Getting phase difference...problems

[whatsup]

I input the identical signal onto both channels and I do in fact have a 180 degree phase shift. The differential inputs do have the correct configuration as well so I guess if I do some math and subtract or 180 I can get a pretty close number. Any idea why it goes - or + and is not allways in the same orientation?

Thanks

[whatsup]

I made up special connectors for the PCI 6023E cards......basically SCSI 2 cables with 2 BNC connectors attached to allow easy input to channel 0 and 1......tried another one to verify 180 phase shift....both the same result. I might be able to make this work by inverting the one channel with leads....or possibly in software?

Any ideas with that?

[Scott Seidman]

whatsup wrote in news:50650000000500000001600100-1077587809000
@exchange.ni.com:

> Thanks for addition to my circuit. I tried it out and unfortunately
> the results don't seem to make much sense. The amplitude part works
> perfectly. The phase, however, jumps around and is consistantly too
> high in value.
>
> Presently I am inputing a 4v sine wave @ 1 khz. Channel 0 is directly
> from the function generator. Channel 1 is thru an RC filter. The
> output at 1khz should be -45 degrees. I am consistently seeing phase
> numbers like 136 or -223 degrees. Is it possible that there is a 180
> degree phase shift added somewhere?
>
> Thanks again for your help
>
> Dave

Your answer is approx
imately 180 degrees out of phase with what you expect.
Somewhere, somehow, you have inverted your signal.

Apart from the 180 degrees, you don't have a problem. Your RC filter must
be off. Since caps aren't high-precision devices, I'm not terribly
surprised.

Apart from having one of your signals plugged in backward, its rather
unlikely that you have any hardware problems with your acquisition system.
If you want to find out what your phase really is, use an oscilloscope to
get the true phase, then determine whether your hardware is screwed up.

Scott

[ramone666]

Hi! I came across this thread having the same problem with the phase and LocalDSP's answer did the trick. Now I just wonder why "360" is included in the formula given:

>- If you are using a multiplexed MIO board, you'll have to correct for the multiplexing delay, that is the slight time difference between the >two acquisitions. This time is reflected by the difference in the two signals timestamp (t0). The resulting phase error depends on the >frequency f of your signal, so overall you'll have to correct your phase difference result with:
>phi_correction(deg.) = (t0_
>1 - t0_0) * f * 360

Thanks in advance!

[Rick_K]

The "360" is a factor to convert the percentage of the cycle phase difference into degrees.

In other words, when you do the difference in time multiplied by the frequency you get the fraction of the cycle representing the phase difference. Multiplying that fraction by 360 gives you the number of degrees that fractional cycle represents (because we know that there is 360 degrees in a fill cycle)

Hope this helps,
Rick Kuhlman