Report Title: Design of Amplitude modulation and demodulation using OrCAD Capture/PSpice
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List of Figures
Figures:
- Circuit diagram
- AM modulation for Depth of modulation 1
- AM modulation for Depth of modulation .5
- The modulating signal, AM, and envelop detector.
List of Tables
- Specification of signals
Table of Contents
Circuit design for Modulator 4
Circuit design for Demodulator 5
Introduction
Amplitude Modulation (AM) is a process in analog communication in which the instantaneous amplitude of the messege signal is used to vary the amplitude of a carrier signal (Simon, 2018). It is one of the three modulation methods in analog communication, the other two have variation in the carrier frequency and phase respectively. AM started as a communication method in the early 20th century and exist today in many forms such as Quadrature Amplitude Modulation (QAM) in digital communication methods which has application in computer modems. The main advantage of using AM as a communication method is the simplicity in its design and it’s also cheap. We can make the receiver of the Am using only a few components. AM has many disadvantages like it is very much susceptible to noise compared to other methods also it require high bandwidth. Am modulated wave is also called Double Side Band AM (DSB-AM) (Urbansky & Zölzer, 2019). This is because AM produce two sidebands along with the carrier. Many varieties of DSB-Am exist today but the bandwidth requirement is high compared to other methods.
Design of AM modulator
We define the modulating signal as
………………… (1)
Where Am is the amplitude of the modulating signal and fm is the frequency in hertz.
Similarly the carrier signal is
……………………. (2)
Where Ac and fcare the amplitude and frequency of the carrier respectively.
The AM signal is defined by
…………………….. (3)
Where m is called the modulation index or depth of modulation. It shows the how much the carrier amplitude is varied according the messege signal. For AM the value of m is always between 0 and 1.
Circuit design for Modulator
As per the equation of AM we have to find the product of Vc with (1+mVm).
The specification of carrier and modulating signal is shown below.
| Carrier wave | Modulating signal |
| Frequency=20k kHz Amplitude=1 | Frequency=100 Hz Amplitude=1 |
Table1. Specification of signals.
Initially we choose the value of m as 1 and an offset of 1volt is added with Vm to make the first part of the equation. These two signals are then fed to a multiplier for AM generation.
The entire circuit diagram is shown below.
Figure 1: Circuit diagram
The result from the simulation is shown below for different Depth of modulation values
Figure 2: AM modulation for Depth of modulation 1.
Figure 3: AM modulation for Depth of modulation 0.5
Circuit design for Demodulator
The demodulation of the modulating signal is done using an envelope detector. The envelop detector takes a Amplitude modulated signal as its input and detect the envelop of the signal. The Am signal is passed through a diode for rectification. For the envelop we need only the positive part. A voltage source is given to compensate for the drop of the diode voltage. It is usually .7v in the case of silicon diodes. A low pass filter at the output eliminates the high frequency components thus reduce the amount of ripples present in the envelop detector. The capacitor C1 in the circuit diagram charges very fast and discharge through the resistor R1 very slow. The capacitor is designed as follows using the condition below.
………………………… (4)
Where W is the bandwidth of the modulating signal.
The equation above states that R1C1 time constant should be greater than 1/fc.
R1= 5k, C1= .32uF satisfies this condition for fc= 20 kHz.
The second stage R2, C2 act as a low pass filter for reducing the high frequency components to the output. The final result is the output resembling the modulating signal.
Figure 4: The modulating signal, AM, and envelop detector.
References
Simon, H. (2018). Introduction to Analog and Digital Communications. India.: Wiley India Pvt. Ltd.
Urbansky, L., & Zölzer, U. (2019). A digital radio-frequency condenser microphone with amplitude modulation. In 2019 IEEE International Symposium on Circuits and Systems (ISCAS) (pp. 1-5). IEEE. DOI: 10.1109/ISCAS.2019.8702611
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