Data Sheet

HMC8364

Rev. A | Page 15 of 17

THEORY OF OPERATION

The HMC8364 consists of four, fundamental VCOs with

overlapping frequency ranges to ensure continuous frequency

coverage from 18.10 GHz to 26.60 GHz over all conditions.

Using four oscillators instead of a single oscillator to span the

frequency range reduces the percent bandwidth and tuning

sensitivity of each oscillator, improving phase noise performance.

Tuning sensitivity flatness across the frequency range is also

improved and simplifies the loop filter design in synthesizer

applications. The tuning sensitivity is similar across the four

VCO cores, which means that the loop bandwidth and phase

margin of the loop filter vary less overall vs. a single oscillator

solution.

The four oscillators share a common tuning port, which means

that even though the active devices in unused VCO cores are

not biased, the resonant tanks of all four VCO cores are in

parallel and tuned simultaneously. A single buffer amplifier is

also shared by all four VCOs.

The upper circuitry of the buffer amplifier is biased by VCB

(Pin 8). The lower portion of the cascode buffer amplifier

remains off and very little current flows until one of the VCO

cores is enabled.

The lower circuitry of the amplifier is enabled when the RF

signal of any one of the four VCOs arrives at its input. If VCB is

biased and any VCO core is enabled, current flows through the

buffer amplifier and the RF signal propagates to RFOUT (Pin 5).

The buffer amplifier is designed to support only one VCO at a

time. Avoid enabling more than one oscillator at a time because

multiple oscillators stress the buffer amplifier enough to reduce

its long-term operating life.