e a low impedance circuit but will be quite useless at high imped- about 10MHz. The larger the components are physically, the lower ances. Conversely, the simple capacitor will give good results at will be the break frequency. As the frequency increases beyond high impedances but will be useless at low ones. The multi-compo- capacitor or inductor self resonance the impedance of the capacitors nent filters will give better results provided that they are configured in the circuit actually rises, or that of the inductors drops, SO that the correctly; the capacitor should face a high impedance and the insertion loss begins to fall. This can be countered by using special inductor a low one. construction for the capacitors. Frequently, ZS and Z are complex and perhaps unknown at the The components are not the only cause of poor h.f. performance. frequencies of interest for suppression. The mains supply imped- Layout is another factor; lead inductance and stray capacitance can ance is fairly predictable and can be quite easily modelled. H.F. contribute as much degradation as component parasitics. Two com- impedances for most signal circuits can be derived, but it is not SO mon faults in filter applications are not to provide an adequate low easy for power supply inputs, as power components such as trans- inductance ground connection, and to wire the input and output formers, diodes and reservoir capacitors are not characterised at h.f., leads in the same loom or at least close to each other, as in Figure and the impedances will either have to be measured or guessed. 23.28. Low Z Low Z High Z High Z Low Z High Z High Z Low Z High-to-medium Z Low-to-medium Z Figure 23.27 Low pass filter 23/22 Safety and EMC Equipment shield Stray coupling capacitance Coupling path through ground impedance filter ground Common ground connected to impedance low inductance earth or screen Figure 23.28 Filter wiring and layout: (a) bad; (b) good A poor ground offers a common impedance which rises with regulations: add on block mains filters a