Amplitude Shift Keying Demodulation

To study ASK (Amplitude Shift keying) Demodulation & regenerate signal.


The process of re - creating original modulating frequencies from the carrier is referred to as DEMODULATION or DETECTION. The circuit in which restoration is achieved is called the DETECTOR or DEMODULATOR. The term demodulator is used because the demodulation process is considered to be the opposite of modulation. The output of an ideal detector must be an exact reproduction of the modulation existing on the wave. Failure to accurately recover this intelligence will result in distortion and degradation of the demodulated signal and intelligence will be lost. The distortion may be in amplitude, frequency, or phase, depending on the nature of the demodulator. A nonlinear device is required for demodulation. This nonlinear device is required to recover the modulating frequencies from the envelope. There are several ways of demodulation depending on how parameters of the base-band signal are transmitted such as amplitude, Frequency or Phase.
The envelope detector is a very simple method of demodulation. It consists of a rectifier (anything that will pass current in one direction only), and a low - pass filter. The rectifier may be in the form of a single diode, or may be more complex. Many natural substances exhibit this rectification behaviour, which is why it was the earliest modulation and demodulation technique used in radio. The filter is usually a RC low - pass type, but the filter function can sometimes be achieved by relying on the limited frequency response of the circuitry following the rectifier. The crystal set exploits the simplicity of AM modulation to produce a receiver with very few parts, using the crystal as the rectifier, and the limited frequency response of the headphones as the filter.
The product detector multiplies the incoming signal by the signal of a local oscillator with the same frequency and phase as the carrier of the incoming signal. After filtering, the original audio signal will result. This method will decode both AM and SSB, although if the phase cannot be determined a more complex setup is required.