Teledyne Lecroy WaveSurfer 3000, Руководство оператора, Страница: 61 / 97

Operator's Manual
Averaging Waveforms
Summed Averaging
Summed Averaging is the repeated addition, with equal weight, of successive source waveform records. If a stable
trigger is available, the resulting average has a random noise component lower than that of a single-shot record.
Whenever the maximum number of sweeps is reached, the averaging process stops. In Summed averaging, you specify
the number of acquisitions to be averaged. The averaged data is updated at regular intervals and presented on the
screen.
An even larger number of records can be accumulated simply by changing the number in the dialog. However, the other
parameters must be left unchanged or a new averaging calculation will be started. You can pause the averaging by
changing the trigger mode from NORM/AUTO to STOP. The instrument resumes averaging when you change the trigger
mode back to NORM/AUTO.
You can reset the accumulated average by pushing the CLEAR SWEEPS button or by changing an acquisition parameter
such as input gain, offset, coupling, trigger condition, timebase, or bandwidth limit. The number of current averaged
waveforms of the function, or its zoom, is shown in the acquisition status dialog. When summed averaging is performed,
the display is updated at a reduced rate to increase the averaging speed (points and events per second).
Continuous Averaging
Continuous Averaging, the default setting, is the repeated addition, with unequal weight, of successive source
waveforms. It is particularly useful for reducing noise on signals that drift very slowly in time or amplitude. The most
recently acquired waveform has more weight than all the previously acquired ones: the continuous average is
dominated by the statistical fluctuations of the most recently acquired waveform. The weight of ‘old' waveforms in the
continuous average tends to zero (following an exponential rule) at a rate that decreases as the weight increases.
You determine the importance of new data vs. old data by assigning a weighting factor. Continuous averaging allows
you to make adjustments to a system under test and to see the results immediately. The formula for continuous
averaging is:
new average = (new data + weight * old average)/( 1)
This is also the formula used to compute summed averaging. But by setting a "sweeps" value, you establish a fixed
weight that is assigned to the old average once the number of "sweeps" is reached. For example, for a sweeps (weight)
value of
4 :
1st sweep (no old average yet) : new average = (new data +0 * old average)/(0 + 1) = new data only
2nd sweep : new average = (new data + 1*old average)/(1 + 1) = 1/2 new data +1/2 old average
3rd sweep : new average = (new data + 2 * old average)/(2 + 1) = 1/3 new data + 2/3 old average
4th sweep : new average = (new data + 3 * old average)/(3 + 1) = 1/4 new data + 3/4 old average
5th sweep : new average = (new data + 4 * old average)/(4 + 1) = 1/5 new data + 4/5 old average
6th sweep : new average = (new data + 4 * old average)/(4 + 1) = 1/5 new data + 4/5 old average
7th sweep : new average = (new data + 4 * old average)/(4 + 1) = 1/5 new data + 4/5 old average
In this way, for sweeps > 4 the importance of the old average begins to decrease exponentially.
NOTE : The number of sweeps used to compute the average is displayed at the bottom of the trace descriptor box.
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