LeCroy DDA 7 Zi series, Operator'S Manual

Page: 411 / 544

Operator's Manual
WP700Zi-OM-E-RevA
410
5. If you selected Custom from the Signal Type menu, touch inside the Bit Rate field and enter a bit rate,
using the pop-up keypad. By touching the Find Bit Rate button, you can have the bit rate determined
automatically. This button is also available when a standard data type is selected.
6. The Pattern Length control sets the repetition length of the data pattern in the signal under test. Serial
data signals generally transmit a repeating data pattern of some sort. For example a PRBS7 pattern
repeats every 127 bits while a K28.5 repeats every 20 bits. Enter the pattern length in the
Pattern Length
control.
Alternatively, the SDA can automatically find the pattern length by means of the Find Pattern Length
button. The Find Pattern Length control determines the pattern repetition length (if any) in the current
acquisition and sets the Pattern length control to this value. The Pj measurement uses the pattern length
value to determine which peaks in the jitter spectrum are caused by the pattern repetition. An incorrect
value in the Pattern Length control will result in overestimation of the Pj and, as a result, underestimation of
the random jitter.
7. The Tx Density control indicates the ratio of transitions in the data record to the total number of bits in the
signal. Normally, an NRZ data stream does not have transitions during every bit interval. For example,
there are cases where adjacent 1 or 0 values exist. On average, approximately 50% of the bit intervals
have transitions, so the Tx density is 500e-3. The actual acquired data set may have a transition density
different from 50%, however. The total jitter measurement uses this transition density to normalize the
measured histogram into a pdf[probability density function] . Many instruments simply assume 50% when
performing this normalization, which can lead to errors. These errors are eliminated in the SDA through the
use of the measured transition density in the normalization process. The Tx density is automatically
measured when the Find Pattern Length button is pressed.
Edge-Edge Jitter Measurements
Jitter measurements are enabled by selecting Jitter in the SDA main menu when the mode is set to Edge-Edge.
The available displays include a jitter bathtub curve, TIE histogram, ISI display, and N-cycle vs. N plot.
The Measurement buttons control which jitter measurements are displayed.
The Basic button displays the total jitter (Tj), Random jitter (Rj), and the deterministic jitter
(Dj).
The Dj Breakdown button enables the display of the components of Dj --- periodic jitter
(Pj), duty cycle distortion (DCD), and data dependent jitter (DDj) --- in addition to the basic
Rj, Dj, and Tj.
The ASDA-J option adds the Advanced button which, when selected, replaces the Tj, Rj,
Dj display with the peak-to-peak and rms values of the filtered jitter.
The SDA measures jitter by determining the timing between the edge crossings of the data signal under test and
comparing these measurements with the nominal unit interval (UI). A flow diagram of the edge to edge jitter
measurement is shown below. The total jitter is derived from the histogram of timing error measurements. The
histogram of measurements is accumulated and the tails of the distribution are extrapolated to form an estimate of
the histogram for a population of 10
16
measurements. The extrapolated histogram is then integrated to determine
the cumulative distribution function, which forms the sides of the bathtub curve. The bathtub curve gives the eye
opening (or 1UI - Tj(UI)) for all bit error rates down to 10
-16
. The total jitter for a given bit error rate is found by
measuring the width of the bathtub curve at the y-axis value corresponding to the desired bit error rate. The
periodic jitter and data dependent jitter are measured directly on the sequence of timing error measurements and
the random jitter is found by measuring the difference between the total jitter and the components of deterministic
jitter.