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  • Ex. No. 1 AMPLITUDE MODULATION

    Aim:

    To construct an Amplitude modulator using SL100 and study its output waveform.

    Apparatus Required:

    Transistor SL100, RPS(0-30V), Inductor, Capacitor, Resistor, Function Generator, CRO, Bread Board,

    Connecting wires.

    Circuit Diagram:

    Design:

    Given: Vcc = 12V,VTh=1.5V, Fc = 70KHz, Fm = 7 KHz,C=0.01F, R2 = 10K

    Fc = 1/2 . So, L= 0.516 mH.

    VTh =

    . So, R1= 70 K.

    Theory:

    Modulation is the process of varying one or more properties of a periodic waveform, called the carrier

    signal, with a modulating signal which typically contains information to be transmitted. The base band signal is

    referred to as the modulating signal and the output of the modulation process is called as the modulation signal. In

    Amplitude modulation, the amplitude of the carrier signal is varied in accordance to the modulating 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.

    In radio communication, a continuous wave radio-frequency signal (a sinusoidal carrier wave) has its

    amplitude modulated by an audio waveform before transmission. The audio waveform modifies the amplitude of the

    carrier wave and determines the envelope of the waveform. In thefrequency domain, amplitude modulation produces

    a signal with power concentrated at the carrier frequency and two adjacent sidebands. Each sideband is equal

    in bandwidth to that of the modulating signal, and is a mirror image of the other. Amplitude modulation resulting in

    two sidebands and a carrier is called "double-sideband amplitude modulation" (DSB-AM).

    Amplitude modulation is inefficient in power usage; at least two-thirds of the power is concentrated in the

    carrier signal. The carrier signal contains none of the original information being transmitted (voice, video, data,

    etc.). However, it does contain information about the frequency, phase and amplitude needed to demodulate the

    received signal most simply and effectively. In some communications systems, lower total cost can be achieved by

    eliminating some of the carrier, thereby lowering electrical power usage even though this requires greater receiver

    complexity and cost. If some carrier is retained (reduced-carrier transmission, or DSB-RC) receivers can be

    designed to recover the frequency, phase, and amplitude information from this "pilot" carrier and use it in the

    demodulation process. If the carrier is eliminated (Double-sideband suppressed-carrier transmission or DSB-SC) the

    receiver must provide a substitute carrier, with inevitable loss of fidelity. Completely suppressing both the carrier

    and one of the sidebands produces single-sideband modulation, widely used in amateur radio and other

    communications applications. SSB occupies less than half the spectrum of AM so it also has greatly improved

    bandwidth efficiency. In AM broadcasting, where there are many receivers for each transmitter, the full carrier is

    provided to allow reception with inexpensive receivers. The broadcaster absorbs the extra power cost to greatly

    increase potential audience.

    Procedure:

    The connections are made as per the circuit diagram.

    The carrier signal is fed to the base and the modulating signal is fed to the emitter of the transistor.

    The waveform of the carrier signal is obtained from CRO.

    Tabulation:

    S.No Parameter Amplitude(V) Time-period(ms)

    1. Message signal

    2. Carrier Signal

    3. Modulated signal-Vmax

    4. Modulated signal Vmin

  • Model Graph:

    Modulating wave

    Vm

    Carrier wave

    Vc

    Modulated wave

    Vmax

    +Vmin

    -Vmin

    Vmax

    t(msec)

    t(msec)

    t(msec)

  • Result: The AM modulator was constructed using SL100 & the detection of the signal was obtained.

  • Ex. No.2 DIODE DETECTION

    Aim:

    To construct a Diode Detector and study its output waveform.

    Apparatus Required:

    Diode OA279, RF signal generator, Capacitor, Resistor, CRO, Bread Board, Connecting wires.

    Circuit Diagram:

    Theory:

    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 envelope detector is an

    electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the

    original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the

    resistor when the signal falls.The simplest form of envelope detector is the diode detector which is shown above. A

    diode detector is simply a diode between the input and output of a circuit, connected to a resistor and capacitor in

    parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of

    this circuit should approximate a voltage-shifted version of the original (baseband) signal. A simple filter can then

    be applied to filter out the DC component.

    Procedure:

    The circuit is connected as per the circuit diagram.

    The output from the RF signal generator is fed as the input to the diode detector circuit.

    This output of the detector is fed t the CRO.

    The CRO is fed to dual mode to see the modulated and the message signal simultaneously

  • Tabulation:

    S.No Parameter Amplitude(V) Time-period(ms)

    1. Modulated signal-Vmax

    2. Modulated signal Vmin

    3. Demodulated signal

    Model Graph: Modulated wave

    Vmax

    +Vmin

    -Vmin

    Vmax

    De-modulated wave

    Vm

    Result: The Diode detector was constructed and the demodulated signal is obtained.

    t(msec)

    t(msec)

  • Ex. No. 3 FREQUENCY MODULATION

    Aim:

    To study the circuit of frequency modulation.

    Apparatus required:

    IC 2206, Capacitors, Resistors, Capacitors, Audio oscillators, DSO, Bread board, Connecting wires.

    Circuit Diagram:

    Theory:

    The modulation systems, namely angle modulation in which the angle of the carrier wave is varied

    according to the Base band signals. In this method of modulation, the amplitude of the carrier wave is maintained

    constant. There are two common forms of angle modulation, namely phase modulation and frequency modulation.

    FM is widely used for broadcasting music and speech, two-way radio systems, magnetic tape-recording systems and

    some video-transmission systems. In radio systems, frequency modulation with sufficient bandwidth provides an

    advantage in cancelling naturally-occurring noise.

    Frequency modulation (FM) conveys information over a carrier wave by varying its instantaneous

    frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its

    frequency remains constant Frequency Modulation is the process in which the frequency of the carrier signal is

    varied by the modulating signal while the amplitude remains constant. The big advantage of frequency modulation is

    its noise reduction ability, because most of noise is appeared as additional amplitude and in FM the amplitude of

    signal is hold fixed. The modulation index is a measure of radian phase shift of the modulated FM signal compared

    to the phase of the un-modulated carrier alone. In most communications systems using FM, maximum limits are put

    on both the frequency deviation and modulating frequency.

    FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Normal

    (analog) TV sound is also broadcast using FM. Narrowband FM is used for voice communications in commercial

    and amateur radio settings. In broadcast services, where audio fidelity is important, wideband FM is generally used.

  • Procedure:

    The connections are given as per the circuit diagram.

    The FM wave is obtained by feeding the modulating signal to the integrator circuit that is designed.

    The output of the integrator is given to the input of the phase modulation kit whose output is the FM signal

    and the output is observed in the oscilloscope.

    The graph is plotted from the output reading .

    Tabulation:

    INPUT

    OUTPUT

    AMPLITUDE(V) Vin Vc Vo(+Vc to -Vc)

    TIME(s)

    Model graph:

    Result:

    The circuit of the frequency modulation is studied and the