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Abstract

An improved circuit for phase noise measurements utilizing a frequency down conversion/multiplier and direct spectrum measurement technique. The circuit is particularly useful for field test environments where laboratory instrumentation is normally not available, and fast and accurate phase noise measurements are required. The phase noise measuring circuit includes a frequency mixer which has a first input signal from a device under test and a second input signal from a reference stable oscillator having ultra low phase noise with a fixed center frequency. The frequency mixer produces a down converted signal comprising the frequency difference signal of the first and second input signals. A lowpass filter passes the down converted signal to a frequency multiplier circuit which produces a second harmonic signal, a fourth harmonic signal, and higher harmonic signals of the down converted signal. A narrow bandpass filter passes a selected one of the second or higher harmonic signals to a low noise amplifier, the output of which is passed to a direct approach spectrum analyzer for measurement of the phase noise of the device under test. A second embodiment includes an additional multiplication stage which includes a second frequency multiplier circuit, a second narrow bandpass filter, and a second low noise amplifier, the output of which is passed to the direct approach spectrum analyzer for measurement of he phase noise of the device under test.

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