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50 The PCB Design Magazine • December 2014 material, the solder mask and the air above. In a stripline configuration, it is dependent on the FR-4 material above and below the trace. So, microstrip signals tend to travel faster than stripline as the effective Er is smaller. Also, an accordion or serpentine pattern will be faster than the equivalent straight trace, due to for- ward crosstalk. So this is not a simple calcula- tion and requires a simulator to compare delays on different layers of the substrate. Rather than matched length, one needs to compare the actual delay as the signals prop- agate through different materials in the sub- strate. This is done in order to determine the skew between clocks (or strobes) and the groups of signals they control. If you must assess using matched length, tighten the tolerance as this will still reduce the skew. Routing critical signals between the planes can reduce emissions by 10 dB or more. There are four constraints to keep in mind: 1. Keep the mark-to-space ratio of the waveform equal as this eliminates all the even harmonics. 2. Route high-speed signals out from the centre of the board where possible, as any radiation will be in the opposite direction and will tend to cancel. 3. Route high-speed signals between the planes, fanout close to the driver (200 mils) dropping to an inner plane and route back up to the load again with a short fanout. 4. Use the same reference plane for the return signals, as this reduces the loop area and hence radiation. Embedding signals between the planes also reduces susceptibility to radiation, as well as providing ESD protection. So, not only does this prevent noise from being radiated, but it also reduces the possibility of being affected by an external noise source. In Figure 6, the noisy waveform produc- es high levels of electromagnetic emissions. With a maximum radiation of 56.94dB @ 7.5GHz, this well exceeds the FCC/CISPR Class B limit. Generally, this is due to unter- minated transmission lines, excessive cross- talk or too much driver strength. Possible so- lutions to try are: 1. Reduce the driver current to the medium strength of 8mA. 2. Check for crosstalk, particularly on long segments. 3. Add series terminators. SIGNAL INTEGRITY, PART 3 continues beyond design Figure 6: noisy waveforms increase radiated emissions.