FM540 Service Manual
3.3 RX Principle
Figure 3-2 RX Schematic
•
Front End of Receiver
The signal received will passTx-Rx switch controller andalso the BPF composed of LC of two levels
toremove undesired out-of-band signals, then it will be sent to the LNA(low noise amplifier) composed of
Q18 and other peripheral components for amplification
The output of LNA passes the BPF composed of LC of three levels to further remove undesired out-of-band
signals and is then sent to the first-level frequency mixer Q19.
•
The First Mixer
The signal from LNA mixes with the first local oscillator signal from the frequency synthesizer to generate
the first intermediate frequency (51.65MHz) signal.
•
IF Circuit
The first IF signal passes the crystal filter (XF700) to remove the signals from the adjacent channel or other
channels.
The first IF signal from the crystal filter is sent to the IF- processing IC(IC6, GT3136) after being amplified
by the first IF amplifier (Q20).
IF IC includes secondary mixer, IF amplifier, limiter, frequency detector, noise amplifier and audio low pass
filter.
The 12.8MHz signal from X1 passes Q300 and the peripheral circuit for amplification and then the second
local oscillator of 51.2MHz is acquired. The second oscillator (51.2MHz) mixes frequency with the first IF
ANT SW
BPF
1st local
osc
BPF
MCF
IF AMP
Q20
IF DET
450K
CODEC
AF PA
D3 D37 D36
D11
MIXER
Q19
XF700
51.65MHz
U6
IC7
ANT
X1
Multiply
Q11
TCXO
MCU
IC19
PC/TV
Q18
RF AMP
Page 7 of 99
Содержание FM540
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