Printed Circuit Design & Fab - December 2007 - (Page 33)

for a high-speed differential voltage is being doubled, and signal. Note that the ripples theoretically, the noise affects are placed near the receiver the tightly coupled traces where the source of length equally. Therefore, they canimbalance lies. cel each other out. In addition to being less Rule #2: The traces susceptible to interference, within a differential pair differential signals also tend need to be routed as close to produce less EMI. This is as possible. because the changes in signal This rule is based upon levels (dV/dt or dI/dt) cre- FIGURE 2. Differential signals are immune to ground offsets the fact that since the differate opposing magnetic fields, that exist between the transmitter and receiver. ential signals are equal and again canceling each other opposite, and if external noise out. equally interferes with these should arrive at the receiver at the same Another benefit provided by difsignals, it (the noise) should nullify. time for its proper operation. For this ferential signaling is the ability to conSimilarly, any unwanted noise induced reason, the traces need to be equal in trol switching timing more precisely. by the differential signals into adjacent length. Remember, that for the single-ended conductors should cancel each other Recent studies have shown that difsignals, the return currents are on an out if traces are routed side by side. ferential circuits can tolerate switching imaginary path on a reference plane. However, if the signals do not have fast timing, and hence, the length mismatch For these signals, the crossover point is rise times, then side-by-side routing to a great deal, and will show little or either a one or a zero, which is subject becomes comparatively unimportant. no jitter. In fact, many people would to crosstalk or outside EMI. Since the Furthermore, sometimes designers fail go to the length of declaring it a myth. differential pairs are referenced to each to understand that there exists some They fail to understand, however, that other, and the pair is routed as equal switching timing is just one of the and opposite, it brings about a more reasons why differential signals are precisely controlled crossover point, as maintained at equal lengths. If there shown in FIGURE 3. are high-speed differential signals that are not exactly equal, then their return currents will not cancel each other, and Differential Signal Design there may be high-speed currents on Rules one of the planes that are unaccounted When the differential signal is launched for. These currents can create serious from the cable to the PCB, interconproblems such as EMI and crosstalk nections may cause problems at high FIGURE 3. Differential pairs, if equal for other circuits. FIGURE 4 shows one speeds because of impedance disconand opposite, allow precise control of tinuities that exist between the cable, way of maintaining an equal length the timing of logic state change. connector or the board. Specialized connectors are used with proper shielding. Special modeling, simulation and measurement validation are required to ensure a smooth transition of the signal from the cable to the trace. Once this transition has been taken care of, the signal comes under the domain of the layout designer, and it is his or her responsibility to ensure its signal integrity. There are some rules that need to be followed while implementing differential signals. Rule # 1: The differential traces in a pair need to be of an equal length. This rule originated from the fact that a differential receiver will detect the time at which the negative and positive signals cross each other – the crossover point. Hence, the signals DECEMBER 2007 PRINTED CIRCUIT DESIGN & FAB 33 http://www.pcb-pool.com http://www.pcb-pool.com

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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