Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 101)

AORM Software Package
923133 Rev A ISSUED: June 2013
99
IMPROVING HORIZONTAL MEASUREMENT ACCURACY
Horizontal measurement accuracy pertains to timing-related measurements. In the AORM
package, these are Dp2c, Dp2cs, edgsh, lper, pwid, t@pit, and timj. In many cases,
measurement accuracy can be improved by considering certain items pertaining to how a DSO
operates and how parameters are measured.
DSOs sample the signal, building a waveform that consists of points at intervals determined by
the sample rate. One obvious consideration for maximizing horizontal measurement accuracy is
to ensure that the highest sample rate possible is used. On low time/divs, waveforms become
long. Thus it is important to set the Max Sample Points value in the SMART Memory dialog to
the largest possible value. This ensures the highest sample rate based on the time/div setting.
Times are calculated for ORM parameters by interpolating between points that straddle the
threshold specified. Measurement accuracy is improved when the edge is:
• fast enough to enable points straddling the threshold that are far from the threshold, and
• slow enough, and the sample rate high enough, to enable points to be sampled on the
edge.
In most cases, these considerations are taken into account by sampling at the highest rate
possible and by ensuring that the volts/div setting is as low as practically possible.
Note on RIS (Random Interleaved Sampling)
RIS is a mechanism used by the oscilloscope to increase the effective sample rate by interleaving samples taken over
multiple waveform acquisitions. The scope enters RIS mode automatically when the time/div setting is set extremely low.
Because multiple acquisitions are interleaved in RIS, a highly stable trigger signal must be maintained, and precisely the
same waveform acquired on each acquisition.
For most ORMs, RIS is neither appropriate nor recommended. If not used properly, it will result in erroneous
measurements.
(For more on RIS, see your scope’s Operator’s Manual.)