Shielded test chamber - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: April 05, 2018, 08:15:08 pm
content: I finished phase I of my test enclosure. It's a pair of 2" x 7" x 11" Bud cadmium plated steel chassis'. The gasket is Laird 4020 conductive cloth material. The piano hinge and hasp were spot welded. Getting the tongs into position was "interesting".Do NOT weld cadmium plated steel unless you understand and take steps to avoid poisoning.Still to go are feed through connectors and ferrite chokes in a small sub-chamber. I plan to test the effectiveness before adding additional shielding such as mu metal sheet and copper foil.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: April 05, 2018, 08:58:47 pm
content: Learn about MuMetal before forming it - separate sheets are less effective. If you try to bend it over a sharp angle you'll need to re-anneal it via hydrogen furnace to maintain full shielding effect.

author: cellularmitosis
author_rank: Unknown
timestamp: April 05, 2018, 09:05:05 pm
content: What is the advantage of cadmium plating on steel?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2018, 10:23:04 pm
content: Corrosion resistance similar to or better than zinc (galvanize). Mostly old fashioned or mil spec, kind of surprising to see it at all these days.Tim

author: rhb
author_rank: Super Contributor
timestamp: April 05, 2018, 11:07:23 pm
content: Cadmium is an extremely common as a plating metal on steel. It's widely used on auto parts and hardware. I have no idea why it is so popular. I've just learned to recognize the 3 common single layer plating materials, zinc, cadmium and tin. Nickel requires a copper undercoat and chrome requires a nickel undercoat. So chrome requires 3 layers unless you're doing "hard chrome" in which case it's a single layer. In the latter case I think here are constraints on surface roughness. It can't be too smooth. I could go look it up in my library, but it's not really germane.Lots of refrigerator shelves were cadmium plated which freaked out a chemist I knew because he was aware that it was toxic.If I apply mu metal it will almost certainly be self stick 1 mm sheet. Naturally that can not be heat treated after forming.But before I spend the money I shall calculate the likely improvement and compare that to a box made of 1/4" annealed 1008 steel. Doing a proper heat treat would be a serious pain for anything larger than about 3" x 5" x 7" as that is about the size of my heat treat furnace, but I've got the tooling to do a small sub-chamber.A more vexing question is what sort of feed throughs and how many should I install? SMA F-F bulkheads seem the most flexible and least costly. I'm still trying to source feed through capacitors at reasonable prices.I plan to construct an insulated box to contain the shielded enclosure. Probably 2 layers of 2" foil face foam with a layer of aluminized mylar bubble insulation between the foam layers and a plywood exterior facing, For precise temperature control I'll make a small oven that fits inside the chamber I built today. Getting the wires through to the shielded box is a bit of a puzzle at the moment. I don't have any really good ideas how to do that so that the connections are easily made and altered and the insulation is continuous.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: April 06, 2018, 12:49:01 am
content: Quote from: rhb on April 05, 2018, 08:15:08 pmIf possible, avoid machining it in any way: filing, drilling, grinding. The fumes and dust are hazardous. Except in the case of some aerospace parts, even the military prefers not to use it these days.

author: rhb
author_rank: Super Contributor
timestamp: April 06, 2018, 01:56:35 pm
content: I'm puzzling over signal feed thrus. For size and frequency considerations I'm leaning towards SMA F-F bulkhead mounts for AC signal lines, But I'm wondering if they might not be a good choice for DMM connections. A pair of M SMAs on two pieces of coax going to a dual banana plug with heatshrink to hold them together. That would shield my leads all the way to the DMM. Ground connection only at the test chamber end. If I use three banana jacks I've got a small unshielded section right as I come out of the test chamber.Comments? Has anyone done anything similar or seen it done?

