Verifying Duty Cycle

[gallen70]

I need to verify the PWM of a circuit is working correctly by using a 6229 DAQ.  The frequency is around 500Khz.  So I need to read in a PWM signal with a DAQ and verify both frequency and Duty cycle are expected values.  I figure NI has a slick way to do this.

Greg

[Graziano]

Hi!
   If I understood correctly, you have a circuit providing a 500 kHz digital signal, sqare-wave like, with variable duty-cycle (that's PWM).  In this case I think you need a DAQ device sampling at, at least, 100 times signal frequency, to verify duty cycle at 1%.  This is to observe the effective wave provided by your circuit.  In this case, the board you have doesn't fulfill your needs.

   But maybe you only need to evaluate duty-cycle, for example with a power measurment.  In this case, I suppose you may low-pass filter the PWM, and sample its "mean" value.  But this works if youu modulate pulses "slowly" in time, for example, you set a duty cicle and you do not modify it, and you verify it's correct, and after you repeat acquisition with a different duty cycle.

   Please, explain better your needs, and tell me if I understand correctly.  Then we can go on on evaluating solutions, maybe i can do some simulations....

   Have a nice day!

graziano 

[AnalogKid2DigitalMan]

Graziano:

Would a counter/timer card (62xx or 66xx) possibly be a better choice given the info provided?

[Tom_W_[DE]]

Hi All-

AnalogKid's suggestion is a better option if the PWM signal is 5V TTL.  Are you using LabVIEW to program?  The M Series (like your 6229) have two counter/timers that can make time and frequency measurements against a precise 80MHz timebase.  This should be more than enough to characterize your 500Hz signal.

LabVIEW shipping examples (Help>>Find Examples) of interest would be Meas Dig Frequency-Low Freq 1 Ctr.vi and Meas Duty Cycle-Buffered-Finite.vi from Hardware Input and Output>>DAQmx>>Counter Measurements

Hopefully this helps-

[AnalogKid2DigitalMan]

500KHz, suppose 80MHz clock is still sufficient

[gallen70]

I would like to use what I have.  So... me being new to Daqs and all how would I be able to use the counters and timers?

Greg

[Tom_W_[DE]]

Hi Greg-

The examples I suggested in my previous post show how to program these types of applications in LabVIEW.  If you're not using LabVIEW let us know.

The unknown signals will need to be applied to the Gate inputs of the counters you choose.  The default counter terminals can be found in the device terminals help in MAX by right-clicking on the device and selecting Device Pinouts.

EDIT:  Thanks for the heads up AnalogKid.  Even with a 500kHz signal the measurement error will be less than 1% with the low frequency 1 counter method.

Hopefully this helps-

Message Edited by Tom W [DE] on 07-12-2006 02:05 PM