I've been playing with a crystal radios, and I have question about the role of the LC circuit used to "tune" the radio. I understand some of the basics, in that a tank circuit's natural frequency, called the resonant frequency, is determined by the size of the inductor and the size of the capacitor. What don't understand is how the matching the resonant freqency helps tune in a station. Does it block freqs out or ampifiy them?

I'm doing a science fair project on Detecting Lighting. I'm trying to use the fact that lighting produces signal bursts over the AM band. I built a crystal set tuned to the upper end of the AM band and then compare the average output looking for and counting bursts of loud static.

Thanks for any help

 

Usually the inductor is fixed and you tune the capacitor. Basically radio stations/tv stations/etc each transmit their signal at a certain frequency. The government has a department that allocates bandwidth (frequencies) for stations to use. In other words, you can't just transmit at any frequency, you have to go to the government and get it registered.

If I want to listen to a radio station which is transmitting at 100kHz, I have to tune my tank so that it's resonant frequency is 100kHz. Then I can pick it up. If I have my tank tuned to 90kHz I probably won't hear it or will get a very low signal.

Basically tanks are filters. You tune the filter so that you get the channel you want and filter out all the ones you don't want. They're not perfect, you can think of the Q of a tank as it's quality. High Q tanks are very sharp filters so when you have them tuned to 100kHz and you can hear the signal well and then you slightly turn the knob - you will most likely hear nothing. On the other hand if you have a really low Q tank, you may be tuned to 100kHz and you will be picking up channels at 110kHz and 90kHz and everything in between! :unsure:

 

THERE ARE TWO MAIN TYPES OF L-C TUNED CIRCUITS.ONE IS PARALLEL TUNED AND THE OTHER IS SERIES TUNED.FOR BOTH OF THESE CIRCUITS TO BE AT RESONANCE THE INDUCTIVE REACTANCE AND CAPACITIVE REACTANCE MUST BE EQUAL.AT RESONANCE A PARALLEL TUNED CIRCUIT HAS A VERY HIGH IMPEDANCE,BUT AT ALL OTHER FREQUANCIES ITS IMPEDANCE IS SIMPLY THE DC RESISTANCE OF THE COIL.THE Q FACTOR OF THE CIRCUIT RELATES TO HOW TIGHTLY TUNED IT IS.A SERIES TUNED CIRCUIT HAS A VERY LOW IMPEDANCE AT RESONANCE AND A VERY HIGH IMPEDANCE AT ALL OTHER FREQUANCIES.THE FORMULA FOR WORKING OUT VALUES OF L AND C IS:- ONE DIVIDED BY 2 PIE ROOT LC.