig. 2 receiving system

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TK-480/481

1. Overview

This transceiver is an 800/900MHz band EFJ LTR™

trunked system compatible FM portable transceiver that can
be programmed to operate on both LTR and conventional
systems.

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 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.

Fig. 1

Frequency configuration

ANT

ANT

SW

RF

AMP

1st MIX

MCF

CF

455kHz

IF SYSTEM

TK-480

TX 806~825MHz

851~870MHz

RX 851~870MHz

TK-481

TX 896~902MHz

935~941MHz

RX 935~941MHz

AF

AMP

44.395

MHz

PA

AMP

TX

AMP

MIC

AMP

44.85MHz

TK-480 :
806.15~825.15MHz
TK-481 :
890.15~896.15MHz

TK-480

806~825MHz
851~870MHz

TK-481

896~902MHz
935~941MHz

PLL

VCO

SP

MIC

3. Receiver System

3-1. RF unit

An incoming RF signal from the antenna terminal is

passed through the antenna switch (D7, D9, and D10 are
off) and then the bandpass filter (L11). The signal is ampli-
fied by RF amplifier Q9, and passed through the bandpass
filter (L20) to remove the spurious signal again. The result-
ing signal is applied to the first mixer (Q6), where it is mixed
with the first local oscillator signal output from the fre-
quency synthesizer to produce the first IF (44.85MHz).

3-2. IF unit

The first IF signal is passed through a four-pole mono-

lithic crystal filter (XF1) to remove a adjacent channel signal.
The filtered first IF signal is amplified by the first IF amplifier
(Q5) and then applied to the IF system IC (IC9). The IF sys-
tem IC provides a second mixer, second local oscillator, lim-
iting amplifier, quadrature detector and RSSI (Received Sig-
nal Strength Indicator). The second mixer mixes the first IF
signal with the 44.395MHz of second local oscillator output
(crystal unit X1) and produces the second IF signal of
455kHz.

The second IF signal is passed through the ceramic filter

(CF1,2) to more remove the adjacent channel signal. The
filtered second IF signal is amplified by the limiting amplifier
and demodulated by the quadrature detector with ceramic
discriminator (CD1). The demodulated signal is routed to
the audio circuit.

Fig. 2

Receiving system

ANT

SW

CF1
CF2

1st local
OSC (PLL)

2nd local
OSC

X1

L30

D9,D10

L11

BPF

Q9

RF AMP

L20

BPF

Q6

1st MIX

XF1

MCF

Q5

1st IF

IC9

MIX,DET,IF

IC8 (2/2)
AF AMP

41

IC3 (2/2)
AF AMP

IC4

VOL

AF AF

Q310

SW

SSW

VC1

VC2

IC300

AF PA

INT.
SP

EXT.
SP

HPF

DE-

EMP

LPF

HPF

BEF

5

2

1

IC12

ANT

EXP

MUTE

CIRCUIT DESCRIPTION