1. Modulation

2. Carrier waves We have seen already that ELECTROMAGNETIC WAVES travel from their source around the world in a variety of ways according to their frequency and strength of magnetic field. In a basic form these waves are called CARRIER WAVES. They can be altered to carry data from point to point rather than being an ordinary sine wave.

3. Modulation Carrier Keying –The simplest way of modulating information into the carrier wave is to switch it on and off according to a code which is understood by the sender and receptor. Morse code is a good example of this.

4. Modulation Amplitude Modulation (AM) –Audio frequency signal waves vary the amplitude of the radio frequency (RF) carrier wave. The frequency of the carrier remains constant. –Depth of modulation = V S x 100% VCVC If V S ever equals V C then depth of modulation will be 100%

5. Modulation Amplitude Modulated carrier wave VSVS VCVC V C is the amplitude of the unmodulated carrier and V S the amplitude of the audio signal. The amplitude of the audio signal should be less than that of the RF carrier. Depth of modulation indicates their relative amplitude. Distortion occurs if depth of modulation reaches 100%.

6. AM Transmitter AUDIO FREQ AMPLIFIER MODULATOR RADIO FREQ AMPLIFIER OSCILLATOR AM Transmitting system This provides the radio broadcast frequency each station operates at MIC Each box is a modular process

7. Modulation Frequency Modulation (FM) –In FM the audio signal varies the frequency of the carrier signal. This time it is the amplitude of the carrier wave that remains constant.

8. Comparison of AM and FM The major disadvantage of FM over AM is the much wider bandwidth requirement. This disadvantage is compensated by the improved performance of FM. FM signals are transmitted at constant amplitude and are therefore immune to noise pickup in the transmission medium. They are also more efficient in terms of transmitter power requirements.