INTRODUCTION TO FILTERS

Introduction


Filter is a frequency selective network designed to attenuate signals of unwanted frequency and passes certain bands of wanted signal frequency. The filters are generally specified in terms of cut off frequencies and the pass band gain for which attenuation is zero. In the communication systems, filters are used to pass the signal containing information and to stop noise. In stereo systems, filters are used to stop particular band of frequency.

Pass band: Pass band is band of frequencies for which attenuation of the wanted signal is zero. In this band, the signal passes smoothly.


Cut-off frequency (fc): The frequency that separates pass and stop band is called cutoff frequency. The filters may have more than one cut off frequencies.

Depending upon the location of the pass band on frequency scale, the filters are:-

i)    Low Pass (LP) filter

ii)    High Pass (HP) filter

iii)    Band Pass (BP) filter

iv)    Band Stop (BS) filter

Depending upon the elements used in the filter network, filter can be of two types,

i)    Passive filter: Uses only R, L, C elements

ii)    Active filter: Filter components could be transistor, OP-AMP in combination with R, L, and C elements.






Low Pass Filter

An ideal low pass characteristic is shown in Figure 1.
•    Passband extending from 
•    Stopband extending from

where   or  is called the cutoff frequency

RC Low Pass Filter

The RC low pas filter can be used as a low pass or high pass. If the output  is taken off the capacitor as shown in the diagram 2, it will respond as a low pass filter.

Let us examine the network at the frequency extremes of f = 0 Hz and very high frequencies to test the response of the circuit.

At f=0

At f = very high frequency

A plot of the magnitude of Vo versus frequency will result in the curve as shown in the figure. The cutoff frequency is defined at the 0.707 level, i.e. the frequency at which the transition takes place from a pass band to stop band

for the RC low pass filter:-