How to get envelope from AM signal without phase shift
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From: Jerry Avins on 21 Mar 2010 22:53 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(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 >> Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi �����������������������������������������������������������������������
From: Jerry Avins on 21 Mar 2010 22:57 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-Gyorgi Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi
From: Jerry Avins on 21 Mar 2010 22:58 WWalker wrote: > 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. Phase shifts don't matter, only relative shifts. A symmetric FIR has none. You are fighting a phantom. Do the math. Jerry -- Discovery consists of seeing what everybody has seen, and thinking what nobody has thought. .. Albert Szent-Gyorgi �����������������������������������������������������������������������
From: Rune Allnor on 22 Mar 2010 04:12 On 22 Mar, 00:55, "WWalker" <william.walker(a)n_o_s_p_a_m.imtek.de> 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. Why didn't you say that first time around? You asked the wrong question. If you want to measure dispersion, ask about how to measure dispersion, not about how to measure obscure quantities that are sensitive to most minute variations in initial conditions or environment parameters. In order to achieve what you want, the ideal set-up is to measure the input and output simultaneously and synchronously. If this is possible, estimate the cross spectrum of the twu signals, and estimate the phase. If it is not possible to measure input and output but you can use an array to measure the output, you can still come up with an estimate of the phase velocity. The one thing *not* to do , is to work directly with phase. But even if you want to do that, it is perfectly possible to use a linear phase FIR filter, and subtract the corresponding delay from the filtered data. But none of this matters until you have answered the real question: Why would you want to verify these predictions? What purpose could it possibly serve? If you don't trust the maths - why not verify it by measuring a quantity that is actually possible to measure with any accuracy, is robust to environmental variation, and can be processed? Rune
From: Rune Allnor on 22 Mar 2010 07:10
On 22 Mar, 11:55, "WWalker" <william.walker(a)n_o_s_p_a_m.imtek.de> wrote: > Hi Rune, > > The question you are asking is complicated but I will try to explain. I am > trying to measure the speed of information transmission in the nearfield of > a dipole source. This can be done by measuring the time delay of the > envelope of an AM signal between two dipole antennas. Theoretical > calculations show that the envelope should deviate about 3 degrees from > light speed for a 50MHz modulated, 500MHz carrier signal. "Deviate 3 degrees from light speed" ??? Again, one of the best ways to measure the effects of the system is to measure both the input and the output and then examine the cross correlation bewteen the two. This standard approach will extract the relative changes through the system while at the same time avoiding questions about absolute phase, which depends on all kinds of details you couldn't possibly track down anyway. Get a copy of the book "Random Data" by Bendat and Piersol. Rune |