Yaesu FT-901DM Maintenance Service Manual page 41

TECHNICAL NOTES
FM UNIT (PB-1707)
In the FM mode, the IF signal from the IF UNIT
is fed to the gate of Qogo, (3SK40M), where it is
heterodyned with the 9442.5 kHz signal delivered
from the FM oscillator Qo92 (2SC1815Y), thus
producing a 455 kHz IF signal. The 455 kHz IF
signal passes through a two-stage ceramic filter
consisting of CF99,; and CF, (LFB-15), and is
amplified by an amplifier limiter consisting of Qgo3
(2SC181SY) and Qo, (TA7061 AP).
A ceramic discriminator consisting of CDoo:
(CFD455S4) and Doo2and Dog; (both 1S188FM)
produces an audio output in response to a cor-
responding frequency shift in the 455 kHz IF
signal. The discriminator output is amplified by
Qoos (2SC1815Y) and fed to the AF UNIT at
pin 6.
When no carrier is present in the 455 kHz IF, the
noise at the discriminator output is amplified by
Qoos and Qoog (both ZSCI815Y) and detected
by Doos and Doos (both 1S188FM) to produce a
DC voltage. This voltage is applied to tum "on"
Qoeo7 (2SC1815Y). With Qyo7 "'on,"" the base of
Qoos is grounded to quiet the audio amplifier.
When a carrier is present, the quieting action of
the receiver removes noise from the discriminator
output and Qgo, is turned "off," permitting
normal action of Qoog. The squelch control VRoo;
sets the squelch threshold level.