Stopband Rejection and Attenuation

The adjacent curves are used to determine the out-of-band or stopband attenuation for ClearComm's Discrete Element Lowpass Filters. These curves show the attenuation as multiples of the normalized stopband frequency to the 3 dB cutoff frequency for filters up to 12 sections. For requirements with greater rejections needed, please contact the factory.

The formula for approximate stopband attenuation:

3 dB Bandwidths from Cutoff Frequency = Reject Frequency / Cutoff Frequency

Example:
Cutoff Frequency (fc) = 230 MHz
40 dB attenuation required at 315 MHz

We must first calculate the ration of the two frequencies:

315 / 230 = 1.37

We can see from the table above that to achieve 40 dB attenuation at 1.37 away that we need a minimum of 5 sections. Always use the higher number of sections.

Insertion Loss

Knowing the number of sections, the insertion loss may be calculated from the formula:

Loss = 0.2 x N

Example:
Number of sections = 9
Insertion Loss = 9 x 0.2 = 1.8 dB

Note: For Lowpass filters, insertion loss is calculated at 0.9 times the cutoff frequency