What is the purpose of notch filter ?

What is the purpose of notch filter ?

a. To block all frequencies that fall within its bandwidth
b. To pass only limited range of frequencies irrespective of active filters.
c. Both a & b
d. None

Correct Answer : a. To block all frequencies that fall within its bandwidth

Explanation :

Notch filter mainly consists of a low pass filter, a high pass filter and a summing amplifier. The circuit configuration is in such a manner that cut-off frequency assigned by low-pass filter is lesser in value as compared to the cut-off frequency allocated by high-pass filter. Therefore, the gap between these both cut-off frequencies represents the bandwidth of the filter.
If the input frequency is lower than cut-off frequency of high-pass filter then output of high pass filter becomes zero. Hence, the total output of the filter will be equal to the output of low pass filter.
On the contrary, if the input frequency is higher than cut-off frequency of high pass filter, then output of low-pass filter tends to zero & hence, the net output under this condition will be definitely equal to the output of high pass filter.
As a result, it is perspicuous that notch filter permits to pass the frequencies below the cut-off frequency of low-pass filter and those above the cut-off frequency of high-pass filter, which can be termed as a crucial function or purpose of the notch filter.