Tektronix Waveform Monitors WFM700 Series - Manual (Page 72)

Reference
WFM700 Series Waveform Monitors User Manual 3- 23
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 distur-
bance 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.
Making Jitter
Measurements