WWalker wrote:> 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?Why do you believe that? Think about what is being filtered. Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi �����������������������������������������������������������������������
How to get envelope from AM signal without phase shift
Started by ●March 7, 2010
Reply by ●March 21, 20102010-03-21
Reply by ●March 21, 20102010-03-21
In the system I am investigating, the phase speed and group speed are not the same and are not constant and change with distance. Because of this, the phase of the carrier is not the same as the phase of the modulation in the signal. As I mentioned one way to get the modulation: without a phase shift, without modulation distortion, and in between oscillations is to simply divide the signal by the carrier which can be obtained by using a PLL. Unfortunatly the technique is very sensitive to noise. But it does show that it is in principle possible. The resultant modulation using the divide technique is plagued with large random spikes. Do you know of any signal processing methods to remove the spikes without distorting the signal or phase shifting the modulation? I have tried using a running average, and mean average but I always get a phase shift. Pehaps a Median filter could be used but my guess is that it will distort the signal and phase shift it. Lastly, I should mention I have come up with another interesting method which is to transmit a modulation signal through the dispersive medium with a sinusoidal carrier (Q) and another modulation signal with a cosinusoidal carrier (I). The instantaneous envelope can then be obtained by squaring each, adding, and taking the square root. This method requires careful alignment of the signals, but it does work and is a lot less sensitive to noise. William>Tim Wescott wrote: >> WWalker wrote: >>> Hi, >>> >>> Does any one know how to extract the envelope of an amplitudemodulated>>> 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 Hilberttransform>>> 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. > >I think W wants to explore the effects of a dispersive channel with >constant group delay in the band of interest. I don't think any kind of >demodulation is useful for that. > >Jerry >-- >Physics is like sex: sure, it may give some practical results, but >that's not why we do it. -- Richard P. Feynman (Nobel Prize, Physics) >������������������������������������������������������������������������ >
Reply by ●March 21, 20102010-03-21
It looks like you don't know what you doing. Matlab is not a substitute for knowledge. From here, you can either seek for professional help or study an ABC=20 book on DSP from cover to cover. VLV WWalker wrote:> In the system I am investigating, the phase speed and group speed are n=ot> the same and are not constant and change with distance. Because of this=,> the phase of the carrier is not the same as the phase of the modulation=in> the signal.=20 >=20 > As I mentioned one way to get the modulation: without a phase shift, > without modulation distortion, and in between oscillations is to simply=> divide the signal by the carrier which can be obtained by using a PLL. > Unfortunatly the technique is very sensitive to noise. But it does show=> that it is in principle possible. The resultant modulation using the di=vide> technique is plagued with large random spikes. Do you know of any signa=l> processing methods to remove the spikes without distorting the signal o=r> phase shifting the modulation? I have tried using a running average, an=d> mean average but I always get a phase shift. Pehaps a Median filter cou=ld> be used but my guess is that it will distort the signal and phase shift=> it. >=20 > Lastly, I should mention I have come up with another interesting method=> which is to transmit a modulation signal through the dispersive medium =with> a sinusoidal carrier (Q) and another modulation signal with a cosinusoi=dal> carrier (I). The instantaneous envelope can then be obtained by squarin=g> each, adding, and taking the square root. This method requires careful > alignment of the signals, but it does work and is a lot less sensitive =to> noise. > =20 > William >=20 >=20 >>Tim Wescott wrote: >> >>>WWalker wrote: >>> >>>>Hi, >>>> >>>>Does any one know how to extract the envelope of an amplitude >=20 > modulated >=20 >>>>signal without a phase shift, distortions, and able to determine the >>>>envelope in between the signal cycles. One way that almost works is t=o>>>>simply devide the signal by the carrier but, this technique is too >>>>sensitive to phase noise. I have also tried using the Hilbert >=20 > transform >=20 >>>>but, I get some leakage distortions. >>> >>>Multiplying by the carrier is an accepted and worthwhile practice. The=re>=20 >=20 >>>are numerous useful extensions of this, many of which are to deal with==20>>>the phase noise issue, and with selective fading that includes the=20 >>>carrier -- search on "exalted carrier" and "synchronous AM" to see the==20>>>variations. >> >>I think W wants to explore the effects of a dispersive channel with=20 >>constant group delay in the band of interest. I don't think any kind of==20>>demodulation is useful for that. >> >>Jerry >>--=20 >>Physics is like sex: sure, it may give some practical results, but >>that's not why we do it. -- Richard P. Feynman (Nobel Prize, Physics)=>>=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD==FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF= =CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF= =CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD= =FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF= =CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF= =CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD= =FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF= =CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD=FF=CF=CD>>
