Transmission Line Theory as Applied to the Design of Low Pass Noise Filters

The insertion loss of a low pass noise filter is dependent upon the impedance of the source of energy and the load as well as the values and number of elements of the filter. In turn, the impedance of the load and source as seen by the filter are a function of the characteristic impedance of the lines leading to and from the filter and the length of the lines with regard to the frequency, as well as the terminating impedances. The determination of the general characteristic impedance of a pair of lines are thoroughly defined, along with the subsequent reflections caused by the difference of impedance between the load, source and the lines. The resultant impedance as seen at the filter is then used to obtain general equations which can be used to determine the insertion loss of filters in a mismatched network. The insertion loss of a pi filter in a matched 50 ohm system and a typically mismatched system is then calculated with the results tabulated in graph form to illustrate the significance of considering the transmission line effect in predicting the insertion loss of a low pass filter.