Amplitude modulation -- misnamed?
in [DSP]
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From: Clay on 3 May 2010 16:09 On May 3, 4:44 am, Jerry Avins <j...(a)ieee.org> wrote: > 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. > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ Actually you are modulating the number of photons emitted by the LED per unit time. Each photon has energy E=h*frequency. So in the case of where your LED is narrowband, then all of the photons have pretty much the same energy. And in the case of a laser diode, then each photon's wavefunctions are also in phase with each other's wavefunctions. Clay
From: Clay on 3 May 2010 16:12 On May 2, 10:45 pm, dvsarwate <dvsarw...(a)gmail.com> wrote: > On May 2, 1:07 am, spop...(a)speedymail.org (Steve Pope) wondered: > > > Shouldn't amplitude modulation really be called "Magnitude > > Modulation"? > > You say to-mato, I say tom-ato.... > > To me, if x(t) = A sin(wt + b) where A > 0, then the > magnitude of x(t) is a time-varying function having > value |A sin(wt_0 + b)| at time t = t_0 while the amplitude > is A and is fixed for all time. So, if A varies slowly with > respect to the carrier frequency w, the result is called > amplitude modulation, and not magnitude modulation. > > 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). > Adding two equal power coherent sinusoids > quadruples the power because the amplitudes > add and the power is proportional to the square > of the amplitude. Adding two equal power > noncoherent sinusoids (in this context, think > orthogonal signals) only doubles the power. > > --Dilip Sarwate > > 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) Try explaining to physics students that carrying a large heavy rock horizontally a distance involves no work! Clay p.s. the optics books most def. use the term "incoherent"
From: glen herrmannsfeldt on 3 May 2010 16:45 Jerry Avins <jya(a)ieee.org> wrote: (snip, someone wrote) >> Just to shake your cage: the 4 quadrant multiplier used in this mode as >> a modulator is a linear system with respect to the modulating signal. >> :-) no troll intended really ...... > Sure. Even a plate-modulated class-C amplifier is linear in that respect. (snip) >> [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".) > 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. See the data sheet for the MC1495. It comes out pretty fast on a google search, from sites like www.alldatasheet.net. -- glen
From: glen herrmannsfeldt on 3 May 2010 17:18 Clay <clay(a)claysturner.com> wrote: (snip) > Actually you are modulating the number of photons emitted by the LED > per unit time. Each photon has energy E=h*frequency. So in the case of > where your LED is narrowband, then all of the photons have pretty much > the same energy. And in the case of a laser diode, then each photon's > wavefunctions are also in phase with each other's wavefunctions. Note that the output of AM and FM radio transmitters is also photons, still with E=h*frequency. So an AM transmitter, as with the LED, modulates the number of photons sent per unit time. Diode lasers have a fairly short (for a laser) coherence time, such that the phase can change with time. (Or consider that the bandwidth isn't so narrow as for gas lasers.) An individual photon, in the time or frequency EM domain, looks like a Gaussian envelope modulation of the appropriate sine. -- glen
From: Jerry Avins on 3 May 2010 18:03
On 5/3/2010 4:45 PM, glen herrmannsfeldt wrote: > Jerry Avins<jya(a)ieee.org> wrote: > (snip, someone wrote) > >>> Just to shake your cage: the 4 quadrant multiplier used in this mode as >>> a modulator is a linear system with respect to the modulating signal. >>> :-) no troll intended really ...... > >> Sure. Even a plate-modulated class-C amplifier is linear in that respect. > (snip) > >>> [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".) > >> 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. > > See the data sheet for the MC1495. It comes out pretty fast > on a google search, from sites like www.alldatasheet.net. I'm puzzled by a familiar circuit. How is that in any way magnetic? Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. ����������������������������������������������������������������������� |