Grounded Coplanar Waveguide PPCB

[ejtough]

Hi,

I want to find a method to create high-frequency grounded coplanar waveguides PCBs quickly using EAGLE. In order to maintain my 50ohm impedance I need to have specific signal widths and gap that I know the dimensions of for the specific dielectric substrate and thickness from simulating in an external software. 

I am a total beginner with EAGLE and would like to know how to do the following:

1. Create a signal line on one edge of the board with a specific width (s1) 

2. Taper the signal line a 2nd larger signal width (s2)

3. Add coplanar ground pour that has clearance (g1) for signal width (s1), continuous increase across the taper length and clearance (g2) for signal width (s2)

4. Add via fences on ground pour, along length of signal lines with specific via separations

The geometry for the PCB is quite complex, requiring multiple G-CPWs at s1,g1 that the radially expand to s2,g2 via a taper length and is used to extend an RF line from our device to edge coupled SMA connectors. Other than the SMA connectors, there are no other electronic components on the board, and I require only G-CPW traces from one edge to the other.

I cannot find any suitable tutorials online and I'm yet to find a software package that allows these sorts of PCBs to be designed without multiple workarounds. So I'm hoping someone can advise me on whether this is possible in EAGLE?

Many Thanks,

ETough

[jorge_garcia]

Hello @ejtough ,

I hope you're doing well. This is possible in EAGLE and Fusion Electronics, but it will require workarounds. There's currently no built in support for tapered traces so you'll have to create the tapers by using polygons that enclose the traces. You'll have to calculate the vertices of all of the tapers in order to create that geometry.

So possible yes but it will require work. You can probably leverage radial symmetry to simplify your work.

Let me know if there's anything else I can do for you.

Best Regards,

[csvanberg]

Using polygons for very tight tolerance pads and traces in board layout is risky. You can make Devices with Symbols and Footprints for "odd" shapes, Eagle has yet to embrace RF design features.
Use curved lines as much as possible. You can order test coupons with your traces (I did) and measure impedance with an RF Vector Network Analyzer.

I needed a tight bend and make a 90 degree bend like this, https://www.everythingrf.com/rf-calculators/microstrip-mitred-bend-calculator

I used another CAD program for setting up dimensions and transferred these to Eagle. I use two small pads to attach traces, and the rest polygons around them. The DRC will complain about this.

[ejtough]

Hi all.

Many thanks for your responses. It's a shame there isn't a full implementation for the design of these types of RF PCBs in the software package. They are very common in my line of work/study and are currently time consuming to design so I was hoping EAGLE may offer some simplified solution. 

I ended up using Fusion 360 CAD to construct the new PCB design. It helpfully has an Offset tool that allowed me maintain the necessary 50 ohm impedance dimensions, create suitable gap features that I could then Cut from the Extruded copper body. It ended up being easier to do this so that my PCB also would be compatible with the other devices that this would be working alongside. 

I hope to see EAGLE embrace the RF solutions in the future. 😀

[salepcb]

Hi,  about Grounded Coplanar Waveguide PCB fabrication, maybe you can ask a PCB manufactuerer for a assistance , their engiener could help on stack up and track width/spacing design.

[csvanberg]

Those tapered sections of the 50 Ohm lines in the pic in this thread are not 50 Ohm. This may be OK, and either compensated by the chip they connect to, or have an acceptable loss. The tapering gives a smooth transition, which is preferable. If the narrow traces were to be 50 Ohm, then the dielectric constant would also need to be much higher in those section, and taper as well. That's not happening in a PCB.