19-2233 PC2
2019-02-01
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ADQ7DC Manual
16-1796 PC2 2019-02-01
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•
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 digitzer.)
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 ADQ7DC sampling with a clock frequency at 5 GSPS. The pretrigger is set to
80 samples and the external trigger is used. The following parameters are returned:
TIME_STAMP
= 5005
RECORD_START
= –645
SAMPLE_PERIOD
= 8
TIME_BASE
= 25 ps
The time for the trigger was then
TRIGGER_TIME
=
TIME_STAMP
*
TIME_BASE
= 125125 ps = 125.125 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
= 200 ps
The time from the record start to the trigger is
RECORD_START
*
TIME_UNIT
= –16.125 ns.
The number of samples between the record start and the trigger event is
(
TRIGGER_TIME
–
RECORD_TIME
) /
SAMPLE_TIME
=
|
RECORD_START
| /
SAMPLE_PERIOD
= 80.625 samples
This is the expected 80 samples set in the pretrigger and 5/8 sample in subsample precision in the
external trigger.
4.3.2
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:
1. The timestamp counter is reset at power-up. This methods does not, however, have absolute preci-
sion, since the timing of the power up is not defined. In a multi-board system, the timestamp will dif-
fer between the boards.
2. With a software reference reset the user has full control of the reset procedure. A reference time
point is created in the users application, which is used for aligning time-stamps in different units.
After power-up the user runs a custom timestamp reset sequence including:
• Apply a reference signal to all boards.
• Trigger a record on the reference signal.
• Read the time-stamps from the records and call this reference;
TIME_STAMP_REFERENCE
.
