Tektronix 11A32 User's Reference Manual page 26

Operating the 11A32 in 11400-series Mainframes
The 11401 and 11402 mainframes do not allow the addition or subtraction of live
waveforms without restriction. The reason for this lies in the manner in which
samples are taken and used by the mainframe. Each plug-in compartment has
an associated sampler in the 11401 or 11402. Samples are taken simultaneously
by each sampler. A channel from one plug-in unit may be added to or subtracted
from that of another plug-in unit accurately by virtue of this simultaneity. Thus
noise and other signals unrelated to the trigger will be canceled when channels
in different plug-in units are subtracted.
Signals from different channels within a plug-in unit are sampled at different
times. Signals that are entirely repetitive and related to the trigger signal may be
successfully added or subtracted in spite of this. However, signals that are not
repetitive or related to the trigger will not be sampled at the same time, and may
not be displayed correctly. Therefore it is preferable that signals to be added or
subtracted be selected from different plug-in compartments.
The best common-mode rejection is obtained with a differential amplifier plug¬
in unit because its hardware subtracts the signals at its two inputs, passing only
the result to the mainframe.
Even when using channels from different plug-in compartments, the following
two problems limit the accuracy of sums or differences:
1. Each waveform in memory is constantly being updated, and therefore
consists of some points that are older than others. Although samples may
be taken simultaneously, there is a chance that a correct sample may be
replaced in memory with a newer value before the waveform is processed
and displayed.
2. Delay differences between channels of different plug-in units due to probe
cable length differences are normally removed by deskewing the probes
and the associated amplifier channel. Samples are still taken simultan¬
eously, but the display is manipulated to remove time differences. Signals
correlated to the trigger signal are correctly restored to their proper time
relationship, but there is no way for signals unrelated to the trigger to be
corrected with this deskewing technique. High-frequency rejection of
signals unrelated to the trigger is not improved by deskewing.
The solution to both problems is to invoke averaging. The averaging process
suppresses all signals and noise unrelated to the trigger.
11A32 User Reference Supplement