Hello,
Napo2 seems to really be asking about the maximum frequency that can be input to the counter from an external source. The best spec for that is the counter's "External Base Clock Frequency", found in the 622x specifications, which claims 20 MHz. The limiting factor here really is the bandwidth of the digital front-end on the PFI lines. Internally, the counter is able to count the 80 MHz timebase directly, but a signal at this rate would be completely attenuated by the digital front-end of the card if you tried to pass it in from an external source. The 20 MHz depends on having a clean square wave coming into the card, although it isn't very specific about rise times or minimum pulse widths. You might be able to go a bit beyond 20 MHz if you have a very clean signal (and conversely, might not be able to hit 20 MHz if the signal is not a clean square wave).
That KB linked previously doesn't seem to relate to Napo2's original question, and it is also kind of misleading in general:
The period measurement is determined by the frequency source that can be acquired. For the Counter/Timer boards, the highest frequency we can accurately measure will depend on the highest frequency timebase for the individual board.
This is true, in the sense that the highest frequency timebase is a factor that plays into the accuracy of a period measurement. However, the KB seems to imply that the maximum period that can be measured is the inverse of the maximum timebase, and doesn't give any further guidance for determining accuracy of a period measurement. A better reference (if you were interested in such things) would be the table found in the X Series User Manual, but again, this does not really relate to the original question.
Best Regards,
John Passiak
