Welcome to the third installment of the Get Connected blog series here on Analog Wire! In this post, we will be discussing jitter from a high level, as this is a very complex topic and could not be completely covered in detail in a single blog post.
To understand jitter, we must first understand the eye diagram. The eye diagram is a representation of a digital signal in the time domain where voltage amplitude is plotted against time. A long string of data is divided into segments called a unit interval (UI), and these UIs are overlaid on an oscilloscope one on top of the other. This enables the oscilloscope to show large amounts of data at once. The unit interval is defined as UI = 1/Bitrate, and an example can be seen in the eye diagram of Figure 1. An eye diagram will consist of 1UI (sometimes engineers plot two side by side), and it is a visual representation of the quality of a digital signal.
Figure 1
Now that we understand the eye diagram we can ask the question, “what is jitter?” Jitter is defined as an undesired high frequency deviation from a signal’s ideal transition location. Jitter can be classified as total jitter (TJ), random jitter (RJ), and deterministic jitter (DJ). Total jitter (TJ) contains the combined effects of all components of jitter as its name would imply. Figure 2 below is a flow chart that shows a breakdown of TJ and all its components:
Figure 2
For more info on TJ, check out the app note on how to measure total jitter.
RJ is unpredictable and it accumulates through random processes that include but are not limited to thermal noise, shot noise, and power supply noise. RJ is unbounded, so its measured value continues to increase as time passes.
DJ is predictable jitter, as it is related to the type of data being transmitted as well as the transmission media. DJ has a non-Gaussian probability density function, and is bounded so its measured value will increase to a point over time but will not continue to increase indefinitely. DJ consists of several components that include data dependent jitter (DDJ), duty cycle distortion (DCD), inter-symbol interference (ISI), and periodic jitter (PJ).
Inter-symbol interference (ISI) is a byproduct of insufficient bandwidth within the transmission media of choice. A PCB trace or cable acts as a low pass filter (LPF) which attenuates the high frequency content in our data signal. This causes voltage offsets and unbalanced data patterns and ultimately leads to bit errors if not dealt with correctly. Duty cycle distortion (DCD) is the result of unequal rise and fall times leading to ones and zeros that have a different duration, causing deviations in the threshold crossing point. Data dependent jitter (DDJ) is a pattern dependent jitter that is caused by the sum effects of ISI (bandwidth limitations) and DCD (deviations from the threshold crossing point). With DDJ, previously transmitted data bits negatively affect the state of the current data bit. Periodic jitter (PJ) is a jitter that repeats in a cyclical fashion and contains no data related jitter components. PJ is any edge deviation that is periodic in nature. One type of PJ is known as sinusoidal jitter, SJ, because it can be decomposed into a Fourier series of harmonically related sinusoids. PJ is typically caused by external deterministic noise sources that couple into our system via a switching power supply, crosstalk, or an unstable clock-recovery PLL. Of course, much more information can be found on each of these jitter components and I encourage you to dig deeper into each one of them to gain a true understanding of jitter and its components.
ISI jitter can be compensated for by design since it is a function of the bandwidth of the transmission media. For backplane or cable transmission paths that experience significant amounts of ISI jitter, devices like the SN65LVCP1414 Linear Equalizer and SN65LVCP114 Linear Quad Channel MUX Equalizer are used to reverse the LPF effects of the channel. These devices can compensate for up to 17dB of loss in your system, extending the reach of your channel by reducing the overall jitter.
For more information on application-specific solutions like this please visit the High Speed Interface Forum in TI’s E2E™ Community and check out existing posts from engineers already using TI equalizers, or create a new thread to address your specific application. Please join me for my next post in the Get Connected series where I will be discussing the fundamentals of measuring jitter. If you are not connected you can get connected with one of the broadest equalizer portfolios in the industry.
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