Circuit Description; Frequency Configuration; Receiver System; Front-End Rf Amplifier/First Mixer - Kenwood TK-7108 Service Manual

TK-7108

Frequency Configuration

The receiver utilizes double conversion. The first IF is
49.95MHz and the second IF is 450kHz. The first local oscil-
lator signal is supplied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary
frequencies. Figure 1 shows the frequencies.

Receiver System

The receiver is double conversion superheterodyne. The
frequency configuration is shown in Figure 1.
■ Front-end RF Amplifier
An incoming signal from the antenna is applied to an RF
amplifier (Q353) after passing through a transmit/receive
switch circuit (D603, D604 are off) and a band pass filter
(L357, L356 and varactor diodes : D353, D354). After the
signal is amplified (Q353), the signal is filtered through a band
pass filter (L354, L355 and varactor diodes: D351, D352) to
eliminate unwanted signals before it is passed to the first
mixer.
The voltage of these diodes are controlled by tracking the
CPU (IC101) center frequency of the band pass filter. (See
Fig. 2)
■ First Mixer
The signal from the RF amplifier is heterodyned with the
first local oscillator signal from the PLL frequency synthesizer
circuit at the first mixer (Q352) to create a 49.95MHz first
intermediate frequency (1st IF) signal. The first IF signal is
then fed through one pair of monolithic crystal filter (MCF :
XF351) to further remove spurious signals.
ANT
D602~604
ANT
SW
IC161
D/A
24

CIRCUIT DESCRIPTION /

ANT
1st
MIX 49.95MHz
ANT RF
SW AMP
RX
POWER RF
AMP AMP
TX
Fig. 1 Frequency configuration /
L357,356 L354,355
Q353 Q352
D353,354 D351,352
RF AMP MIX
BPF BPF
IC402
TV
1/2 divider
CPU
1st local
OSC (VCO/PLL)
Fig. 2 Receiver system /
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