how to get the envelope of a signal accurately?

[zientist]

Hi all, I am using the NI DAQ 6251 and I need to get the envelope of a signal (you can think of it as sin(t./480).*cos(t./15)).   I acquire this signal from a device and need to only look at the envelope of the signal.  In Matlab I can just use hilbert transfrom functin which gives me the analytic signal, equivalently, I can just do the fft, remove the negative frequency components and take the modulus of the analytic signal to get the envelope.  However in labview, Hilbert transform just gives the imaginary bit of the analytic signal which is just the original signal phase shifted by 90 degrees.   I am not able to easily implement the algorithm to give me the analytic signal.  Can anyone please help me out.  My code is attached (I am using labview 8.2 version). Thanks 


1 Synch-Sine-Acq-Save-FFT-7.vi:
http://forums.ni.com/ni/attachments/ni/250/42362/1/1 Synch-Sine-Acq-Save-FFT-7.vi

[rpursley8]

Just use the complex number functions to build the analytic signal.  Original signal is the real part, the Hilbert Transform output is the imaginary part.  The magnitude of this signal will be the envelope.  


Envelope.PNG:
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