Circuit Description; Frequency Configuration; Receiver System; Front-End Rf Amplifier - Kenwood 762G Service Manual

TK-760G/762G

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 neces-
sary frequencies. Figure 1 shows the frequencies.
ANT
1st
MIX 49.95MHz
ANT RF
SW AMP
RX
PA TX
AMP AMP
TX
Fig. 1 Frequency configuration

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 (Q27) after passing through a transmit/receive
switch circuit (D33 and D34 are off) and a band pass filter
(L18). After the signal is amplified (Q27), the signal is fil-
tered through a band pass filter (L11, L13 and L15) to elimi-
nate unwanted signals before it is passed to the first mixer.
Band pass filters (L18, L11, L13 and L15) have varactor di-
odes (D30, D31, D18, D23 and D28).
The voltage of these diodes are controlled by to track the
CPU (IC502) 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 synthe-
sizer circuit at the first mixer (Q15) to create a 49.95MHz
first intermediate frequency (1st IF) signal. The first IF signal
is then fed through two monolithic crystal filters (MCFs :
XF1) to further remove spurious signals.
24

CIRCUIT DESCRIPTION

CF 450kHz
MCF
AF
IF SYSTEM
AMP
50.4MHz
X3
multiply
TCXO 16.8MHz
MIC
1/2 PLL/VCO
AMP
ANT
L18 Q27 L11,13,15
D33,34
BPF RF AMP BPF
ANT
SW
TUNE
APC
IC6
CPU
D/A
Fig. 2 Receiver system
IF Amplifier
The first IF signal is amplified by Q13, and the enters IC5
(FM processing IC). The signal is heterodyned again with a
second local oscillator signal within IC5 to create a 450kHz
second IF signal. The second IF signal is then fed through a
450kHz ceramic filter (Narrow : CF1, Wide : CF2) to further
eliminate unwanted signals before it si amplified and FM
detected in IC5.
Item
Nominal center frequency
SP
Pass bandwidth
35dB stop bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminal impedance
MIC
Table 1 Crystal filter (L71-0551-15) : XF1
Item
Nominal center frequency
6dB bandwidth
50dB bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminal impedance
Table 2 Ceramic filter (L71-0959-05) : CF1
Item
Nominal center frequency
6dB bandwidth
50dB bandwidth
Ripple
Insertion loss
Guaranteed attenuation
Terminal impedance
Table 3 Ceramic filter (L72-0973-05) : CF2
CF1 (Narrow)
CF2 (Wide)
Q15 XF1 Q13
MIX MCF IF AMP
MIX,IF,DET
IC8 X1
VCXO X3 multiply
1/2 divider
1st local
OSC (PLL)
Rating
49.95MHz
5.0kHz or more at 3dB
20.0kHz or less
1.0dB or less
5.0dB or less
80dB or more at fo 1MHz
Spurious : 40dB or more within fo 1MHz
350 5% / 5.5pF 0.5pF
Rating
450kHz
4.5kHz or more
10.0kHz or less
2.0dB or less
6.0dB or less
55.0dB or more within fo 100kHz
2.0k
Rating
450kHz
6.0kHz or more
12.5kHz or less
3.0dB or less
6.0dB or less
35.0dB or more within fo 100kHz
2.0k
IC4
AF AMP
IC5
DEO
Q7
Q35
Wide/
Narrow SW