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Analog & Digital Communication Lab Experiments

To Construct and Verify Pre-emphasis and De-emphasis and Plot the Waveforms



Aim

To Construct and Verify Pre-emphasis and De-emphasis and Plot the Waveforms.

(a) To generate frequency modulated signal and determine the modulation index and Bandwidth.

Apparatus Required:

  1. Resistors (10 K-2, 47K, 75K, 1K)
  2. Capacitors (22μF, 0.1μ-2,)
  3. Transistor BC107
  4. Function generators
  5. CRO
  6. Connecting Wires
  7. RPS (15V)
  8. Connecting Wires

Theory

The noise has a effect on the higher modulating frequencies than on the lower ones. Thus, if the higher frequencies were artificially boosted at the transmitter and correspondingly cut at the receiver, an improvement in noise immunity could be expected, thereby increasing the SNR ratio. This boosting of the higher modulating frequencies at the transmitter is known as pre-emphasis and the compensation at the receiver is called de-emphasis

Circuit Diagram

pre-emphasis
de-emphasis

Procedure:-

  1. Connect the circuit as per circuit diagram as shown in Fig.
  2. Apply the sinusoidal signal of amplitude 20mV as input signal to pre emphasis circuit.
  3. Then by increasing the input signal frequency from 500Hz to 20 KHz, observe the output voltage (VO) and calculate gain 20 log (vo/vi).
  4. Plot the graph between gain Vs frequency.
  5. Repeat above steps 2 to 4 for de-emphasis circuit (shown in Fig.2). by applying the sinusoidal signal of 5V as input signal./li>

Tabular form:

Pre-emphasis:

S.No. Frequency (Hz) I/P Voltage Vi O/P Voltage Vo Gain in dB 20log(Vo /V i)
1
2
3
4
5
6
7
8
9

Pre-De-emphasis:

S.No. Frequency (Hz) I/P Voltage Vi O/P Voltage Vo Gain in dB 20log(Vo /V i)
1
2
3
4
5
6
7
8
9

Model Graph

75-us-emphasis-curves

Result:Thus the pre-emphasis and de-emphasis characteristics are studied