Measurement Computing WaveBook rev.3.0 - Manual (Page 149)

WaveBook User’s Manual. 6-24-99 Theory of Operation 8-15
Trigger Latency & Jitter
Trigger latency and jitter depend on the trigger source and the acquisition mode:
• Trigger latency is the duration between the valid trigger and the start of the acquisition.
• Trigger jitter is the variation of the latency, how much time the latency can vary from trigger to
trigger.
As discussed, WaveBook has post-trigger and pre/post-trigger acquisition modes. Post-trigger modes (N-
shot, N-shot with re-arm, and infinite-post-trigger) collect scans only after the trigger has occurred. They
are different from the pre/post-trigger mode that collects scans both before and after the trigger. This
difference affects the trigger latency and jitter.
In a post-trigger mode, WaveBook is not scanning while waiting for the trigger. Thus, it is free to respond
to the trigger as soon as it occurs. This minimizes the trigger latency and jitter.
In the pre/post-trigger mode, pre-trigger data is being collected while WaveBook waits for the trigger, and
WaveBook will not respond to a trigger until the current scan is complete. The pre-trigger scan period
separates the first scan after the trigger from the last scan before the trigger. All the scans (up through the
one immediately following the trigger) are collected at the pre-trigger rate; and all subsequent scans are
collected at the post-trigger rate. This preserves the integrity of the acquisition timebase as shown in the
figure below:
Start
No acquisitions
before start
Pre-Trigger
Scan Period
Pre-Trigger Scan Count
Pre/Post-Trigger Acquisition
Time
Trigger
Armed
Trigger
Pre-Trigger
Scan Period Post-Trigger
Scan Period
Post-Trigger Scan Count
The time needed to complete the final pre-trigger scan is part of the trigger latency; and so, in the pre/post-
trigger mode, the trigger latency may be greatly increased.
Not only do the trigger latency and jitter depend on the pre- vs post-trigger type of acquisition, they also
depend on the trigger source: Software, digital (TTL), single-channel (Channel 1 analog), or multi-channel.
The following table gives the trigger latency and jitter for each of the different trigger sources and
acquisition modes:
Acquisition Type Trigger Source Max. Trigger Latency Trigger Jitter Notes
Software 100 µs + T 100 µs + T a, c
Digital (TTL) 200 ns + T 50 ns + T c
Single-Channel 300 ns + T 50 ns + T c
Multi-Channel 2 * T - NS µs T c, d
Digital Pattern 300 ns + T 50 ns + T e
Pre-Trigger
Pulse 300 ns + T 50 ns + T e
Software 100 µs 100 µs a
Digital (TTL) 200 ns 50 ns
Single-Channel 300 ns 50 ns
Multi-Channel 2 * NC + 3 µs NC +2 µs b
Digital Pattern 300 ns 50 ns e
Post-Trigger (N-Shot,
N-Shot with re-arm, or
infinite-post-trigger)
Pulse 300 ns 50 ns e
Notes: a) Software trigger latency and jitter depend greatly on the host computer's speed, operating system, and
printer-port protocol. Most systems should take much less than 100 µs.
b) NC is the number of channel samples used for multi-channel triggering, from 1 to 72, as specified by the trigger
configuration.
c) T is the pre-trigger scan period.
d) NS is the number of samples in a scan including, if present, the first "dummy" sample, from 1 to 128.
e) WaveBook/516 series only