RF PCB design - Page 1
Forum section: electronics / general

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 27, 2016, 11:54:54 pm
content: I have lots of gripes about how CAD seems to ignore the realities of RF PCB design, but one thing that drives me nuts is the IPC-356 netlist.Lots of RF boards have RF copper parts that are DC shorts, but have various ports for RF signals. I keep getting giant short reports from the PCB fab. I'm not upset at them about that, but what am I missing here? There seems to be no easy way to create RF PCB copper parts/features that don't show up as shorts or DRC violations at one level or another.Then some designs have separate boards on one schematic and there's always one last ground connection that isn't made; there's no way to flag the last GND connection as OK?Other designs have cavities with parts that drop in, and the ground comes from the enclosure. The CAD package then complains about unconnected pins on RF parts.So once you define the "bottom" of your PCB stackup in the CAD package, it doesn't seem to understand there might be an enclosure for RF ground, another "bottom" that isn't part of the PCB but should count for DRC.What CAD package do you use for PCB design of RF boards and do you have gripes/annoyances?I'm just fed up of all the DRCs and hacks you need to do to CAD to get RF boards done, especially when you inherit someone else's layouts and you have to recheck everything to be sure.

author: nctnico
author_rank: Super Contributor
timestamp: April 28, 2016, 11:34:46 pm
content: I usually give the same net name to everything which is connected together.

author: uncle_bob
author_rank: Unknown
timestamp: April 29, 2016, 12:23:18 am
content: HiWith almost no exceptions, every board I do has full ground pours on all layers. There may be other planes on the inner layers, but they all get ground. I don't believe I have ever gotten a fault back from the PC fab on any of my boards. My guess is that you have a naming convention (multiple names for the ground net) issue in your setup.If you are *relying* on an external frame / plate / enclosure to provide ground, a DRC is not going to be happy with that. Some software will let you flag it as "accepted" and then move on. It's likely to get you later on (keep popping up). I guess that's another reason to put off board grounds in the "don't do that" pile ...Bob

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 29, 2016, 12:25:00 am
content: Quote from: nctnico on April 28, 2016, 11:34:46 pmSure, but when the "connected together" happens in the enclosure and not the PCB, it doesn't understand that. Or if you have more than one PCB on one design, there's always one connection that can't be routed...Check it out:The PCB will have a cavity for that transistor, the source just sits in a cutout with no copper. The connection happens after when it's screwed to the enclosure and the PCB soldered to the enclosure.Anyways, guess I'll just endure the silliness.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 29, 2016, 12:31:08 am
content: Quote from: uncle_bob on April 29, 2016, 12:23:18 amReally? Where do you put the traces?Kidding. RF PCBs are another kettle of fish with weird outlines, bizarre shapes, holes, slots, odd devices sitting at all kinds of heights, etc.I just like things nice and neat because my scripts rely on various checks before making gerbers and some designs just always report unrouted connections.One design is 4 separate boards from one schematic that are soldered to the enclosure. The enclosure is the ground for the system.Either that or you make 4 schematics, 4 part numbers, 4 layouts, 4 sets of gerbers, 4 POs, 4 BOMs for something that is really one design.

author: uncle_bob
author_rank: Unknown
timestamp: April 29, 2016, 12:38:03 am
content: Quote from: Alex Eisenhut on April 29, 2016, 12:31:08 amQuote from: Alex Eisenhut on April 29, 2016, 12:25:00 amHiTwo basic answers:1) simply put the part in with no ground. As far as the PCB layout software is concerned, that's what you are doing. You can edit the library for the part to make that happen.2) If the PC board bolts on top of the part / under the part / around the part, make the ground tie there as a fixed part of the layout.(not quite sure how the two messages got quoted above ... weird ..).Bob

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 29, 2016, 12:52:34 am
content: Yeah, I'll have to resign myself to that. I just like things nice and simple. Having to create an entire species of symbols with no actual ground pins because CAD software is silly annoys me.