PXI-6120 Frequency Generation Pulse Width

[Blane63580]

Has anyone tried to generate a 4 MHz frequency outputed on either counter line that is 50/50 duty cycle on a PXI-6120 card?

I have tried this card using a LabVIEW RT program and all I get is a 4 MHz signal with a duty cycle of 60/40. I am using the DAQmx drivers for this signal. The weird thing is that if I do it for 1MHz, 2MHz, 3MHz and 5MHz it works perfectly.

Any help would be appreciated.

[Kevin_Price]

I'm pretty sure this'll explain it:

Your board has a 20 MHz timebase for its counters.  When generating output pulsetrains, it can only divide this down to integer #'s of cycles for the low time and the high time respectively.  You're fine to generate 1, 2, or 5 MHz pulsetrains as you get (low,high) times of (10,10), (5,5), (2,2) respectively.

When you request 4 MHz, you need to divide the 20 MHz timebase down by 5 total.  So you get 2 low and 3 high or vice versa.

Incidentally, your request for 3 MHz may give you a square wave with equal high and low times, but it can't be at exactly 3 MHz.  Most likely, the driver picks the closest possible freq of 3.3333... MHz by dividing down by 6.  But you *do* get a square wave because (low, high) = (3,3).

-Kevin P.