Then, the audio input is applied to the transistor s emitter through a transformer. Naturally, the amplitude of the current pulses through transistor q1 will vary. Amplitude modulation is mostly used in the form of electronic communication. It was developed in the first few decades of the twentieth century. The theoretical frequency spectrum of the am signal of fig. The message signal is, for example, a function of the sound to be reproduced by a. The wave form of am modulation is shown in figure 3. Quadrature amplitude modulation qam is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. This is a short presentation video of my self made amplitude modulator. Amplitude modulation is defined as the process in which the amplitude of the carrier wave ct is varied about a mean value, linearly with the baseband signal. Fig 3 a fig 3 b this diode is assumed to be operating as a switch. Note also that an optimum carrier level is recommended in figure 22 for good.
Chapter 7 amplitude modulation techniquesgeneration and. Amplitude modulation is a very basic form of modulation, and although its simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Basic concept of modulation the information source typically a low frequency signal referred to as the baseband signal carrier a higher frequency sinusoid example. The focus of this lab is to familiarize the student with the modulation and demodulation of an. Frequency division multiplexing, fdma amplitude modulation and demodulation. What results is a new modulated signal, given by yt cftcos t. The switching modulator using a diode has been shown in fig 3 a. It may be necessary to adjust the level and frequency of both v c and v m to achieve a suitable waveform and index of modulation. Collector modulation method is high level amplitude modulation c. Update cancel a npgd d w xtbu b ckx y ka lagf b eqv h wdoq m iut d mcoqv tar d qv e hyaa r oht m g a jrd l zmbc. Pulse amplitude modulation is the simplest form of modulation. Amplitude modulation or am is a method to transmit signals via electromagnetic transmission.
Am modulated signal vamt with fm500 hz, fc5khz and m0. An am wave may thus be dscribed, in the most general form, as a function of time as follows. How do i make a circuit of amplitude modulation using transistor. Amplitude modulation circuit implementation for use in a. The method were using for amplitude modulation is possibly the worst working method for modulating an rf signal. As in the circuit diagram, the left hand side is the varactor. The circuits carrier generator, modulator and demodulator are provided with the. These signals carry the information through modulation. Amplitude modulation is defined as a process in which the amplitude of the carrier wave ct is varied linearly with the instantaneous amplitude of the message signal mt. With the modulating voltage adjusted to zero amplitude, increase the carrier amplitude until v o has a value of 2 v pp. T he circuit connection are made as shown in the circuit diagram. Frequency modulation fm the process of combining the message signal with the carrier signal that causes the message signal to vary the frequency of the carrier signal.
Communication is achieved over a transmitter and a receiver through signals. The capacitive reactance of c at the oscillator frequency is made five to ten times larger than the value of series resistance. There are two types of devices in which it may be necessary to the generation of amplitude modulation. Am modulators base, collector, grid and plate modulators. Before we study in detail about pam lets know the concepts of. Understanding the principle of amplitude modulation am. Amplitude modulation is the process of transferring information signals to the amplitude of a highfrequency continuouswave carrier. Essentially an amplitude modulated wave consists of a radio frequency carrier a sine wave at one frequency, typically in the radio frequency portion of the spectrum. How does one select the values of components used in amplitude modulation circuit using a transistor common emitter amplifier circuit. The original method of electronically encoding sound information on radio waves is called amplitude modulation, or am. Understanding the waveform and frequency spectrum of am signal and calculating the percent of. This nonlinear device is required to recover the modulating frequencies from the rf envelope. The basic theory and equations behind amplitude modulation are relatively straightforward and can be handled using straightforward trigonometric calculations and manipulation.
You can publish your book online for free in a few minutes. Generation of am waves using the switching modulator could be understood in a better way by observing the switching modulator diagram. What we do in am, very roughly,is to throw away the frequency content of. In equation 37, we notice that after the audio signal and the carrier signal input into the base and emitter. In this modulation, the signal is sampled at regular intervals and each sample is made proportional to the amplitude of the modulating signal. We give the theory and the experimental results of the. As an example, the following figure shows the amplitude modulation with. As before, this may require a manual adjustment of the voltsdiv and. Introduction ang man shun december 20, 2012 to send multiple signals at one time, one way is to use qam. Its still used in radio systems to transmit audio signals although it has lesser in popularity compared to fm due to its lower signal to noise ratio. The demonstration centers around a number of signals displayed in both the time domain and the frequency domain using a signal generator, an oscilloscope to view the signal.
In this system we have a fixed amplitude and starting time of each pulse. T1 first patch up according to figure 6, but omit the input x and y connections to the multiplier. Department of electronics and communication engineering. In order to understand the theory along with the experiments behind this course, the. Modulation is done at high power of carrier and modulating signal b. Switching is an easily attained function with diodes and transistors in electronic circuits. In amplitude modulation, the amplitude signal strength of the carrier wave is varied in proportion to that of the message signal being transmitted. Amplitude modulation and demodulation electrical and computer. Hence, the square law modulator produces an am wave. Connect to the two oscilloscope channels using the scope selector, as shown.
