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48 The PCB Design Magazine • September 2017 Signal paths are designed to be low Q reso- nators to dampen the ringing and to increase the bandwidth. However, cavities composed of a power and ground plane, can have very high Qs. This means even a slight amount of cou- pling, from signal paths, can drive resonances and give rise to long range noise voltages with- in the cavity. Knowing the cavity resonant fre- quencies, that one might encounter, can high- light potential problems. When the cavity has open end boundary conditions, resonances arise when a multiple of half wavelengths can fit between the ends of the cavity. Figure 3 shows the cavity reso- nance of a plane pair with a resonant frequen- cy of 1GHz. If the signal clock frequency (or harmonics) are multiples of 1GHz, then noise can be injected into the plane cavity. When the clock or data harmonics overlap with the cav- ity resonant frequencies, there is the potential for long range coupling between any signals that run through the cavity. This is one reason why all return planes should be GND layers, so that stitching vias between GND planes can be placed adjacent to each signal via transition to minimize the possibility of exciting the cavity resonance. Cavity resonances are (at first) a signal in- tegrity issue but the amplification of cavity resonance excited by fast rise time signals, at high frequencies, can also contribute to electro- magnetic emissions (EMI). The frequency com- ponents of the voltage noise are related to the peak impedance of the cavity and the frequency components of the return currents. In any com- plex system, with typical interconnect density, avoiding signal layer transitions is not practi- cable and is an issue that designers must live with. However, one can learn to avoid injecting excessive noise into the cavity or at least mini- mize the impact. The goal of designing a high-performance cavity is to reduce the impedance peaks below the target impedance level and to push the peak PLANE CAVITY RESONANCE Figure 2: Impedance profile of a plane cavity with VRM (Source: iCD Design Integrity).

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