Amplitude Modulation and Demodulation

Topics: Amplitude modulation, Modulation, Carrier wave Pages: 6 (591 words) Published: December 6, 2014

Study how to perform the amplitude modulation and demodulation and to calculate the modulation index for various modulating voltages.

Theory :

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. 3. Modulation index = (A-B)/(A+B). where A and B as shown in the figure below.

Figure 1: Designing modulation index

Fig2: Amplitude modulation model graph
Design Procedure :
Modulated Signal (Under Modulation) :
Given VC = 300mV, fc = 500 KHz, fm = 1KHz.
Set modulating voltage Vm = 200mV.
Emax = 220 V, Emin = 60 V
Modulation index (m) = E max - E min/ E max + Emin X 100 = 0.57%
Modulated Signal (Over Modulation) :
Given VC = 300mV, fc = 500 KHz, fm = 1KHz.
Set modulating voltage Vm = 200mV.
Emax = 320 V, Emin = 70 V
Modulation index (m) = E max - E min/ E max + Emin X 100 = 1.56%
Test Procedure :
1. The power supply is connected to the collector of the Transistor. 2. Modulated Output is taken from the collector of the Transistor. 3. Calculate Emax and Emin from the output waveform.

The process of detection provides a means of recovering the modulating Signal from modulating signal. Demodulation is the reverse process of modulation. The detector circuit is employed to separate the carrier wave and eliminate the side bands. Since the envelope of an AM wave has the same shape as the message, independent of the carrier frequency and phase, demodulation can be accomplished by extracting envelope. An increased time constant RC results in a marginal output follows the modulation envelope. A further increase in time constant the discharge curve become horizontal if the rate of modulation envelope during negative half cycle of the modulation voltage is faster than the rate of voltage RC combination ,the output fails to follow the modulation resulting distorted output is called as diagonal clipping : this will occur even high modulation index. The depth of modulation at the detector output greater than unity and circuit impedance is less than circuit load (Rl > Zm) results in clipping of negative peaks of modulating signal. It is called “negative clipping “

Fig. 3- Amplitude Demodulation Waveform
Test Procedure :
1. The amplitude modulated signal from AM generator is give as input to the circuit. 2. The demodulated output is observed on the CRO.
3. The various values of modulating voltage signal frequency corresponding demodulated voltage and frequency are noted and the readings are tabulated. Lab Result :
Thus the amplitude modulation and demodulation were performed and the modulation indexfor various modulating voltage were calculated. Questions :
What is Modulation Index ?
Ans - The modulation index (or modulation depth) of a modulation scheme describes by how much the modulated variable of the carrier signal varies around its unmodulated level. It is defined differently in each modulation scheme.

Draw the AM Spectrum.

Fig. 4: Frequency spectrum of a modulated singnal
What is DSB...
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