For a perfect modulation, the value of modulation index should be 1, which implies the percentage of modulation should be 100%. University of north carolina at charlotte department of. It conveys two analog message signals, or two digital bit streams, by changing modulating the amplitudes of two carrier waves, using the amplitudeshift keying ask digital modulation. This experiment examines the characteristics of the amplitude modulation am. How to make a circuit of amplitude modulation using. The pulse width modulation pwm varies the width of individual pulses according to the amplitude of the modulating signal.
Communication systemsamplitude modulation wikibooks. The spectrum of the modulated signal yt can be found by using the modulation property of the fourier transform. In pulse amplitude modulation system the amplitude of a carrier pulse train is varied in accordance with the instantaneous level of the modulating signal. Power amplifiers are used to boost the carrier and modulating signals before modulation d.
The standard form of an amplitude modulated am wave is defined by st a c 1 k a mt cos2f c t where k a is a constant called the amplitude sensitivity of the modulator. The carrier frequency, fc, should be larger than the highest spectral component. In amplitude modulation am, we utilize the amplitude of the audio signal to modulate the amplitude of carrier signal, which means that the amplitude of carrier signal will be varied with the amplitude of the audio signal. I ntroduction an amplitude modulated signal is composed of both low frequency and high frequency components. The pulse position modulation ppm varies the position of individual pulses from their mean position according to the amplitude of the modulating signal. The simplest form of the pam modulator is an analog switch that is turned on and off at the rf carrier pulse rate. There are different kinds of modulation technique based on varying the properties of a pulse train. When the modulation signal occurs, the ac voltage across the secondary of the modulation transformer will be added to and subtracted from the collector supply voltage. Multiple choice questions and answers on amplitude modulation. Today communication is the heart of the technology. The focus of this lab is to familiarize the student with the modulation and demodulation of an am signal. Block diagram of pulse amplitude modulation pam 1 variable frequency sine wave generator. Qam structure consider the yt in the transimitter yt x1tcos.
In practical tuned amplifier circuits, an intermediate frequency transformer ift is. Modulation the amplitude of the carrier wave ct is varied linearly with the. However, for the communication process to be completed or to be useful, the intelligence must be recovered in its original form at the receiving site. Reduce the frequency of e c to 1 khz, observe and draw the waveform as e m is adjusted from 0 to 100% modulation. Figure 35 circuit diagram of transistor am modulation. The demodulation of the pam signals can be done easily using a lowpass filter. This varying supply voltage is applied to the class c amplifier. Chapter 7 amplitude modulation techniques generation and detection chapter outline 7. Carrier modulation allows the transmission of modulating frequencies without the use of transmission wire as a medium. The goal here is to learn something, not to make a high. Theory pulse width modulation is also referred as pulse duration modulation or less often pulse length modulation.
It is often called as am and is commonly used in transmitting a piece of information through a radio carrier wave. Increase the modulated signal until an am waveform appears. The message signal used here is a pure sine wave generated using wien bridge oscillator wbo and an fet based modulator circuit is used for the pam modulation. But the width of each pulse is made properly proportional to the amplitude of the signal at that instant. Amplitude modulation am is a modulation technique where the. Modulation allows us to transmit multiple signals in the same medium i. The first of these, the am transmitter, generates such high powers that its prime requirement is efficiency, so quite complex means of. To study the amplitude modulation and demodulation and to calculate the modulation index values. Pulse amplitude modulation is the basic form of pulse modulation. The circuit that i designed ends up giving a weird waveform but i am pretty sure my professor gave us the same circuit. Amplitude modulation is one of the earliest radio modulation techniques. Amplitude modulation circuit and how am works youtube. In this circuit, the carrierwave generated by an oscillator that isnt shown in the circuit is applied to the base of a transistor. The receivers used to listen to amdsbc are perhaps the simplest receivers of any radio modulation technique.
To understand the basic theory of amplitude modulation am. Pulse amplitude modulation pam is the simplest pulse modulation scheme. Amplitude modulation with simple am radio transmitter. The output is the if which is a fixed frequency from which original information is detected using filters and amplifiers. Explain the generation of am waves using square law.
Pulse amplitude modulation is one of the kinds of modulation techniques used in signal transmission. In the above circuit the transistor is using as switch for performing the operation of on and off. Am is a relatively simple way to add audio information to a carrier wave so that sounds can be transmitted. Lab amplitude modulated radio frequency transmission s. Amplitude modulation am, demodulation and their circuit. Modulation is defined as the process by which some characteristics of a carrier signal is varied in. To design and implement the am modulator by using transistor. Generation of amplitude modulation basic requirements.
Amplitude modulation am is a modulation technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. Amplitude modulation am, demodulation and their circuit implementations. The am signal is taken from the transistor s collector. Using your data from steps 3 and 4, determine the min and max iabc currents for 50% and 100% modulation. Amplitude shift keying refer to a type of amplitude modulation in whi ch the amplitude of carrier sinusoidal is modified according to binary input data. Amplitude modulation am the process of combining the message signal with the carrier signal and the two sidebands. To generate amplitude modulation am waveform and to measure modulation index of am wave using waveform method. Amplitude modulation is a process by which the wave signal is transmitted by modulating the amplitude of the signal. Please point me to resources that can help me decide what values of components to use. Amplitude modulation circuit implementation for use in. Amplitude modulation definition, types, solved examples.
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