Analog amplitude, frequency and phase modulation and demodulation with spectrum analysis
The purpose of a communication system is to transmit signals that carry information from a transmitter to a receiver through a communication channel. These signals, which represent the original message or information from the source, are sometimes referred to as low-frequency or original signals.
To transmit these signals efficiently, their frequency range is shifted to higher frequencies suitable for the communication channel, and then shifted back to the original range at the receiver. For example, a radio system operates at frequencies above 30 kHz, whereas the original signal typically contains audio frequencies. This frequency shift is necessary for successful transmission.
This shift is achieved using modulation, which is the process of varying a characteristic of a carrier signal according to the original message signal. A common carrier is a sinusoidal wave, and such a process is called continuous-wave modulation. The output of this process is called the modulated signal. Modulation is performed at the transmitting end of the communication system.
At the receiver, the original message signal is recovered using demodulation, which reverses the modulation process.
In Amplitude Modulation (AM), the amplitude of the sinusoidal carrier is varied according to the message signal. Frequency Modulation (FM) and Phase Modulation (PM) involve varying the instantaneous frequency or phase of the carrier according to the message signal.