17-2000 C
2020-09-17
14(50)
ADQ8-8C Manual
17-2000 C 2020-09-17
14(50)
4.4 Timestamp
4.4.1
Timestamp definitions
The timestamp counter enables real-time measurement of a trigger event. It is used for tagging an
event, sorting events in time or comparing timing between events.
The timestamp information consist of three parts, which uniquely defines the timing:
•
TIME_STAMP
measures the time of the trigger event relative to other trigger events.
•
RECORD_START
is the time between the trigger event and the start of the record. For a pretrigger,
this is a negative value. When trigger delay is used, this is a positive value.
•
SAMPLE_PERIOD
is the length of a sample period. The sample period may vary with sample skip
setting and clock frequency of the digitizer.
The
TIME_STAMP
,
RECORD_START
, and
SAMPLE_PERIOD
are measured in the unit
TIME_BASE
= 25 ps. See
on how to use these parameters. These parameters are available
in the record header, see
The timestamp counter is based on the internal clock of the digitizer. The internal clock is based on the
selected clock reference. The timestamp is thus also related to the clock reference. When the clock ref-
erence is phase-locked to an external source, the timestamp counter is running synchronized with the
external source. On the other hand, if the digitizer is free running, the timestamp counter also free run-
ning. (See
for all details about the clock system of digitizer.)
The timestamp counter measures the time from a reference time point to the trigger event. The refer-
ence time point is when the counter is started or reset. See
for information on how to
reset the timestamp counter.
Example 3: Assume an ADQ8-8C sampling with a clock frequency at 1 GSPS. The pretrigger is set to
16 samples and the external trigger is used. The following parameters are returned:
TIME_STAMP
= 5010
RECORD_START
= –650
SAMPLE_PERIOD
= 40
TIME_BASE
= 25 ps
The time for the trigger was then
TRIGGER_TIME
=
TIME_STAMP
*
TIME_BASE
= 125250 ps = 125.25 ns
The time for the first sample in the record is
RECORD_TIME
= (
TIME_STAMP
+
RECORD_START
) *
TIME_BASE
= 109.000 ps = 109 ns
The time between two samples are
SAMPLE_TIME
=
SAMPLE_PERIOD
*
TIME_BASE
= 1 ns
The time from the record start to the trigger is
RECORD_START
*
TIME_UNIT
= –16.25 ns.
The number of samples between the record start and the trigger event is
(
TRIGGER_TIME
–
RECORD_TIME
) /
SAMPLE_TIME
=
|
RECORD_START
| /
SAMPLE_PERIOD
= 16.25 samples
This is the expected 16 samples set in the pretrigger and 1/4 sample in subsample precision in the
external trigger.
4.4.2
Timestamp reset
When powering up a system with many boards, the timestamp counter in each board will start. But the
counters start at different times in different physical digitizers. There are four methods for resetting the
timestamp and get a common time reference in all the digitizers in the system:
