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52 SMT Magazine • May 2014 It is important to realise that this doesn't just cover the cables between circuit boards, but also the route of the current around each board. To minimise the loop area, the physical area bounded by the signal and its return current should be minimised. Ideally the signal and its return will be routed directly next to one an- other, which can be easily achieved in a ribbon cable (Figure 1a) or a traditional twisted pair (Figure 1b). Both of these arrangements have small loop areas, and so will help improve sig- nal quality. As soon as the return path for the ground current takes a different route to the signal cur- rent then a much larger loop area is formed causing three problems: 1. The signal becomes much more suscep- tible to electromagnetic interference (EMI)—the cable becomes an aerial picking up electrical noise. 2. The signal is radiated away from the cable, creating additional electrical noise that may in- terfere with other signals. 3. There is a change in characteristic imped- ance as the signal propagates down the cable. This degrades the quality of the signal. Figure 1c shows an obvious case where there is a larger loop area, and demonstrates how eas- ily this can occur if care is not taken to keep the signal and return wires close together. How to minimise the Loop area for Test Fixtures Test fixtures are often created using many individual wires, one for each active signal. Al- though this makes it easier to wire up the fix- ture, it can also lead to an electrically noisy environment and consequently signal integrity problems which are particularly evident when using JTAG. Figure 2 shows some common mistakes seen in test fixture design: The ground connection to the JTAG controller is connected to the DUT a long way away from the TAP signal connec- tions; and the ground and TAP signal cables do not run close to each other. This can be easily, feATuRe SIGNaL INTEGRITy IN TEST FIXTURES continues Figure 1: The arrangement of signal and return cables using (a) ribbon cables, (b) twisted pairs, and (c) individual wires, can have a significant impact on a signal's loop area.