Trace between RF module and SMA connector

Question

I am designing a simple board that uses a TI CC1101 based radio module operating at 868MHz or 915MHz.

Due to layout constraints I would like to locate an SMA connector some distance from the module, like this:

Don't worry about the unconnected GND, I am going to expand the ground planes when I'm happy with the trace from the module to the SMA connector, and also move the GND vias to surround that trace. This is an FR4, 4 layer board with 2oz copper on the top and bottom, 1oz on the middle layers.

It's unfortunate that having the connector centred places the RF trace close to the SPI bus digital lines, but the chip has an interrupt feature so those lines will not toggle during transmission or reception.

Wavelength on this PCB at 915MHz will be around 170mm. The trace is 11.5mm long and 0.25mm wide. My understanding is that if the trace is less than 1/10th the wavelength, things like an impedance mismatch aren't going to have any significant effect on signal strength. Is that all correct?

Other than moving the connector to the left, is there any way to improve this layout? I can probably move the module down about 1-2mm, so will do that. Unfortunately Kicad doesn't natively do rounded traces, but I could round off the corners manually.

Answer 1

Ti usually provides reference designs for some of their evaluation boards. Your first go to place would be the CC1101 reference design files such as the Gerber files and layer stackup etc. Otherwise I can say that your trace length is too short to be of concern. Better to distance the RF trace away from other traces and pads, and if you can move the connector to the left 2 mm or the IC right 2mm would be better. Your 50 Ohms trace width will be dependent on the stackup that your manufacturer will use to manufacture the board. In this case you have to be aware of what is their standard stackup for the PCB thickness you require and work with that to get your trace width. Rounding off RF traces is generally advised and should be a reflex rather than something to think about later, but in your case its not a big deal and of secondary concern (more of an issue at higher frequencies and for longer RF traces ...) Finally, your ground plane will need to be completely solid copper with no breaks or slots if possible... good luck !