Tektronix WFM 601i, User Manual

Page: 87 / 135

Measurement Applications
WFM 601i Serial Component Monitor
3–9
To make a jitter measurement, select the EYE display. Choose the OVERLAY
display mode (located in the EYE PATTERN CONFIGure menu). In this mode,
each of the ten bits of a scrambled serial word are overlaid in the same location,
showing peak jitter at each eye crossing. Select the appropriate jitter high pass
filter. Normally, two measurements are made: one with the 10 Hz filter, showing
total broadband jitter, and one with the 1 kHz filter, removing the low frequency
jitter. Use the timing cursors to measure the width of the eye zero-crossing. This
is often easier to do if X5 vertical gain is used. Use the horizontal MAG for more
resolution.
Jitter measurement is the subject of continuing standards committee work. EBU
tech 3267 specifies 740 ps p-p, measured over one horizontal line. Interim
SMPTE standards specify a maximum jitter of 500 ps p-p, measured over one
horizontal line. This specification has created some confusion and is currently
under review. Many experts favor a revised specification, where jitter is
measured over specified frequency bands. This is the approach taken by the
WFM 601i. Suggested jitter limits are 740 ps p-p, measured with the 1 kHz jitter
HPF. If composite D/A conversion will be done, a 740 ps p-p limit with the 10
Hz jitter HPF is suggested.
To date, many of the jitter problems in systems have been the result of
genlocking clocks to other references such as horizontal sync. House sync may
contain several nanoseconds of jitter, and genlocks typically have control loop
bandwidths between 20 and 100 Hz. Because of this, reference jitter transferred
by genlocks into a serial system is generally between 20 and several hundred
Hertz. Also, the phase detection process used by the genlock may be noisy, and
this noise is transferred to the output up through the control loop bandwidth of
the genlock phase lock loop. The net result is added jitter in the 10 Hz to 1 kHz
range. By using the appropriate jitter high pass filter in the WFM 601i, genlock
jitter can be included or rejected from a jitter measurement.