 be done by lapped or flanged joints. guard against either or both of these, but some knowledge is re- d = 0.25mm 2.5cm 0.25cm d = 25cm 10kHz 100kHz 10MHz 100MHz 10GHz Frequency Figure 23.26 Attenuation through an aperture Filtering 23/21 quired of the characteristics of the circuit in which the filter will be When the source is taken to be a cable acting as an antenna any embedded to design or select the best filtering device. analytical method for deriving its impedance will be irrelevant Filters fall broadly into three categories: those intended for mains when applied to the real situation, in which cable orientation and terminals, for input/output connections and for individual power or positioning is uncontrolled, and a nominal value has to be assumed, signal wires. The basic principle is the same for all of these. typically 150S. 50Q is usually specified as the test impedance for filter units, but it should be remembered that published insertion loss figures in a 500 system are not likely to be obtained in the real 23.14.1 The low pass filter application. This does not necessarily matter, provided the filter and circuit combination is carefully characterised and tested as a whole. The simpler circuit arrangements which offer a low pass response are shown in Figure 23.27. The attenuation of any given filter is conventionally quoted in terms of its insertion loss, that is the 23.14.1.1 Components and layout difference between the voltage across the load with the filter in and out of circuit. The insertion loss depends not only on the filter Filter components, like all others, are imperfect. Inductors have self components but also on the source and load impedances. The simple capacitance, capacitors have self inductance. This complicates the inductor circuit, in which the inductor may be nothing more than a equivalent circuit at high frequencies and means that a typical filter ferrite bead will give good results, better than 40dB attenuation, in using discrete components will start to lose its performance abov