ACTIVE NOTCH FILTER

LIC Lab Manual

ACTIVE NOTCH FILTER

AIM

To design and setup a notch filter with a notch frequency fN = 1 KHz.

COMPONENTS AND EQUIPMENTS REQUIREDSl. No.Name of Component or EquipmentValueQuantity

1

2

3

4

5

6

7Op-amp IC

Resistors

Capacitors

Dual Power Supply

Function Generator

CRO

Bread BoardA741

15K

100K

10K

8.2K

0.01F

0.02F

(15-0-15)V

(0-1)MHz

(0-20)MHz2

21

1

1

2

1

1

1

1

1

PRINCIPLE

The narrow band reject filter is commonly called a notch filter and is useful for the rejection of single frequency. It is used to filter 60Hz power line frequency hum. The most commonly used notch filter is the twin-T network. This is a passive filter composed of two T-shaped networks. One T network is made up of two resistors and a capacitor, while the other uses two capacitors and a resistor. The notch-out frequency is the frequency at which maximum attenuation occurs; it is given by

fN =

We have to select a particular notch frequency. At the notch frequency the output of both the T networks will have a phase difference of 180. Thus they will cancel each other. Thus there will be no output from the filter at notch frequency. This way we can remove a particular frequency and hence the name notch filters. The passive twin-T network has a relatively low figure of merit, Q. The Q of the network can be increased significantly if it is used with the voltage follower. The most common use of notch filters is in communications and biomedical instruments for eliminating undesired frequencies.

CIRCUIT DIAGRAM

Frequency Response

DESIGN

Let be the centre frequency. Also

BW = fH - fLQ-factor =

Let 1KHz.fN = = 1KHz.Take C = 0.01F then R 15K

Then 2C = 0.02F and R/2 = 8.2K.PROCEDURE

1. Set up the connections as shown in the circuit diagram on a bread board.

2. Connect the power supply to the circuit and switch ON.

3. Feed sine wave of 1Vpp to the input of the circuit.

4. Vary the input signal frequency of the circuit.

5. Measure the amplitude of the corresponding output wave from CRO.

6. Tabulate the readings.

7. Plot the frequency response graph with log f along X-axis and gain along Y-axis.

OBSERVATIONS

Vin = 1VppF in HzVoIn voltsLog fGain in db20log Vo/Vin

RESULT

Designed and setup a notch filter and plotted the frequency response.

Sl. NoParameterTheoreticalPractical

1Resonant Frequency

2Band width

3Q - factor

R 15K

EMBED Equation.3EMBED Equation.3

R 15K

EMBED Equation.3EMBED Equation.3

+15V

3

7

2

4

6

-15V

741

-

+

+15V

2

7

3

4

6

-15V

+

-

741

C 0.01EMBED Equation.3F

EMBED Equation.3EMBED Equation.3

C 0.01EMBED Equation.3F

EMBED Equation.3EMBED Equation.3

2C 0.02EMBED Equation.3F

EMBED Equation.3EMBED Equation.3

R/2 8.7K

EMBED Equation.3EMBED Equation.3

R1 10K

EMBED Equation.3EMBED Equation.3

R2 100K

EMBED Equation.3EMBED Equation.3

Vin

1Vpp

fL fN fH

logf

Gainin dB

3dB

PAGE 58Dept. of ECE

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