HP 1660CS-Series User Manual page 260

Transitional Mode Theory
250-MHz transitional mode
Minimum transitions stored
Minimum transitions stored
The figure above shows what data is stored from a data stream with
transitions that occur at a slow rate (more than 24 ns apart).
As shown, transitions are stored in two different ways, depending strictly on
chance. Remember that the transition detector only looks at the full 34 bits
while the data is stored as two 17-bit samples. So, the transition detector will
not see time tag 3 (101/000) as a transition. However, when the detector
compares the transition to time tags 2 (101/101) or 4 (000/000), it sees a
difference and detects them as transitions. For this first set of time tags, the
transition detector sees more transitions than are really there. This causes
the analyzer to store 6 samples per transition (three 34-bit sample pairs),
instead of just two, as in the 125-MHz mode. If all the transitions will be
stored in this way throughout the trace, the minimum number of stored
transitions is 682 (4096/6).
However, as you see with time tags 7 (000/000) and 8 (001/001), transitions
can fall between the pairs of samples. When this happens, only one transition
is detected and only 4 samples (two sample pairs) are stored. If all transitions
will be stored in this way, 1023 (4096/4) transitions are stored.
From run to run, the actual number of transitions stored for transitions that
occur at a slower rate will fall between these two numbers, based on the
probability of a transition falling between a sample pair or falling within a
sample pair.
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