mesuring negative tones of a signal

[obarriel]

I am processing the armonics of a variable signal that always has two main frequencies: 50Khz and 100Khz. I am using a modified version of "extract multiple tone information" that is giving me the magnitude and the phase difference of  these two frequencies. This is working pretty well.

Now I would like to do a thing that I don't know whether it can be done or not. Let's see... when I measure the magnitude of the tones I suppose that the vi is giving me the absolute value of the tone, but as far as I know one somebody performs the fourier transform of a function, the fourier coefficients (that are the tones I suppose) can be either positive or negative. As the vi is only giving me positive values, I am losing sign information, and I need it!

So my question is... is there any way of measuring negative tones. So I would like to be capable  of distinguish negative and positive tones.

Any help or explanation is welcomed. Sorry for my poor english.

[JaimeC]

HI,

Hopefully these 2 tutorials will help you to understand the different VI's for spectral analysis and the way they work:

http://digital.ni.com/public.nsf/websearch/8B09A4DE6805112186256D36007943D4?OpenDocument

http://www.ni.com/support/labview/toolkits/analysis/analy3.htm

[Mellobuck]

obarriel,

Like the above links say, the Fourier transform of a signal will be a symmetric graph. All of the negative harmonics will be same as the positive (same amplitude, shape and frequency offset) only in the negative portion of the frequency domain. They really only have Mathematical significance (how can you have a tone at -3000 Hz?), but if you need them then repeat the set of positive harmonics, keep their amplitude identical and make their frequencies negative.

Or, you might already have the negative harmonics and not realize it. If you are getting only positive signals, the 'positive' side is the frequencies up to your Nyquist frequency (half your sampling frequency) and the signals above that, are the 'negative' half of your harmonics, which have wrapped around.

Message Edited by Mellobuck on 02-15-2007 08:17 AM