Giga-tronics GT 9000, Operation & Maintenance Manual

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used, the variable oscillator can be twice the frequency of the reference and still be
phase-locked to it and the variable oscillator acquires the stability of the reference without
equaling its frequency. A 100 MHz VCO can be controlled by a PLL using a 10 MHz reference,
provided a divide-by-ten circuit intervenes between the VCO output and the variable input to
the PLL. Then both inputs to the phase detector will be 10 MHz when the loop is locked (see
Figure 3-4).
If the prescaler is a programmable divider, a number of different frequencies can be
phase-locked to the same reference, with the limitation that all must be exact multiples of that
reference. One of the PLLs employed in this instrument, for example, has a divide-by-N circuit
that can be programmed with 300 different integer divisors; therefore it can phase-lock 300
different frequencies to the same reference.
The phase lock loops used in Giga-tronics instruments employ integrated circuit phase
detectors, with external filter components and amplifiers designed to minimize the sidebands,
which the output pulses of the phase detector tend to contribute to the variable frequency.
Figure 3-4. Phase lock loop circuit, with prescaler
Model GT 9000 Microwave Synthesizer
3-4
Manual No. 120AM00250, Rev C, September 1998