![]() ![]() Once the jitter signal is gathered, you can apply a Fourier transform to get a jitter spectrum. By treating the intended bitstream as a carrier signal, you can unwrap the jitter signal (which creates the observed modulation) using heterodyning. When measuring a bitstream in an eye diagram measurement or simulation, we can extract a “jitter signal” through demodulation. The flowchart below nicely summarizes how phase noise and jitter in digital electronics are related. However, jitter is very clearly visible in an eye diagram, so it’s best to start from time-domain measurements. Digital signals are broadband, making it difficult to determine the contribution from phase noise to a signal’s power spectrum. The second reason jitter is a more useful noise metric is that all jitter measurements come from the time domain. Contrast this with a sinusoidal signal, where jitter is only seen around the band edge as the signal’s power spectrum rolls off to the noise floor. First, jitter may affect different portions of a signal’s power spectrum with different magnitudes, which requires an extraction and visualization process to determine the effects of jitter on a digital signal. Jitter is a more useful metric in digital signals for several reasons. In contrast, jitter is measured statistically in the time domain from an eye diagram by looking at crossing edge points in a bitstream. Phase noise is calculated from a power spectrum measurement in the frequency domain, which is why it is normally associated with sinusoidal oscillators. This causes variations in timing in a digital signal, meaning the time at which a signal level rises above its 50% span. ![]() Phase noise refers to the random fluctuations in the phase of an oscillator, which creates variations in the edge rate of a digital signal. Jitter in Digital ElectronicsĪs was mentioned above, these two quantities are related to each other. Let’s look at these to better understand the link between phase noise and jitter, and explore the various sources of jitter in a digital design. Jitter has additional sources beyond random fluctuations in phase due to the edge rate of digital signals. When dealing with digital signals, it’s best to use jitter to quantify signal quality for several reasons. When we look in the frequency domain for arbitrary periodic signals, such as pulse trains in PAM-4, we sometimes use phase noise to define the rolloff to the noise floor in the design. Eye diagram measurements are a starting point for qualifying channel designs, and you’ll want to extract the jitter value from your measurements and compare it with specs in your signaling standards. These metrics are related to each other by a Fourier transform and they are half of the information you’ll collect from an eye diagram simulation or measurement. With digital signals, jitter is the primary metric used to understand signal integrity and stability.ĭigital signals require precise timing that can be quantified using a calculation of jitterĪmong all the possible signal integrity metrics you can formulate to quantify signal quality, phase noise and jitter in digital electronics are of prime importance. Phase noise in the frequency domain appears as jitter in the time domain. Phase noise is one way to quantify timing noise in a signal, which is typically used in analog signals. ![]()
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