Issue link: https://iconnect007.uberflip.com/i/1478618
28 DESIGN007 MAGAZINE I SEPTEMBER 2022 e electric fields surrounding the microstrip exist partially within the dielectric material(s) and partially within the surrounding air. Since air has a dielectric constant of one, which is always lower than that of FR-4 (typically 4.3), mixing a little air into the equation will speed up the signal propagation. Even if the trace widths are adjusted on each layer, so that the impedance is identical, the propagation speed of microstrip is always faster than stripline, typically 13–17% (Figure 5). e speed of propagation of digital signals is independent of trace geometry and impedance. If you are aware of this issue, then the trace delays can be matched to compensate for the varying flight time, so that at the nomi- nal temperature, all signals running on either microstrip or stripline will arrive at the receiver simultaneously. Unfortunately, drivers do not have the same impedance as the transmission line (typically 10–35 ohms) so terminations are used to bal- ance the impedance, match the line, and minimize reflections. Reflections occur when- ever the impedance of the transmission line changes along its length. is can be caused by unmatched drivers/loads, layer transitions, different dielectric mate- rials, stubs, vias, connec- tors and IC packages. By understanding the causes of these reflections and eliminating the source of the mismatch, a design can be engineered with reliable performance. As shown in Figure 6, using a 12 mA LVCMOS 1.8V driver of a Spartan 6 F P G A , a n 1 8 . 7 - o h m series resistor is required to match the driver to the 51.67 ohm trace on the outer layer. is is auto- matically derived from the IV curves of the Spartan 6, IBIS model by the iCD Termination Planner (Figure 6). Correct termination is extremely impor tant as it reduces transmission line reflections that can cause multiple issues for the design integrity. So, apart from the accurate determination of single-ended, differential and broadside cou- ple impedance, a field solver can offer further insight to enhance the signal and power integ- rity of a design. Key Points • Although power planes can be used as reference planes, the ground is more effective. • To calculate the layer count, look at a reference design with similar characteristics and then add two layers. • With no tweaking, the autorouter needs to complete at least 85% of the routes to indicate the selected stackup is routable. • It is much easier to increase the number of layers than to reduce them. • A field solver can be used to determine the unknown variables for an established impedance goal. Figure 5: Relative signal propagation of microstrip and stripline, simulated in iCD Design Integrity.