What is 3 dB Bandwidth of a Filter?


What is 3 dB bandwidth in a band pass filter? What is 1 dB bandwidth?

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Editorial Team - everything RF

Nov 4, 2019

An RF filter is used to block unwanted signals in a wireless system. It is designed to allow signals in a particular frequency range to pass through with minimal losses while blocking all other signals. 

The bandwidth of a band pass filter is the frequency range that is allowed to pass through with minimal attenuation. The frequency at which the power level of the signal decreases by 3 dB from its maximum value is called the 3 dB bandwidth. A 3 dB decrease in power means the signal power becomes half of its maximum value.

If we draw the graph of an ideal bandpass filter, there is no signal loss near the center frequency of the filter. As we go further away from the center frequency the signal starts to attenuate on both the lower and upper end of the passband frequency range. The 3 dB bandwidth is the frequency at which the signal amplitude reduces by 3 dB i.e. becomes half its value. 

The bandwidth of a bandpass filter is usually defined as the 3 dB bandwidth.

Similarly, the 1 dB bandwidth is the point at which the signal amplitude decreases by 1 dB from its maximum value (above and below the center frequency).

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