Issue link: https://iconnect007.uberflip.com/i/310350
40 The PCB Design Magazine • May 2014 the propagation of signals along the media pro- viding the following: 1. A well-defined uniform path that exists for the flow of both signal and return current. 2. Conductors that are closely spaced by comparison to the wavelength of the signals. 3. Conductors that have a long length, compared to the space between the conductors. In 1995 at the Interop Expo, Broadcom en- gineers demonstrated their T4 Ethernet chipset operating over the worst possible cable—barbed wire. During the show, 100Mbps of data was successfully transferred through rusty barbed wire. Figure 2 shows the setup. Wideband Cor- poration later effectively demonstrated 1Gbps over barbed wire with their transceiver. Provid- ing the impedance and delay are constant along the length, the dielectric loss is low and the crosstalk is low (due to the large space between the pairs), the performance is not impaired. I wonder if razor wire would make it perform faster, as it is more cutting-edge. You may wonder how this actually works, since it is obvious that two uninsulated twisted wires would touch. The trick is that they used rusty barbed wire; the ferric-oxide coating acts as an insulator and the twisted pair looks like 100 ohms of impedance. So, it you try this at home, don't use new wire. Telegrapher's equations are a pair of linear differential equations that describe the volt- age and current present on a transmission line relative to distance and time. Oliver Heaviside developed the transmission line model in the 1880s. Remarkably, the theory still applies to transmission lines, of all frequencies, including high-frequency transmission lines, as seen in the multilayer PCB. The telegrapher's equations have many derivatives and the math employed is beyond the scope of this column, not to men- tion beyond the ability of the author. Contrary to common belief, the transmission line does not carry the signal itself but rather guides electromagnetic energy from one point to another. It is the movement of the electromag- netic field or energy, not voltage or current that transfers the signal. The voltage and current ex- ist in the conductor, but only as a consequence of the field being present as it moves past. The ICD Stackup Planner [5] , in Figure 3, il- lustrates the three most common transmission line structures of a multilayer PCB. For embed- ded microstrip (solder mask coated microstrip), the electromagnetic field propagates partially in the dielectric material (Isola 370HR), the sol- der mask and the air. Whereas, in both stripline structures, the electromagnetic field propagates in the dielectric material sandwiched between the planes. The traces simply guide the signals beyond design Figure 2: Broadcom Fast ethernet 100Base-T4 over barbed wire (courtesy Broadcom). TRANSMISSION LINES continues