- Buyers Guide
Signal Loss: A Big Picture Approach to RF/MW PCB Design
Oct 21, 2011
Judy Warner is currently the Director of Sales and Marketing for Transline Technology, Inc. in Anaheim, CA. Judy has been in the Printed Circuit Board industry for nearly two decades. Her career began with Details, Inc. (later to become DDi). She was a Top-Producing Sales Professional for 10 years for Electroetch Circuits (later to become Tyco, then TTM). She has also spent several years as an Independent Sales Representative including time as the owner of her own Rep firm, Outsource Solutions. This blog is part of Microwave Journal's guest blog series.
A few weeks back, I had the opportunity and pleasure to co-present a talk at PCB West in Santa Clara, CA. The topic we chose was: Navigating the Pitfalls of RF/MW and High Perfomance PCB Design and Fabrication. My co-presenter was Michael Ingham, of Spectrum Integrity, whose design firm is highly focused on challenging RF/MW and High Performance PCBs.
Since we only had an hour for our talk, we created a supplementary paper to elaborate on our talking points. One of the ideas that intrigued me was a point that Michael made about what creates losses in a PCB. We often talk about loss tangent as if it were The source of loss (with a capital “T”) and focus on this number on data sheets, more than all other factors present. Below is a portion of the paper we created, where Michael discusses the 4 main components of loss:
It is very important to look at the big picture when selecting materials for use with critical signals. There are four main components of loss: conductive, dielectric, radiation, and “design induced”. Taken collectively together, these determine the total loss for a given signal path.
Figure 1 below shows a general loss calculation accounting for both the dielectric and conductive loss. Note that the conductive loss is a significant contributor to overall loss.
Dielectric Loss – this is the component of loss contributed by the dielectric material. Choosing materials with low Dissipation Factors (or Loss Tangents) help to reduce this component. However, this is just
one component of overall loss. By also attending to the other loss factors, it may be highly likely a material with a higher loss tangent may be acceptable. In this way, one can expect increased yields, lower costs and broader available supplier base.
Conductive Loss – this is due to the resistance of the metal of signal traces. It is important to note that for RF and higher signal speeds, the surface finish is a significant factor due to skin effect properties. The graph below in Figure 2 shows a simulation of Insertion Loss for two types of metal finish for the same dielectric material. The smooth finish performs substantially better.
Radiation Loss – this is due to the type of transmission line used. As a general rule, microstrip types of traces on outer layers will have more radiation loss compared to coplanar types of lines. Lines routed on internal layers, such as stripline types, have substantially less radiation loss.
Design induced Loss – these are generally due to discontinuities in signal paths. Care must be taken during the design phase to keep this at a bare minimum. Common ongoing problems seen include not properly transitioning between different types of transmission line structures, having gaps in ground planes underneath signals, not optimizing connector footprints to PCB (field match and impedance match), and many more.
He made an excellent point, by later stating, that we don’t always have to pick the material with the lowest loss tangent to achieve the desired performance. By being mindful of these other areas of loss while designing a board, we may be able to choose an alternative material that is less costly, more readily available, and easier to fabricate with. Makes sense to me!
I thought this was a simple, yet very helpful point to those of you designing these unique and challenging boards. Let me know if this was helpful!