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# Signal Chain filtering - Filter Q value

Why are the Q values for practical bandpass filters greater than the Q Values for practical low pass and high pass filters?

## Top Replies

• A Bandpass filter (BPF), can be wide band or narrow band. An arbitrary definition of wide band BPF is the minus 3dB passband is greater than the center frequency (fc). A narrow band BPF is defined by its selectivity. Selectivity is the magnitude of the stopband attenuation at an arbitrary frequency higher or lower than fc. The amplitude plot of a second order BPF illustrates the selectivity. For example, the stopband attenuation at twice fc is -5dB, -10dB, -17dB, and -23dB for Q equal to 1, 2, 5, and 10 respectively. The BPF selectivity for a Q equal to 5 or 10 is much higher than for a Q equal to 1 or 2. For a LPF, Q values of 1 to 2 are sufficient and for a high selectivity narrow band BPF, Q values of 5 or higher are required. • A Bandpass filter (BPF), can be wide band or narrow band. An arbitrary definition of wide band BPF is the minus 3dB passband is greater than the center frequency (fc). A narrow band BPF is defined by its selectivity. Selectivity is the magnitude of the stopband attenuation at an arbitrary frequency higher or lower than fc. The amplitude plot of a second order BPF illustrates the selectivity. For example, the stopband attenuation at twice fc is -5dB, -10dB, -17dB, and -23dB for Q equal to 1, 2, 5, and 10 respectively. The BPF selectivity for a Q equal to 5 or 10 is much higher than for a Q equal to 1 or 2. For a LPF, Q values of 1 to 2 are sufficient and for a high selectivity narrow band BPF, Q values of 5 or higher are required. 