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: April 06, 2018, 04:34:48 pm
content: A) If you're doing Vref's, all feed thurs do is add thermal EMF errors (potentially, otherwise keep them at same temp.). That's where careful application of MuMetal braid over shielded cable comes into play, and the outer cable shield becomes part of your can outguard. Ferrite noise blocks are used (and carefully pass the cable to the inner can), but typically -not- feed thrus if you're doing low-level DCV measures.B) Keep in mind those "RF gaskets" on your lid are almost completely transparent to low freq H-fields.C) MuMetal sheilding is calculated based on your environment, so measure that first...If all you're using is a 5.5 digit meter, steel may be good enough especially in multilayers. Like every piece of quality test equipment is made, consider going with two layers of steel, inguard and outguard. Those guards are always electrically separate, except via a resistor and/or cap (as required). If you're using a high accuracy 8 digit meter, then MuMetal will almost always improve results.D) You might be making this way too complex. Learn from every low-noise, low-freq measurement pro's and get a supply of steel cans, and then you if you find you need multilayer you can nest them for your inner guard + outer earth ground shield. Mount your board on the lid...look at Jim Williams app notes, and other examples would be like Geller's JCan design for testing resistor noise.https://bascousa.com/1-2-gallon-metal-paint-can.htmlhttp://www.gellerlabs.com/JCan.htmAll of this has been done before, so you don't have to re-invent the wheel here.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 06, 2018, 04:57:02 pm
content: @Mr.Diodes,thanks for the information.how about breaking a TO-3 can (2N3055) and using that as a hermetically sealed feedthrough and the shield can be machined / fixed instead of the cap?best regards.-zia

author: rhb
author_rank: Super Contributor
timestamp: April 06, 2018, 05:56:53 pm
content: As much as possible I'm working with what I've got which is why the 60 year old Bud chassis', etc. I'm doing this as cheaply as is consistent with actual requirements. I may well weld an overlapping flange on the outside to cover the 0.065" gap filled by the Laird gasket. That *should* have been done before welding the latch, but I didn't think of it, so it will be a bit more fiddly to do. Finding the steel stock will probably take me more time. I want to use things up, not buy more fo things I have already.If I can find a couple of smaller steel chassis which will fit, I can mount an inner chamber pretty easily. I just need to dig through Dad's stuff.This is intended as a general purpose enclosure, not just for Vrefs. At the moment I'm trying to decide how to mount arbitrary PCBs without making holes in the enclosure. The intended application is testing prototypes before committing to a package design. I have a 20 ton press brake, lathe, mill, welders, etc. So I can fabricate pretty much anything I need so long as it is not too large.The feed through capacitors are for power and control signals. Most of the signals coming out of the box will be RF. If I find I need mu metal braid cable I'll make a taper thread steel fitting that clamps onto a cable passing thought the compartment wall. I can't imagine a source of strong magnetic fields that I can't just turn off to make a measurement. But I don't have a good means of measuring magnetic fields. I've been meaning to build a magnetometer for 30 years but never quite got around to doing it.Bear in mind, I'm just an old man playing with toys. I can't think of any reason any of this matters. It's just things I always wanted to do, but would not spend the money for what is just an indulgence.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 06, 2018, 06:02:39 pm
content: Mount with glue or VHB tape?Ah, but that's rather permanent, and doesn't give a ground. Well, you could tape in a bracket with mounting holes or studs for the boards of interest. Maybe even make a bracket with slots so you can slide things in? And ground could be had with metal tape (with conductive adhesive), or putting some EMI gasket around the bracket.Tim

author: ogden
author_rank: Super Contributor
timestamp: April 06, 2018, 07:08:15 pm
content: Quote from: T3sl4co1l on April 06, 2018, 06:02:39 pmDuring tests my box is not mobile so I just use insulator on the box floor and (plastic) standoffs for test boards. To be honest, usually I just drop board on the insulator w/o standoffs because bottom of the PCB is ground plane more than oftenObviously in case of RF applications it is advised to inlay all inner surfaces of the box with RF absorbent foam. Oh, and mine test "chamber" is Ferrero Raffaello 300g (10.6 oz) candy box populated with 5 SMA f-f bulkhead connectors

author: rhb
author_rank: Super Contributor
timestamp: April 06, 2018, 09:51:59 pm
content: The chassis' have two holes on each side. I've mounted 1/2" aluminum angle on the standoffs in the photo with a couple of pieces of angle that span between them and can be moved and clamped at the board width. Then some aluminum clamps that actually clamp a board to the angles, probably on low standoffs.I still need to make the clamps, but I think this will prove satisfactory for most purposes. I went with the tall standoffs so I can easily reach the SMA F-F bulkhead connectors when they arrive and get installed.One set of standoffs will hold a steel plate for enclosing the EMI filters for power and control signal lines.

