How to get envelope from AM signal without phase shift
in [DSP]
|
Prev: How Rune Allnor can attend the COMP.DSP Conference
Next: Inertial navigation (anyone familiar with this stuff here?)
From: WWalker on 7 Mar 2010 20:53 Hi, Does any one know how to extract the envelope of an amplitude modulated signal without a phase shift, distortions, and able to determine the envelope in between the signal cycles. One way that almost works is to simply devide the signal by the carrier but, this technique is too sensitive to phase noise. I have also tried using the Hilbert transform but, I get some leakage distortions. Thanks
From: Jerry Avins on 7 Mar 2010 22:37 WWalker wrote: > Hi, > > Does any one know how to extract the envelope of an amplitude modulated > signal without a phase shift, distortions, and able to determine the > envelope in between the signal cycles. One way that almost works is to > simply devide the signal by the carrier but, this technique is too > sensitive to phase noise. I have also tried using the Hilbert transform > but, I get some leakage distortions. Extracting the envelope of an AM signal is called "demodulation" or less rigorously, "detection". There are many reasons for dividing by the carrier not working, but extreme non-linearity is probably the best one. What do you do with your Hilbert transform? It is part of a technique that works very well provided the needed computation can be done in time. Jerry -- It matters little to a goat whether it be dedicated to God or consigned to Azazel. The critical turning was having been chosen to participate. �����������������������������������������������������������������������
From: robert bristow-johnson on 7 Mar 2010 23:54 On Mar 7, 10:37 pm, Jerry Avins <j...(a)ieee.org> wrote: > WWalker wrote: > > Hi, > > > Does any one know how to extract the envelope of an amplitude modulated > > signal without a phase shift, distortions, and able to determine the > > envelope in between the signal cycles. One way that almost works is to > > simply devide the signal by the carrier but, this technique is too > > sensitive to phase noise. I have also tried using the Hilbert transform > > but, I get some leakage distortions. > > Extracting the envelope of an AM signal is called "demodulation" or less > rigorously, "detection". There are many reasons for dividing by the > carrier not working, but extreme non-linearity is probably the best one. dunno why you can't rectify (abs value) and LPF. unless overmodulated or it's suppressed carrier. > What do you do with your Hilbert transform? It is part of a technique > that works very well provided the needed computation can be done in time. one thing i would like to figure out is what the OP means by "without phase shift". if he/she means no delay in the detection alg, then Hilbert is out of the picture completely. r b-j
From: Jerry Avins on 8 Mar 2010 00:14 robert bristow-johnson wrote: > On Mar 7, 10:37 pm, Jerry Avins <j...(a)ieee.org> wrote: >> WWalker wrote: >>> Hi, >>> Does any one know how to extract the envelope of an amplitude modulated >>> signal without a phase shift, distortions, and able to determine the >>> envelope in between the signal cycles. One way that almost works is to >>> simply devide the signal by the carrier but, this technique is too >>> sensitive to phase noise. I have also tried using the Hilbert transform >>> but, I get some leakage distortions. >> Extracting the envelope of an AM signal is called "demodulation" or less >> rigorously, "detection". There are many reasons for dividing by the >> carrier not working, but extreme non-linearity is probably the best one. > > dunno why you can't rectify (abs value) and LPF. unless overmodulated > or it's suppressed carrier. What guarantee is there that the sample instants will coincide with the peaks of the carrier? It's conceivable (but most unlikely) that all the samples will nearly coincide with zero crossings of the carrier. >> What do you do with your Hilbert transform? It is part of a technique >> that works very well provided the needed computation can be done in time. > > one thing i would like to figure out is what the OP means by "without > phase shift". if he/she means no delay in the detection alg, then > Hilbert is out of the picture completely. I would like to understand what dividing by the carrier would do. I almost get it, but the carrier is zero twice a cycle. Jerry -- It matters little to a goat whether it be dedicated to God or consigned to Azazel. The critical turning was having been chosen to participate. �����������������������������������������������������������������������
From: WWalker on 8 Mar 2010 05:26
>WWalker wrote: >> Hi, >> >> Does any one know how to extract the envelope of an amplitude modulated >> signal without a phase shift, distortions, and able to determine the >> envelope in between the signal cycles. One way that almost works is to >> simply devide the signal by the carrier but, this technique is too >> sensitive to phase noise. I have also tried using the Hilbert transform >> but, I get some leakage distortions. > >Extracting the envelope of an AM signal is called "demodulation" or less >rigorously, "detection". There are many reasons for dividing by the >carrier not working, but extreme non-linearity is probably the best one. > >What do you do with your Hilbert transform? It is part of a technique >that works very well provided the needed computation can be done in time. > >Jerry >-- >It matters little to a goat whether it be dedicated to God or consigned >to Azazel. The critical turning was having been chosen to participate. ---------------------------------------------------- > First I should say I am tryting to extract the envelope of an amplitude modulated signal that has been captured by an oscilloscope. I am doing some wave propagation experiments and I need to measure the time delay of the envelope very accurately. As you mentioned, dividing by the carrier is not a good way to do it, but it does demonstrate that it should be possible to come up with a technique to extract the envelope without a phase shift of the envelope, negligible distortions, and able to determine the envelope in between signal cycles. Regarding the Hilbert Transform method, I squared the signal and added it to the square of the Hilbert transform of the signal. Then I took the square root of the result. This technique extracts the envelope without a phase shift, but it does introduce problematic oscillations near the beginning and end of the signal. I do not want to use a filter to get rid of the oscillations because it will add a phase shift to the envelope. Another method I am considering is to curvefit the known form of the AM signal, provided the everything is known about the signal except the unkown modulation amplitude. But I am not sure if this technnique will work with real signals that have some noise. William |