Amplitude modulation -- misnamed?
in [DSP]
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From: glen herrmannsfeldt on 3 May 2010 00:09 dvsarwate <dvsarwate(a)gmail.com> wrote: (snip) > I don't agree with Glen Herrmansfeldt's expressions >> "for the coherent case, add amplitude, for the >> incoherent case add magnitude." > either. Usually, it is the squared amplitudes that > get added (square-law detectors, remember). Yes, the term is supposed to be intensity, the (more or less) time averaged square of the signal. Somehow I was thinking about that when reading magnitude in the OP. In the case of vectors, magnitude is the square root of the square (dot product with itself). I don't know that it would be used for the square root of the intensity. > Adding two equal power coherent sinusoids > quadruples the power because the amplitudes > add and the power is proportional to the square > of the amplitude. Anywhere from 0 to 4, depending on phase. > Adding two equal power > noncoherent sinusoids (in this context, think > orthogonal signals) only doubles the power. So LED output is intensity modulated, not amplitude modulated? > P.S. My students have requested that the word > incoherent not be used in such contexts, only > noncoherent. They want to reserve the word > incoherent to describe my lectures (and postings > to comp.dsp) You will have to change a lot of optics books. -- glen
From: Jerry Avins on 3 May 2010 04:44 On 5/3/2010 12:09 AM, glen herrmannsfeldt wrote: ... > So LED output is intensity modulated, not amplitude modulated? Strictly, yes. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Rune Allnor on 3 May 2010 05:58 On 3 Mai, 04:45, dvsarwate <dvsarw...(a)gmail.com> wrote: > P.S. My students have requested that the word > incoherent not be used in such contexts, only > noncoherent. They want to reserve the word > incoherent to describe my lectures (and postings > to comp.dsp) Your students might be interested in a similar game we played here a few years ago: http://groups.google.no/group/comp.dsp/browse_frm/thread/46174ccbd583b5bb/34a749d251ddfc0e?hl=no&lnk=gst&q=dictionary#34a749d251ddfc0e Rune
From: Tauno Voipio on 3 May 2010 14:07 On 3.5.10 5:49 , Jerry Avins wrote: > >> [The "4 quadrant multipliers" I used were really modulators that >> expected a sinusoidal input for one of the inputs - so in that sense I >> believe they were "tuned". The specs on the output were really tight >> with respect to phase, distortion and gain WRT the "dc" input. They were >> electromagnetic devices called "magnetic modulators".) For balanced modulators there were special beam-deflection tubes, a thing like a cross between an amplifier tube and a CRT, e.g. the RCA 7360. > That's new to me. Diode modulators work best when the carrier was strong > enough so that it might as well have been a square wave. The carrier > switches the polarity, washing out any diode drop that the signal might > see. A more modern way of this approcah is to use CMOS analog switches with the selection inputs driven by hard-limited carrier. -- Tauno Voipio tauno voipio (at) iki fi
From: Jerry Avins on 3 May 2010 15:34
On 5/3/2010 2:07 PM, Tauno Voipio wrote: > On 3.5.10 5:49 , Jerry Avins wrote: >> >>> [The "4 quadrant multipliers" I used were really modulators that >>> expected a sinusoidal input for one of the inputs - so in that sense I >>> believe they were "tuned". The specs on the output were really tight >>> with respect to phase, distortion and gain WRT the "dc" input. They were >>> electromagnetic devices called "magnetic modulators".) > > For balanced modulators there were special beam-deflection tubes, > a thing like a cross between an amplifier tube and a CRT, e.g. > the RCA 7360. > >> That's new to me. Diode modulators work best when the carrier was strong >> enough so that it might as well have been a square wave. The carrier >> switches the polarity, washing out any diode drop that the signal might > > see. > > A more modern way of this approcah is to use CMOS analog switches > with the selection inputs driven by hard-limited carrier. I've done that too. my "hard-limited carrier" was the square wave output of a CD4046. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. ����������������������������������������������������������������������� |