RTS DV 2400 Series, Руководство по обслуживанию

Страница: 3 / 40

DV-2400
2
CIRCUIT DESCRIPTION
„
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 (Q205) to create a 38.850MHz first intermediate frequency
(1st IF) signal. The first IF signal is then fed through one pair of monolithic crystal filter (MCF : XF200 and
XF201 ) to further remove spurious signals.
„
IF amplifier
The first IF signal is amplified by Q204, and then goes to U200 (FM processing IC). The signal is
heterodyned again with a second local oscillator signal within U200 to create a 450kHz second IF signal.
The second IF signal is then fed through a 450kHz ceramic filter (Wide : CF201, Narrow : CF200) to
further eliminate unwanted signals before it is amplified and FM detected in U200.
„
Wide/Narrow Switching Circuit
The Wide port and Narrow port (pin 35) of the CPU is used to switch between ceramic filters. When the
Wide port is high, the ceramic filter SW diodes (D200, D201) cause CF201 to turn on to receive a Wide
signal.
When the Narrow port is low, the ceramic filter SW diodes (D200, D201) cause CF200 to turn on to
receive a Narrow signal. (See Fig. 3.)
IF_IN
U200
IF SYSTEM
MIX_O
D201
D200
CF200
(NARROW)
CF201
(WIDE)
R227
R228
SW
SW
Q203
Q201
WIDE/NARROW SW
U2 35pin
.
R2 29
R2 30
Fig. 3 Wide/Narrow Switching Circuit
„
AF Signal System
The detection signal from IF IC (U200) goes to D/A converter (U505) for adjusting the gain and is output
to AF filter (U502) for characterizing the signal. It is also amplified by entering compander amplifier (U500).
The AF signal output from U500 and the TONE signal are summed up and the resulting signal goes to the
D/A converter (U505). The AFO output level is adjusted by the D/A converter. The signal output from the
D/A converter is input to the audio power amplifier (U506). The AF signal from U505 was inputted into
(U506).The AF signal from (U506) switches between the internal speaker and speaker jack (J502) output.
(See Fig. 4.)
U501, U502
AF Filter
U505
D/A
CONV.
U200
IF IC
U505
D/A
CONV.
AF PA
U506
U500
COMPANDE SP
Fig. 4 AF signal system