CLOCK JITTER

Presented By:

Maj AASP Athuldora arachchiSri Lanka Army

• Jitter is the timing variations of a set of signal edges from their ideal values.

• Jitters in clock signals are typically caused by noise or other disturbances in the system. Contributing factors include thermal noise, power supply variations, loading conditions, device noise, and interference coupled from nearby circuits.

What is Jitter?

Clock jitter • most jitter in a electrical circuit is caused by thermal noise, which has a

Gaussian distribution, random jitter also follows a Gaussian distribution(Normal distribution).

• Jitter can be quantified in the same terms as all time-varying signals, e.g.,RMS.

• Also like other time-varying signals, jitter can be expressed in terms ofspectral density, e.g. ,phase noise. Sampling frequency is normally assumed tobe constant.

• Samples should be taken at regular intervals. If some jitter is present on theclock signal, then it will produce an error proportional to the slew rate of theinput signal and to the absolute value of the clock error.

• Clock jitter originates from noise inside clock generator and from noiseinherent to the PLL(Phase-locked loop). Clock jitter directly translates intosampling noise and adds to the overall sampling noise.

• As well as expressing the amount of jitter in time units, such as Pico seconds and Nano seconds.

• jitter can also be expressed as the Unit Interval (UI).

• jitter amount is represented as Tj [ps], and the interval per bit is represented as Tbit [ps]

• For example, using a 10 Gbit/s signal, the interval per bit is 100ps. If there is jitter of 10 ps in this signal, the amount of jitter iscalculated as 0.1 UI.

Types of Jitter Jitter can be measured in a number of ways; the following are the major types of jitter:

• Period Jitter.

• Cycle to Cycle Period Jitter.

• Long Term Jitter.

• Phase Jitter.

• Time Interval Error (TIE).

Period Jitter Period jitter is the deviation in cycle time of a clock signal

with respect to the ideal period over a number ofrandomly selected cycles.

If we were given a number of individual clock periods, wecan measure each one and calculate the average clockperiod as well as the standard deviation and the peak-to-peak value.

Cycle-to-Cycle Jitter• Cycle-to-cycle jitter is the difference in a clock’s

period from one cycle to the next. Cycle-to-cyclejitter is the most difficult to measure usuallyrequiring a timing interval analyzer.

• As shown in Figure , J1 and J2 are the measuredjitter values. The maximum value measured overmultiple cycles is the maximum cycle-to-cyclejitter.

Long-Term Jitter• Long-term jitter measures the change in a clock’s output from the

ideal position, over several consecutive cycles.

• The actual number of cycles used in the measurement isapplication dependent.

• Long-term jitter is different from period jitter and cycle-to-cyclejitter because it represents the cumulative effect of jitter on acontinuous stream of clock cycles over a long time interval.

• That is why long-term jitter is sometimes referred to as theaccumulated jitter. Long term jitter is typically useful ingraphics/video displays and long-range telemetry applicationssuch as range finders.

Phase Jitter • In a square wave, most of the energies are located at the

carrier frequency. However, some signal energies are“leaked-out” over a range of frequencies on both sides ofthe carrier.

• Phase jitter is the amount of phase noise energycontained between two offset frequencies relative to thecarrier (fc). Figure is an unfiltered phase noise plot andthe shaded areas represent the phase jitter betweenfrequencies f1 and f2.

Time Interval Error (TIE)

• Time Interval Error (TIE) of an edge is the timedeviation of that edge from its ideal positionmeasured from a reference point. In effect, TIE isthe discrete time domain representation ofphase noise expressed in seconds or pico-seconds.