"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message news:xK6dndFXrqGMd7jeRVn-oQ@comcast.com...> >> Then what's the Heisenberg ambiguity principle;) > > Whether the cat died or not. >According to T. S. Elliot, the cat knows ;-) Clay "If there is no more uncertainty, there is no more knowledge." -Heisenberg
minimum cycles for fft, and limits of filling short sample out with zeros
Started by ●September 4, 2005
Reply by ●September 13, 20052005-09-13
Reply by ●September 13, 20052005-09-13
"Clay S. Turner" <Physics@Bellsouth.net> writes:> > According to T. S. Elliot, the cat knows ;-) >And will never confess!!!! Ciao, Peter K.
Reply by ●September 13, 20052005-09-13
glen herrmannsfeldt wrote:> Stan Pawlukiewicz wrote: > >> glen herrmannsfeldt wrote: > > >>> Stan Pawlukiewicz wrote: > > >>> (snip) > > > > (snip) > >> Then what's the Heisenberg ambiguity principle;) > > > Whether the cat died or not.The size of the box doesn't matter. The amount of time you can look at the box doesn't matter.> >> Schroderinger's Drag Queen? > > >>> In real systems you never find a single sine wave, that is, from >>> t= negative infinity to positive infinity. > > >> Agreed, in real life, stuff is more complicated. > > > And this problem makes it hard to get right. > > The discussion was of a noiseless source, but do we also require > a noiseless detector? For a heterodyne detector, a noiseless > mixer and local oscillator? How do we get the right amount of > real life into the problem, even with a noiseless source. > > In considering zero crossing of an optical signal, how do we > know where zero is? Optical signals are always AC coupled, > and so require a baseline restore. Any errors in the baseline > restore shift the reference for zero crossing. > > -- glen >Heisenburgs bound is a hard bound. Additional information doesn't budge it. For a classical measurement, making a few assumptions lets you do better. Even in real life, for some situations, you can make those assumptions. Optics is a special case because the technology is very different from say audio or ultrasonic processing.
Reply by ●September 13, 20052005-09-13
glen herrmannsfeldt wrote: ...> In considering zero crossing of an optical signal, how do we > know where zero is? Optical signals are always AC coupled, > and so require a baseline restore. Any errors in the baseline > restore shift the reference for zero crossing.Whatever applies to optics in this regard applies to all electromagnetic radiation. There are zero crossings of the E and H waves, but it's hard to measure them. What's more, the instantaneous power remains constant. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 13, 20052005-09-13
"Jerry Avins" <jya@ieee.org> wrote in message news:orydnfiM27QwSrveRVn-gg@rcn.net...> glen herrmannsfeldt wrote: > > ... > >> In considering zero crossing of an optical signal, how do we >> know where zero is? Optical signals are always AC coupled, >> and so require a baseline restore. Any errors in the baseline >> restore shift the reference for zero crossing. > > Whatever applies to optics in this regard applies to all electromagnetic > radiation. There are zero crossings of the E and H waves, but it's hard to > measure them. What's more, the instantaneous power remains constant. >Hello Jerry, You actually have to be careful here. For plane waves in space, the instantaneous power as viewed by a stationary observer pulses. If you recall the Poynting vector is simply S = E cross H. In free space both E and H are in phase but both are time varying at the frequency of the E-M wave. Hence S varies at twice the frequency of the wave. If a 30 Hz E-M wave passes through you, you are actually hit with 60 pulses of energy per second. For all but the lowest frequencies, this pulsing is almost unobservable. So most deal with the time averaged power, which for a plane wave is constant. And the average works out to be 1/2 of the peak. Clay
Reply by ●September 13, 20052005-09-13
