29 Apr '16 11:41>
Originally posted by sonhouseThis one is cool too.
https://www.youtube.com/watch?v=ayovPr_WjSM
Originally posted by lemon limeSo all you need for a tuner is an atomic force microscope then.....
I couldn't find a youtube video for this, but a few years ago Scientific American had a story about the worlds smallest radio. It was so tiny you had to look through a microscope to see it. The only components were one carbon nanotube connected to a power source and tiny amplifier. The speaker was probably visible... I'm working off memory here so I'm not ...[text shortened]... une in one frequency, and whatever frequency it could tune in depended on how the tube was bent.
Originally posted by lemon limeFor amplitude modulated signals the carrier wave is filtered out with a capacitor. The diode provides rectification, without it the loudspeaker will just see a high frequency AC signal which it can't respond to and not do anything. For FM it's more complex, but one way of getting the signal off the carrier wave is to use a tuning circuit as the voltage response is frequency dependent and more or less linear near the resonant frequency. I'd guess that the nanotube does its filtering this way.
I couldn't find a youtube video for this, but a few years ago Scientific American had a story about the worlds smallest radio. It was so tiny you had to look through a microscope to see it. The only components were one carbon nanotube connected to a power source and tiny amplifier. The speaker was probably visible... I'm working off memory here so I'm not ...[text shortened]... une in one frequency, and whatever frequency it could tune in depended on how the tube was bent.
Originally posted by DeepThoughtDoes the same thing happen in a simple crystal radio set, does the crystal provide rectification without removing the carrier wave? If so then how is the carrier wave removed in a simple crystal radio receiver before reaching the speaker? Is it the tuner that does that?
For amplitude modulated signals the carrier wave is filtered out with a capacitor. The diode provides rectification, without it the loudspeaker will just see a high frequency AC signal which it can't respond to and not do anything. For FM it's more complex, but one way of getting the signal off the carrier wave is to use a tuning circuit as the voltage ...[text shortened]... ss linear near the resonant frequency. I'd guess that the nanotube does its filtering this way.
Originally posted by lemon limeThe diode does half-wave rectification - it cuts off the negative voltage. Handily there's a circuit diagram on wikipedia on the page [1], take a look at the diagram [2]. The diode D1 does half wave rectification and the capacitor C2 allows high frequency through but not the low frequency signal - it acts as a low pass filter. As it happens you can get away without the capacitor C2 as the speaker in the headphones can't respond to the high frequency of the carrier wave so it will tend to just follow the envelope.
Does the same thing happen in a simple crystal radio set, does the crystal provide rectification without removing the carrier wave? If so then how is the carrier wave removed in a simple crystal radio receiver before reaching the speaker? Is it the tuner that does that?
I once built my own little version of a crystal radio set using parts from a small ...[text shortened]... or Fm. The switch was set to Am, but for some reason I was getting a station broadcasting on Fm.
Originally posted by DeepThoughtI'm not exactly an electronics expert myself, basically all I was doing was following the same design templete of a toy crystal radio set. I later learned we were living very close to a transmission tower, so that's probably why I was able to tune in that one station.
The diode does half-wave rectification - it cuts off the negative voltage. Handily there's a circuit diagram on wikipedia on the page [1], take a look at the diagram [2]. The diode D1 does half wave rectification and the capacitor C2 allows high frequency through but not the low frequency signal - it acts as a low pass filter. As it happens you can ge ...[text shortened]... ttps://en.wikipedia.org/wiki/Crystal_radio#/media/File:Crystal_radio_with_impedance_matching.svg
Originally posted by lemon limeActually, the capacitor works in conjunction with the coil to respond to a certain frequency range, hopefully in the AM broadcast band, 550 to 1700 Khz. Crystal sets don't have a lot of "Q" and so are really wide banded so you can't expect to separate two stations 40 Khz apart. The diode DOES the demodulation because it cuts off the bottom or top half of the modulated CW wave, modulated with AM so the audio is riding on the carrier wave, which would be at something like 1000 Khz, for example.
I'm not exactly an electronics expert myself, basically all I was doing was following the same design templete of a toy crystal radio set. I later learned we were living very close to a transmission tower, so that's probably why I was able to tune in that one station.
If I used a tin can attached to a wire running to the metal stem of an umbrella, I might have been able to hear that same station... that's how close we were to that tower.
Originally posted by sonhouseIt's possible because mammalian ears are so sensitive. You can make a just-about-functional radio with some headphones, copper wire, a rusty razor blade, and a needle (the aerial will provide a bit of capacitance). We seem to have a slight difference of opinion on what constitutes demodulation - I'd regard the capacitor C2, or just the headphones, as doing the demodulation because after it's gone through the diode the carrier wave is still present.
Actually, the capacitor works in conjunction with the coil to respond to a certain frequency range, hopefully in the AM broadcast band, 550 to 1700 Khz. Crystal sets don't have a lot of "Q" and so are really wide banded so you can't expect to separate two stations 40 Khz apart. The diode DOES the demodulation because it cuts off the bottom or top half of t ...[text shortened]... with the zener diode and then go to a simple one transistor audio amp but that is another story.
Originally posted by DeepThoughtThe key is the carrier wave, although technically still there, it is now pulsating DC and now can do some work using the modulation. The earphones could not vibrate very much at 1 megahertz but can at 1 kilohertz. The diode is the demodulator that allows that to happen. The capacitor is either in series with the coil or in parallel with it, either way it is an LC circuit which will have its frequency response curve somewhere, hopefully in the 1 megahertz band.
It's possible because mammalian ears are so sensitive. You can make a just-about-functional radio with some headphones, copper wire, a rusty razor blade, and a needle (the aerial will provide a bit of capacitance). We seem to have a slight difference of opinion on what constitutes demodulation - I'd regard the capacitor C2, or just the headphones, as d ...[text shortened]... ng the demodulation because after it's gone through the diode the carrier wave is still present.
Originally posted by sonhouseRight, I agree that a crystal set won't work without a diode, but you've missed what I'm saying slightly. There are two capacitors. One is in the tuning circuit, that is not the one I'm talking about. I'm thinking of a second one, in parallel with the earphones and after the diode whose job is to filter out the carrier wave. It's not absolutely necessary but stops the earphones trying to respond to the carrier wave. It's labelled C2 in the diagram in link [2] from my earlier post.
The key is the carrier wave, although technically still there, it is now pulsating DC and now can do some work using the modulation. The earphones could not vibrate very much at 1 megahertz but can at 1 kilohertz. The diode is the demodulator that allows that to happen. The capacitor is either in series with the coil or in parallel with it, either way it is ...[text shortened]... Here is a wiki about the diode used as demod:
https://en.wikipedia.org/wiki/Envelope_detector
Originally posted by DeepThoughtYes, that would work. It's called a bypass cap. An earphone would never be able to respond to megahertz signals so it is really not needed. But the diode is what does the actual demod. Did you see my link?
Right, I agree that a crystal set won't work without a diode, but you've missed what I'm saying slightly. There are two capacitors. One is in the tuning circuit, that is not the one I'm talking about. I'm thinking of a second one, in parallel with the earphones and after the diode whose job is to filter out the carrier wave. It's not absolutely neces ...[text shortened]... respond to the carrier wave. It's labelled C2 in the diagram in link [2] from my earlier post.