ts are expected the lead Feedthrough capacitors are available in a wide range of voltage and wiring inductance has a significant effect on the circuit's ability and capacitance ratings but their cost increases with size. Cheap to clamp the edge of the pulse. In these cases a combined capaci- solder-in types between 100pF and 1000pF may be had for a few tor/varistor component, in which the ceramic dielectric is treated to tens of pence, but larger screw mounting components will cost give it a predictable low voltage breakdown characteristic, can offer £1-£2. If good performance is required down to the low-MHz a solution. region then the filters will be more expensive. A cheaper solution is to parallel a small feedthrough component with a larger, cheaper 23.14.4 Feedthrough and 3-terminal capacitors conventional unit which suppresses the lower frequencies at which physical construction is less critical. Any low pass filter configuration, except for the simple inductor, uses a capacitor in parallel with the signal path. A perfect capacitor would give an attenuation increasing at a constant 20dB per decade 23.14.4.2 Circuit considerations as the frequency increased, but a practical wire ended capacitor has some inherent lead inductance which in the conventional configu- When using any form of capacitive filtering, one has to be sure that ration puts a limit to its high frequency performance as a filter. This the circuit can handle the extra capacitance to ground. This factor lead inductance can be put to some use if the capacitor is given a can be particularly troublesome when one needs to filter an isolated three terminal construction (Figure 23.31). circuit at radio frequencies. The r.f. filter capacitance provides a The lead inductance now forms a T-filter with the capacitor, ready made a.c. path to ground for the signal circuit and will greatly improving its high frequency performance. Lead inductance seriously degrade the a.c. isolation, to such an extent that an r.f. filter can be enhanced by incorporating a fe