RF notch filters, also known as RF bandstop filter or reject filter, is a device that rejects or blocks radio frequencies within a specific frequency range and allows other frequencies above and below that range. There’s multiple different types of RF notch filters with unique features used for different applications. As it eliminates frequencies, it’s sometimes also referred to as a band elimination filter. 

Two of the most common types of RF notch filters are what is referred to as lowpass and highpass filters. In this article we’ll explain what these terms mean, how they work, and what types of applications they are used for:

The definitions of LPF and HPF

  • Lowpass filter, LPF for short, is a filter that allows signals through that has a frequency below the cut-off frequency.
  • Highpass filter, HPF for short, is a filter that allows signals through that has a frequency above the cut-off frequency.

You might be thinking: “What range of frequency would be considered high or low?”. Well, this is determined by the so-called “cut-off frequency”, which will vary depending on the filter.

How do they work?

Highpass RF filters offer a low reactance to signals with a frequency above the cut-off frequency, while providing a high reactance to the frequency range below the cut-off frequencies. 

Lowpass RF filters do the complete opposite. It offers low reactance to signals with frequencies lower than the cut-off frequency so they can pass, but has a high reactance to the high-frequency signals and therefore block them.

Applications of LPF and HPF filters

Lowpass filters

Lowpass RF notch filters are most commonly used in communications circuits as an anti-aliasing filter as it’s significant in removing aliasing effects. 

Highpass filters

Highpass RF notch filters are most commonly used in audio amplifiers, low noise amplifiers, etc. It is significant when the distortion due to low frequency signals such as noise is to be removed .

Active vs. Passive filters

Depending on if the filter is using passive components, such as resistors, capacitors, inductors, etc., the filter will be termed as a passive filter. While it has an amplifier included to increase the gain of the filtered signal, the filter will be termed as an active filter. 

In conclusion

The RF notch filters discussed above are passive filters as they use passive components. If we want to increase the gain of the signal, we can use amplifiers in the filter circuit which will create an active filter.