Yaesu FT-901DM Maintenance Service Manual page 39

TECHNICAL NOTES
FILTER UNIT (PB-1716C)
The IF signal received at pin 2 of the FILTER
UNIT (PB-1716) is amplified by parallel-connected
Qs: and Q3o; (both 2SK19GR), and fed through
a monolithic filter XF39, (8.9M-20A), which has
a +10 kHz bandwidth, providing additional selec-
tivity and enough delay time to match the noise
blanker gating pulses. The signal is then fed to the
second IF amplifier, Q3o2 (2SK19GR).
The noise blanker diode D3; (181007) is placed
between T392 and T4393, and it functions as an
ON/OFF switch which is controlled by the noise
blanker driver Q393 (2SC1815Y). The output
from the source of Q392 is passed through the SSB
filter XF39, (or the optional AM or CW filters).
Selection of the filter to be used is performed by
one of the diode switches D393 - D3 (181007),
depending on the mode of operation. The IF
signal is then transferred to the IF UNIT. In the
FM mode, the IF signal is coupled directly through
D3io (181555) to pin 5 of the IF UNIT.
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[FILTER UNIT
IF UNIT (PB-1704)
The IF signal from pin 14 is fed to the gate of the
IF first mixer, Qo: (3SK51), where the IF signal
is heterodyned with a 19.7475 MHz Af local signal
delivered from crystal oscillator Qzos (2SC535A)
and buffer amplifier Qaoe (2SC1815Y). resulting
in a signal of 10.76 MHz +Af.
The new 10.76 MHz + Af signal is fed through filter
XF301 to the IF second mixer, Qso2 (3SK51),
where the filtered signal is heterodyned with a
19.7475 MHz +Af signal, producing an 8.9875 MHz
signal, the same as the original IF. This process
moves the IF signal across the passband of XF4o1.
The combination of filters XF304 and XFao1
provides continuously variable width of the IF
passband.
The frequency of crystal oscillator Qsos is varied
by varactor diode Dao; (182209).
On the AM and FM modes, the IF signal is passed
through diode switches Dgo2 and Dao3 (181007),
and does not pass through filter XF4o1.
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