What is Jitter Tolerance and its Effects on Signal Integrity

December 3, 2019 Cadence PCB Solutions

Components influencing the signal integrity of a PCB

 

In humans, the number one cause of undesirable nervousness (jitters) and bad moods, usually in the mornings, is the lack of caffeine, with sleep coming in at a close second. Whether you get your caffeine fix from soft drinks, coffee, or tea (me), the need is real. So, is the irritability caused by premature withdrawals?

Furthermore, many of us are not ourselves until we have our caffeine fix. The effects of these jitters also affect our ability to work, concentrate, and even function in a productive manner. From a medical perspective, if your body is fluidly accustomed to caffeine, you will actually develop headaches if you do not receive caffeine for more than 30 hours.

Well, in electronics, jitters are equally undesirable, but unlike humans, drinking a cup of coffee or a glass of your favorite tea, will not fix the problem. Also, unlike humans, in PCB design, we must adhere to the acceptable level of tolerance for jitter, or functionality will suffer.

What is Jitter in Electronics?

When referring to electronics, by definition, jitter is the variance from exact or true periodicity of a presumably periodic signal, and often this is in relation to a reference clock signal. Also, in clock recovery applications, it is called timing jitter. As a whole, jitter is a substantial factor when considering the overall PCB design when applicable.

In terms of quantification, jitter, like all time-variance signals, it can be quantified with terms such as root mean square (RMS), or peak-to-peak displacement, as well as in terms of spectral density. As I mentioned earlier, jitter is an undesirable distortion of the signal. It occurs in the digital realm, and it pertains to the timing of digital pulses. In general, when there is jitter, the data stream, as conveyed to the receiver, will contain inaccurate information.

Furthermore, the cause of jitter is often the result of electromagnetic interference or crosstalk. Both of these can be allayed by first detecting the sources and secondly by applying one of the following remedies. For example, lowering the power of the source, or by creating separation through rerouting the wiring, by relocating the equipment itself, or by placing shielding between the source and the affected component.

Note: Overall, jitter affects computer monitors (flickering), degrades the operation of processors in computers as well as test instrumentation, distorts audio signals, and garbles data in networks. Therefore, you should mitigate jitter wherever possible.

IC and PCB on motherboard

Ensure your PCB or IC maintains the proper design for jitter tolerance standards.

 

What Are Some Types and Categories of Jitter?

The following are types of jitter you may encounter in the world of electronics:

Absolute Jitter: It is a measure of the deviation in time of a clock pulse edge from its ideal location.

Period Jitter: It refers to a variation between the ideal clock periods and actual clock periods. The effects of Period jitter are especially disruptive in synchronous circuitry, i.e., a central processing unit.

Inter-cycle jitter or Cycle-to-Cycle Jitter: This type of jitter refers to the difference in duration between successive clock periods. Also, excessive Inter-cycle jitter will nullify a microprocessor’s functionality.

Whenever jitter has a Gaussian distribution, it is usually calculated using the standard deviation of this distribution. Thus, translating to a root mean squared measurement for a zero-mean distribution. However, if the jitter distribution is considerably non-Gaussian, then a peak-to-peak measurement may be in order. Also, a non-Gaussian distribution is indicative of an external source, i.e., power supply noise. Note: Generally, the reference point for jitter is defined such that the mean jitter is 0.

There is one other way to categorize jitter. This method is by distinguishing between random jitter and deterministic jitter. Random jitter is a by-product of noises such as receiver thermal noise, quantum noise, and amplified spontaneous emission noise accumulated throughout the system. Due to its origination from omnipresent thermal noise in the electronic circuitry, it conforms to a Gaussian distribution. Whereas, Deterministic jitter (non-Gaussian) is non-random, bounded, and caused by systematic occurrences in a design. Note: These two varieties constitute total jitter.

How is Jitter Tolerance Tested?

With the ever-growing demands of cloud computing and high definition video streaming, the need for faster server and storage transmission speeds is critical. Therefore, it stands to reason that to meet those needs, the transmission speeds of physical layer devices and modules must increase as well. Thus, the need for an increase in signal integrity and an increase in signal integrity analysis to meet these demands.

As previously mentioned, the testing for jitter and its measurement is steadily growing in importance within the field of electronics. The increases in clock frequencies in digital electronic circuitry continues as a means to achieve higher device performance. These higher clock frequencies also have smaller eye openings, and thus impose tighter tolerances on jitter. For example, PC or laptop mobos use serial bus-architectures with eye-openings of approximately 160 pico-seconds or less. This is exceptionally small compared to parallel bus-architectures with similar performance, which may have eye openings of around 1000 picoseconds.

Furthermore, testing of device performance for jitter tolerance often involves the injection of jitter into electronic components with specialized test equipment. The measurement of jitter is evaluated in a variety of methods or ways, depending on the type of circuit in question. For example, you measure jitter in serial bus architectures using eye diagrams, per industry-accepted guidelines. 

A less direct approach, in which analog waveforms are digitized, and the resulting data stream analyzed is employed when measuring pixel jitter in frame grabbers. However, in each case, the goal of jitter measurement is to verify that the jitter will not disrupt the normal operation of the circuit.

Printed circuit board deployed in an electronic device

Testing is invaluable when it comes to avoiding jitter disruption on your boards.

 

Jitter is a measure of short-term, significant variations of a digital signal from its ideal position in time. These disruptive variations affect the extraction of the clock and network timing. This, in turn, negatively affects signal integrity, which in PCBs, this equates to failure. Failure in the design, manufacturing, and the end product.

Have your designers and production teams working together towards implementing strategies for proper jitter tolerance in all of your PCB designs with Cadence’s suite of design and analysis tools. Allegro PCB Designer unquestionably facilitates the implementation of jitter tolerance strategies into your current and future PCB designs. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

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