How to measure frequency of non-TTL pulses?

[querty999]

Google says the "on" threshold for TTL pulses is 2.0 volts (Vih). But I need to measure frequency of pulses that don't all reach 2.0 volts. Is that possible? My (limited) understanding is that the counter/timer input to my DAQ device (USB-6221) requires TTL signals. Does that mean I can't count anything but TTL pulses? Or is there a way to adjust the threshold?

[Dennis_Knutson]

The specs list the minimum Vhi as 2.2 volts and you cannot adjust this. Either use an external circuit (i.e. comparator) to convert your signal to ttl levels or use analog acquisition and determine the frequency from that.

[querty999]

Analog acquisition would be much slower, right? That is, the frequencies that could be measured would be lower because of the processing (comparing numbers) and lower sample rate, no? More error prone as well, I imagine.

[Dennis_Knutson]

You would be limited to half the maximum sampling rate. The external shifting to ttl levels would be better, in my opinion.

[querty999]

Where can I get a (comparator/shifting) circuit? Can they keep up, ie are they as fast as the counter input?

Will this do? I don't see anything in the spec sheet that says what level the input signal has to be in order to create TTL output.

I'm not an electronics expert, just somewhat knowlegeable. I understand the concept, just not sure I can make it happen.

thanks!

[Dennis_Knutson]

At first glance, that should work. A 10ns switching speed is 100MHz.