What is Cut-off Frequency in an RF Filter?

The cut-off frequency of a filter is the frequency characterizing a boundary between a passband and a stopband. Passband consists of the range of frequencies the filter lets through (minimal attenuation), and the stopband consists of the range of frequencies the filter rejects (high attenuation). The cut-off frequency is sometimes taken to be the point in the filter response where a transition band and passband meet, for example, as defined as the point at which the output level from the filter falls/rises by 50% (i.e., ±3 dB, since a fall/rise of 3 dB corresponds approximately to half power) of the in-band level, assuming a constant input level. It is also sometimes referred to as the half power or ±3 dB frequency.

The stopband of the filter is essentially the band of frequencies that is rejected by the filter. It is taken as starting at the point where the filter reaches its required level of rejection. Thus RF filters may have one or two cut-off frequencies depending upon their frequency rejection type.

An RF filter can have one or more cut-off frequencies, depending on the type of filter. In a low pass filter (frequencies below the cut-off are let through) and in a high pass filter (frequencies above the cut-off are let through), in both these filters there is only one cut-off frequency.

Figure 1. Generic low pass filter response

Figure 2. Generic high pass filter response

RF filters that allow through a range of frequencies like the bandpass filter (frequencies within the accepted range is let through) and band-reject filter (frequencies outside the specified range is let through) have two cut-off frequencies corresponding to the upper and lower frequency limits.

Figure 3. Generic bandpass filter response

Figure 4. Generic Band-reject filter response

An ideal RF filter should exhibit no loss within the passband and completely attenuate the signals in the stopband. However, in real-world applications, it is not possible to achieve an ideal RF filter and there is always some loss within the passband, and it is not possible to achieve infinite rejection in the stopband. Also, in the transition between the passband and the stopband, where the response curve falls away, the level of rejection usually rises as the frequency moves from the passband to the stopband.