Teledyne LeCroy SDA II - Manual (Page 9)

Operator's Manual
Serial Data Inputs
There are several scenarios for the configuration of the Seria Data Input(s) section.
If you are using a differential probe or if your signal is connected by one coaxial cable, use the 1 Input (or
Diff. Probe) button and select the input source.
If you have a differential signal transmitted on two two coaxial cables or two single-ended probes., use
the Input1-Input2 button and select the input channels used.
NOTE: There is no need to configure a math function to calculate the difference between to inputs. Doing
so adds additional computational steps, and unnecessarily uses RAM.
Lastly, any math function, memory trace, etc. can be used as an input, as well as any input channel.
When in Input1-Input2 mode, and when using traces that may have been the result of other processing
steps, be sure that both traces have the same record length and sample rate.
You can choose to upsample rate in the Serial Data Inputs dialog by a factor of two in order to provide a
higher sample density for analysis. In SDAII this was typically done to facilitate formation of eye diagrams
without gaps for bit rates integrally related to the sampling rate (for example, 20 GS/s is exactly eight
times 2.5 Gb/s), and especially for relatively short acquisitions. This, however, unnecessarily slowed
down the analysis process. When needing to upsample to remove gaps in the eye use the upsample con-
trol in the Eye Diagram dialog.
Signal Types
The signal type defines the compliance masks and bit rate for the selected standard.
When you touch the Signal Type field, a pop-up table of standard signal types is shown. Touch the
desired signal type to populate the Signal Type field. The list of signal types is taken from the SDA Mask
Database. As new serial data standards are invented, masks are added to the database.
When choosing Custom from the Signal Type pop-up menu, always enter a bit rate in the Nominal Rate
entry box. Using an incorrect Nominal Rate can lead to incorrect equalizer results, since this value is used
by several of the equalizers that are available the oscilloscope includes Eye Doctor II functionality. The
Custom selection does not specify any eye masks, and the Mask Type selector in the Eye dialog will be
greyed out.
Crossing Levels
The Crossing level section of the Signal Input dialog determines the voltage level where the arrival time
of each edge of the signal is measured. The crossing level is set separately for the data and clock (if an
external clock is selected).. The Crossing Level on this dialog is for the data. Setting the crossing level to a
value that is not optimal can result in higher than expected deterministic jitter, since the error in the tim-
ing of the edges will be different for rising and falling edges.
The Level Type can be either absolute or relative.
The Absolute crossing level can be set directly in volts (or watts for an optical signal) , or you can click the
Find Level button to automatically find the level. The level is found by locating the midpoint between the
highest and lowest signal levels in the current acquisition. When you select the Absolute crossing level,
the crossing time used by both the jitter and eye pattern measurements is determined as the time at
which the signal level crosses the specified threshold. When Relative level is selected, the level is auto-
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