Study of AM modulation and Demodulation techniques (Transmitter and Receiver) Calculate of parameters.
(a) To Generate Amplitude Modulated Wave and Determine the Percentage modulation.
Modulation is defined as the process by which some characteristics of a carrier signal is varied in accordance with a modulating signal. The base band signal is referred to as the modulating signal and the output of the modulation process is called as the modulation signal.
Amplitude modulation is defined as the process in which is the amplitude of the carrier wave is varied about a means values linearly with the base band signal. The envelope of the modulating wave has the same shape as the base band signal provided the following two requirements are satisfied
(1) The carrier frequency fc must be much greater then the highest frequency components fm of the message signal m (t)
I.e. fc >> fm
(2) The modulation index must be less than unity. If the modulation index is greater than unity, the carrier wave becomes over modulated
% of Modulation (ma)= Vmax-Vmin/Vmax+Vmin x 100
or Modulation factor (ma) = Vmax-Vmin/Vmax+Vmin
Result:Amplitude modulated wave generated and studied.
(b)To Demodulate the Amplitude modulated wave using envelope detector.
Demodulation is the act of extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer program in a software-defined radio) that is used to recover the information content from the modulated carrier wave. There are many types of modulation so there are many types of demodulators. The signal output from a demodulator may represent sound (an analog audio signal), images (an analog video signal) or binary data (a digital signal).
These terms are traditionally used in connection with radio receivers, but many other systems use many kinds of demodulators. For example in a modem, which is a contraction of the terms modulator/demodulator. a demodulator is used to extract a serial digital data stream from a carrier signal which is used to carry it through a telephone line, coaxial cable, or optical fiber
There are two methods used to demodulate AM signals.
The envelope detector is a very simple method of demodulation. It consists of a rectifier (anything that will pass current in one direction only) or other non-linear that enhances one half of the received signal over the other, 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 behavior, 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.
An AM signal can be rectified without requiring a coherent demodulator. For example, the signal can be passed through an envelope detector (a diode rectifier and a low-pass filter). The output will follow the same curve as the input baseband signal.
Result:Original baseband signal recovered from Amplitude modulated wave.
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