19
A stable and a jittery clock signal
We can also look at the problem spectrally. An ideal clock has only a single frequen-
cy components (and harmonics). All energy is concentrated on an infinitely narrow
frequency band, see the top picture below. When jitter is present, side bands occur.
Some spectral energy is located away from the clock. The faster the timing chatters,
the further away from the main frequency you’ll find energy.
frequency
frequency
Amplitude
Amplitude
Jittery clock
Jitter-free clock,
slightly off the
ideal frequency
The red curve in the bottom graph shows only random jitter. Sometimes jitter
is periodical and then you wouldn’t see a smooth slope but sharp peaks. An im-
portant note is that frequency accuracy has absolutely nothing to do with jitter
at all. The red graph shows a jittery clock with an exactly correct frequency, say
44100.0000000Hz, the blue one shows a clock that’s somewhat off but otherwise
brilliantly stable. A frequency error can be annoying from a practical perspective,
Spectra of clock signals
Summary of Contents for CC1
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