I'm trying to use the counter on the 6321 as a frequency divider that takes an 80 MHz sin wave input and generates a 10 MHz square wave output by digitizing the output of a 50 ohm device on PFI0. The specifications to for my board list a counter "Max Source Frequency" of 100 MHz, leading me to believe that 80 MHz should be possible. However, in my experimenting while the counter based divider works very well at lower frequencies, it fails at about 35 MHz by missing edges. The result is an unstable output frequency that bounces between 8:1 division, 9:1, 10:1... depending on how many edges are missed.
To explore this problem further, I swapped out my 2 meter 68-pin VHDCI for a 1 meter and plugged my function generator into the break out board with a < 30 cm cable, thinking that perhaps impedance mismatch was an issue. However, even with < 1.5 m, the device still works at 30 MHz and fails at 35 MHz. This suggests to me that impedance mismatch is not the problem, as the maximum frequency was unchanged even though the cable length was nearly halved.
Is there something else I'm missing? Is there a faster interface than PFI? Or someway to 50 ohm terminate the device?