How to get envelope from AM signal without phase shift
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From: WWalker on 21 Mar 2010 19:24 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(a)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(a)ieee.org % got no one..." >http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO >
From: Rune Allnor on 21 Mar 2010 19:30 On 21 Mar, 23:15, "WWalker" <william.walker(a)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
From: WWalker on 21 Mar 2010 19:44 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 >> >> >> >
From: glen herrmannsfeldt on 21 Mar 2010 19:48 Rune Allnor <allnor(a)tele.ntnu.no> wrote: > On 21 Mar, 23:15, "WWalker" <william.walker(a)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. Note that the previous post indicated that they "change with distance". It is possible to have a system where they vary in such a way that the overall system is not dispersive. One way this has been done in optical systems is with a phase conjugate mirror. After a signal goes through a dispersive medium (such as optical fiber), it then goes through a phase conjugation device. That reverses the effect such that passing through the same amount of fiber restores the original signal. That is, dispersive fiber+phase conjugation+dispersive fiber is, overall, not dispersive! > 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? Now that is a good question! -- glen
From: WWalker on 21 Mar 2010 19:55
Hi Rune, Although the system is dispersive, provided the phase and amplitude reponse of the system are linear over the bandwidth of the signal, the signal will propagate undistorted. This is satisfied in my system with a 50MHz Modulation, 500MHz Carrier AM signal. I simply want to measure a predicted 3 degree phase shift of the Modulation. In order to do that I need to extract the modulation and compare it to the modulation before the propagation. I do not know if this can be done. This is why I am asking. William >On 21 Mar, 23:15, "WWalker" <william.walker(a)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 > |