Reference

Making Jitter Measurements

The Jitter display mode plots jitter versus time as a trace and measures peak-to- peak time jitter on the active input signal. The Jitter mode converts any phase modulation to amplitude and plots it against time.

Timing jitter is the deviation of signal transitions compared to those of a reference clock. Jitter results in Eye closure along the time axis, narrowing the window in which the data values can be accurately determined.

Jitter is characterized by both its magnitude and frequency. Signal transitions deviate from their ideal position by a peak amount and at one or more frequen- cies, depending on the sources. Typically, only high frequency jitter affects data recovery. But low frequency jitter can affect time-critical operations such as signal multiplexing and D/A conversion.

Jitter Demodulation. The Jitter measurement uses a demodulator method to determine signal jitter. The serial clock is recovered from the input signal and demodulated against a very stable oscillator, which translates any phase modulation (jitter) into a DC value. This DC value represents the phase difference between the input signal and the reference oscillator.

The resulting DC values plotted against time are proportional to jitter in the serial signal. This jitter waveform is passed through a high-pass filter and applied to a peak detector. The peak detector measurement is presented in the jitter measurement box. The demodulator can detect jitter up to 5 MHz.

Observing Word Correlated Behavior. The Eye Pattern display allows you to analyze word correlated jitter in video signals. Use the 10-Eye mode to analyze SD format signals and use the 20-Eye mode to analyze HD format signals.

When video is serialized, a 270 MHz serial clock is derived from the 27 MHz rate parallel word clock. Often there is slight phase modulation of the serial clock between the transitions of the parallel clock producing jitter at data-bit transitions. This jitter is not random; it is correlated to the parallel word rate. Also, the video pattern applied to the serializer changes at a 27 MHz rate or at an integer fraction of this rate. Any video pattern related effects in the serial system typically appear at fixed data-bit locations with respect to the parallel word.

In the 10-Eye or 20-Eye display modes, the trigger is on the parallel word boundaries, with 10 or 20 Eyes shown per sweep. Parallel word and TV-line correlated behavior can be seen in these modes. If a serial system has a disturbance that appears related to video patterns, either word or TV-line, use the

10-Eye (SD) or 20-Eye (HD) display mode to analyze the problem. Use the Line Select function to place the area of interest in the Eye pattern display.

WFM700 Series Waveform Monitors User Manual

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