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42 The PCB Design Magazine • February 2016 how effectively I can speak, and how well you can hear me. If we are in a quiet room, I can probably speak very quickly and you can still hear me. If we are far apart, or the environment is noisy, I will have to speak louder, more slowly and clear- ly. With electrical communications the situation is much the same. The speed limit is not due to the latency, but rather how fast one end can transmit with the other end still being able to reliably receive. This is limited by noise picked up from the environment and distortions intro- duced by the medium—the noise margin. It is the signal-to-noise ratio (SNR) that be- stows optical fibers with a higher bandwidth than other transmission mediums, given the same speed of transmission. One of the most important ways to determine the quality of a digital transmission system is to measure its bit error ratio (BER). The BER is calculated by comparing the transmitted sequence of bits to the received bits and counting the number of errors. Very small changes of the SNR (in the order of a dB) can cause very large changes in the BER. An eye diagram is a common indicator of the quality of signals in high-speed digital transmis- sions. An oscilloscope generates an eye diagram by overlaying sweeps of different segments of a long data stream driven by a master clock. In a simulation tool such as Mentor Graphics' HyperLynx, a pseudo-random bit stream is gen- erally used to produce the overlapping sweeps as in Figure 3. In an ideal world, eye diagrams would look like rectangular boxes. In reality, communications are imperfect, so the transi- tions do not lie precisely on top of each other, and an eye-shaped pattern results. However, in Figure 3, we can see the jitter (horizontal miss- alignment) and the distortion set by the SNR (vertical miss-alignment). In high-speed multilayer PCBs, we need to select the material with the lowest dielectric constant (Dk) and the lowest dielectric loss (Df) in order to achieve the maximum bandwidth which is the 5 th harmonic of the fundamen- tal frequency. (The dielectric materials library integrated in to the ICD Stackup Planner, has 20,000 rigid-flex materials up to 100GHz to choose from. This makes selecting the right ma- terial for your application easy.) As frequency increases, so does the band- width. However, we must select the most ef- ficient frequency for the particular transmis- sion channel. If the frequency is too low or too high, we lose the signal's power. This is due to how the medium responds to different levels of charge energy. In general, the amount of infor- mation you can transmit is proportional to the rate the channel can respond. Basically, one has to stay within a certain limit depending on the medium. It just so happens that the higher the operating frequency and the lower the loss, the easier it is to get wider bandwidths and hence more data reliably through the channel. Points to Remember: • A transmission line does not carry the sig- nal itself; it guides electromagnetic energy from one point to another. • Electron flow in a multilayer PCB, is ex- tremely slow—a few meters per second. • Photons and electrons transmit data at the same speed. The limiting factor is the relative permittivity of the medium in which the signal propagates. • Fibers have a number of advantages com- pared to copper, including higher band- width, lower loss, and immunity to elec- tromagnetic interference. • The charge carriers (electrons) move very slowly; however the knock-on effect is very fast as it follows the electromagnetic field. • DC and low-frequency circuits have a single point ground reference. However, above 100KHz parasitic capacitance and inductance become significant allowing current to flow in multiple paths. • The speed at which the signal travels, down the conductor, really has nothing to do with the drift speed of the electrons. The signal is an electromagnetic wave that travels at about half of the speed of light. The electrons serve to guide the wave down the wire. • It is the signal to noise ratio (SNR) that bestows optical fibers with a higher band- width than other transmission mediums, given the same speed of transmission. faster than a speeding Bullet