AN-1177

Application Note

Rev. 0 | Page 8 of 12

JITTER, SKEW, DATA ENCODING, AND SYNCHRONIZATION

With high speed differential signaling, such as LVDS and

M-LVDS, accurate timing is critical to the performance of a

system. PCB traces, connectors, and cabling can degrade the

performance of data and clock signals, requiring that a margin

for error is also present in system timing. This means that

careful timing analysis may be required to achieve the maxi-

mum throughput on an LVDS or M-LVDS communication link.

Modern FPGAs and processors also have built-in capabilities to

correct for timing errors, although there may be clearly defined

limits to the amount of jitter tolerated, for example.

WHAT IS JITTER?

Jitter refers to the apparent movement of a signal edge with

respect to the ideal time position of that signal edge. If a

periodic signal is observed on an oscilloscope, the edges

literally jitter back and forth with respect to the reference point.

TIE

EYE

JITTER

(PEAK-

TO-PEAK)

IDEAL

ACTUAL

(ONE

PASS)

ACTUAL

(MULTIPLE

PASSES)

Figure 16. Waveforms Showing Time Interval Error, Jitter and Eye

Jitter can be quantified simply as time interval error, the time

difference between when a signal edge occurs, and when it

should occur. Usually in order to determine the sources of

jitter, a large number of TIE samples are recorded to build a

histogram, from which deterministic jitter can be separated

from random jitter. Total jitter can be quantified as a peak-to-

peak value when bounded to a specific quantity of samples.

The peak-to-peak value means the time difference between

the earliest and latest edge observed during sampling.

Peak-to-peak jitter can be seen visually if multiple waveform

samples are overlaid on an oscilloscope display (infinite

persistence), as shown in Figure 16. The width of the overlaid

transitions is the peak-to-peak jitter, with the clear area in-

between referred to as the eye. This eye is the area available for

sampling by a receiver.

Random jitter occurs due to noise, both electrical and thermal.

The result is a Gaussian distribution to the time error, with this

error introduced as random jitter. The jitter is unbounded;

when more samples are recorded, the probability function

continues to grow.

Deterministic jitter is, by contrast, bounded. There is a fixed

amount of this jitter in the system due to specific factors, such

as the board layout and driver performance. Periodic jitter is

one type of deterministic jitter and refers to the time difference

between each cycle compared to the ideal. Periodic jitter is also

recorded as a peak-to-peak value, that is, the difference between

the longest and shortest periods observed

WHAT IS SKEW?

There are different definitions for skew, several of which are

typically considered in designing high speed LVDS links. The

most basic definition of skew is the difference in propagation

time between the two signals in a differential pair. This means

that edge transitions on one signal in a pair will not match up

exactly with transitions on the complementary signal (the

crossover will be asymmetric).

INPUT

ACTUAL

OUTPUT

IDEAL

OUTPUT

PULSE SKEW

(

D–

D+

D–

D+

D–

D+

Figure 17. Waveforms Illustrating Pulse Skew Calculation

Pulse skew on a differential signal refers to the difference

that is, the bit period is longer or shorter for a Logic 1 or Logic 0.

Pulse skew is illustrated in Figure 17. The blue waveform

corresponds to an input signal, the green waveform to an ideal

output (where propagation times on high-to-low and low-to-

high transitions are matched), and the red waveform to an

in pulse skew.

Channel-to-channel skew and part-to-part skew are some of

the most important parameters in typical LVDS applications

because they have multiple data channels that need to remain

synchronized. Channel-to-channel skew refers to the difference,

across all channels in a part, between the fastest and slowest

low-to-high transition, or the fastest and slowest high-to-low

transition (whichever is larger). Part-to-part skew extends this

concept to channels across multiple parts.

Skew across multiple channels (on one or multiple parts) is

illustrated in Figure 18. The blue waveform corresponds to an

input signal, with the four red waveforms comprising output

channels on one or more parts. The difference between the