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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
For a remote control project, I am interested in building a simple AM radio receiver like those used on toy cars. I would like to use a pair of crystals to do the tuning of the transmitter and receiver circuits. The transmitter is easy, Ill just use a simple circuit like a collpits oscilator. The receiver, however, really confuses me. I have no idea how to take a simple crystal and make a tuned circuit with it to receive a radio signal, and then say, turn on a transistor whenever a signal is present on the crystals resonant frequency. Any help?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Given the cost of a used Radio-control transmitter and receiver for toys or new 2.4 GHz transmitter/receiver pairs from eBay, it's not worth the time and effort designing/building.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, since half of the moderating team are radio hams, we do encourage the learning of RF electronics It is difficult to use a crystal in the way you describe, so it's not usually done that way at all. Instead, the crystal in the receiver is made a slightly different frequency to the one in the transmitter, and used as an oscillator (which we call the local oscillator, or LO)
The signal from the antenna is then combined with the LO in a "mixer", and what comes out is the difference between the two frequencies, amongst other things. The difference, or intermediate frequency (IF) is then filtered, amplified, and detected using a rectifier or whatever.
This is the classic superhet receiver, but it may well be too complicated for your purpose. The simplest receiver is a TRF (tuned radio frequency) which is just an RF amplifier with a tuned circuit that resonates at the frequency you want to receive. Like a crystal set, but with an amplifier between the coil thingy and the diode.
PS. As far as I know, the sets of crystals you buy to change the frequencies on your radio controlled model are different for the transmitter and receiver, because the receiver is a superhet. If you took the receiver crystal and put it in the transmitter, and put the transmit crystal in the receiver, it would probably stop working.
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Simple fixed frequency AM radio receiver
It is difficult to use a crystal in the way you describe, so it's not usually done that way at all. Instead, the crystal in the receiver is made a slightly different frequency to the one in the transmitter, and used as an oscillator (which we call the local oscillator, or LO)