RCA BR200DV, Service Manual, Page: 23 / 79

BR200DV/BR300DV
Service Manual
23
Signals from LNA are filtered again by BPF before entering the 1st mixer (Q202).
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 D202 and D204 to control the center
frequency of BPF.
1
st
mixer
The first IF (49.95MHz) 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
，
NJM2591V).
IF IC consists of the 2
nd
mixer, 2
nd
local oscillation, IF amplifier, limiter, frequency discriminator and noise
amplifier.
Frequency (16.8MHz) produced by TCXO(X1) is amplified and then selects 3
rd
harmonics (51.4MHZ) as
the second local oscillator signal source. The second IF signals (450kHz) are generated after signals
mixing of the second local oscillation (51.4MHz) and the first IF (49.95MHz) 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).
Figure 3 Schematic Diagram for IF System