Transmitter Circuits - Icom IC-25A Instruction Manual

When there аге по audio signals, rectified DC voltage from 016 and 017 is applied to the base of
Q19, turning it on. Since the collector of Q19 is connected to the base bias network of AF
amplifier Q15, base voltage of Q15 falls and Q15 is turned off, thus squelch action is applied, and
no audio is amplified by IC4. At this time, signal lamp control IC, | СЗ of DRIVER unit, does not
conduct, and so the RECEIVE lamp goes off.
When incoming signals are received, noise is suppressed, the base voltage of 019 falls, and 019 is
turned off. Therefore, normal voltage is applied to the base of 015, the squelch circuit is opened,
and audio signals are heard from thespeaker. 1СЗ alsois turned on, and the RECEIVE lamp lights up.
The point at which squelch becomes operative (squelch threshold) is adjusted by R2.
During transmit, Q19's collector is grounded through 018 and PTT switch, and the squelch circuit
is operative, so squelch action is started the moment there is switchover from transmit to receive,
and no loud crackling or similar noise is heard.
AF POWER
SPEAKER AMP. AF FILTER AF AMP.
IF
UNIT
FM
DETECT.
SQL.
SQUELCH
CONTROL
AF/SQUELCH CIRCUITS
TRANSMITTER CIRCUITS
16.9MHz OSCILLATOR, MODULATION CIRCUITS
16.9MHz signals are oscillated by Q6 and amplified audio signals from the microphone are supplied
to varactor diode D10 connected in series with the crystal unit. Voltage of these signals causes the
capacitance of 010 to vary, and frequency modulation is effected. Since this transceiver is a
heterodyne type, any frequency deviation that occurs in this circuit appears unchanged as a fre-
quency deviation in Ше 144MHz band, and so use is made of a crystal unit with special characteris-
tics to ensure suitable frequency deviation and stability.
AUDIO FREQUENCY CIRCUIT
Audio signals from the microphone are fed to the audio amplifier O4. Amplified signals are sent
through a differential circuit C24 and R26 to Pin 3 of IC1, which is a limiter amplifier and has
6dB/Octave response between 300Hz and 3KHz. This output is fed to low-pass filter ОБ, and then
the FM modulator D10, through level adjust trimmer R36.
FM MOD BUFFER
MIC AMP. LIMITER CRYSTAL OSC AMP
| MIXER
CIRCUIT
СЛ
AF/MODULATION CIRCUIT 1
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