What are crystals used for in radio? What are crystals used for in radio? I think I sort of know the answer to this but I would like a more detailed explanation. To my current understanding, piezoelectric crystals are used to modulate between mechanical energy and electric energy.
I may not have stated that entirely correctly but what it essentially does is take a sound wave and convert it to an electrical signal to be broadcast. Then, on the receiving end, a crystal of similar frequency is used to reverse this process, turning electrical waves into mechanical waves that can then be made audible?
So, the crystals aren't actually used to receive any signal, correct? What I mean by that is that the signal is taken in by the antenna, not any crystal? Don't the crystals determine the carrier frequency being broadcast on though?
What got me interested in this is I read that there are piezoelectric crystals in the inner ear. Would it be possible to broadcast an RF signal to the inner ear? Trying to get the details of RF systems down.
 A: John Rennie is correct, the crystal serves to act as a diode but furthermore to function as a detector component for an amplitude modulated radio signal. A detector detects the modulated signal from the carrier that's received from the antenna and amplified by a resonant tank (coil and capacitor). The detector or diode provides half wave rectification; a preservation of positive signal and rejection of negative. This results in the positive half of the envelope of the carrier wave remaining. Subsequently a capacitor smooths the peaks of the carrier within the carrier envelope resulting in the audio communication signal.
The earliest hobby crystal radios used a crystal of galena and a 'cat's whisker' contact terminal, a sharp metal wire used to contact a single point on the crystal which was usually supported below by a metal ring to serve as the other contact terminal. I believe the crystal and its effectiveness as a diode was anisotropic, meaning that you had to rotate, position the crystal in the right orientation for maximum effect.
Later the crystal radios used germanium diodes as the detector.
And John also correct - no piezoelectric components in the ear. Sound modulates cliliary-like structures within the cochlea of the inner ear and these structures stimulate nerve endings. The cochlea serves sort of serves as a spectrum analyzer where specific sound frequencies stimulate specific areas in the cochlea. The brain combines/sorts out all of these individual signals to comprise intelligible sound.
I don't know of any RF devices. But I do know that you can transmit an ultrasonic sound carrier signal with audible sound into the skull and demodulate the sound virtually 'inside the head'. Not sure if this would allow the deaf to 'hear' but it does have other applications. Google 'Hypersonic Sound', 'Holosonics' and 'American Technology Corporation' to learn more.
A: These answers are correct, so far as they go, but it must be correctly noted that we are discussing two different types and uses of crystals. The two are not interchangeable. 
The piezoelectric crystal is usually made of quartz. The material is cut, like a diamond, in  a particular plane or direction to give it a particular mode of operation, but this is a technical matter and I’m taking enough space and time just discussing this. The key to its operation is that its physical dimensions determine its resonant frequency. As this should remain fixed throughout its existence the resonant frequency of its molecular vibrations must remain  constant within a tight tolerance, making it valuable as a frequency reference in radio. Passing too much electrical power through it can cause damage, and the resonant frequency can be affected by extreme ambient temperature changes but in most uses it is one of the most stable circuit-tuning devices available.
The “crystal radio” type of crystal is of a different material, often galena (lead sulfide). This was one of the first semiconductors in general use. A crystal set would use as long a wire for an antenna as could be had, as every radiowave photon as possible must be gathered. Unless you were located very near an (AM) transmitter you would note that the signal strength of a programme falls off with distance. Also if you are in a strong signal (metropolitan) area the selectivity between stations isn’t that great but back in the day there were fewer of them. A stiff piece of wire colloquially called a “cat whisker” would be used to probe the crystal to find the best spot for signal pickup. It was often left up to the user to fasten it into place. High impedance headphones would be used to listen in. 
This rather finicky system fell out of favour once amplifying tubes were developed to boost the signals. Not until the discovery of germanium and silicon semiconductor junctions did “solid state” detection return
A: Another use for crystals in a radio is in a crystal oscillator.
A crystal oscillator uses the piezoelectric effect to couple the mechanical vibrations of a small slab of crystal to the oscillations of an electric circuit.
Back when I was a kid, you changed the channel(s) on which a two-way radio operated by changing out pairs of pluggable crystals.
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In modern radio equipment, there usually is a single crystal, soldered to the circuit board, that serves as the timebase for a frequency synthesizer that can be programmed to generate any desired radio frequency.
