Phase Noise In Crystal Oscillators

By Barry Kleinle, Greenray Industries, Inc.

The spectrum of crystal oscillator output signals can be made up of various components. Besides the fundamental frequency that can be seen on a spectrum analyzer, other signals such as harmonics, sub-harmonics, and spurious signals appear. Ideally, we would like to have a single spectral line and one that does not drift with time. The harmonic components caused by distortion of the waveform can be filtered to very low levels. The spurious signals are a result of amplitude and frequency modulated signals from other discrete sources within the oscillator. These also may to be reduced significantly. Another unwanted contribution to the output could come in the form of phase-modulated signals that can be caused by a variety of sources and may be random or definite in nature. Random phase fluctuations cause phase noise or short-term instability. Most spectrum analyzers don’t have the dynamic range or the 1 Hz bandwidth capability to measure phase noise directly. A different measurement system will be discussed later.

What is phase noise?

To describe what phase noise is, let’s start with the basic sine wave output and a few fundamental facts.

A sine wave oscillator has a frequency specified as the number of cycles in one second. The frequency stability is the ability of the signal to repeat the sine wave signal each period throughout a specified period of time. The period of one cycle is the time of one cycle or 1/f. A perfect sine wave oscillator produces a signal with amplitude and phase, which repeat exactly each cycle. What causes variations in these stabilities? Generally, noise sources affect the oscillator to cause small random perturbations in the sine wave from cycle to cycle.

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•Technical Article: Phase Noise In Crystal Oscillators