Clay S. Turner wrote:> "Jerry Avins" <jya@ieee.org> wrote in message > news:orydnfiM27QwSrveRVn-gg@rcn.net... > >>glen herrmannsfeldt wrote: >> >> ... >> >> >>>In considering zero crossing of an optical signal, how do we >>>know where zero is? Optical signals are always AC coupled, >>>and so require a baseline restore. Any errors in the baseline >>>restore shift the reference for zero crossing. >> >>Whatever applies to optics in this regard applies to all electromagnetic >>radiation. There are zero crossings of the E and H waves, but it's hard to >>measure them. What's more, the instantaneous power remains constant. >> > > > Hello Jerry, > > You actually have to be careful here. For plane waves in space, the > instantaneous power as viewed by a stationary observer pulses. > > If you recall the Poynting vector is simply S = E cross H. In free space > both E and H are in phase but both are time varying at the frequency of the > E-M wave. Hence S varies at twice the frequency of the wave. If a 30 Hz E-M > wave passes through you, you are actually hit with 60 pulses of energy per > second. For all but the lowest frequencies, this pulsing is almost > unobservable. So most deal with the time averaged power, which for a plane > wave is constant. And the average works out to be 1/2 of the peak.I wasn't thinking of the Poynting vector, but of the cyclic energy transfer between the E and H fields. The energy relations in a lossless tank circuit (an idealization, to be sure) in which the energy remains constant but shuttles between the electric field in the capacitor and the magnetic field of the inductor the analog of what I meant to convey. The analogy goes further. Just as the voltage on a tank's capacitor crosses zero, so does the EM wave's E field. There is no DC (the wavelength would be too long :-)!) but there is a "DC" reference. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●September 13, 20052005-09-13
Rune Allnor wrote:> kroger@princeton.edu wrote: > > 2. If I sample less than one period, then zero-fill up to one period > > (or more?), can I get a good result with an FFT? > > It depends what you want to do. You will not be able to separate > several close sinusoidals by zero-filling.Depends on how close. If the sinusoids are farther apart than a few bins, then zero-filling and using a longer FFT may tell you how far apart they are with a higher resolution, perhaps even with more accuracy that a 3-point polynomial interpolation. Whether the S/N ratio is high enough for that difference to be statistically significant, and whether it's worth the computational cost is another question. IMHO. YMMV. -- rhn A.T nicholson d.O.t C-o-M
Reply by ●September 13, 20052005-09-13
Jerry Avins wrote: (snip)>> If you recall the Poynting vector is simply S = E cross H. In free >> space both E and H are in phase but both are time varying at the >> frequency of the E-M wave. Hence S varies at twice the frequency of >> the wave. If a 30 Hz E-M wave passes through you, you are actually hit >> with 60 pulses of energy per second. For all but the lowest >> frequencies, this pulsing is almost unobservable. So most deal with >> the time averaged power, which for a plane wave is constant. And the >> average works out to be 1/2 of the peak.Also, the frequency that most people think of as "60Hz" is actually 120Hz. Most vibrating systems, such as transformer windings, move on both halves of the cycle. -- glen
Reply by ●September 13, 20052005-09-13
Jerry Avins wrote: (snip regarding electromagnetic fields and zero crossing)> The analogy goes further. Just as the voltage on a tank's capacitor > crosses zero, so does the EM wave's E field. There is no DC (the > wavelength would be too long :-)!) but there is a "DC" reference.There is, but how do you find it in a time varying envelope of the wave? As in my previous example, say a gaussian wave packet. Any inaccuracy in finding the DC reference is an uncertainty in the frequency measured by zero crossings. That is, even with a noiseless signal. -- glen
Reply by ●September 13, 20052005-09-13
glen herrmannsfeldt wrote:> Jerry Avins wrote: > > (snip regarding electromagnetic fields and zero crossing) > >> The analogy goes further. Just as the voltage on a tank's capacitor >> crosses zero, so does the EM wave's E field. There is no DC (the >> wavelength would be too long :-)!) but there is a "DC" reference. > > > There is, but how do you find it in a time varying envelope of > the wave? As in my previous example, say a gaussian wave packet. > Any inaccuracy in finding the DC reference is an uncertainty in the > frequency measured by zero crossings. That is, even with a noiseless > signal.Certainly, but not being able to use it isn't the same as its not being there. I don't mind loose terminology as long as everyone involved knows what's real. I hate it when a sidekick complains, "But you said ..." so I've gotten uptight in that department. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
