But what is this phase noise all about and why is this value so important for a clock in the audiophile world? Intuitively, we know that low noise in a system is better than high noise. Walter Schottky explained the physical phenomenon as a measurable irregular current fluctuation. If we amplify this fluctuation and make it audible via a loudspeaker, we hear the typical noise that we commonly understand as noise and which also gave the phenomenon its name. But this rather analog understanding of noise has only a partial connection with the term used in modern electronics. Here, noise is generally characterized much more as any unwanted signal that interferes with the main signal. It can interfere with any parameter such as voltage, current, phase or frequency.

In the case of an oscillator, we are primarily interested in the frequency stability of its signal. Here we distinguish between the long-term stability and the short-term stability. The long-term stability refers to the amount by which the absolute clock frequency drifts over a longer period of time. Causes can be for example aging processes or temperature fluctuations of components. Even if this is important for telecommunications or studio applications, for example, it has, according to MUTEC, no influence on the quality of the reproduction of digital audio material.

The decisive factor is the short-term stability, which is described by fluctuations of amplitude and phase in a very short time range. The phase noise is a measure for the short-term stability of the oscillator in the frequency domain and is given in decibels of carrier (dBc) for a certain offset from the carrier frequency. A value that decreases with increasing distance from the carrier frequency. For example, MUTEC specifies as standard for the REF10 SE120 a phase noise of -120dBc measured at 1 Hertz offset and -148dBc measured at 10 Hertz offset from the carrier frequency of 10 Megahertz.

Sounds very abstract, but phase noise has a sibling more familiar to us audiophiles and that is called jitter. Jitter is also a measure of the short-term stability of an oscillator, but this time in the time domain. Phase noise and jitter are linked. One can say that with increasing phase noise of an oscillator, generally the jitter effect increases as well. And we all know that low jitter values are decisive for the sound quality of digital audio devices. And now you are at it again. Let's keep in mind: Changes in the short-term stability of an oscillator are described by phase noise and jitter. Especially important here is the measured value at 1 Hertz offset from the carrier frequency of 10 Megahertz. Surprisingly, just this value is sometimes not indicated at all, or very hidden only. This may have to do with the fact that, for technical reasons, this value is always worse than the one that can be measured at of 10 Hertz offset from the carrier frequency of 10 Megahertz.