Audio Circuit - Icom IC-25H Instruction Manual

The 16.9MHz, first IF signals are taken from the other port of the DBM through a monolithic
crystal filter F11, then fed to the IF circuit.
IF CIRCUIT
16.9MHz 15 IF signals from the crystal filter Р11 аге fed to the 1st gate of the dual gate MOS FET
02. Signals amplified by 02 are supplied together with 17.355MHz signals from the second local
oscillator Q1, to the base of second mixer Q10, for conversion to the 455K Hz second IF.
In the second IF amplifier, which has excellent selectivity due to а CFU455E ceramic filter, signals
are amplified by 011 ~ 013 and 014. After removal of noise and other AM components Бу IC3
functioning as a limiter, the signals are detected in terms of audio frequency signals by a ceramic
discriminator.
This ceramic discriminator has outstanding temperature characteristics, linearity and detection
sensitivity, which guarantee clear, stable reception.
Audio signals from the discriminator are divided into audio signals and noise component signals to
operate the squelch circuit.
IF CIRCUIT
RF
UNIT
455KHZ
S METER
DETECT.
LED DRIVER S METER
AUDIO CIRCUIT
In the audio amplifier, audio signals are passed through the de-emphasis circuit consisting of R85
and C78, and amplified by Q15. High frequency components are cut by a low-pass filter Q16, in
order to improve the signal-to-noise ratio. Then the audio signals are adjusted to a suitable level
by volume control R1, amplified by AF power amplifier IC4 to 2W or more, and fed to the
speaker.
SQUELCH CIRCUIT
This is a noise circuit that suppresses noise when signals enter the set. To avoid erroneous opera-
tion due to audio signals, noise components of about 25KHz are selectively amplified.
Squelch control R2 is located immediately after the discriminator, thus increasing the dynamic
range of the circuit.
Noise components from squelch control R2 are amplified by Q17 and Q18, rectified by D16 and
D17, and with C103, C105, and R112 ensure correct timing sequence for smooth squelching
supplied to the base of squelch control transistor Q19.
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