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Experiment#6

Frequency response of a low pass filter

Objective:

Study the characteristics and frequency response of passive Low Pass RC filterTheory:

The inductive reactance is directly proportional to frequency and capacitive reactance is inverselyproportional to frequency. These properties can be used to select or reject certain frequencies of aninput signal. This selection and rejection of frequencies is called filtering, and a circuit which doesthis is called afilter. If a filter passes low frequencies and rejects high ones, it is called a low-passfilter. An RC low pass filter is shown in Figure 1.

Figure 1

Filters, like most things, arent perfect. They dont absolutely pass some frequencies andabsolutely reject others. A frequency is considered passed if its magnitude (voltage amplitude) is

within 70% (or 1/2) of the maximum amplitude passed and rejected otherwise. The 70%frequency is called corner frequency, roll-off frequency, break frequency, cutoff frequency or half-power frequency. The corner frequency for the RC filter is given as:

fC = 1 / 2RC

At cut off frequency, R=XC i.e. voltage VR=VC and phase angle between input and output voltagewill be 450.

Figure 2

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Equipment:

Function generator, oscilloscope, probes, bread board, jumper wires, 1.2 k, 1F

Circuit Diagram:

AC Voltage Source

1.2 kohm

1 micro farad

Procedure:

Connect the components on bread board according the circuit diagram.

Apply sinusoidal input voltage of 5 V peak to peak with 20 Hz frequency from functiongenerator and note the peak to peak voltage across capacitor from oscilloscope.

Also measure the phase difference () between input and output voltage by oscilloscope.

Increase the frequency at regular steps and fill the table shown below.

Draw the graph between output voltage and frequency.

Mark the cut off frequency on the graph.

Draw the graph between and frequency.

Mark the value of at cut off frequency.

Results:

Sr. No. Peak to peakvalue of input

voltage

Frequency ofinput voltage

(Hz)

Peak to peak valueof output voltage

from oscilloscope(V0=VC)

Phase angle between inputand output voltage

( degrees)

1 5 Vp-p 102 5 Vp-p 203 5 Vp-p 304 5 Vp-p 405 5 Vp-p 506 5 Vp-p 60

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7 5 Vp-p 708 5 Vp-p 809 5 Vp-p 9010 5 Vp-p 1011 5 Vp-p 110

12 5 Vp-p 12013 5 Vp-p 13014 5 Vp-p 14015 5 Vp-p 150

Graph between Output Voltage and Frequency:

Graph between and Frequency:

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Discussion:

Differentiate between passive and active filters?

What is the ratio of output and input voltage levels in db at cutoff frequency ?

What do you mean the term frequency octave and frequency decade ?

Write some real applications in which low pass filter is used?

Your viewpoint regarding the results.

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