Trace width effect on signal strength - Page 1
Forum section: electronics / rf-microwave

author: Shimonu
author_rank: Regular Contributor
timestamp: March 26, 2025, 06:53:26 pm
content: I made my first PCB for RF, a relatively simple board for the CC1101 with an SMA-connector. I've got it running and analyzed a bit with the tinySA. I see that I'm getting quite a weak output around -60 dBm at best which is also confirmed when trying to transmit to the second unit as it shows a quite low RSSI, around -82 over the air and around -60 with a cable.I realized I didn't pay attention with the trace width I used for the output and used the 0.125 mm I used for some other traces. An online calculator says this gives me about 65 ohm impedance but I suspect it might be closer to 70. Comparing to the trace width the PCB manufacturer gives for 50 ohm the calculator shows about 45 ohm so I think the real world impedance might be a bit higher.I'd like to understand if the incorrect trace width could explain the poor performance as I can easily order some new boards then. Otherwise I would imagine it would be a software issue because I've followed the layout example from TI and used their recommended components exactly.I'll be receiving a nanoVNA tomorrow and I'll double check that the antennas are correct but as I said I still get poor performance with a cable.

author: tchicago
author_rank: Regular Contributor
timestamp: March 26, 2025, 07:33:10 pm
content: The question is what signal level are you expecting instead of -60dBm?A mismatched 70 ohm load instead of 50 ohm would give you a loss of -15dB. Using this calculator as an examplehttps://chemandy.com/calculators/return-loss-and-mismatch-calculator.htmSo, if you are expecting, say -40dBm, then yes it can be explained with the mismatched line in the order of 70 vs 50 ohms. But if you are expecting 0dBm and getting -60dBm, then probably this is something different.As for calculating the trace width, it is better to do several experiments with the specific PCB vendor and their specific materials. Order a test pcb with traces calculated using various formulas and various assumed parameters of the material. Maybe even add some variety of like +/-5%. Terminate those traces with SMA footprints, measure them using a VNA and then use the best one for the actual PCB design.

author: Shimonu
author_rank: Regular Contributor
timestamp: March 26, 2025, 07:59:04 pm
content: Thanks, that's very helpful then I know I can should be able to get more out of the chip, even with a thinner trace.As for my expectations, unless I'm misunderstanding something, the chip should be able to output +10 dBm and the lowest setting given by the datasheet should give -30 dBm, so I would expect at least that. But I'm still early in the development so it's very possible I've made a few mistakes. I'm also basing my code on some github code I found that wasn't too impressive. So there could very likely be some more settings to fiddle with and bugs to iron out.EDIT: Found my issue, a silly software omission is all I'm going to sayI'm actually not getting that much of a loss, maybe a up to -3dBm, I haven't checked too accurately but I could essentially see the output power I was programming minus the attenuator I had.It was really easy to track down once I could move my focus away from the PCB side, thanks again!