Filters

A crystal filter is also known as a Band Pass filter. These filters pass signals in a specific frequency band called pass band frequency range and reject/attenuate signals in a specific frequency band called the stop band frequency range. An crystal filter can use quartz crystals as resonator components of a filter circuit.

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

Ripples are the fluctuations (measured in dB) in the pass band, or stop band, of a filter’s frequency magnitude response curve. Elliptic and Chebyshev-based filters have constant ripple across their pass bands, while Bessel and Butterworth derived filters have no ripple in their pass band responses. Ripples in the stop band response are called as out-of-band ripple.