3 dB Cutoff

The 3dB cutoff of a lowpass filter is defined as the highest frequency where 3dB or less of attenuation occurs. Conversely, the 3dB cutoff of a highpass filter is defined as the lowest frequency where 3dB or less of attenuation occurs. On bandpass and bandstop filters the 3dB low cutoff and 3dB high cutoff refer to the low and high edges of the 3dB passband respectively. The reference is usually taken at the point of minimum insertion loss. Normally the 3dB cutoff is the minimum pass bandwidth of a filter. In some cases, a filter passband cutoff can also be specified as 0.5 dB cutoff, 1.0 dB cutoff or 6dB cutoff.

The term 3dB cutoff is used in conjunction with lowpass and high pass filters when defining the passband of the filter. However, the more generic term of x dB cut-off (x-usually greater than 6dB) is used when specifying the stopband of a lowpass or highpass filter. Above the cut-off frequency, for a lowpass filter, the attenuation should be greater than x dB up to some point of "spurious". Above this frequency other modes of resonance occur which degenerates the attenuation. Below the cut-off frequency, for a highpass filter, the attenuation should be greater than x dB.

Filtronetics, Inc. can supply both lowpass and highpass filters with a 3 dB cut-off from 60 Hz to 3 GHz. Most highpass and lowpass filters are realized using LC, microstrip, stripline, or coaxial technology. They can be monotonic or elliptic designs with characteristic responses of Buterworth, Chebyshev, Cauer, Gaussian, Bessel (maximally flat delay), Equal ripple delay, Legendre, minimum insertion loss, synchronously tuned, or m derived image parameter responses.