Yaesu FT-902DM, Instruction Manual

Page: 24 / 55

The output from the IF second mixer is fed
th rough a mono Ii thic fil ter
XF
402 to the two-stage
IF amplifier consisting of Q 403 and Q404 (both
3 SK5 1 - 03) .Th e outpu t from Q403 is coupled to
the rejection tuning circuit which eliminates in ter­
fererice within the
I F
passband. This rejection
function is accomplished by varying the resonan t
frequency of a cry stal with varac tor diode
0406
( 1 S2209)
on the S S B , CW , and FSK modes.
The amplified IF signal is fed to the CARRIER
U NIT. In the AM mode, the signal is fed through
a buffer amplifier Q410
(2SC 1 8 1 SY) to the AM
d e tector D407 ( 1 N60). The signal is then fed to
the AF UNIT.
A portion of the ou tput from Q41 0 is rectified
by D4 1 1
and
D4 1 2 ( 1 N270)
to p roduce AGC voltage .
The
AGC vol tage is amplified by DC amplifier
Q409 (2SC 1 8 1 5GR)
and fed to the gate of RF
ampl fier
Q , 01 and to I F amplifiers Q403 and Q404 to
con trol the gain of these stages for A GC purposes.
The time constant of the AGC voltage d ecay is
selectable
(FAST /SLOW)
by swi tch S 2003 . The AGC
vol tage is further amplified by Q408 (2SK 1 9GR)
and Q407
(2SA564A)
for signal strength indica­
tion on the front panel S-meter.
D402 R413 R414 040 3
15100 . 7 180 180 151007
AF AM
9V
The RF GAIN con trol on the fron t p anel varies
the AGC vol tage leve l , providing manual con trol
of the gain of the RF and IF stages.
Q41 1 ( 2SC1 8 1 SGR) works as a relay driver for the
IF rejection tuning circui t. Delay transistor
Q4 1 2
(2SC73 5Y) supplies the voltage to the drain of
Q401 and Q402 when the transceiver re turns to the
receive mode after transmission.
0403 3S K 5!-03
20
30
40
50
60
- 2
-
I (f to•I
t-
2
1sss1 ; c w i
0404 3SK51-03
10
REJECT
v O
8V
5 wtTER
-22-
0409
2SC l 8 1 5 G R
04 1 0
2SCl8 1 5 Y
04 1 1
I
2SC l 8 1 5 G R
I
I
I