The original method of electronically encoding sound information on radio waves is called amplitude modulation, or AM. It was developed in the first few decades of the twentieth century. AM is a relatively simple way to add audio information to a carrier wave so that sounds can be transmitted.
One of the simplest forms of AM modulators simply runs the power supply for an oscillator circuit through an audio transformer that is coupled to a microphone or other sound source.
This circuit uses a 1 MHz crystal oscillator, which is often used to generate the clock frequencies for microprocessor circuits. 1 MHz is perfect for a simple AM transmitter circuit because 1 MHz falls right in the middle of the band that’s used for AM radio transmissions.
Although you can’t buy a crystal oscillator at your local RadioShack, you can get it on the Internet. Use Google to search for 1 MHz crystal oscillator, and you should find several online sources that will sell you one for under $2.
The crystal oscillator is contained in a metal can that has three pins. One pin is for ground, the second pin is the supply voltage (typically 9 VDC), and the third is the oscillator output.
By running the Vss supply through the secondary coil of a transformer whose primary coil is connected to an audio input source such as a microphone, the actual voltage supplied to the oscillator will fluctuate based on the variations in the input signal.
Because crystal oscillators are very stable, these voltage variations won’t affect the frequency generated by the oscillator, but they will affect the voltage of the oscillator output. Thus, the audio input signal will be reflected as voltage changes in the oscillator’s output signal.
A better AM modulation circuit uses a transistor. In this circuit, the carrier-wave generated by an oscillator that isn’t shown in the circuit is applied to the base of a transistor. Then, the audio input is applied to the transistor’s emitter through a transformer. The AM signal is taken from the transistor’s collector.
So how does this circuit work? The transistor amplifies the input from the oscillator through the emitter-collector circuit. However, as the audio input varies, it induces a small current in the secondary coil of the transformer. This, in turn, affects the amount of current that flows through the collector-emitter circuit. In this way, the intensity of the output varies with the audio input.
As you can see, the carrier wave is a constant frequency and amplitude. In other words, each cycle of the sine wave is of the same intensity. The current of the audio wave varies, however.
When the two are combined by the modulator circuit, the result is a signal with a steady frequency, but the intensity of each cycle of the sine wave varies depending on the intensity of the audio signal.