In theory, the impedance, Z, of a perfect capacitor should decrease indefinitely with increasing frequency, following the relationship Z = 1/2 fC. Similarly, the suppression performance, i.e. insertion loss, of a perfect capacitor should increase indefinitely with frequency. However, this is not the case in practice, and conventional two-wire capacitors will not operate as effective suppressors over a wide frequency range.

Every capacitor has an intrinsic value of inductance which, together with the inductance of internal connections and terminal leads, forms a series resonant circuit with the capacitance. The self-resonant frequency occurs where capacitive reactance (1/2 fC) and inductive reactance (2 fL) are equal.

The typical frequency response of a conventional 1µF capacitor with 20mm lead length is illustrated - in simplified form - in Figure 1. The actual and theoretical graphs can each be considered to represent both impedance and insertion loss as indicated.

As the frequency increases above the self-resonant frequency, the capacitor impedance becomes inductive and starts to increase, causing the suppression effectiveness of the two-wire capacitor to diminish rapidly. The self-resonant frequency is dependent on the length of the connecting leads; a lower self-resonant frequency and lower overall performance will result if longer leads are used.

In general, a conventional two-wire capacitor has very limited use as a suppressor beyond its self-resonant frequency. If suppression performance is required above this frequency, a feedthrough suppressor must be used.

Feedthrough capacitors
The feedthrough construction of capacitor has a very low internal series inductance and, in effect, no external lead inductance. This provides a suppression performance over a much wider frequency range than a conventional two-wire capacitor of equivalent value.

As the term implies, a feedthrough capacitor has a current-carrying conductor passing through its center. This co-axial conductor forms one terminal of the capacitor. The other terminal is the metal outer case of the capacitor, which is specifically designed for mounting through an earthed metal bulkhead. This design feature is common to all feedthrough capacitors and ensures that any radio frequency currents carried on the central conductor are shunted to earth by the capacitor.

Because of the extremely low series inductance resulting from this type of construction, the self-resonant frequency of a feedthrough capacitor will be very high. A typical frequency response is shown in Figure 2.

As frequency increases, the impedance of the feedthrough capacitor decreases steadily to provide excellent performance to beyond 1GHz. Some small resonances can be expected in the performance characteristics of feedthrough capacitors, as shown in Figure 2. These are usually attributable to distributed inductance within the capacitor and can cause its high frequency response to vary slightly from the theoretical.

When the performance exceeds a certain level of insertion loss, it will level out instead of increasing further. This is due to the series resistance within the circuit (capacitor e.s.r. and lead resistance) becoming a limiting factor, instead of the capacitor impedance. For the type of feedthrough filters and capacitors covered by this catalog, the figure at which the insertion loss levels off (in a 50 ohm system) can be well in excess of 90dB.