What is the primary mechanism of attenuation in a notch filter tuned to a target frequency f0?

• A. It uses an RC network tuned to f0 to create a deep notch at that frequency.
• B. An LC tank tuned to f0 presents a high impedance at f0, resulting in attenuation.
• C. It relies on a transformer to block a single frequency.
• D. It relies on digital subtraction to remove components at f0 from the signal.

Answer: (B)

A notch filter creates a sharp attenuation at a specific frequency by exploiting resonance in an LC circuit. When the tank is tuned to the target frequency f0, its impedance becomes very large at that frequency. In the notch topology, this resonant tank is arranged so that at f0 the signal can no longer pass through the main path in the intended way, effectively removing energy at that single frequency while leaving other frequencies largely unaffected. The deep, narrow suppression comes from the high Q of the LC resonance, which is why an LC-based notch is typically preferred for precise single-frequency attenuation.

The other ideas don’t capture this mechanism: an RC network doesn’t provide as sharp a notch, a transformer isn’t used to block a single frequency in this way, and digital subtraction operates in the digital realm rather than the analog resonance-based approach of a notch.