RF bench design question? ground. - Page 1
Forum section: electronics / projects

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 02, 2015, 11:34:44 pm
content: So I remembered I saw a picture a while back that showed someones work bench laid out with copper sheet.I think this is interesting, but how much use does it give? Have you ever seen this setup in a professional environment? At what frequency ranges is such a design choice effective in? Are there downsides?I have noticed some odd things happening due to having a ESD mat near poorly designed electronics (I think high impedance pins on a MCU were picking up something emanating from my ESD mat, the device seemed to work well when placed on top several books, however, if you put it on a sheet of paper and then the ESD mat it would frequently crash). I imagine a big copper plane might make some high frequency/microwave circuits behave funny, I have little experience in this as of yet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2015, 09:49:29 am
content: Commonly seen in EMC test labs. Can't imagine you'd be using one as a bench work surface. If nothing else, it would dirty so quickly (what happens when you flick solder onto clean copper?..), and damage so much stuff (accidental shorting, machine model ESD..).Typically, EMC standards call for some combination of ground plane as a floor, table or wall, and/or with coupling planes as well (e.g. 61000-4-2 ESD where ESD strikes a nearby 0.5m square plate -- this is surprisingly destructive, a much better test than it sounds). Typically, emissions tests use a typical to worst-case-ish sort of distance (equipment sitting on a nonconductive table, or freestanding on the floor), which will somewhat increase the possible voltage that might be radiated or conducted off the equipment. Susceptibility tests typically use a consistent height, to control the common mode impedance of cables and such, also increasing the RF current flowing in and out of connections.Tim

author: Berni
author_rank: Super Contributor
timestamp: December 03, 2015, 11:27:30 am
content: You don't want a copper sheet on your bench. Not only makes it really easy to short things out, it would not stay nice and shiny for long. Copper is soft so it scratches and dings easily and will oxidize unevenly. Areas that you touch more with your hands will oxidize much quicker and chemicals used in electronics will leave marks on it(especially flux).What you are most likely experiencing with your board is the slightly conductive ESD mat working as an antenna. Because we run 50/60Hz mains all over our houses this puts a lot of RF energy in to the at that frequency. Large conductive objects pick it up, including humans (Thats why touching the end of a scope probe gives you a sinewave-ish signal). So you can imagine the conductive ESD mat going up and down by a few volts at 50 Hz. Putting a floating digital input pin close to it could capacitively pick up that signal. I suggest just connecting the ESD mat to ground at a few spots, or perhaps put a grounded sheet of aluminum foil under it.But really if your project crashes when it encounters a bit of 50Hz hum then there is something wrong with your design. Another good way to test is to wave a cellphone around it when it in a call.

author: dom0
author_rank: Super Contributor
timestamp: December 04, 2015, 04:13:35 pm
content: @ESD mat moving around: There are different kinds of ESD mats, mainly conductive and dissipative. With a conductive ESD mat this might happen, but it would really surprise me with a dissipative one. They have surface resistances >>10 M?. The ESD mat on my bench is specified as "less than 1e9 ? [1 G?] surface resistance".QuoteI 'wipe' my iron against a bent copper wire of my solder iron stand to clean off excess solder. Or I just flick it at the copper-clad base of it. It never sticks.I think it's a bad idea because of the metallic smell, maybe ESD issues, cost and it's "reflectivity" which would probably be very unergonomic.

author: rdl
author_rank: Super Contributor
timestamp: December 04, 2015, 05:13:59 pm
content: A few years back I was having problems with RF interference. Somebody who works on RF stuff suggested putting a sheet of grounded copper under my antistatic mat (2 layer dissipative/conductive type). They said many of the benches at their work were set up that way. I was going to try it, but a sheet of copper 2x5 ft isn't cheap, and I tracked the interference problem to a local college FM station, so it was never done.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 05, 2015, 01:28:21 am
content: Quote from: rdl on December 04, 2015, 05:13:59 pmyea, this is what I had in mind. i forgot to mention putting a ESD mat ontop of itmy esd mat is very high z on top layer (i think I measure it with electrometer before and its very high) and like measurable in the megaohm on bottom layer (darker conductive material coated with blue grooved rubber)