What is the amplitude of a photon? I read somewhere that amplitude is meaningless in this context, but if that is the case, then how is amplitude modulation possible?
I was reading (in a Tom Clancy novel - sue me) that bandwidth in the ELF spectrum was about 1 character every 30 minutes or so. Why so low and how is the signal modulated?
Ok, so maybe that is more than two questions. Please fight ignorance anyway.
Thanks for your help,
Rob
As you note, the amplitude of a photon is not a meaningful concept. Amplitude modulation doesn’t change anything about individual photons, but rather it works by changing the number of photons emitted over time. Lots of photons = high amplitude; not so many photons = low amplitude.
I’m sure you must have misread that information (or Tom uncharacteristically screwed the pooch quite severely). ELF occupies the EM spectrum between 3 and 30 Hz. Wiki indicates that it could support data rates of a few characters per minute.
That is why I didn’t understand why it wasn’t meaningful. Doesn’t the above mean that each individual photon has a particular amplitude?
It may have been 30 seconds, not 30 minutes.
Thanks,
Rob
No, each photon has a particular energy. That’s not really the same thing.
That sounds a little closer to the mark, although it’s probably still on the low side.
BTW, the wiki article didn’t mention how the signal was modulated. Also, was it an analog or a digital signal?
Thanks,
Rob
IIRC, energy is proportional to wavelength but has nothing to do with amplitude. Does that mean that the amplitude being modulated in an AM signal is the voltage or current amplitude? Is that also true with FM modulation?
Thanks,
Rob
Right, for photons, wavelength is related to energy by Planck’s constant, h-bar. Basically, the shorter the wavelength, the higher the energy.
When we speak of “amplitude” in relation to a radio signal, what we really mean is power level. At the fundamental level, more power is the result of more photons being transmitted during a given timespan.
No, FM is frequency modulation; the power level (amplitude) remains constant, while the frequency varies. This does mean the energy of the individual photons making up the radio signal changes, as well, but the difference in energy in a typical FM broadcast is too small to be significant.
It’s a digital signal. There’s more information on this page, which was linked to in the previously posted Wiki article.
(Emphasis added.)
Well, it’s not a *requirement * that it remain constant. It might be more correct to simply say that, when using frequency modulation, the amplitude is not a variable in the modulation scheme. Theoretically speaking, the receiver is immune to amplitude changes in the signal. In reality, though, amplitude *does * play a part in FM transmission due range and SNR issues.
Tha amplitude of a photon is meaningful, if you are already thinking about the strength of a beam of light or of a radio transmission. The beam and the transmission are both radiating energy. There is some total amount of energy, and it can be varied, but only in steps. It is quantized, as they say. The quanta, the steps in which a light beam or radio transmission can be varied in energy, are what photons are. This elaborates on Q. E. D’s “At the fundamental level…” sentence.
You can make electrical frequencies as low as you want. You could even conceive of what mathematically was clearly a fraction of a cycle of a sine wave, but that had been occurring over the life of the universe. Stars and planets that have a magnetic field not parallel to their rotational axis emit EMR whose frequency is the inverse of their rotational period.
>Does that mean that the amplitude being modulated in an AM signal is the voltage or current amplitude?
Typically, there’s a power amplifier sending signal along a cable to the antenna, and the antenna couples cable signal with free signal. Antennas have some impedance, which is a chatacteristic voltage to current ratio, but it is more complicated than simple resistance because there’s generally some shift between the current and voltage waveforms, and this complicated impedance is a function of frequency but not of power. Doubling the voltage the amplifier puts out also doubles the current it has to put out, given that it’s feeding something whose voltage to current ratio is constant. So, both voltage and current would be modulated, and the power would be proportional to the product of the two. This is a bit misleading - typically they’d design an amplifier to modulate the voltage or the current, and it’s the behavior of the antenna (in part) that chooses the other one.
How meaningful would a photon be for such a low frequency? It seems to me that the location for such a photon would be effectively undefined, though I guess that would depend on how much you knew about the wavelength.
I think that’s sort of the point of ELF for communicating with submarines. The wavelength is so long that little things like the Earth and ocean don’t impede its progress much.
As I recall, Tom Clancy has a sub receiving a rather complex message, but in fact all they can do in any reasonable time is simple three-character codes. For anything more complex, they just would probably just send the code for “surface and call home.”
My two cents on AM vs. FM: In a sense, FM modulates the amplitude (energy) of the photons, but it’s called FM because we think of that particular amplitude as the signal’s frequency (or wavelength). AM modulates the signal’s strength, i.e., the number of photons you generate from one moment to the next.
>The wavelength is so long that little things like the Earth and ocean don’t impede its progress much.
Yes, something like that, I guess. There are, to hear hams tell it, two different kinds of electromagnetic radiation - the electric field wave and the magnetic field wave. Generally they go together, and it’s the fact that charge variously appears at one end of the antenna versus the other that causes changes in the electric field to radiate away, and it’s the fact that the charge is traveling along the length of the antenna that causes changes in the magnetic field to radiate away. The two fields work together in a very straightforward way at big distances from the antenna and other conductive objects, I think, but they ways in which the two fields are different very close to the antenna are important enough that antenna designers think about them. At least, that’s the impression I got from reading books like the ARRL Antenna Design Guide, which is really a very fun book and a worthy browse for anyone who likes technology even if they’ve no interest at all in radio.
So, in the case of submarines, I’m speculating that they might be using ELF because its magnetic field can travel through ionically conductive saltwater. But I don’t know. Maybe we need to find a Smith chart or something…
Y’all may ignore this hijack if you wish, but this made me think of something.
Is it possible to amplitude-modulater an FM carrier, and thus put another signal on the carrier? You’d have to limit the AM modulation to, say, fifty percent since with AM 100% modulation (on the positive side of the modulating signal) essentially clips the carrier, which would mess up the FM side of the equation. I’m guessing nobody ever figured out a way to make an AM receiver ignore carrier frequency variations.
Yep.