How AM and FM Works

and transmitter. The information or intelligence to be transmitted starts as found waves which travel through air. The microphone picks up these waves and transforms them into electrical energy. Now the intelligence travels into the transmitter, taking the form of audio frequency voltages. Audio frequencies generally fall within a range of from 300 to 3000 cycles per second.

However, it is not practical to transmit these audio frequencies directly. An oscillator provides a much higher frequency. This radio frequency wave is called a carrier by means of a modulator and one or more RF amplifiers.

The amplitude variations of the audio signal and the radio frequency carrier are combined. Now the intelligence takes the form of amplitude variations in the carrier. The amplitude modulated RF carrier is fed to the antenna from which it is radiated through space through the receiving antenna.

Once the signal has left the antenna, however, it can be affected by many disturbances. For example, one set of disturbance can be lightning. Let's freeze the signal at the receiver antenna and see what the interference has done to it.

Our primary concern is with the amplitude variations, for they carry the intelligence. To a point, the variations are just as we transmitted them, and the AM system was working fine. But the lightning has added new variations.

These are superimposed upon the variations carrying intelligence. There are many man-made devices and natural elements or occurrences that can affect the amplitude of the RF carrier. Each one adds more hash, more static, more interference.

The receiving antenna will pick up both the composite RF waves from the transmitting antenna and the random noises generated by the lightning. These two will combine and distort the amplitude of our desired signal. Now a partial review of what happens in the AM receiver.