Frequency measurement clock teminology.

[querty999]

An NI document explains "high frequency" two counter measurement using this diagram:

If I understand the Device Specifications correctly, my USB-6259 BNC has "Internal base clocks" with frequencies of 80 MHz, 20 MHz, 0.1 MHz.  Is an "Internal base clock" the same as the "Internal Timebase" shown in the diagram above?

How do I know which clock/frequency is used in a given measurement? Can I specify which one to use?

The document also says to use two counter measurements for "high frequency" measurements. What's a "high frequency" measurement?

Why does the device have more than one clock/frequency? Why would I want to use any frequency other than the fastest one?

thanks!

[GatorChomp]

Hi querty999,

The Internal Timebase can 

I would suggest taking a look at the DAQ M Series User Manual pages, as some more details on the implementaion of Counter Input tasks can be found there. Specifically, I would look at the formulas used to determine frequency for each type of frequency measurement, on pages 7-9 throught 7-14. There is also a section on page 7-13 which covers how to choose which method to use for measuring frequency with several example cases.

http://www.ni.com/pdf/manuals/371022k.pdf

To answer some of your questions more directly:
The Internal Base Clock is typically the same as the Internal Timebase. In a 2 counter measurement, this timebase is used to generate the "Internal Signal" from one counter, with that signal then tied to the gate of a second counter. The source of the second counter is tied to the Input Signal, and counts how many pulses are received while the Internal Signal is high, then uses the known timing of the Internal Signal and the number of Input Signal pulses counted to calculate the frequency. 

You can specify which frequency is used with a DAQmx Channel Property Node. You can find this specific option under Counter Input -> General Properties -> Counter Timebase -> Source on that property node. Simply select the timebase you wish to use.

The M Series User Manual does a great job of explaining how to determine which method to use, I would recommend using that to decide what constitutes a "high frequency measurement." Essentially, what constitutes a "high frequency" will depend on your application and the amount of error which is reasonable for said application.

Multiple timebases exist in general for certain applications which require them. I think in the case of measuring frequency, you will always be better served by the higher frequency timebase. If you were trying to generate a counter output at a specific frequency, certain timebases might work better depending on the application.