Amplitude modulation -- misnamed?
in [DSP]
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From: Mark on 3 May 2010 18:13 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. > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ If I recall correctly, the electrical to light conversion is typically non-linear and a 1 dB change in the electrical input to the modulator created a 2 dB change to the light (or was it the other way round)? And vice versa at the receiver so when you got your electrical signal back, all was well. But if you look at it in the light domain, it was NOT the same as if it were simply a very high frequency RF carrier. Mark
From: Clay on 3 May 2010 18:33 On May 3, 5:18 pm, glen herrmannsfeldt <g...(a)ugcs.caltech.edu> wrote: > Clay <c...(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 It all comes down to detecting energy although we see the effects of amplitudes via interference. You would be suprised at the coherence length of a diode laser these days. Since they tend to operate in single mode you can make holograms with a depth 100s of time greater than the diode's cavity size. While gas lasers due to longer cavities exhibit narrow spectra, the gain envelope oftens spans more than one mode of the cavity. For example an HeNe laser will be single mode if its cavity is around 10cm in length, but such a laser will barely osc. My 10mW HeNe has multiple modes that caused coherence beats every 27 inches (twice the cavity length). Clay
From: glen herrmannsfeldt on 3 May 2010 19:17 Jerry Avins <jya(a)ieee.org> wrote: > On 5/3/2010 4:45 PM, glen herrmannsfeldt wrote: >> Jerry Avins<jya(a)ieee.org> wrote: >> (snip, someone 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".) >>> 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? Sorry, I wasn't trying to reference the magnetic version. Putting magnetic modulator into google results in a good number of hits, mostly using saturating core (and so nonlinear) transformers. One reference is to books.google.com for a book on microwave tube transmitters. -- glen
From: Fred Marshall on 3 May 2010 20:18 Jerry Avins wrote: > 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 hmmmmm ... why do I think we've segued into a chopper application to get around dc amps. Not the same thing..... Fred
From: Fred Marshall on 4 May 2010 15:29
Jerry Avins wrote: > On 5/3/2010 4:45 PM, glen herrmannsfeldt wrote: >> Jerry Avins<jya(a)ieee.org> wrote: >> (snip, someone wrote) >> > I'm puzzled by a familiar circuit. How is that in any way magnetic? > > Jerry The new ones aren't magnetic. The ones I worked with in the early 60's were. Then around 1968, a fellow named Barrie Gilbert (at Tektronix at the time and then Analog Devices) invented an integrated circuit version. It beat the heck out of one that I built. At issue is always the nonlinearities that you don't want. At about the same time that Gilbert was publishing his multiplier, I was trying to build a more discrete semiconductor version with dual/matched transistors. I didn't work where there was a fab and had to buy parts off the shelf. I used a matched amplifier approach borrowed from vacuum tube designs. You took two matched amplifiers and varied their gain with a more or less low frequency signal "X". One amp had a constant input and the other had the "Y" input. The one with constant input generated an error signal that was applied to both amplifiers so that the gain variation with "X" on "Y" would be linearized. I must say it wasn't very good but it worked to a point. Gilbert describes what he did with bipolar junction transistors here: http://www.ieee.org/organizations/pubs/newsletters/sscs/jan03/jssc_classic.html Fred |