Matched Length Does Not Always Equal Matched Delay


Reading time ( words)

In previous columns, I have discussed matched length routing and how matched length does not necessarily mean matched delay. But, all design rules, specified by chip manufacturers regarding high-speed routing, specify matched length--not matched delay. In this month’s column we’ll take a look at the actual differences between the two.

Typically, more than one layer change is required when routing traces to matched length. Figure 1 illustrates the DDR2 address bus routing I did in Altium Designer, my preferred layout tool. In this case, each address signal has four layer changes. The red and green traces are the top and bottom layers--which should be kept as short as possible--and the yellow and orange traces are inner layers embedded between the planes. This was a particularly difficult route as there were two DDR2 memory chips placed on both the top and bottom sides of the board, so each address signal had to go to four different chips and still maintain the correct delay.

 Olney_Delay.jpg

Figure 1: Matched delay T-section DDR2 address routing in Altium Designer.

The longest routes should be placed on the inner layers as this reduces electromagnetic radiation. With all other factors being equal, generally, a trace routed on the inner stripline layer exhibits 4-10 dB less noise than a trace routed on the outer microstrip  layer. Also, please note that there are more high harmonics on the top layer routing. The high-frequency components radiate more readily because their shorter wavelengths are comparable to trace lengths, which act as antennas. Consequently, although the amplitude of the harmonic frequency components decreases as the frequency increases, the radiated frequency varies depending on the trace’s characteristics.

Read the full column here.


Editor's Note: This column originally appeared in the March 2014 issue of The PCB Design Magazine.

Share


Suggested Items

Signal Integrity: The Experts Weigh In

11/14/2017 | The I-Connect007 Team
When we began planning the October issue on signal integrity, we arranged a conference call with a variety of industry experts. Mike Steinberger of SiSoft, Mark Thompson of Prototron Circuits, and Yogen and Sunny Patel of Candor Industries joined editors Andy Shaughnessy, Patty Goldman, Happy Holden and Publisher Barry Matties on the call for a spirited discussion about the challenges related to signal integrity and some of the tricks of the trade for helping ensure SI.

Nine Dot Connects: Good Design Instruction is a True Value-Add

11/09/2017 | Andy Shaughnessy, PCBDesign007
Nine Dot Connects has certainly blazed an interesting trail. The company started out as an Altium reseller, but in less than a decade, Nine Dot Connects has also become a design service bureau and a provider of PCB design instruction, training, and consulting services. I recently interviewed Paul Taubman, technical services director for Nine Dot Connects. We discussed the company’s expansion from VAR to service bureau and content provider, and the changing landscape of PCB design instruction.

AltiumLive Summit—Munich, Germany, Part 1

11/07/2017 | Pete Starkey, I-Connect007
Altium held a very successful AltiumLive PCB Design Summit in San Diego, California at the beginning of October for the benefit of their North American design community, and followed it three weeks later with a counterpart European event in Munich. And what an eye-opener it proved to be—literally hundreds of delegates, a superbly organised and managed programme, billed as a completely immersive two-day interactive design experience on a theme of learning, connecting and getting inspired.



Copyright © 2017 I-Connect007. All rights reserved.