First TIme creating BLE PCB - Page 1
Forum section: electronics / rf-microwave

author: RisingRedstone
author_rank: Newbie
timestamp: January 29, 2022, 11:25:37 am
content: This is my first time designing a BLE PCB.This uses Efr32BG22 QFN32 rf MCU and is powered either by a micro USB or 3.3v cr2032 button Cell. I'm not sure if this will work or not. Tips and suggestion would be helpful, thanks.Modify 1: I had't uploaded proper images or renders, sorry about that.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: January 29, 2022, 12:29:03 pm
content: What is going on with the antenna feed? Where are the traces, and what are those stippled copper lines?If I interpret your renders correctly, this will not work.Maybe double check those? (Which CAD is this?)Edit: do you have an internal ground plane?

author: RisingRedstone
author_rank: Newbie
timestamp: January 29, 2022, 01:37:14 pm
content: This is a two layer PCB so the bottom layer is continuous ground plane(atleast near the antenna and the matching circuit). Those dotted lines are ratsnests. I used ratsnest instead of routing tool because I needed a 1.5mm trace and the rounting tool would create problems. So I used the ratsnest to create the traces. This is autocad eagle.

author: RisingRedstone
author_rank: Newbie
timestamp: January 29, 2022, 01:46:10 pm
content: Oh, I forgot to click ratsnest before exporting those models, I've changed it now.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: January 29, 2022, 08:35:21 pm
content: Typically, you should impedance match traces to an antenna.If I read your new render correctly it seems each trace is just a zone?If you want good performance, this is not the way to do it. RF is quite particular in terms of impedance and also ground plane area/return. Glad to hear you have a solid GND plane.For me to give any tips & think i know your reasoning behind the current rats nest thing (which in kicad means unconnected nets).Hearing your arguments for the current design decisions will make for better feedback.Routing RF is almost like routing a differential pair, but the GND return is the other trace. You will want the signal trace to match impedance as closely as you can. (Typically the RF chip/frontend will have a target impedance, and you should get as close as you can to that).

author: RisingRedstone
author_rank: Newbie
timestamp: January 30, 2022, 07:33:16 pm
content: I have used the exact same RF pi-matching circuit as given on the datasheet, and have set the trace width to be about 1.5mm to the best of my ability to get a 50 ohm trace.

author: ThomasDK
author_rank: Regular Contributor
timestamp: January 30, 2022, 08:02:25 pm
content: Trap for young players:You need a matching filter for the antenna as well. The one in the datasheet is just for matching the radio to 50ohms. At the same time it acts as a harmonic suppression filter.The BG22 is a real pain to get through emc - even SiLabs' own eval boards passes with only a few dB margin...

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: January 30, 2022, 08:29:34 pm
content: You will have a much better time going to 4 layers. You would be looking at about 0.3mm trace.Data sheets/app notes for PCB antennas typically has something to say about the ground plane size too, though with a good pi filter a lot can be done.It may very well be that you would get a working BLE on your bench. The unknown spec is what you need in terms of acceptable dB loss. What kind of minimum range do you target?

author: RisingRedstone
author_rank: Newbie
timestamp: January 31, 2022, 07:31:56 am
content: I think the trace antenna I used is already 50 ohm matched, so I didn't include another matching ciruit.

author: thinkfat
author_rank: Unknown
timestamp: January 31, 2022, 08:09:00 am
content: So the antenna is 50 Ohms. okay, but for which PCB material and thickness? I guess you use regular 1.6mm FR-4? Is that what the reference antenna was designed for?

author: RisingRedstone
author_rank: Newbie
timestamp: January 31, 2022, 08:14:25 am
content: The datasheet said that a 0.3mm(4 layer board) height between the matching circuit and the ground plane is ideal but it could work for upto 0.8mm(2 layer board). I don't have many requirements from this board, a range of 10-20m would work for me. Although I would have a much easier time with a 4 layer pcb board, this is just a test to see if I can make a 2.4ghz bluetooth module on a 2 layer board.

author: RisingRedstone
author_rank: Newbie
timestamp: January 31, 2022, 08:53:48 am
content: Yeah that is something that they haven't mentioned properly in the datasheet. They said the material and board thickness information is in the "refernece design" and the antenna could be made slightly longer/shorter depending on board thickness. But the link is not working anymore. But reading other people's posts using this antenna I think my board (0.8mm and FR-4) should work.

author: thinkfat
author_rank: Unknown
timestamp: January 31, 2022, 10:04:41 am
content: It might be a good idea to simulate the antenna and at least your feed-in structure after the matching network.

author: mayor
author_rank: Regular Contributor
timestamp: January 31, 2022, 11:48:07 am
content: Hi,I've designed similar boards with TI chips. From my experience, you can't use the same values for your Pi network as those from another board. Those values need to be tuned with a VNA for your particular board/enclosure. In my opinion, trying to match otherwise is just a stab in the dark.

author: RisingRedstone
author_rank: Newbie
timestamp: February 01, 2022, 06:19:51 am
content: Thanks a lot, I'll see if I can simulate this.