Measuring frequency with NI USB 6009

[J3lda]

Hello, I have problem with counting edges. I have voltage-frequency conterter, so I change voltage and measure frequency by counting edges. I use DAQmx assistant (prepared block) and everything works good until voltage is 3,8V and frequency 70 kHz. Then counter shows 0 ...frequency = 0. Hardware is all right. It´s very strange that it´s working and then not.

Do you know where could by mistake? (... I know that it is probably imposible )

Thank you very much for ideas.

[crossrulz]

3.8V is on the edge of the TTL input threshold.  You need to increase your voltage.

EDIT: Nevermind.  I just found the spec and 2V is the threshold voltage.

[Norbert_B.]

So if I understand correctly, all you are doing is measuring edges with ctr0 on the usb-6009. It works ok, but when you exceed 70kHz, then the DAQ Assistant stops counting edges. The 6009 should work up until 5MHz.

Are you absolutely sure, that the signal source is providing the correct waveform? Please refer to the 23rd page of the 6009's manual: http://www.ni.com/pdf/manuals/371303m.pdf, and check if everything is fine with your signal.

Right now the only idea that comes to mind is that something must be wrong with the signal.

Norbert

[johnsold]

When you talk about changing voltages, that raises red flags! Digital (logic) signals do not change voltage with frequency or anything else.

Does your V/F converter have a logic IC or circuit as its output stage, or is it an op amp acting as a comparator? If it is the latter or something similar, you may have slew rate limitations keeping your signal from meeting the logic input requirements of the counter.

Lynn

[J3lda]

Thanks for ideas. Signal is all right. I checked it by osciloskope. According manual everything is in order. It's really strange.

[Norbert_B.]

Just to make sure, can you please post a screenshot form the scope, with the signal that produces this weird phenomenon?

If everithing seems to be alright, and the device keeps acting strange, you should contact your local NI branch and let them know about it.

Norbert

[J3lda]

You have probably right. There is probably something wrong with signal from V-f converter. Here is screenshot from scope:

[johnsold]

Yes.  The leading "edges" of that signal look more like slow ramps than an edge to a logic circuit.

If the V-f converter does not have a logic gate at its output, try inserting a Schmidt trigger inverter such as the 47xx14 or 40106 type devices between the V-f and the USB-6009.

Lynn