Transmitter Circuits - Icom IC-T70A Service Manual

2ND IF AND DEMODULATOR CIRCUITS
The signal from the 1st IF circuits is applied to the IF
demodulator IC (IC313), which contains the 2nd IF mixer,
2nd IF AMP, FM detector, etc. in its package.
The 1st IF signal is applied to the 2nd IF mixer and mixed
with the 2nd LO signal, resulting in the 450 kHz 2nd IF
signal. The 2nd LO signal is produced by tripling the
15.3 MHz reference frequency signal that is generated by
the reference frequency oscillator (TCXO; X300).
The 2nd IF signal is filtered by the crystal filter (FI300) to
eliminate unwanted signals, and amplified by the 2nd IF
AMP, and then demodulated by the quadrature detector
circuit, which employs a discriminator (X301) as the phase
shifter.
The demodulated AF signal is applied to the RX AF circuits.
• 2ND IF CIRCUITS
Q305
TCXO
15.3 MHz 45.9 MHz
X3
X300
To the AF circuits
IF DEMODULATOR IC
RX AF CIRCUITS
The demodulated AF signal from the IF demodulator IC
(IC313) is passed through the mute SW (IC904), LPF and
AF filter (Q8), which de-emphasizes the signal to obtain
–6 dB/oct of frequency response. The de-emphasized AF
signal is adjusted in level by the D/A converter (IC4), and
then applied to the AF power AMP (IC1) to obtain the AF
output signal.
The amplified AF signal is applied to the internal or external
speaker.
• RX AF CIRCUITS
IC4
IC1
Ext. speaker jack
AF
D/A
AMP
Int. speaker
FI300
CERAMIC
BPF
IC313
From the
1st IF circuits
X301
DISCRI
Q8 IC904
MUTE
BPF LPF
SW
From the
demodulator circuit

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
The audio signal from the internal or external microphone
(MIC signal) is passed through the MIC gain VR (IC4) which
adjusts the microphone sensitivity, and then applied to the
MIC AMP (IC800B).
The amplified MIC signal is passed through the MIC mute
SW (IC909), HPF, BPF (IC2A) and LPF (IC2D). The filtered
MIC signal is applied to the D/A converter (IC5) to be
adjusted in level (=deviation).
The level-adjusted MIC signal is applied to the modulation
circuit as the modulation signal.
• TX AF CIRCUITS
IC4
Int. MIC
MIC gain
VR
Ext. MIC JACK
MODULATION CIRCUIT
The modulation signal from the TX AF circuits is applied to
both of the VHF/UHF VCOs and to the reference oscillator
(X300), to frequency-modulate it. The frequency-modulated
signal from the VHF/UHF VCOs is applied to the TX AMP
circuits through the buffer (Q314/Q313), LO SW (D311/
D312), LO AMP (Q512) and another LO SW (D319), as the
TX signal.
• MODULATION CIRCUITS
Q311
D304-D306
From the
TX AF circuits
VHF
VCO
Q312
D309, D310
D307,
UHF
VCO
4 - 2
IC909
IC800B
MIC MIC
HPF
AMP MUTE
IC2D
IC5
To the modulation circuit
MOD
LPF
D/A
converter
D311,D312
D319
Q314 Q512
LO
LO LO
BUFF
AMP SW
SW
Q313
BUFF
IC2A
BPF
To the TX AMPs