RED PT500-2, Руководство по обслуживанию

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PT500-2 Service Manual
18
The PWM wave is output by MCU composed of 58 foot and then commutated to adjustable
voltage after filtering to change the capacity of varactor diode D905 D203 D206 D201 D202
and D204 to control the center frequency of BPF.
1
st
mixer
The first IF (51.65MHz) signal is produced after mixing of the receiving signal from LNA and
the 1
st
local oscillation signal from frequency synthesizer. The first IF signal is filtered out
adjacent channel and other useless signals by crystal filter (XF201).
IF Circuit
The 1
st
IF signal from crystal filter is amplified by the first IF amplifier (Q201) before
processing of IC in IF( IC200
，
GT3136).
IF IC consists of the 2
nd
mixer, 2
nd
local oscillation, IF amplifier, limiter, frequency
discriminator and noise amplifier.
Frequency (12.8MHz) produced by TCXO(X1) is amplified and then selects 3
rd
harmonics
(38.4MHZ) as the second local oscillator signal source. The second IF signals (450kHz) are
generated after signals mixing of the second local oscillation (38.4MHz) and the first IF
(38.85MHz) in IC200. Audio signals are demodulated and outputted by IC200 after the
second IF signals are amplified and limited in IC200 and then filtered by ceramic
filter(450kHz).