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iO1000 Detailed Service Manual - THEORY OF OPERATION
Main VCO Synthesizer
This synthesizer consists of the crystal-based reference oscillator, Low Voltage
Fractional ASIC, and main VCO circuit. It produces the RF signal to down-con-
vert the received signal to the first IF frequency (109.65 MHz) and up-convert the
transmitter IF frequency (150.9 MHz) to the transmit RF frequency.
The main VCO receives its control voltage from the LV Frac-N to determine the
oscillation frequency. It outputs its local oscillation frequency through a discrete
buffer circuit that allows matching and gain. Part of this frequency is fed back to
the pre-scalar input of the LV Frac-N to complete the loop.
Second Local Oscillator (LO)
This circuit supplies the LO signal to the quadrature down conversion mixer
inside the iZIF in the receiver lineup (see “iZIF IC” on page 15).
The second LO consists of a VCO, loop divider, phase detector, reference fre-
quency, and loop filter. The loop divider and phase detector are internal to the
iZIF. The reference frequency is obtained by taking the 16.8 MHz crystal refer-
ence signal into pin 21 of the iZIF and then dividing it down inside the IC to get
2.1 MHz. The loop filter and VCO circuits are external to the iZIF. Although the
IF frequency is 109.65 MHz, the VCO oscillates at 219.3 MHz. The VCO signal is
divided by two inside the iZIF to get the desired 109.65 MHz.
The VCO is run at twice the LO frequency to avoid the coupling of its signal into
the iZIF preamp to eliminate the possibility that it will mix with itself at the down
mixer. The VCO resonator circuit consists of L607, C639, CR601, and C627.
The loop filter determines how fast the second LO locks on frequency and how
big the LO spurs are. This circuit consists of R601, R602, C625, and C626.
