Phase noise in crystal oscillators

24 June 2015

The spectrum of crystal oscillator output signals can comprise various components. As well as the fundamental frequency – which can be seen on a spectrum analyser – harmonics, sub-harmonics and spurious signals may also be present.

It is possible to use filtering techniques to reduce the harmonic components, caused by distortion of the waveform, to very low levels. Spurious signals resulting from AM and FM signals from other discrete sources within the oscillator are reduced significantly in Greenray oscillators.

Random phase fluctuations cause phase noise and short-term instability but the difficulty is that most analysers do not have the dynamic range or the 1Hz bandwidth capability to actually measure the phase noise directly.

Phase noise 

A sine wave oscillator has a frequency specified in cycles per second and the frequency stability is the repeatability of the sine wave signal throughout a specified period of time.

Sources within the oscillator cause small random perturbations in the sine wave from cycle to cycle. The composite output of the oscillator is made up of all of the component frequencies and these noise sources affect the signal purity in two ways. These are amplitude fluctuations – the Amplitude Modulated (AM) power spectral density – and phase fluctuations or phase spectral density.

Noise may be from several sources including component generated noise such as the noise profile of active components, thermal noise in passive components and flicker noise in active components, sometimes called pink noise. There will also be noise processes within the oscillator itself.

Other factors contributing to phase noise include crystals with a low “Q” which have an adverse effect and grounding and impedance matching. 

It is important to take all the above into account along with the operational environment when designing oscillators. Temperature stability will only impact on phase noise if the oscillator is a TCXO or VCXO compared to a stable OCXO.

Phase noise measurement

Taking two oscillators with equal or better phase noise, phase locking them together and mixing their outputs 90° out of phase (phase quadrature) will eliminate the carrier and provide a better measure of phase noise to be taken, see Figure 2.

A lowpass filter is used after the mixer and the difference in frequency is the measurement used.

Vibrational effects on phase noise

Vibration can affect the oscillator due to the g-sensitivity of the crystal assuming no resonance occurs in the oscillator package. The phase noise at a particular vibration can be calculated and taken into account during the oscillator design.

Successful oscillator design can be achieved by building-in isolation systems in both the oscillator and the system, designing electronic cancellation within the oscillator and optimising the crystals for low g-sensitivity.

A white paper covering calculations and more detailed discussion is available from Euroquartz Ltd.

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