The Crystal Radio Receiver
We are going to use a very simple crystal radio for this explanation. Figure
below shows a very simple crystal set. A small amount of energy
from the radio wave is captured by the antenna wire and is taken to the coil.
The coil has to be designed just right to capture only the frequency we are
trying to receive. In our case we are trying to receive our radio station above
at 610 kHz. By winding just the right amount of wire on just the right diameter
coil form, the coil will be what we call "resonant" and
"ring". In other words it will be able to store the energy of
the radio wave we want to hear. All other radio waves not
"resonant" will pass through the coil and out the other
side to the ground.
A small amount of the radio wave energy stored in the coil (our 610 kHz or 610,000 cycles per second) moves to the detector or the device called a diode. The energy is an alternating current signal (AC) at this point. The detector (diode) rejects half of the alternating current signal and the signal looks like figure #5 below. Now the signal is a pulsating direct current (DC) signal.
This allows the earphone to use the energy. If both sides of the wave were used, they would cancel each other out as they are opposite of each other. As this energy go into the earphone, the amplitude or strength of the signal varies because the wave is "modulated". This energy is converted by mechanical means in the internal workings of the earphone. The sound waves exit the earphone which you perceive as the original sounds from the radio station.
Not discussed here is tuning the coil to get different frequencies. That can be done by adding more turns of wire around the coil form. Or removing some. On a simple radio, this is done by moving the diode up or down the taps on the radio in effect making the coil longer or shorter. This changes the "inductance" of the coil or makes it resonant to different frequencies.
Another way to do this is to add a variable capacitor across the coil. This adds or removes capacitance to the coil and changed the resonant frequency the coil will tune to.
