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Programmer’s Guide Page 56 of 66
uncertainty of ± 0.5 samples. For implementation reasons, the acquired waveform in fact has a timing uncertainty
that is twice as large, i.e. ± 1 samples. In this case, the trigger time stamps of the sequence acquisition mode are not
available.
Depending on the ratio of sampFrequency/inputFrequency , a waveform is sampled either on negative-going
transitions of the external clock signal through the user-defined threshold or, when the ratio is > 1, on both of the
transitions.
NOTE: First generation digitizers that have more than one converter/channel (DC240, DP210, and DP211) will
generate two data samples for each sampling interval. You must dimension your acquisition and readout for twice
the normal amount of data and can then either, drop every other data sample from the record, or average the two
data values which could enhance the signal to noise ratio.
3.17.3. External Clock (Start/Stop)
The start/stop external clock mode (clockType = 4) permits the application of a (variable) external clock. It should
not be used for the 10-bit-FAMILY, 12-bit-FAMILY, or U1071A-FAMILY digitizers. The clock can be setup to
give bursts during which the frequency is between 10 MHz and 500 MHz. The first sample of each burst may have to
be ignored. The waveform is sampled on positive-going transitions of the external clock signal through the user-
defined threshold. Thus, the sampling rate is equal to the input frequency.
For digitizers and Averagers/Analyzers in the digitizer mode, there is no concept of trigger when a Start/Stop clock is
used. Therefore, all trigger parameters will be ignored. This also means that there is no concept of sequence
acquisition. Operation in a channel combined mode is not possible.
The AC/SC Analyzers can be used in this mode. A continuous clock frequency of up to 800 MHz, to give 800 MS/s
sampling, will work.
In this mode, the horizontal control parameters sampInterval and delayTime are completely ignored, as well as the
value of delayNbrSamples. The waveform length is, as usual, controlled by the number of samples in the function
AcqrsD1_configMemory. Careful synchronization between the function calls to the driver and the generation of the
clock burst is required.
There are 2 ways of terminating an acquisition in the start/stop mode:
1. Generate a number of clock transitions that corresponds exactly to the requested number of samples, and
stop the acquisition with the function AcqrsD1_stopAcquisition. This requires that the host computer
obtain some external signal when the clock sequence is terminated.
2. Generate some extra clock transitions, which will fully terminate the acquisition. You can then use the
functions AcqrsD1_acqDone or AcqrsD1_waitForEndOfAcquisition to detect the end of acquisition.
Example for Termination (1): if you wanted to acquire 20 waveforms of 2000 data points each, at a sampling rate of
33.3 MHz, and a time distance of 5 µs between the waveforms, you would use:
AcqrsD1_configMemory(instrID, 40000, 1);
AcqrsD1_configExtClock(instrID, 4, threshold, 0, 0., 0.);
AcqrsD1_acquire(instrID); // start the acquisition
 Generate 20 bursts of 2000 clock pulses at 33.3 MHz. At the end, you need
to inform the host computer to terminate the acquisition and:
AcqrsD1_stopAcquisition(instrID);
AcqrsD1_readXXXWform(instrID, . . .);
Note that the sampling rate and the time between bursts have no incidence on the configuration parameters of the
digitizer, i.e. they appear nowhere.
Example for Termination (2): if you wanted to acquire 5000 waveforms of 200 data points each, you would write: