Transmitter Circuits - Icom IC-F70DT Service Manual

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifier circuit amplifies the audio signals
from microphone within +6 dB/oct pre-emphasis characteris-
tic. The microphone signals are processed in the base band
IC which contains microphone amplifier, compressor, scram-
bler, limiter, splatter filter, etc. in its package.
The audio signals from the microphone (FRONT UNIT;
MC201) are passed through the microphone mute switch
(FRONT UNIT; IC204). The switched signals are amplified at
the microphone amplifiers (FRONT UNIT; IC203, pins 1, 2,
13, 14) to obtain within +6 dB/oct pre-emphasis characteris-
tics. The amplified signals are applied to the MAIN UNIT via
J1 (pin 2).
The amplified MIC signals from the FRONT UNIT are
applied to the base band IC (IC301, pin 3). The applied
MIC signals are amplified at the amplifier section, and level
adjusted at the volume control section. The level adjusted
MIC signals are applied or bypassed the compressor sec-
tion, pre-emphasis section, TX/RX HPF, de-scrambler, lim-
iter, splatter, in sequence, then applied to another volume
controller.
The compressor compresses the MIC signals to provide
high S/N ratio for receive side, and the pre-emphasis obtains
+6 dB/oct audio characteristics. The TX/RX HPF filters out
250 Hz and lower audio signals, the limiter limits its level
and the splatter filters out 3 kHz and higher audio signals.
The filtered MIC signals are level adjusted at another vol-
ume control section and amplified at the amplifier section,
and then output from pin 7 via smoothing section (SMF).
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
with the audio signals from the microphone.
MIC signals from the base band IC (IC301) are passed
through the MIC switch (IC302, pins 5, 4), PM filter (C338,
R327), FM/PM switch (IC302, pins 1, 15), and then applied
to the AF mixer (IC12, pin 2) to be mixed with CTCSS/DTCS
signals.
The mixed MIC signals are applied to the D/A converter
(IC303, pin 4) to be adjusted its level. The level adjusted AF
signals are output from pin 3 and applied to the modulation
circuit (D8) to modulate the VCO oscillating signal by chang-
ing the reactance of D8 at the TX VCO (Q5, D6, D7, D25).
• ALC CIRCUIT
VCC
T5V
from TX/RX switch
(D9, D10)
+
T2
ALC
amp.
–
TMUT
Q11 Q12
YGR Drive
amp. amp.
IC5
The CTCSS/DTCS signals are generated by the CPU (IC307)
and output from pins 89–91 ("CENC0," "CENC1," "CENC2").
The CTCSS/DTCS signals are passed through 3 regis-
ters (R374–R376) to change its wave form. The wave form
changed CTCSS/DTCS signals are then passed through the
LPF (IC12, pins 8, 10) and applied to the converter (IC303,
pin 9) to be adjusted its level, and output from pin 10.
The level adjusted CTCSS/DTCS signals are applied to
the AF mixer (IC12, pin 2) to be mixed with MIC signals.
The mixed CTCSS/DTCS signals are output from pin 1 and
applied to the D/A converter (IC303, pin 4) to be adjusted its
level again, then output from pin 3. The CTCSS/DTCS sig-
nals from the D/A converter are applied to the both of refer-
ence frequency oscillator (X1) and modulation circuit (D8) to
modulate the reference frequency signal and VCO oscillating
signal.
The modulated VCO output signal is amplified at the buffer
amplifiers (Q6, Q10) and is then applied to the YGR ampli-
fier (Q11) via the TX/RX switch (D10).
4-2-3 TRANSMIT AMPLIFIERS
The VCO output signal is amplified to transmit output power
level by the transmit amplifiers .
The buffer-amplified signal from the TX/RX switch is applied
to the YGR (Q11), the driver (Q12), and power (Q13)
amplifiers, to be amplified to the transmit output power level.
The power amplified transmit signal is passed through the
power detector (D11), antenna switch (D12), and two-stage
LPFs (L22, L23, C204–C207, C209), and then applied to the
antenna connector (CHASSIS UNIT; J1).
4-2-4 ALC CIRCUIT
The ALC (Automatic Level Control) circuit stabilizes transmit
output power and controls transmit output power High or Low.
The power detector circuit (D11) detects the transmit output
signal and converts it into DC voltage.
The detected voltage is applied to the ALC amplifier (IC5,
pin 3). The "T2" signal from the D/A converter (IC310, pin
2), controlled by the CPU (IC307), is applied to the another
input (pin 1) for reference, and the "T2" signal also controls
transmit output power (5 W or 1 W).
The output voltage from the ALC amplifier controls the bias
of the YGR (Q11), driver (Q12) and power amplifier (Q13) to
reduce the output power by comparing the detected voltage
and the reference voltage. Thus the ALC circuit maintains a
constant transmit output power.
Power detecter
Q13
Power
LPF
amp.
D11
4 - 3
LPF
to anntena switch
(D12, D22)