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This is a "crystal radio", an AM radio receiver with no amplifier.  The raw antenna feed is shown in the first scope slot in the lower left.  The inductor and the capacitor C1 are tuned to 3 kHz, the frequency shown in the lower right as "res.f".  This picks up the carrier wave shown in the middle scope slot.  A diode is used to rectify this, and the C2 capacitor smoothes it out to generate the broadcast signal in the last scope slot (which is simply a 12 Hz sine wave in this example).  By experimenting with the value of C1's capacitance, you can pick up two other "stations" at 2.71 kHz and 2.43 kHz.

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Generated Tue Feb 23 2010