Reply by ●March 21, 20102010-03-21
Hi Jerry, The low pass filter is used to filter out the higher harmonic terms generated by the mixer. But unfortunately, the filter phase shifts the wanted modulation. In my experiment I am transmitting a 50MHz Modulation signl with a 500MHz Carrier. If I use a simple 3rd order filter (1/(j(f/fc)+1)^3), with a 100MHz cutoff, the resultant modulation is phase shifted about 90 degrees. But, the effect I am trying to measure is a 3 degree change in modulation. William> >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 >>WWalker wrote: >> Hi Hardy, >> >> Unfortunately, the LPF will phase shift the modulation. So thistechnique>> will not work for me. Do you know of any other way to extract the >> modulation without using a filter? > >Why do you believe that? Think about what is being filtered. > >Jerry >-- >Discovery consists of seeing what everybody has seen, and thinking what >nobody has thought. .. Albert Szent-Gyorgi >����������������������������������������������������������������������� >
Reply by ●March 21, 20102010-03-21
"WWalker" <william.walker@n_o_s_p_a_m.imtek.de> writes:> Hi Hardy, > > Unfortunately, the LPF will phase shift the modulation.Not if you do it with a digital linear-phase filter. -- Randy Yates % "So now it's getting late, Digital Signal Labs % and those who hesitate mailto://yates@ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
Reply by ●March 21, 20102010-03-21
I am a professional. Say something intelligent and perhaps we can talk about it. But being rude does not help. William> >It looks like you don't know what you doing. >Matlab is not a substitute for knowledge. > From here, you can either seek for professional help or study an ABC=20 >book on DSP from cover to cover. > >VLV
Reply by ●March 21, 20102010-03-21
I am *the* professional. I need to know the problem statement. I.e. what is the input, what should be the output, what is the accuracy and what hardware is available. Your problem will probably take 3-4 hours of work. The cost is going to be $1000. Is this OK ? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com WWalker wrote:> I am a professional. Say something intelligent and perhaps we can talk > about it. But being rude does not help. > > William > > >>It looks like you don't know what you doing. >>Matlab is not a substitute for knowledge. >>From here, you can either seek for professional help or study an ABC=20 >>book on DSP from cover to cover. >> >>VLV > > >
Reply by ●March 21, 20102010-03-21
Hi Hardy, A (FIR) linear phase filter will phase shift the modulation a small amount without distorting the signal in the pass band. As I mentioned in a previous post. I am trying to measure a 3 degree shift of a 50MHz modulation, 500MHz carrier signal. But, I should mention, that the following technique does work. Fourier Transform the signal. Replace the higher harmonics mixer terms with zeros, and then inverse Fourier Transform back to the time domain. William William>"WWalker" <william.walker@n_o_s_p_a_m.imtek.de> writes: > >> Hi Hardy, >> >> Unfortunately, the LPF will phase shift the modulation. > >Not if you do it with a digital linear-phase filter. >-- >Randy Yates % "So now it's getting late, >Digital Signal Labs % and those who hesitate >mailto://yates@ieee.org % got no one..." >http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO >
Reply by ●March 21, 20102010-03-21
On 21 Mar, 23:15, "WWalker" <william.walker@n_o_s_p_a_m.imtek.de> wrote:> In the system I am investigating, the phase speed and group speed are not > the same and are not constant and change with distance. Because of this, > the phase of the carrier is not the same as the phase of the modulation in > the signal.If the phase and group velocities are different, the system is dispersive. If you have a dispersive system, you are in far worse trouble than a mere filter or AM demodulator, irrespective of phase responses, can handle. What are you doing? What do you want to achieve? Why do you think *you* are able to handle whatever it is you are up to? Rune
Reply by ●March 21, 20102010-03-21
Vladimir, Thanks for the offer. I will think about it. In the meantime I would like to know if there are solutions for this type of problem. From my experience this is not an easy problem to solve and may require comming up with something new as I have indicated in my posts. The problem is to measure a 3 degree change in the envelope of an AM Signal (50MHz modulation, 500MHz Carrier) captured on a 1GHz digital scope. The envelope needs to be extracted from the signal and compared to the envelope before the signal propagated. I am trying to measure the group speed. William> > >I am *the* professional. >I need to know the problem statement. I.e. what is the input, what >should be the output, what is the accuracy and what hardware is >available. Your problem will probably take 3-4 hours of work. The cost >is going to be $1000. Is this OK ? > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com > > > >WWalker wrote: > >> I am a professional. Say something intelligent and perhaps we can talk >> about it. But being rude does not help. >> >> William >> >> >>>It looks like you don't know what you doing. >>>Matlab is not a substitute for knowledge. >>>From here, you can either seek for professional help or study an ABC=20 >>>book on DSP from cover to cover. >>> >>>VLV >> >> >> >
