LeCroy DDA 7 Zi series, Operator'S Manual

Page: 488 / 544

WavePro 7Zi
487
WP700Zi-OM-E-RevA
acsn . An ACSN measurement is performed for each pattern, and the results averaged.
All individual ACSN measurements can be observed by histogramming the acsn parameter.
ACSN is limited to measuring signal-to-noise ratios of 9.6 dB or greater.
ACSN results are presented in dB. All averaging, including statistics and trend average, is
performed on linear units. Averaged results are then converted to dB.
Parameter
Settings
When you select acsn from the PRML dialog's With parameter field (Drive Analysis
Measure Parameter Set PRML ) an acsn Pattern Length field appears also.
The pattern length should be set as an integral number of waveform periods, and must be at
least 50 samples at the DDAs sample rate. (The sample rate is shown in the
TimeBase label
box.) Because these periods will be correlated with the same number of following periods, the
pattern length must be no more than half the number of full periods available in the sweep.
Notes on How Settings Affect ACSN Measurement
Pattern length should be close enough to the signal period (or multiple thereof) that it is on the
correlation peak. The code measures correlation at 3 positions: the length, 6 samples less and
6 samples more. It will walk up a slope until the center sample is highest, tighten the spacing
until the two side positions have correlation above half the correlation at the center position, and
eventually perform a parabolic fit. But if pattern length misses the peak entirely then this search
may not work; if the smallest of the three correlations is at the center and the two side position
correlations are equal, it stops. For a sinusoidal signal, almost any setting of pattern length
should work.
One limitation to know about at this stage: the code checks for a "reasonable" peak. The
correlation at the center position must be > 0.9, or the parameter computation is canceled. This
is done without the benefit of the parabolic fit, but the intent is only to make sure of a
reasonable peak.
The total number of cycles in the acquisition does not affect the ACSN result for a stationary
signal. Note that horizontal scale and the
Max Sample Points entry on the TIMEBASE "Smart
Memory" dialog interact. As you increase the time of an acquisition, the sample rate may
decrease to keep the acquisition smaller than
Max Sample Points . Although ACSN is not
affected by horizontal scale, it is affected by sample rate. Be sure to keep it high enough.
For the same signal, ACSN will fall as vertical scale is increased, because the signal fills less of
the code space of the ADC and is therefore more contaminated by quantization noise and any
other internal noise. In general, when displayed on a channel (or a reset trace), the signal
should fill as much of the grid vertically as possible, but should never clip against the top or
bottom of the grid. If high ACSN readings (greater than or equal to 30 dB) are expected, this is
especially important.
The pattern length should be kept to the minimum number of cycles that will satisfy the 50
samples per pattern requirement. In general, with a somewhat noisy signal, as the number of
cycles in one pattern length is increased, ACSN is slightly reduced. For example, with an 80
MHz 75 mV sine wave with 0.6% (of p-p) rms noise, sampled at 1GS/s,
acsn result varied with
pattern length as follows:
As ACSN readings get higher, small amounts of noise cause greater changes in dB. This is
because high readings mean correlation was very close to 1. Examples:
Pattern Length (Seconds) Cycles in One Pattern Length ACSN
5.00E-8 4 27.54 dB