How do I do tow signals time delay measurement?

[Robert_Mensah]

Hello,

I am using Ni 5112 Scope card, LabWindows/CVI 8.5, in WinXp OS. I have two identical 790Hz sine waves (one at channel 1 and the other at channel 2) but are 180 degrees out of phase. What I want to do is measure the time delay between the two signals.

 NOTE: Using the the scope Soft Front Panel, I selected "Time Delay" as my measurement function but could not get the expected 0.0006329sec Time Delay. I also selected the "Phase Delay" measurement function and did not get the expected 180 degrees. What am I doing wrong? How do I set up the scope to do this using the Scope Soft Front Panel? Any help would be appreciated.

Thank you.

Robert

[DFGray]

There are a few things that may be going wrong:

1. You said you did not get the expected values, but you didn't say how far off they were.  Unless you have calibrated your scope card lately, you may just be seeing calibration errors.  This is not self-calibration (which also may help - in the soft front panel, select Utility>>Self Calibration or just press Ctrl-Shift-S).  The timebase on the 5112 is a 50ppm clock, so if it is calibrated, it should not contribute much to errors.
   
2. If you have a lot of noise on your signal, the level crossing detector may not be working well.  This would result in errors.  Noise can be caused by external influences or can be caused by using a range that is too high.  The soft front panel emulates an oscilloscope, so has ranges in the standard 1-2-5 sequence.  However, the 5112 has ranges at 10% intervals - much more frequent.  You can reduce the digitizing noise associated with your measurement by picking the range closest to your signal in CVI, the soft front panel does not support this feature.
3. The zero crossing level is being calculated slightly differently for both signals.  This can be effected by noise, as well.  By default, the measurement determines the crossing point by analyzing the waveform to determine the base and top, then takes the middle as the crossing point.  With sine waves, this process is very subject to noise.  You can tweak the process by expanding the Add Scalar Measurement dialog with the right arrow button.  A possible tweak is to set the level units to Volts and the Mid Level to 0.  Use AC coupling to ensure your waveform is centered at 0 (although calibration rears its ugly head at this time).
4. Your time/division may be too large.  Decrease your time/division to the minimum that will still give you a fill waveform.
5. You may be taking too few points.  Under Edit>>Device Configuration, choose the Horizontal tab and change Record Length to something larger.  This will change the available Time/Div settings and can change update rates.  If you set it high enough, your device will go into RIS mode, which may actually be good, since it will average the waveforms, reducing noise.
   

If this doesn't help, let us know.  If you need more help, please post some example waveforms, either as an HWS file or a screenshot from the soft front panel.

[Henrik_Volkers]

Depending on your resolution needs 
If you make a four parameter sine fit on both channels over say 20 - 200 periodes you can get a subsample phase resolution (BTW what is your samplerate??).