A. 121.5 MHz carrier B. Varying DC audio information C. Amplitude modulated carrier leaving transmitter + − 0 + − 0 A. Amplitude modulated carrier in receiver B. Detected modulated carrier C. Demodulated signal D. Audio frequency signal in speaker + 0 + 0 + 0 + − 0 11-35 Figure 11-80.  Demodulation of a received radio signal involves  separating the carrier wave from the information signal.  Figure 11-79.  A DC audio signal modifies the 121.5 MHz carrier  wave as shown in C. The amplitude of the carrier wave (A) is  changed in relation to modifier (B). This is known as amplitude  modulation (AM).  AM has limited fidelity. Atmospheric noises or static alter  the amplitude of a carrier wave making it difficult to separate  the intended amplitude modulation caused by the information  signal and that which is caused by static. It is used in aircraft  VHF communication radios.  When the modulated carrier wave strikes the receiving  antenna, voltage is generated that is the same as that which  A radio wave can be altered to carry useful information by  modulating the amplitude of the wave. A DC signal, for example  from a microphone, is amplified and then superimposed over  the AC carrier wave signal. As the varying DC information  signal is amplified, the amplifier output current varies  proportionally. The oscillator that creates the carrier wave does  so with this varying current. The oscillator frequency output  is consistent because it is built into the oscillator circuit. But  the amplitude of the oscillator output varies in relation to the  fluctuating current input.  [Figure 11-79] modulated carrier wave then carries the information from the  transmitting radio to the receiving radio via their respective  antennas. Two common methods of modulating carrier waves  are amplitude modulation and frequency modulation. was applied to the transmitter antenna. However, the signal  is weaker. It is amplified so that it can be demodulated.  Demodulation is the process of removing the original  information signal from the carrier wave. Electronic circuits  containing capacitors, inductors, diodes, filters, etc., remove  all but the desired information signal identical to the original  input signal. Then, the information signal is typically  amplified again to drive speakers or other output devices.  [Figure 11-80] Amplitude Modulation (AM)