Appendix A
558
• However, all the samples contained in one UTP describe just one
signal cycle.
You should at least
capture one UTP.
• Capturing one UTP is the minimal requirement for dynamic
mixed-signal tests. One UTP is the shortest valid test window.
Redundant sampling
means capturing
more than one UTP.
• Capturing more than one UTP can be useful for averaging in order
to increase the measurement accuracy. It can also be useful for
obtaining the signal-to-noise ratio more precisely. Capturing more
than one UTP is also called redundant sampling.
• Coherence requires that you capture a whole number of UTPs.
The Rules of Coherence
Coherent testing, with relatively prime M and N, maximizes the
average information per sample.
Three equations
describe coherence
The relations in coherence can be described by three equations
(shown on the left-hand side):
The Fundamental Requirements for Coherence
M and N must have no common divisor other than 1. ∆f is called
the primitive frequency.
Dividing the second equation by the third yields the fundamental
rule for coherence:
F
in
/ F
s
= M / N
An M/N ratio without any common divisor ensures that each
sample contributes unique, independent information. These
samples can be spread out over an arbitrarily long UTP.
Under coherent test conditions, we can acquire 1/Mth of the signal
information each signal cycle until all M cycles have been sampled.
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