Icom IC-E92D Service Manual Download

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• FREQUENCY SYNTHESIZER CIRCUITS (A BAND)

4-3 FREQUENCY SYNTHESIZER CIRCUITS

VCOs

This transceiver has total of five VCOs; one RX VCO and
RX/TX VCOs on the VCO UNIT, and two RX VCOs on the
MAIN UNIT.

<A BAND>
• BC BAND VCO (VCO UNIT; Q1, D1, 3, 5, 6

)

The BC BAND VCO generates the 1st LO for BC band (0.495
–75.995 MHz) RX.

• VHF BAND VCO (VCO UNIT; Q51, D51, 54)

The VHF BAND VCO generates the 1st LO for VHF band (76
–173.995 MHz) RX, and also TX signal for the operation on
the VHF band.

• UHF BAND VCO (VCO UNIT; Q101, D101, 104)

The UHF BAND VCO generates the LO for UHF band (174
–599.995 MHz) RX, and also TX signal for the operation on
the UHF band.

When receiving 600 MHz and higher signals, the UHF BAND
VCO oscillates 269.175–469.17 MHz 1st LO signals, and the
output signal is doubled by the doubler (MAIN UNIT; Q850,
D851) before being applied to the mixer (MAIN UNIT; IC900).

<B BAND>
• VHF BAND VCO (MAIN UNIT; Q301, D300, 301)

The VHF BAND VCO generates the 1st LO for VHF band
(118–174 MHz) RX for B BAND.

• UHF BAND VCO (MAIN UNIT; Q351, D350, 352)

The UHF BAND VCO generates the 1st LO for UHF band
(350–470 MHz) RX for B BAND.

PLLs

The PLL circuit provides stable oscillation for both of the
transmit and 1st LO frequencies. By comparing the fedback
VCO output and the reference frequency signal, the oscillating
frequency is stabilized. The PLL output frequency is controlled
by the serial data including divide ratio from the CPU.

<A BAND> (VCO AND MAIN UNITS)

A portion of output signals from each VCO are fedback
to the PLL IC (M: IC800) via buffers (V: Q200, 202) and
LPF. The applied VCO outputs are divided by the prescaler
and programmable divider, then phase-compared with
divided reference frequency from X450 (MAIN). The phase
difference is output via the charge pump, and applied to the
VCOs as lock voltage via the loop filter.

<B BAND> (MAIN UNIT)

A portion of output signals from each VCO are fedback
to the PLL IC (IC400) via buffer (Q353). The applied VCO
outputs are divided by the prescaler and programmable
divider, then phase-compared with divided reference
frequency from X450. The phase difference is output via the
charge pump, and applied to the VCOs as lock voltage via
the loop filter.

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.

APC CIRCUIT (RF UNIT)

The APC (Automatic Power Control) circuit stabilizes
transmit output power to prevent transmit output power level
change which is caused by load mismatching or heat effect,
etc.

TX signal is passed through the power detector (VHF; D154
–156/UHF; D150, 152,153). The power detector rectifies
a portion of the TX signal and converts it into DC voltage
which is in proportion to the transmit output power. The
detected voltage is applied to the APC controller (IC150B).
The TX power setting voltage “PSET” is applied to another
input terminal as the reference voltage.

The output voltage is applied to the ATT (D158, 159) to con-
trol the attenuation level, to adjust the input level of the YGR
AMP (Q102) so that the TX output power is stable.

The setting of TX power is carried out by applying voltage
“DSET” to the APC controller (IC150A). The output voltage of
the controller controls the bias of the drive and power AMPs
to reduce/increase the gain of these amplifi ers to set the TX
output power to High, Mid., Low and S-Low.

ATT

ANT

PWR

DET

PWR
AM P

LPF

APC

CTRL

YGR

AM P

DRIVE

AM P