RC Low Pass Filter Experiment

Aim

To design and set up a low pass filter circuit for a 3dB frequency of 1KHz and study the frequency response.

Components And Equipments Required

Capacitor, resistor,signal generator,breadboard and CRO

Theory

Filters are the networks designed to pass only certain desired frequency band. It can be broadly classified as passive or active filters according to the devices used to implement them. Filters can also be classified according to the frequency spectrum it passes, such as low pass, high pass, band pass, and band reject filters.

A Passive low pass filter is shown in the figure. It passes low frequency readily and attenuates high frequencies. Since the reactance of the capacitor C decreases with increasing frequency, at high frequencies the capacitor acts as a virtual short and output falls to zero. For a sinusoidal input Vin, the output signal Vo decreases with increasing frequency. The magnitude of the ratio of output voltage to input voltage of the circuit is given by A=1/√1+(f/fh)^2. Where fh is the higher cut off frequency given by the expression fh=1/2πRC and f=input signal frequency. 

At the frequency fh, the gain falls to 0.707 of its value at low frequency. Hence fh is called upper3dB frequency.

Procedure

1.      Wire up the circuit and set the input sine wave voltage at 5V peak to peak and observe the input and output on the two channel of the CRO

2.      Vary the input frequency from 10Hz to 100Khz or more and note down the output voltage in tabular column.

3.      Plot the graph on a semilog graph sheet with f(or logf on ordinary graph sheet) along X-axis and gain in dB along Y-axis.

4.      Mark a point on graph at 3dB less than the maximum gain. Extend the point to X-axis and mark the upper 3dB frequency.

Design

Let the upper cut off frequency fh be 1KHz. We have fh=1/2πRC. To avoid loading, as a rule of thumb, select R=6000Ω. Use 6.8k std.

Substituting this in the above expression, we get C=0.023µF. Use 0,022µF std.

Result

RC low pass filter is designed for 3dB frequency of 1KHz and hence studied the frequency response.

Theoretical 3dB frequency = 1 KHz

Observed 3dB frequency = 1.25 KHz