Insertion Loss: The loss at center frequency, normally expressed in dB, resulting from the insertion of a device in a transmission system. 

Flat Loss: The insertion loss at the frequency of minimum loss within the passband. 

Attenuation: The loss of a filter at a given frequency measured in dB. 

Passband (BW): The range of frequencies attenuated less than a specified value, typically 3 or 6 dB. 

Center Frequency (F_o): The arithmetic mean of the passband limits. 

Fractional Bandwidth: The ratio of the passband bandwidth to the center frequency.

Reference Frequency: A specified frequency, typically the minimum loss point of F_o, from which all attenuation measurements are made. 

Ripple: The amplitude difference, in dB, between the maximum passband peak and minimum passband valley. Both the peak and the valley are defined by a surrounding change in slope, i.e. sign of the amplitude response.

Attenuation: The output of a filter at a given frequency relative to the defined insertion loss reference. 

Stopband: The range of frequencies attenuated greater than some specified minimum level of attenuation. 

Transition Band (BWT): The range of frequencies differentially attenuated between the passband and stopband limits.

Shape Factor: The ratio of the bandwidth at some point within the transition region, typically 60 dB, to the specified passband bandwidth.

Spurious Attenuation: The specified minimum level of attenuation received by all non harmonic related resonances of each crystal resonator within the filter network.

Insertion Phase: The phase shift at the output load (measured at the reference frequency) resulting from insertion of the filter. 

Differential Phase: The measurement of phase at a given frequency relative to the phase at the reference frequency. 

Fig 1-2: Amplitude and Group Delay of two pole Chebyshev, Butterworth and Bessel responses.

Phase Linearity: The phase error in degrees between the phase points and the best straight line drawn through the phase points.

Group Delay: The time by which an input signal will be delayed before it appears at the filter output, i.e. the derivative of phase with respect to frequency. 

Differential Delay: The measurement of delay at a given frequency relative to the reference frequency.

Source Impedance: The impe-dance of the circuit driving /loading the filter, measured at the reference frequency. .(Rp / Cp)

Fig 1-3: General Design Limits. crystal filters are available in both tandem monolithic and discrete formats. Choice of realization is a function of applicable design contraints. Approximate bandwidth limits for filters, as a function of center frequency, are shown above.

Load Impedance: The impedance of the circuit terminating the filter at its output, measured at the reference frequency. (Rp / Cp)

Maximum Level: The driving point power, voltage or current level above which intolerable signal distortion or damage to the device will result.

Drive Level Stability: The ability of the crystal filter to return within a specified tolerance of its original insertion loss, at a specified drive level, after experiencing changing environmental and/or drive level conditions. 

Drive Level Linearity: The maximum permissable variation in insertion loss, per dB change in drive level, measured over a specified dynamic range. 

Inband Intermodulation Distortion: The attenuation, in dB, of 3rd and higher order signal products, inband, relative to the power level of two signals placed within the passband.