Typically, all connections in a digital circuit board are impedance controlled. This is by virtue of the fact that most layout designers will define, for example, 50 ohms as the default routing model for single ended connections.
The significance of defining the transmission line physical parameters prior to layout commencing is that the type of transmission line structure chosen will have a major effect on the component placement in RF circuits, particularly in terms of placement density. Layout designers will most commonly use one or more of the following types of transmission line structures.
- Embedded Microstrip
- Coplanar Wave Guide (CPW)
- Grounded Coplanar Wave Guide. (GCPW or CPWG)
There are many other transmission line structures that can be implemented in a circuit board, including:
- Parallel strips transmission line (sometimes called paired strips).
- Micro Coplanar Stripline.
- Coplanar Strips.
- Asymmetric Coplanar Strips.
- Asymmetric Coplanar Waveguide.
- Grounded Differential Coplanar Wave Guide.
And there are quite a few others….
The choice of which transmission line structures to use should be made collectively by the layout, engineering, EMC, fabrication, assembly and test personnel. All of these, and others, may have valuable input as to why a particular transmission line structure is or is not suitable for a given design. There are advantages and potential disadvantages to each of these transmission line structures as discussed below.
Dispersion is a measure of the property of transmission lines that have different group velocity versus frequency. A wide bandwidth signal is composed of a range of frequency components. RF designs are often intended to support a wide bandwidth, and in digital designs the actual frequency content of a square wave is also wide band, it is made up of a fundamental frequency plus a theoretically infinite number of odd harmonics of that frequency. As operating frequencies and edge rates continue to increase, so does the high frequency content of the signal.
Since the dielectric medium surrounding the microstrip trace is non uniform, above the trace is air while below is a substrate of a different dielectric, a portion of the high frequency electromagnetic field will be in air and a portion will be in the substrate. Since propagation delay is related to dielectric constant, the high frequency energy in air will see a different propagation delay to the energy in the substrate. This difference in delay along the line for different frequency content of the signal is called dispersion.
What to do about it?
Use stripline transmission lines wherever possible. Even though they are more expensive to fabricate because a multilayer circuit board is required. Stripline transmission lines do not exhibit the multimodal dispersion issues that microstrip transmission lines do, because they are fully embedded between two solid reference planes and in a homogenous dielectric. The electromagnetic field patterns in a stripline transmission line environment are exclusively TEM.
Do not use excessively wide signal traces in the microstrip transmission line model, it forces the signal trace to be too far from the reference plane. Of course, this can be a bit of a balancing act as well, because if the signal trace is too narrow then the skin effect losses will be greater. Signal trace to reference plane distances up to 20mils have been demonstrated to work fine up to frequencies in the 10-20GHz ranges.
This post is a condensed snippet from A Practical Guide to RF and Mixed Technology Printed Circuit Board Layout, written by Optimum's Brendon Parise. Available for purchase by following this link.