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4-5

4-3 FREQUENCY SYNTHESIZER CIRCUIT

(MAIN UNIT)

VCOs

The ID-5100A/E has total of fi ve VCOs: three VCOs for Band
A and another two for Band B.

• Band A
VHF VCO #1
(Used when the Heterodyne function is set to "Normal")

The VHF VCO #1 (Q211, D204, D206 and D228) generates
both 1st LO signal (for receiving a VHF signal on Band A)
and the VHF TX signal. The output of buffer (Q215) is ampli-
fi ed by the LO AMP (IC203), and then used as the VHF TX/
RX LO signal.

While receiving, the LO signal is applied to the 1st IF mixer
(IC125) for receiving a signal on the VHF band, through the
RX LO SW (D212 and D217) and LPF (L314, L316, C986,
C992 and C997).

While transmitting, the LO signal is applied to the TX AMP
circuits, through the LO SW (D50).

VHF VCO #2
(Used when the Heterodyne function is set to "Reverse")

The VHF VCO #2 (Q237, D238 and D239) generates the 1st
LO signal (for receiving a VHF signal on Band A). The output
of buffer (Q215) is amplifi ed by the LO AMP (IC203), and
then applied to the 1st IF mixer (IC125) for receiving a signal
on the VHF band, through the RX LO SW (D212 and D217)
and LPF (L314, L316, C986, C992 and C997).

UHF VCO

The UHF VCO (Q205, Q210, D201, D202 and D205) gener-
ates the 1st LO signal (for receiving a UHF signal on Band
A). The output of buffer (Q214) is amplifi ed by the LO AMP
(IC203), and then applied to the 1st IF mixer (IC125), through
the RX LO SW (D213 and D216) and LPF (L315, C988 and
C995).

• Band B
VHF VCO

The VHF VCO (Q228, Q233, D225, D226 and D230) gener-
ates the 1st LO signal (for receiving a UHF signal on Band
A). The output of buffer (Q236) is amplifi ed by the LO AMP
(IC208), and then applied to the 1st IF mixer (IC124), through
the RX LO SW (D235 and D237) and LPF (L338, L339,
C1103, C1105 and C1106).

UHF VCO

The UHF VCO (Q232, Q227, D223, D224, D227 and D229)
generates both 1st LO signal (for receiving a UHF signal on
Band B) and the UHF TX signal. The output of buffer (Q235)
is amplifi ed by the LO AMP (IC208), and then used as the
UHF TX/RX LO signal.

While receiving, the LO signal is applied to the 1st IF mixer
(IC124) for receiving a signal on the UHF band, through the
RX LO SW (D234 and D236) and LPF (L337, C1102 and
C1104).

While transmitting, the LO signal is applied to the TX AMP
circuits, through the LO SW (D52).

PLL

•

Band A

A portion of VHF and UHF VCOs output signal is amplifi ed by
the buffer (Q213), and then fed back to the PLL IC (IC201).

The PLL IC (IC201) phase-compares the output of reference
frequency oscillator (TCXO: X200) and VCO, and the phase
difference is output as the charge pump current. The current
is passed though the loop fi lter (R715, R717, R719, R722,
C924, C926 to C928 and C930) to be converted into the lock
voltage, which controls the oscillating frequency of VCO.

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.

• Band B

A portion of VHF and UHF VCOs output signal is amplifi ed by
the buffer (Q234), and then fed back to the PLL IC (IC206).

The PLL IC (IC206) phase-compares the output of reference
frequency oscillator (TCXO: X201) and VCO, and the phase
difference is output as the charge pump current. The current
is passed though the loop fi lter (R797, R799, R802, R805,
C1041, C1043 to C1045 and C1048) to be converted into the
lock voltage, which controls the oscillating frequency of VCO.

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.

TX/RX

AMP

FIL

LOOP

BUFF

PLL

LPF

TX/RX

FIL

LOOP

PLL

RX
SW