Education Library

Frequently Asked Question

How do harmonic spurs in the output spectrum affect jitter (random or deterministic)?

The short answer is that harmonic spurs have no impact on jitter.

However, what about non-harmonic spurs? Non-harmonic spurs do affect jitter, but they do so in a periodic (i.e., deterministic) manner. This is because a non-harmonic spur causes deviations in edge timing that wander about the ideal timing point in a sinusoidal manner; unlike random noise, which causes the edge timing to wander unpredictably.

Non-harmonic spurs become an important consideration when the signal is the result of a sampled system (e.g., a DAC). This is because harmonics of the fundamental signal may be sufficiently high enough in frequency to appear as an aliased image. Unfortunately, the frequency of an aliased image is generally not harmonic. For example, suppose a DAC operates at a sampling frequency of 100MHz, which puts the Nyquist frequency at 50MHz. Furthermore, suppose that the DAC's fundamental output frequency is 15MHz. In this case, the 4th harmonic occurs at 60MHz. However, 60MHz is greater than the Nyquist frequency of 50MHz, which means that the 4th harmonic actually appears at the aliased image frequency of 40MHz (100MHz-60MHz). Note that 40MHz is NOT a harmonic of 15MHz. Therefore, in this case, the 4th harmonic produces a non-harmonic spur, which means that the 4th harmonic would produce deterministic jitter.