This deviation in a clock’s output transition from its ideal position can negatively impact data transmission quality. In many cases, other signal deviations, like signal skew and coupled noise are combined and labeled as jitter.
Deviation (expressed in ±ps) can occur on either the leading edge or the trailing edge of a signal. Jitter may be induced and coupled onto a clock signal from several different sources and is not uniform over all frequencies.
Excessive jitter can increase the bit error rate (BER) of a communications signal by incorrectly transmitting a data bit stream. In digital systems, jitter can lead to a violation of timing margins, causing circuits to behave improperly. Accurate measurement of jitter is necessary for ensuring the reliability of a system.
Sources of Jitter
Common sources of jitter include:
Internal circuitry of the phase-locked loop (PLL)
Random thermal noise from a crystal
Other resonating devices
Random mechanical noise from crystal vibration
Traces and cables
Beyond these sources, termination dependency, cross talk, reflection, proximity effects, VCC sag, ground bounce, and electromagnetic interference (EMI) from nearby devices and equipment can also increase the amount of jitter in a device.
Reflection and cross-talk frequency-dependent effects may be amplified if an adjacent signal is synchronous and in phase. Aside from noise caused by power supplies and ground, changes in circuit impedance are responsible for most of the jitter in data transmission circuits.
The two major components of jitter are random jitter, and deterministic jitter.
The random component in jitter is due to the noise inherent in electrical circuits and typically exhibits a Gaussian distribution. Random jitter (RJ) is due to stochastic sources, such as substrate and power supply. Electrical noise interacts with the slew rate of signals to produce timing errors at the switching points.
RJ is additive as the sum of squares, and follows a bell curve. Since random jitter is not bounded, it is characterized by its standard deviation (rms) value.
Deterministic jitter (DJ) is data pattern dependant jitter, attributed to a unique source. Sources are generally related to imperfections in the behavior of a device or transmission media but may also be due to power supply noise, cross-talk, or signal modulation.
DJ is linearly additive and always has a specific source. This jitter component has a non-Gaussian probability density function and is always bounded in amplitude. DJ is characterized by its bounded, peak-to-peak, value.
Types of Jitter
There are many different types of jitter. Period jitter, cycle-to-cycle jitter and half-period jitter are described below.
Period jitter is the change in a clock’s output transition (typically the rising edge) from its ideal position over consecutive clock edges. Period jitter is measured and expressed in time or frequency. Period jitter measurements are used to calculate timing margins in systems, such as tSU and tCO.
Cycle-to-cycle jitter is the difference in a clock’s period from one cycle to the next. Cycle-to-cycle jitter is the most difficult to measure usually requiring a timing interval analyzer.
As shown in Figure 2, J1 and J2 are the measured jitter values. The maximum value measured over multiple cycles is the maximum cycle-to-cycle jitter.