measuring frequency of TTL input with counte inputs on a USB 6363 device

[jac2015]

hi all,

Is there a simple LabView vi to measure frequency of TTL input wired to the four available counter input/s on a USB 6363 device?  I have LabView 2014 FDS installed. I guess utilizing 2 of the 4 counters would be better than 1 for 32-bit resolution, correct?

thanks,

john

[QueenSelene · Accepted solution]

Hi jac2015,

There is an example measuring frequency located in LabVIEW Help->Find Examples-> Hardware Input and Output->DAQmx->Counter Input->Counter - Read Pulse Width and Frequency (Continuous).vi

Take a look at this and see if this helps. There is also a pretty useful White Paper on considerations to make when measuring frequency, which you can find here: 

"Frequency Measurements: How-To Guide"

http://www.ni.com/tutorial/7111/en/

The main considerations is what frequency is expected of the signal you are trying to measure? 

Hope this helps!

[jac2015]

hi Selene,

thanks for your response!   i found the example you referred to and applied it to my application and i believe it is working nicely.   🙂

thanks for the white paper - i will review it.

also, just not clear on the example's frequency min and max inputs - am i to input in units of counts/sec or cycles/sec and what are the output data units ???  how sensitive is the output to the input min and max freq values I choose, i.e., do i need to be w/in +-10%, or other amt, to assure some degree of output accuracy?  My application is:  measuring an electric motor's speed via its encoder which outputs a 5V TTL squarewave digital signal, whereby, square wave frequency correlates to motor speed.  Once i apply the target speed into the its motor controller program (not LabView) and speed is ramped up to target, the target speed is meant to stay [fairly] constant.  I am only monitoring the motor speed with LabView which is using the same signal from motor's encoder that the motor controller program is using as feedback signal.

thanks again for your assistance,

John

[Kevin_Price]

From my observation, the task is not highly sensitive to the min/max inputs, just try to be in the right order of magnitude.  I believe they're mainly used to help DAQmx choose a timebase for the frequency measurement.   If the min freq value is extremely low, DAQmx would recognize that the 100 MHz timebase could cause a 32-bit count rollover within a single interval, and will then choose a slower timebase.  The potential trouble is that the slower timebase results in higher quantization error, especially for the higher frequencies.

Note: this is all in reference to the single-counter freq measurement.  Analogous reasoning probably holds for the 2-counter method that I haven't generally used.

-Kevin P

[jac2015]

thank-you for the info, Kevin !  Great forum !!!

-john