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Radios, how do they work?
11–20 of 114 posts
Re: Radios, how do they work?
#12Would it be possible to construct a rudimentary FM radio receiver with only the most basic parts ala Masters of the Air?
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Re: Radios, how do they work?
#13I think this undersells the trick behind radio. Say we have the technology to broadcast a signal from an antenna to receivers, with some bandwidth B. Without getting clever, we can only send or receive one signal, since any others would interfere with each other. The trick is, can we do something to shift the bandwidth B to some other base frequency F such that B + F > B? Or B + (N - 1)F > B? And if we can do that, a…
> divide that bandwidth by frequency (like AM and FM) or time Ah. The real magic is when we separate by space (beyond just frequency or time). The ability to do this was discovered relatively recently, in 1996, by a guy called Foschini, though radio astronomers will say "Meh". By adding multiple antennas and doing space-time coding engineers found they could pump an order of magnitude more data through a radio channe…
Fascinating how we keep being inspired by fundamental physics and astronomy to keep cramming mode information in our channels. I'm still trying to understand Orbital Angular Momentum multiplexing https://en.m.wikipedia.org/wiki/Orbital_angular_momentum_mul...
Re: Radios, how do they work?
#14Would it be possible to construct a rudimentary FM radio receiver with only the most basic parts ala Masters of the Air?
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
Re: Radios, how do they work?
#15Earlier quoted context omitted.
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
If you are building an AM crystal radio. [1] You will also need a high-impedance speaker [2] if you want to operate it without a power supply, otherwise you will need an amplifier. You can avoid using a commercial diode by making your own point contact diode as done in Foxhole radios [3] and you can make your own piezoelectric speaker from Rochelle salt [4]. Here [5] is one personal projects site touching all those topics.
In conclusion, you should be able to build a simple radio from copper wire, aluminium foil, a pencil, a razor blade, and baking powder.
[1] https://en.wikipedia.org/wiki/Crystal_radio
[2] https://en.wikipedia.org/wiki/Crystal_earpiece
[3] https://en.wikipedia.org/wiki/Foxhole_radio
[4] https://en.wikipedia.org/wiki/Potassium_sodium_tartrate
[5] https://rimstar.org/science_electronics_projects/index.htm#S...
Re: Radios, how do they work?
#16Earlier quoted context omitted.
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
By coiling wires separately to form an inductor
Re: Radios, how do they work?
#17Earlier quoted context omitted.
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
Very high impedance transducer, and very low forward voltage diode.
Re: Radios, how do they work?
#18Earlier quoted context omitted.
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
A Foxhole radio was often made from a coil of wire (inductor), a razor blade and pencil lead (diode):
https://en.wikipedia.org/wiki/Foxhole_radio
> The aerial is connected to the grounded inductor. The coil has an internal parasitic capacitance which, along with the capacitance of the antenna forms a resonant circuit (tuned circuit) with the inductance of the coil, resonating at a specific resonant frequency. The coil has a high impedance at its resonant frequency, and passes radio signals from the antenna at that frequency along to the detector, while conducting signals at all other frequencies to ground. By varying the inductance with a sliding contact arm, a commercial crystal radio can be tuned to receive different frequencies. Most of these wartime sets did not have a sliding contact and were only built to receive one frequency, the frequency of the nearest broadcast station. The detector and earphones were connected in series across the coil, which applied the radio signal of the received radio station. The detector acted as a rectifier, allowing current to flow through it in only one direction. It rectified the oscillating radio carrier wave, extracting the audio modulation, which passed through the earphones. The earphones converted the audio signal to sound waves.
Re: Radios, how do they work?
#19I think this undersells the trick behind radio. Say we have the technology to broadcast a signal from an antenna to receivers, with some bandwidth B. Without getting clever, we can only send or receive one signal, since any others would interfere with each other. The trick is, can we do something to shift the bandwidth B to some other base frequency F such that B + F > B? Or B + (N - 1)F > B? And if we can do that, a…
And for those even deeper into the theory, one question you might ask is, if we can divide spectrum and time to get some bandwidth B per channel, how many bits can we send/receive over a distinct channel? The answer is C = Blog2(1 + S/N) where B is the bandwidth and S/N is the signal to noise ratio determined by the environment (how much noise is present relative to the signal being transmitted). The crazy thing is t…
Shannon was pretty ridiculous. He basically invented information theory, proved all the major theorems involved, and applied it to communications and error-correction codes. If you work in RF you can't do much without encountering his work. (It did take a while before anyone figured out how to get close in practice to the limits he proved, though)
Re: Radios, how do they work?
#20Earlier quoted context omitted.
AM is quite easy (a diode and a capacitor can be enough), an FM receiver need a local oscillator that require some active elements (transistors) and a more complex circuit.
Under what circumstances is a diode and a capacitor enough to make a radio receiver?
AM peak detector is probably the easiest and primitive AM demodulator: it's basically made by a diode, a capacitor and a resistance. I implemented it when I was in the high school and I was making the first physics experiments.
The idea behind this demodulator is quite easy: the diode filters out all of the negative part of the signal, then the positive signal charge the capacitor and the energy is released in a quite constant way (R*C must be several order of magnitude higher than 1/f where f is the carrier frequency) during the negative signal "hole".