Such equipotentiality enforcement is not possible if two reference planes of the trace cannot be connected or stitched. If a few stitching vias are placed electrically close (within quarter of wavelength in dielectric at Nyquist frequency) to each signal via, the PPW waves will be substantially reduced, and the signal will be transformed into the TEM wave again (see Figure 5). The most critical location for such enforcement is the location of the signal via. Planes are connected with 3 vias around each port, to enforce the equipotentiality. Both ports are via-ports with signal terminal at the trace and reference terminals at the bottom plane only. The trace segment is excited with a 1 V 50 Ohm, 20 GHz harmonic source at the bottom left port. Power flow density in a trace with enforced equipotentiality. The vias make planes equipotential locally, and the signal goes mostly into the TEM mode of the trace as illustrated in Figure 5 (click on graphic to see animation).įigure 5. We can avoid or reduce such transformations in real life by short-circuiting or stitching vias, connecting the reference planes and enforcing the equipotentiality as shown at the bottom picture on Figure 4. On the other hand, the port from the trace to just the bottom plane with +1 V excitation (as shown in the middle picture of Figure 4) produces waves in both transmission line and transmission plane. That produces no voltage between the planes. The excitation port up has a – 1 V source, and the port down is +1 V. Such port construction removes the interaction between the TEM and PPW completely. Trace with ports to two planes (top), with ports to a bottom plane (middle) and with a port to bottom plane with short-circuiting vias around each port (bottom). Port construction is illustrated in the top picture in Figure 4.įigure 4. The first case from Figure 1 with ideally equipotential planes is just a model – 2 ports from the trace to the planes are used to excite only a TEM wave. TDR plot of a trace with two equal-potential reference planes (red plot) and a TDR plot of a trace with reference to just one plane at the excitation location (blue plot). In this simple case, the impedance of the trace referencing just one plane will be about 1 Ohm off on a time domain reflectometry (TDR) plot, as shown in Figure 3.įigure 3. In addition, the signal transformation changes the observable trace impedance. Unfortunately, those effects are practically unpredictable for complex designs. Waves in PPW are unwanted noise that can cause electromagnetic compatibility or interference (EMC/EMI) problems as well as unwanted propagation of signals between the components through parallel planes (multi-path propagation). Referencing the excitation to just one of the planes (GND) causes the signal to transform into a useful TEM wave as well as into radial waves of PPW. The trace segment is excited with a 1 V, 50 Ohm, 20 GHz harmonic source at the bottom left port. Power flow density in a trace as referenced to one plane. Figure 2 illustrates what happens with the signal in this case (click on graphic to see animation).įigure 2. What happens if one of the planes is a power plane and the other is a ground plane? Then the equipotentiality cannot be enforced. Related ResourcesĢ24 Gb/s Per Lane: Options and Challenges Both ports are via-ports with signal terminal at the trace and reference terminals simultaneously at the top and bottom planes. Power flow density in a trace with equipotential planes. For instance, if we launch a TEM wave into a trace with ideal equipotentiality of the parallel planes, we can observe the wave propagating strictly along the traces as in Figure 1 (click on graphic to see animation).įigure 1. Both waves are useful, but they have to be separated. TEM waves are used to transmit the signal between components while PPW waves are used to deliver power to the components. What can be simpler than that? Well, the fact that it is a three-conductor structure that supports propagation of transverse electromagnetic (TEM) waves as well as waves of the parallel plate waveguide (PPW) –superposition of radial or plane waves-complicates things. Traces on PCBs are often routed between parallel planes.
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