why does my labview filter cause a 360 phase shift?

[weatherhound]

I'm looking at the phase difference between two channels on my 4472. how can I avoid the -360 degrees added to my relative phase. the -360 is not consistent and is only added as I go up in frequency with my input signals (around 8k). also, why don't all my channels start out at 0 degrees phase. sometimes they start at 180 and sometime 0 (at DC).

thankyou very much
Jim

[Jack_A.]

Jim -

I don't have a very concrete answer for you, but rather a debugging suggestion.

I would recommend testing your filter and phase difference algortihms with some LabVIEW generated data such as a simple single tone sine wave. This way, you should be about to easily determine exactly what the results should be. If you still see strange results, then we can stop looking into the filter and phase calculation.

Also, it's always easier to give more specific suggestion when you can provide code which is small and hardware independent so everyone can reproduce what you're seeing.

Jack Arnold
Application Engineer
National Instruemnts

[ANDRÉ_CARVALHO]

Hi!

I had a similar problem... I use the single tone VI to measure the phase of two signals and calculate the diference of phases. Sometimes the difference between them was positive, and sometimes negative ( = difference - 360 ). I solved this problem puting a case structure: if diff < 0 do diff = diff + 360. I know it is not a very elegant process, but it worked. Hope it helps! ; )

André