Kenwood NX-840, Руководство по обслуживанию

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1-10 (No.RA011<Rev.001>)
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.
Fig.1 Frequency configuration
2.4.2 Receiver System
The receiver is double conversion superheterodyne.
The frequency configuration is shown in Figure 1.
Fig.2 Receiver system
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 (L506, L507, L508 and varactor diodes : D501, D502, D503) to
eliminate unwanted 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.
RF
AMP
1st
MIX
AF
PA
TCXO
MIC
AMP
x3
Tripler
RF
AMP
POWER
AMP
MCF
49.95MHz
IF SYSTEM
PLL/VCO
16.8MHz
50.4MHz
ANT
ANT
SW
RX
TX
SP
MIC
IC503
DC OP AMP
Q500
Tripler
X1
TCXO
IC501
FM IC system
IC709
Baseband IC
1st local
OSC (VCO/PLL)
TV
TVC
ANT
SW
D305,D306
D309,D310
L512,L513
D504,D505
BPF
Q506
RF AMP
L507,L508
L506,D501
D502,D503
BPF
Q503
MIX
XF500
MCF
Q501
IF AMP