System User Guide / Owners Manual

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

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