New-Tech Europe | November 2016 | Digital edition

induced by the irregularities in the weave of the glass cloth required to provide mechanical strength in the PCB can cause a signal path to fail. 4. Signal loss along the data paths is still an issue but, in most cases, can be handled with the materials currently available used to fabricate PCBs and backplanes. However, as the shift is made to 56 Gb/s and higher, loss in the data path comes back into the equation as a major issue. Solving the first three problems has met with varied success. The first problem (excess capacitance in the plated through holes) has been dealt with by using a technique called back-drilling to remove the excess capacitance of the connector plated through holes that extend below the layer in which the signal traces are routed. In thick backplanes this has resulted in very complex manufacturing procedures to ensure enough of the plated through hole copper has been removed to achieve proper performance while at the same avoiding drilling so deep that the connection is severed. Figure 1 is a loss vs. frequency plot of two 8” (20 cm) traces. The red trace is routed near the bottom of the PCB so that the signal travels nearly the full length of the vias used to connect to the trace. The blue trace is routed near the top of the PCB so that the signal travels only part of the way along the vias leaving small parasitic capacitors attached to each end of the trace. Clearly, the bandwidth of the blue trace has been severely affected by these parasitic capacitors. Back drilling removes this unwanted parasitic with the risks noted earlier. The second problem (excess crosstalk) has been dealt with by routing the signals farther and farther apart from each other so this problem is

Figure 1. Block diagram of hybrid beamforming transmitter.

Figure 2. 1080 glass weave and a 3.5 mil wire

These changes have exposed a number of microdefects in the signal path that were of little consequence in previous products running at lower data rates. These microdefects include: 1. The parasitic capacitance of the plated through holes required to mount the connectors can introduce substantial bandwidth degradation.

2. Crosstalk between transmit and receive signals can be severe because those signals that tolerate 38 dB of loss at the receivers are far more susceptible to interference from a signal leaving a transmitter at full amplitude. 3. The difference in travel time of the two sides of a differential pair (skew)

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