Hi Bob,
The 6602 board has 4 built-in filters. They are 5ms, 1ms, 500ns, 100ns. These filters guarantee to pass all signals with a period shorter than these values and they guarantee to block all signals with a period shorter than 1/2 these values. Signals with periods in between these values might or might not get blocked depending on the phase. This means if you have an 80MHz signal, it's period will be 12.5ns. This is much shorter than the 1/2 the smallest filter setting and thus it will be blocked.
You can however use your own custom filter clock settings but you would need a clock of 160MHz to guarantee that all 80MHz signals pass. This is well beyond the minimum custom filter frequency. When using your own custom clock, all frequencies ab
ove your clock frequency will be blocked. Similarly, all frequencies below 1/2 your clock frequency will pass.
Anyway, the point is you are looking for a filter than blocks everything above 80MHz and passes everything equal or below 80MHz. This is not possible with the filters on the 6602. You will need a custom circuit at the input of the counter pin to filter out the signals.
Another consideration is that the 6602 can handle frequencies up to 125MHz using prescalers. Prescalers are mini counters that can scale your frequency by 1/2 or 1/8 depending on your setting. It is esentially a mini 3-bit counter that can handle frequencies up to 125MHz.
Anyway, you can use the LabVIEW VI Counter Set Attribute.vi to enable prescaling. You would use the function GPCTR_Change_Parameter() in C. To enable and use filtering you would use Line Set Attribute.vi in LabVIEW or you can use the function Select_Signal() in C programming.
Hope that helps. Have a good day.
Ron
A
pplications Engineering
National Instruments
