24
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.
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.
■ 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 Rating
Nominal center frequency 49.95MHz
Pass bandwidth ±5.0kHz or more at 3dB
35dB stop bandwidth ±20.0kHz or less
Ripple 1.0dB or less
Insertion loss 5.0dB or less
Guaranteed attenuation 80dB or more at fo±1MHz
Spurious : 40dB or more within fo±1MHz
Terminal impedance 350Ω±5% / 5.5pF±0.5pF
Table 1 Crystal filter (L71-0551-15) : XF1
Item Rating
Nominal center frequency 450kHz
6dB bandwidth ±4.5kHz or more
50dB bandwidth ±10.0kHz or less
Ripple 2.0dB or less
Insertion loss 6.0dB or less
Guaranteed attenuation 55.0dB or more within fo±100kHz
Terminal impedance 2.0kΩ
Table 2 Ceramic filter (L71-0959-05) : CF1
Item Rating
Nominal center frequency 450kHz
6dB bandwidth ±6.0kHz or more
50dB bandwidth ±12.5kHz or less
Ripple 3.0dB or less
Insertion loss 6.0dB or less
Guaranteed attenuation 35.0dB or more within fo±100kHz
Terminal impedance 2.0kΩ
Table 3 Ceramic filter (L72-0973-05) : CF2
Fig. 2 Receiver system
CIRCUIT DESCRIPTION
ANT
SW
RF
AMP
1st
MIX
AF
AMP
TCXO
MIC
AMP
X3
multiply
TX
AMP
PA
AMP
CF 450kHz
MCF
49.95MHz
IF SYSTEM
PLL/VCO
16.8MHz
50.4MHz
ANT
RX
TX
SP
MIC
1/2
ANT
L18
BPF
L11,13,15
BPF
Q27
RF AMP
Q13
IF AMP
IC4
AF AMP
Q15
MIX
XF1
MCF
D33,34
ANT
SW
IC8
1/2 divider
IC6
D/A
Q7
X3 multiply
X1
VCXO
IC5
MIX,IF,DET
1st local
OSC (PLL)
DEO
CF1 (Narrow)
CF2 (Wide)
APC
TUNE
CPU
Q35
Wide/
Narrow SW
