Band-Reject Filter (BRF) is a type of filter that allows most frequency signals to pass while strongly attenuating a specific frequency range (stopband). It functions opposite to a bandpass filter and is used to suppress interference or unwanted frequency components.  

Key Applications  

1. Interference Rejection: In communication systems, it eliminates noise or interference in specific bands (e.g., power-line hum, harmonic interference).  

2. Signal Conditioning: In audio or RF systems, it removes spurious signals to improve signal-to-noise ratio.  

3. Equipment Protection: Prevents strong interfering signals from damaging sensitive electronics (e.g., radar, medical devices).  

4. Spectrum Management: In wireless communications, it avoids crosstalk between different frequency bands. 

When to Use It?  

A band-reject filter is ideal when a system has fixed-frequency interference and needs to preserve signals in other bands. Examples include removing 50Hz power-line noise or suppressing interference in a specific radio frequency band.

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