The reference level for the relative attenuation a rel of TFS374D is the minimum of the pass band attenuation a min. This value is defined as the insertion loss ae . The centre frequency fc is the arithmetic mean value of the upper and lower frequencies at the 3 dB filter attenuation level relative to the insertion loss a e. The given values for the relative attenuation a rel and the group delay ripple have to be reached at the frequencies given below, even if the centre frequency f c is shifted due to the temperature coefficient of frequency TCf in the operating temperature range and due to a production tolerance for the centre frequency fc.