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: Tim Wescott on 8 Mar 2010 10:50 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. Multiplying by the carrier is an accepted and worthwhile practice. There are numerous useful extensions of this, many of which are to deal with the phase noise issue, and with selective fading that includes the carrier -- search on "exalted carrier" and "synchronous AM" to see the variations. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Dirk Bell on 8 Mar 2010 10:57 On Mar 8, 5:57 am, "WWalker" <william.wal...(a)imtek.de> wrote: > >On Mar 7, 10:37=A0pm, Jerry Avins <j...(a)ieee.org> wrote: > >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 > > ------------------------ > I simply want a very good match when I overlay the AM Signal with the > calculated envelope. In order for this to work the calculated envelope can > not be phase shifted. > > William When you say it can not be "phase shifted", do you include a simple time delay (known number of samples) as a "phase shift"? Dirk
From: HardySpicer on 10 Mar 2010 13:28 On Mar 8, 2:53 pm, "WWalker" <william.wal...(a)imtek.de> 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. > > Thanks Use a PLL to get the carrier frequency and multiply and then low-pass filter. Synchronous demodulation. For supressed carrier you need to square the signal first then lock onto 2f then divide by two and multiple - filter. For low carrier to noise ratios you may need a different method. Hardy
From: WWalker on 21 Mar 2010 15:38 Hi Hardy, Unfortunately, the LPF will phase shift the modulation. So this technique will not work for me. Do you know of any other way to extract the modulation without using a filter? William >On Mar 8, 2:53=A0pm, "WWalker" <william.wal...(a)imtek.de> 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. >> >> Thanks > >Use a PLL to get the carrier frequency and multiply and then low-pass >filter. Synchronous demodulation. >For supressed carrier you need to square the signal first then lock >onto 2f then divide by two and multiple - filter. >For low carrier to noise ratios you may need a different method. > >Hardy >
From: Rune Allnor on 21 Mar 2010 15:55
On 21 Mar, 20:38, "WWalker" <william.walker(a)n_o_s_p_a_m.imtek.de> wrote: > Hi Hardy, > > Unfortunately, the LPF will phase shift the modulation. So this technique > will not work for me. That's a claim that needs justification. A lot of DSP newbies and amateurs as similar questions as yours, because the term "phase shift" somehow seems scary, imperfect, or awkward. Too bad - it's a fact of life. So make sure you understand *why* you want to avoid phase shifts: "Sounds scary", "don't want to deal with them" or "don't understand what they are or what causes them" are perfectly valid *subjective* reasons. Which, of course, can be attributed to poor education. However, "phase shifts invalidates my analysis" is a totally different cup of tea: It *sounds* as being objective (but is almost always really caused by one or more of the subjective reasons listed), and as such it requires objective, substantiated arguments in support. Rune |