How to get envelope from AM signal without phase shift
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From: WWalker on 21 Mar 2010 20:13 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(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: Randy Yates on 21 Mar 2010 22:23 Hi WWalker, "WWalker" <william.walker(a)n_o_s_p_a_m.imtek.de> writes: > Hi Hardy, My name is Randy. > A (FIR) linear phase filter will phase shift the modulation Your 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(a)ieee.org % got no one..." http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
From: Randy Yates on 21 Mar 2010 22:30 "WWalker" <william.walker(a)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(a)ieee.org % the coin will fall." http://www.digitalsignallabs.com % 'Big Wheels', *Out of the Blue*, ELO
From: Jerry Avins on 21 Mar 2010 22:42 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 �����������������������������������������������������������������������
From: Jerry Avins on 21 Mar 2010 22:51
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 |