Kenwood NX-740H Service Manual page 10

2.4 CIRCUIT DESCRIPTION
2.4.1 Frequency Configuration
The receiver utilizes double conversion. The first IF is 49.95MHz and the second IF is 450kHz. The first local oscillator signal is sup-
plied from the PLL circuit.
The PLL circuit in the transmitter generates the necessary frequencies. Figure 1 shows the frequencies.
2.4.2 Receiver System
The receiver is double conversion superheterodyne.
The frequency configuration is shown in Figure 1.
L512,L513
D305,D306 D504,D505
D309,D310
ANT
SW
DC OP AMP
2.4.2.1 Front-end RF Amplifier
An incoming signal from the antenna is applied to an RF amplifier (Q506) after passing through a transmit/receive switch circuit (D305,
D306, D309 and D310) , BPF (L512, L513, and varactor diodes : D504, D505).
After the signal is amplified (Q506), the signal is filtered by a BPF (L507, L508 and varactor diodes : D502, D503) to eliminate unwant-
ed signals before it is passed to the first mixer.
The voltage of these diodes are controlled by the TVC according to the channel frequency. (See Figure 2)
2.4.2.2 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 (Q503) 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 : XF500) to further remove spurious signals.
2.4.2.3 IF Amplifier Circuit
The first IF signal is amplified by Q501, and the enters IC501 (FM processing IC). The signal is heterodyned again with a second local
oscillator signal within IC501 to create a 450kHz second IF signal. The second IF signal is fed through a 450 kHz filter in IC501 to
further eliminate unwanted signals before it is amplified and FM detected in IC501.
1-10 (No.RA010<Rev.001>)
ANT
1st
MIX
ANT
RF
AMP
SW
POWER RF
AMP AMP
Fig.1 Frequency configuration
L507,L508
D502,D503
Q506
BPF BPF
RF AMP
TV
IC503
OSC (VCO/PLL)
TVC
Fig.2 Receiver system
MCF
49.95MHz
IF SYSTEM
50.4MHz
x3
Tripler
TCXO 16.8MHz
RX
MIC
PLL/VCO
AMP
TX
Q503 XF500 Q501
MIX MCF IF AMP
X1
TCXO
1st local
SP
AF
PA
MIC
IC501
FM IC system Baseband IC
Q500
Tripler
IC709