To Make Jitter Measurements - Tektronix WFM8200 User Manual

To Make Jitter Measurements

WFM8200 and WFM8300 Waveform Monitors
This section explains the procedures for taking jitter measurements. After you
have configured the instrument for Eye or Jitter measurements, the instrument is
also ready to take jitter measurements using one of the following displays:
Measurement cursors on the Eye display
SDI Status display
Jitter display (Option PHY)
NOTE.
Many jitter problems in systems have resulted from genlocking clocks to
other references, such as the horizontal synchronization pulse. Reference jitter
transferred by genlocks into a serial system is typically between 20 and several
hundred Hertz. Also, the phase detection process used by the genlock can add
noise, which results in jitter in the 10 Hz to 1 kHz range. Use the appropriate BW
Limit filter selection to include or reject genlock jitter from a measurement.
The simplest way to measure jitter is to use the jitter readout and jitter
thermometer in the Eye display or SDI Status display. These are derived from
the jitter waveform. For more qualitative jitter information, use the Eye and
Jitter displays to view the presence and magnitude of jitter and its potential for
causing data errors as the eye closes. The Jitter display provides further analysis
information including added time-domain information such as the wave shape of
the jitter and whether there are jitter components that are synchronous or nearly
synchronous to the video line or frame (these components appear as stationary
or near-stationary artifacts in line or field sweeps).
Looking at jitter in both displays can help you separate jitter sources to determine
whether they are within a single circuit on a circuit board or from various pieces
of equipment in a system. If you are planning a composite D/A conversion, select
the 10 Hz bandwidth filter to measure total broadband jitter.
Following is an example displaying a signal with a lot of jitter. (See Figure 55
on page 110.) In the top two tiles, the high-pass filter is set for 10 Hz so that all
jitter above 10 Hz is displayed. In the lower two tiles, the high-pass filter is set
for 100 Hz, removing most of the 30 Hz jitter and leaving the spikes. Both Jitter
displays are set for two-field sweep. The upper Eye display shows a fairly even
jitter spread, which suggests a moderately uniform statistical distribution of jitter.
This jitter is from a sine wave component. Notice that there is no indication of
jitter spikes. In the lower Eye display, the jitter is of a low-density haze, which
suggests a less uniform jitter distribution. This is indicated by the spikes.
Monitoring the SDI Physical Layer
109