Shields are your friend, except when... (Part 3)
I've written about how to use Coplanar Waveguide Over Ground (GPWG) and its benefits before  and there are several reasons to use CPWG (Figure 3):
- It allows us to narrow the trace almost arbitrarily for a given layer thickness while maintaining a 50- or 75-ohm impedance. This helps us to more closely match our component width with the PCB trace width. This not only makes our RF designs perform better it saves PCB real estate.
- Less current flows into the air above the transmission line with CPWG compared to a microstrip structure. This is because the topside ground copper shunts the field current locally to the topside ground.
Figure 3: Coplanar Waveguide Over Ground adds top copper to a Microstrip structure. This allows the transmission line trace to be narrowed almost arbitrary for a given substrate thickness by adjusting the gap between the transmission line and the topside ground as shown. Also see reference 1.
Point #2 is the aspect that will be exploited this time. If we can keep the field from our PCB trace from interacting too much with the top cover of our shield we will mitigate the problem.
Most RF designers use rules of thumb to space things appropriately. For instance, in microstrip design, many designers use a rule of thumb to keep topside copper away from their microstrip by four times the substrate layer thickness so as to not affect their microstrip impedance. With CPWG you don't have to worry about this since the topside ground is purposefully brought up next to the topside RF trace.
But do you have to worry about that close board shield?
To find out, I conducted a series of simulations using the usual textbook formulas to see the effect of the shield height over the PCB.
For two different substrate thicknesses I compared microstrip with CPWG versus cover height just to get a feel on the sensitivity.
Let's take a look at a comparison of trace impedance vs cover height for a microstrip and CPWG on a 62-mil thick substrate (Figure 4).
In Figure 4, it is easy to see that the microstrip trace impedance (red trace) is much more sensitive than the CPWG trace (blue trace) to the cover height.
Why? Well, this is proof that the microstrip trace has much more field in the air above the trace than the CPWG trace, whose field is more concentrated around the topside copper ground return path.
This is great, not only because of its lack of sensitivity to close-in metal objects, but also because there are less radiated emissions with CPWG, which makes our shielding job even easier – a true win-win.
What about the sensitivity to substrate thickness? Figure 5 shows a comparison of this. The least sensitive trace is the red one – this is a CPWG trace on a 10-mil thick substrate, while the blue trace is a CPWG on a 62-mil thick substrate.
We can say that this shows that the thinner the substrate is, then the less field there is in the air above the CPWG trace. The cover only starts to affect the trace by a significant amount when it gets closer than about a substrate's distance away.
Perhaps a better way to look at the data in Figure 3 and 4 is to normalize the distance to the substrate height. Figure 6 shows the data presented this way.
By comparing the CPWG and microstrip cases together on a normalized cover height plot, you can see that the CPWG trace (blue) only gets a really significant shift to about 45 Ohms at 0.4 cover heights away, whereas the microstrip trace (red) reaches the same low impedance when the cover height is a whopping 3X the substrate height.