Actual Record Length Changes with Requested Sample Rate When Using NI-SCOPE

Issue Details

• I noticed in the NI-SCOPE Soft Front Panel that when I change the requested sample rate of my acquisition, the actual record length of my acquisition changes as well. This happens even if the actual sample rate for the acquisition stays constant. Why does the actual record length depend on the requested sample rate? 
• When I change the Reference Position property of my scope, it has no effect unless I change the same rate too. Why is this?

Solution

The way in which the NI-SCOPE driver calculates the actual record length is defined by the IVI standard. IVI specifies that the time per record of an acquisition must remain constant. Since NI-SCOPE is IVI compliant, it tries to keep the time per record at a constant value. 

Therefore, if you request 1000 samples at 24 MS/s, the total record time is 0.0416 ms. If your digitizer cannot sample at exactly 24 MS/s, it coerces the requested sampling rate to nearest sample rate, for example, 25 MS/s,  which is now the actual sample rate. Since IVI specifies that the time per record must remain constant, NI-SCOPE calculates the new record length to be (0.0416 ms) * (25 MS/s) which is 1042 samples.

The Reference Position property is a percentage of the record length. Therefore, if the actual record length changes with sample rate, the Reference Position is also affected.

Additional Information

More information can be found in the NI High-Speed Digitizer Help document which is installed with the NI-SCOPE driver.
The default directory is: C:\Users\Public\Documents\National Instruments\NI-SCOPE\Documentation