Page 46 - Power Electronics News - December 2020
P. 46
Test & Measurements Test & Measurements
that is uncorrelated, such as PJ
(periodic jitter).
Widespread crossings on the eye
diagrams can indicate RJ. Eyes
that appear to consist of many
nearly distinct lines indicate
DDJ, likely from impedance mis-
matches in the signal path. More
detailed measurements are
needed to identify the types of
Figure 3: Jitter summary screenshot, clockwise from top left: bathtub plot, eye jitter that can indicate hardware
diagram, TIE spectrum, jitter analysis results, waveform, TIE histogram. bugs: TIE, RJ, DJ, DDJ, PJ, TJ (to-
step is to perform a jitter analysis. Figure 2 maps tal jitter), EH (eye height), EW (eye width), eye high,
the breakdown of jitter into its components and and eye low. Table 1 lists different types of jitter
Figure 2: The breakdown of jitter into its components.
sub-components, and Figure 3 shows a Jitter Sum- and some causes. Power rail ripple is a common
SI & PI CONTRIBUTE TO ERRORS channel, and receiver in terms of the BER (bit mary measurement including the bathtub plot, eye cause of PJ and sometimes RJ, too.
Digital errors are caused by jitter and noise. Noise is error rate). Power integrity focuses on the PDN’s diagram, TIE spectrum and histogram, jitter meas-
a broad term for variations in the signal amplitude. ability to provide constant voltage power rails and urement results, and waveform.
Jitter is the variation in the timing of bit transitions low impedance return paths. SI and PI have broad JITTER AND THE POWER
with respect to the data-rate clock, the so-called interdependence. The PDN can cause noise and The breakdown of jitter starts with the separation DISTRIBUTION NETWORK
time interval error. Jitter is caused by both phase jitter. The circuit design and components—chip of the TIE distribution into its random and deter- The PDN’s job is to sustain a constant voltage and
noise and amplitude noise-to-jitter conversion. package, pins, traces, vias, connectors—affect the ministic components, RJ (random jitter) and DJ provide sufficient current to the components in
Noise-to-jitter introduces problems from crosstalk, impedance of the PDN and hence the quality of the (deterministic jitter). DJ is further separated into the system. It impacts the performance of every
EMI (electromagnetic interference), and random power supplied. jitter that is correlated to the sequence of bits in element, active or passive. The PDN includes the
noise. the data—DDJ (data-dependent jitter)—and jitter whole system, not just the DC-to-DC converters
Signal integrity analysis concentrates on the per- DEBUGGING SI PROBLEMS
formance of the transmitter, reference clock, STARTS WITH THE EYE DIAGRAM
Hardware debugging can start
with eye diagram analysis. The
eye diagram consists of over-
lapping waveforms relative to a
clock, Figure 1.
The horizontal width of the
crossing points indicates jitter
and the vertical width of the top
and bottom of the eye indicates
noise. A wide-open eye should
correspond to a low BER. If
Figure 1: Eye diagram with mask test, top, and corresponding waveform, bottom. the BER is too high, the next Table 1: Jitter measurements with examples of common causes.
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