An eye on the challenge

If you’re doing digital design and incorporating a high speed digital bus, then jitter is a key parameter you need to measure. How you measure and characterise jitter depends on the type of bus implemented and the equipment you use, but you do need to understand jitter, its components and the different measurement techniques. Jitter is important because it can cause bit errors. It is typically defined as a measure of the time deviation the waveform exhibits at its significant instants from an ideal reference. In a digital signal, the significant instants are the transition points that define the bit period or Unit Interval. There are many interpretations of jitter, but time interval error (TIE) is the most common as it expresses the deviation in time using either the actual transmitter clock or one reconstructed from the data. There are three main sources for jitter. The first includes all random noise processes. The second source includes systematic effects, such as crosstalk and spurious from adjacent lines and circuitry, and duty cycle distortion (DCD) – a measure of the symmetry of the high and low driving functions. The third source includes data dependency effects and mechanisms such as intersymbol interference (ISI), dispersion and word synchronised distortion.