Frequency Synthesizer Circuits - Icom IC-T70A Service Manual

TRANSMIT AMPLIFIERS
The TX signal from the LO SW (D319) is passed through
the attenuator (D500 and D501), which continuously adjusts
the TX signal in level to keep it constant. The level-adjusted
signal is amplified by the YGR AMP (Q502), drive AMP
(Q503) and power AMP (Q507) in sequence.
The gate voltage of the drive AMP (Q503) is adjusted by the
TX power reference voltage from the D/A converter (IC5),
depending on the TX power.
The output signal of the power AMP (Q507) is passed
through one of the following paths, depending on it's
frequency.
• TX AMPLIFIERS
D500,D501
Q502
From the VCO
YGR
ATT
AMP
IC501B
APC
CTRL
PSET

4-3 FREQUENCY SYNTHESIZER CIRCUITS

VCO
• VHF VCO
The VHF VCO is composed of Q311, D304–D306, and
generates both the TX signal and RX 1st LO signals. This
VCO is frequency-modulated by applying the modulation
signal to D306.
During receive, the output signal is passed through the
buffer AMP (Q314) and LO SW (D311), and then applied to
the LO AMP (Q512). The amplified signal is passed through
another LO SW (D314), attenuator and LPF, and then
applied to the VHF band 1st IF mixer (Q500).
During transmit, the output signal is applied to the TX AMP
circuits through the buffer (Q314), LO SW (D311) and LO
AMP (Q512).
• UHF VCO
The UHF VCO is composed of Q312, D307, D309 and
D310, and generates both the TX signal and RX 1st LO
signals. This VCO is frequency-modulated by applying the
modulation signal to D307.
During receive, the output signal is passed through the
buffer AMP (Q313) and LO SW (D312), and then applied to
the LO AMP (Q512). The amplified signal is passed through
another LO SW (D315), attenuator and LPF, and then
applied to the UHF band 1st IF mixer (Q509).
During transmit, the output signal is applied to the TX AMP
circuits through the buffer (Q313), LO SW (D312) and LO
AMP (Q512).
Q507
Q503
UHF band
PWR
DRIVE
AMP AMP
IC5
D/A
LPF
VHF RX circuits(136-174 MHz)
UHF RX circuits(400-479 MHz)
• VHF BAND
The TX signal is passed through the LPFs and ANT SW
(D509–D511), before being fed to the antenna.
• UHF BAND
The TX signal is passed through the HPFs, ANT SW (D506,
D524) and the LPFs, before being fed to the antenna.
APC CIRCUIT
The TX power detector circuits (VHF band: D514 and D517/
UHF band: D507 and D516) rectify a portion of the TX signal
to direct current, and the APC AMP (IC501B) compares the
voltage and the TX power control reference voltage, "PSET."
The resulting voltage controls the attenuation level of the
attenuator (D500 and D501) to keep the TX power constant.
D506,D524
ANT
HPF LPF
SW
D509-D511
ANT
SW
PLL
A portion of generated signal from each VCO is amplified
by the buffer (Q313/Q314), and then fed back to the PLL IC
(IC305) through the buffer (Q310) and LPF.
The applied VCO output signal is divided and phase-compared
with a 15.3 MHz reference frequency signal from the TCXO
(X300), which is also divided. The resulting signal is output from
the PLL IC through the internal charge pump, and DC-converted
by the loop filter, and then applied to the VCO as the lock
voltage which controls the oscillation frequency of the VCOs.
When the oscillation frequency drifts, its phase changes from
that of the reference frequency, causing a lock voltage change
to compensate for the drift in the VCO oscillating frequency.
• FREQUENCY SYNTHESIZER CIRCUITS
From the modulation circuit
IC305
X300
PLL LOOP
IC FILTER
15.3 MHz
Q305
45.9MHz
BUFF
4 - 3
400-479 MHz
LPF HPF
D507,D516
PWR
DET
PWR
DET
D514,D517
136-174 MHz
LPF LPF
Q311
D304-D306
D311,D312
Q314
VHF LO
BUFF
VCO SW
Q313
UHF
BUFF
VCO
Q312
D307,D309,D310
Q310
BUFF
LPF
IC311
D314
IF demodulator IC
LO
(2nd mixer)
ATT
SW
D315
LO
ATT
SW
Q512
LO To the TX AMPs
AMP
Q500
LPF
VHF mixer
Q509
LPF
UHF mixer