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.
Best Answer
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.