Question

Explain how a bandpass filter works and indicate by a block diagram what procedure should be followed to verify its correct operation.

Answer 1

Band pass filter is a device which is a combination of low pass filter and high pass filter. In band pass filter the low frequencies and high frequencies are blocked and only certain predefined frequencies are allowed to pass through the filter, this predefined set of frequencies are called as pass band and the set of lower frequencies which is blocked by the filter is termed as lower stop/block band and the set of higher frequencies which is blocked by the filter is termed as higher stop/block band. The frequencies which saperatesthese stop bands to pass band is called lower and upper corner/cutoff/cutin frequencies respectively. The band pass filters can be designed by using active elements such as semiconductor devices such filters are called active band pass filters another variety of band pass filter is designed by passive elements such as resistor, inductors, capacitor such filters are called as passive band pass filters. The band pass filter's block diagram can also be made by two basic filters blocks low pass filter and high pass filter, these filters can be connected in series. In case of series connection the low pass filter is connected prior to high pass filter. Hence the high pass filter has lower cutoff frequency and lower frequencies are blocked by high pass filter and passed the higher frequencies and the low pass filter has higher cutoff frequency which passes lower frequency band and blocks higher frequency band.

the circuit diagram of passive band pass filter is shown as follows

the active band pass filter is shown as follows

To test the performance of band pass filter one should have a signal generator or function generator whose frequency to be generated can be controlled at the input to band pass filter and a oscilloscope or CRO at the output of band pass filter. Different frequency signals are applied to the filter circuit and their corresponding responses are observed on CRO and no. Of divisions on time/x axis is counted and multiplied by time/division dial setting and then taking the inverse will give the frequency.