rrite bead on each of the upper may actually increase susceptibility to lower frequency common leads. The 3-terminal configuration can extend the effectiveness of mode interference. This is a function of the capacitance imbalance a small ceramic capacitor from below 50MHz to upwards of between the isolated signal and return lines (Figure 23.33), and it 200MHz, which is particularly useful for interference in the v.h.f. may restrict the allowable r.f. filter capacitance to a few tens of pF. band. Another problem may arise if several signal lines are being fil- tered together and a common earth point is being used, as is the case for example with the filtered-D range of connectors. Provided that 23.14.4.1 Feedthroughs the earth connection is low impedance there is no problem, but any series impedance in the earth path not only degrades the filtering but will also couple signals from one line into another (Figure 23.34), Any leaded capacitor is still limited in effectiveness by the induct- leading to designed in crosstalk. For this reason filtered connectors ance of the connection to the ground point. For the ultimate perfor- should be used with care, and they must always be well grounded to mance, and especially where penetration of a screened enclosure the case, as long as the case is the signal ground. must be protected at u.h.f. and above (this is more often the case for military equipment) then a feedthrough construction is essential. Here, the ground connection is made by the outer body of the capacitor being screwed or soldered directly to the metal screening 23.15 Cables and connectors or bulkhead (Figure 23.32). There is effectively no inductance associated with this terminal and the capacitor performance is The EMC of any given product is always affected by the configura- maintained well into the GHz region. The inductance of the through tion of the cables that are connected to it. Stray inductance limits h.f. attenuation Figure 23.31 Two and three terminal capacitors: (a) two terminal; (b) three terminal 