Agilent Technologies 93000 SOC Series Training Manual page 548

Appendix A
Some Notes on Digital Signal Processing
The samples captured from the DUT are stored as contiguous
blocks of sampled points, also called vectors. Sampling and
digitizing of analog data brings us from the continuous to the
discrete signals domain.
Continuous Signals Domain vs. Discrete Signals Domain
Synchronization Requirements
Most analog instruments for measuring AC parameters work
without being synchronized to the stimulating or received signal.
This is different for a digital test system.
Let us have a look at a simple RMS (root-mean-square) voltmeter:
Functional Blocks of an Analog RMS Voltmeter
The analog instrument determines the mean value by low-pass
filtering the squared received signal. The filter's time constant
must somehow be adjusted to the signal period, but after some
settling time this method provides a reliable mean value.
A computer, on the other hand, performs a numerical integration.
It uses discrete values:
548