How do the piezoelectric crystals work with copper antennas in CB radios?
In: Physics
Crystals are like little high frequency tuning forks. They like to resonate at just one frequency which is determined by their size and shape. They provide the local oscillator (LO) frequency, either directly like in old CBs (one crystal/channel) or as part of a programmable oscillator. The LO is combined with the incoming signal in a mixer which gives a difference frequency in a process known as heterodyning. That difference is always the same frequency, usually 455 kHz for AM/CB. It is called the IF (intermediate frequency). So to receive 30.000 MHz an LO of 30.455 MHz is used and the difference is 0.455 MHz. The advantage is that the IF amplifier is always working at the same lower frequency and can be optimized. This is a simplified description believe it or not.
TLDR the crystals help to tune the radio. The antenna is another subject.
Piezoelectric crystals work two ways. You can apply mechanical stress, and they will create a voltage. Or you can apply a voltage, and they will mechanically deform.
A crystal in an oscillator circuit uses the latter of these. If you carefully make the crystal, it will have a frequency that it “wants” to resonate at. So if you have a 15.315 MHz crystal, for example, you put it in an unstable circuit. It will quickly (if the circuit is designed right) make the whole circuit oscillate at the crystal’s natural frequency of 15.315 MHz.
This doesn’t really have much to directly do with the antenna in a radio. The crystal provides a fairly stable reference frequency that the radio circuits can then use for tuning.
IIRC, some old-fashioned CB radios will use multiple crystals to tune in specific frequencies more directly, but that would be an unusual thing to do these days.
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