Measure Frequency gives wrong value

[Eyal_S.]

Hi,

I recently got the USB-6210 and I also have the pci-6251. 

I'm trying to measure the frequency of a TTL signal. I'm using a signal generator as my signal source.

I'm using a continuous frequency measurement task and averaging the results. When I look at the signal on an oscilloscope and the pci-6251 I get the correct frequency but when I use the USB-6210 I keep getting a higher frequency.

For example:

50kHz --> 50.0313 kHz

100kHz --> 100.125 kHz

500kHz --> 503.145 kHz

Can anyone explain what is going on? Is there a way to fix it?

Thanks,

Eyal

[Eyal_S.]

I forgot to mention that I don't think that the +-1 clock cycle error explains the results since I'm averaging N samples (N is very big ~ 1Mcounts) and it should cancel out. Although it is suspicious that the results I'm getting correspond to exactly -1 clock cycles per measurement (I'm using the 80MHz clock).

[Dan_K]

Hello Eyal S,

I have a couple of suggestions for further troubleshooting this issue.

1. Try a frequency measurement of the internal 100 kHz timebase.
2. Generate a pulse train from the USB-6210 and read the frequency using the PCI-6251.
3. Verify that the behavior is consistent between the 2 counters on the USB-6210.

These tests will help to shed light as to what is going on with your USB-6210.

[Eyal_S.]

Hi Dan_K,

Thanks for your reply.

I actually added one cycle to all of the data points and the problem is fixed. There must be a bug somewhere...

Eyal