Rune Allnor <allnor@tele.ntnu.no> wrote:> 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.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
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
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@n_o_s_p_a_m.imtek.de> >wrote: >> In the system I am investigating, the phase speed and group speed arenot>> the same and are not constant and change with distance. Because ofthis,>> the phase of the carrier is not the same as the phase of the modulationin>> 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
Hi Glen, I am simply trying to measure the group speed of a dispersive system by transmitting an AM signal (50MHz modulation, 500MHz carrier) and comparing the modulation before and after the signal has propagated. The signal is expected to shift by about 3 degrees and it is not expected to distort as it propagates. William>Rune Allnor <allnor@tele.ntnu.no> wrote: >> 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 arenot>>> the same and are not constant and change with distance. Because ofthis,>>> the phase of the carrier is not the same as the phase of the modulationin>>> 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 >
Reply by ●March 21, 20102010-03-21
Hi WWalker, "WWalker" <william.walker@n_o_s_p_a_m.imtek.de> writes:> Hi Hardy,My name is Randy.> A (FIR) linear phase filter will phase shift the modulationYour choice of terms do not make sense to me. "Modulation" doesn't get "phase shifted" by a specific number of degrees; rather, a specific frequency component of a signal gets phase shifted by some number of degrees. If you're actually referring to time delay, then I'd use the term "time delay" and not "phase shift." -- 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
"WWalker" <william.walker@n_o_s_p_a_m.imtek.de> writes:> 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.I see now from one of your previous responses that you are indeed attempting to measure time delay. There still should not be any problem whatsoever with using a linear-phase FIR. Sure, it introduces a time delay, but the time delay is constant and known and thus can be eliminated. -- Randy Yates % "...the answer lies within your soul Digital Signal Labs % 'cause no one knows which side mailto://yates@ieee.org % the coin will fall." http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO
Reply by ●March 21, 20102010-03-21
WWalker 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.Phase and group velocities are in general functions of frequency, but what kind of system makes the depend on distance? (To provide useful bandwidth, group velocity must be substantially constant --with or without equalization -- over the band of interest.)> 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.The carrier goes to zero twice per cycle, even without noise. Division by zero is difficult when it can't be accomplished by algebraic cancellation of terms.> 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.The dispersive medium delays upper and lower sidebands by different amounts of time. The envelope will be distorted by this differential delay no matter what demodulation scheme is used. Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi> William > >> Tim Wescott 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. >>> 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) >> ������������������������������������������������������������������������ >>-- 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 wrote:> I am a professional. Say something intelligent and perhaps we can talk > about it. But being rude does not help.Unfortunately, Vlad was accurate. Do the math. 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 wrote:> Hi Glen, > > I am simply trying to measure the group speed of a dispersive system by > transmitting an AM signal (50MHz modulation, 500MHz carrier) and comparing > the modulation before and after the signal has propagated. The signal is > expected to shift by about 3 degrees and it is not expected to distort as > it propagates.Then you want to test the path with two frequencies. An AM signal -- the kind that has an envelope -- has at least three frequencies. Consider what happens when the distance shifts the upper sideband -90 degrees and the upper sideband +90 degrees relative to the carrier. Then the envelope disappears and the signal becomes FM. (Undistorted FM if the original modulation percentage is low enough.) *Do the math* 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 wrote:> 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.Of course it can. There will be delays due the processing, but they can be made equal for all components.> William > > > > >> 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 >>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 wrote:> 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.You are trying to isolate the carrier with the PLL. You can make a zero-degree lock.> > > >> 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 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-GyorgiJerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi