New York, NY IMG HI 58° LO 56° Home About Contact
IMG-LOGO
Home Electrical and Electronics Analog & Digital Communication Study of AM modulation and Demodulation techniques (Transmitter and Receiver) Calculate parameters
Analog & Digital Communication Lab Experiments

Study of AM modulation and Demodulation techniques (Transmitter and Receiver) Calculate parameters



Aim

Study of AM modulation and Demodulation techniques (Transmitter and Receiver) Calculate of parameters.

(a) To Generate Amplitude Modulated Wave and Determine the Percentage modulation.

Apparatus Required:

  1. Amplitude Modulation and Demodulation Trainer
  2. Function Generator
  3. Oscilloscope
  4. Connecting Wires

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

Procedure:-

  1. Switch on the trainer and check the O/P of carrier generator on oscilloscope.
  2. Connect 1 KHz with 2 Volts A.F signal at AF I/P to the modulator circuit.
  3. Connect the carrier signal at carrier I/P of modulator circuit.
  4. Observe the modulator output signal at AM O/P by making necessary changes in A.F. Signal.
  5. Vary the modulating frequency and amplitude and observe the effects on the modulated waveform.
  6. The depth of modulation can be varied using the variable knob (potentiometer) provided at A.F. input.
  7. The percentage of modulation or modulation factor can be calculated using the following formulas.

% of Modulation (ma)= Vmax-Vmin/Vmax+Vmin x 100

or Modulation factor (ma) = Vmax-Vmin/Vmax+Vmin

circuit-diagram-for-modulation
waveform-a
waveform-b
waveform-c

Precautions:

  1. Connect the circuit as shown in the circuit diagram.
  2. Apply the required voltages wherever needed.
  • Do not apply stress on the components.
  • Result:Amplitude modulated wave generated and studied.

    (b)To Demodulate the Amplitude modulated wave using envelope detector.

    Apparatus Required:

    1. Amplitude Modulation and Demodulation Trainer
    2. Function Generator
    3. Oscilloscope
    4. Connecting Wires

    Theory

    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.

    Procedure:-

    1. Switch on the trainer and check the O/P of carrier generator on oscilloscope.
    2. Connect 1 KHz with 2 Volts A.F signal at AF I/P to the modulator circuit.
    3. Connect the carrier signal at carrier I/P of modulator circuit.
    4. Observe the modulator output signal at AM O/P Spring by making necessary changes in A.F. Signal.
    5. Vary the modulating frequency and amplitude and observe the effects on the modulated waveform.
    6. The depth of modulation can be varied using the variable knob (potentiometer) provided at A.F. input.
    7. The percentage of modulation or modulation factor can be calculated using the following formulas.
    8. equation-2
    9. Find the value of R from fm=1/ (2*Pi*R*C) , C=0.1μF
    10. Connect the circuit diagram as shown in Fig.
    11. Feed the AM wave to the demodulator circuit and observe the output
    12. Note down frequency and amplitude of the demodulated output waveform.
    13. Draw the demodulated wave form.
    circuit-diagram-for-demodulation
    expected-waveform

    Precautions:

    1. Connect the circuit as shown in the circuit diagram.
    2. Apply the required voltages wherever needed.
    3. Do not apply stress on the components.

    Result:Original baseband signal recovered from Amplitude modulated wave.