author: ogden
author_rank: Super Contributor
timestamp: April 06, 2018, 10:28:14 pm
content: Quote from: rhb on April 06, 2018, 09:51:59 pmClamps? - Make sure they are gold-platedLooks kind of nice, but w/o offense - IMHO it'soverengineering. It does not change anything - your board is nicely mounted in that box or not, unless you are going to measure electrical parameters and mechanical at the same time. Just my 2 cents obviously.

author: CopperCone
author_rank: Super Contributor
timestamp: April 06, 2018, 10:44:58 pm
content: well I would recommend you get nylon, wood or other kind of plastic angle parts rather then aluminum. If you do it wrong you can short out the PCB. It also adds kinda weird capacitance to the circuit, though small.And if you want easy, you just lay a strip ontop of the inside bit of the L bracket, with some springs, so you basically lift the part up and it wants to come down naturally with spring forceand i forgot to draw a handle on top of the press down piece so you can draw it back like a crossbowI gues you can use wing nuts instead of springs if you wanna make the thing resistant to being knocked off the table completely, but I assume you would need to be completely hammered to fuck up that experiment, or have an earth quakeAnd I would cut grooves into the other L brackets, so the nonconductive L-brackets have a track to slide on, with some kinda peg sticking out of em. tapping them and putting a bolt in it so the bolt slides along the cam would work. You can do this the painstaking way by drilling alot of holes in a row then using a dremel or aluminum file to make the slots if you don't have a milling machine. Then the nonconductive brackets can also use wing nuts or spring tension

author: rhb
author_rank: Super Contributor
timestamp: April 06, 2018, 11:21:25 pm
content: The point is to be able to install any board under about 5" x 7" for testing in a shielded enclosure without having to drill holes in the aluminum angle to mount a particular board. Drilling holes works for the first few boards, but eventually you have to replace the angle if you drill holes. And I *really* don't want to drill holes in the outer surface. I considered magnets, but that would compromise the magnetic shielding. Sticky stuff becomes a mess over time.It's large enough that I could actually mount two 5" x 7" boards. One in the top half and one in the bottom half with a shielding plate between them. I'm not likely to do that, though, unless it's a very exotic project. It's intended as part of a very general purpose lab setup. So it's over engineered for some things and under engineered for others. It is intended to simplify building and testing circuits that need electrical and magnetic shielding.The immediate application is measuring 3 LM399 voltage references and other metrology projects, but a lot of my interests fall in the realm of RF. I've got a LimeSDR and I recently picked up a couple of LPC-link2s which have an LPC4370 with an 80 MS/S 12 bit ADC. Those are capable of logging the entire RF spectrum from DC to 30 MHz to disk. But that requires a LP filter which is 72 dB down at 40 MHz. If it's not really well shielded you're not going to make it work.A lot of the design is driven by bits and pieces I have on hand. I happen to have a scrap of aluminum chassis that has a folded edge. So it's perfect for making clamps that attach to the end of a standoff. Drill holes, add screw and standoff, done. Twist the standoff to tighten or loosen the clamp.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 07, 2018, 01:16:15 am
content: Hi.The shielded box you have looks great. There may be a simpler solution waiting at the grocery store. The late Jim William used to use cookie (biscuit) tins for shielded enclosures. These have the additional benefit that you get to eat the cookiesHere is a picture from AN-83Regards,Jay_Diddy_B

author: rhb
author_rank: Super Contributor
timestamp: April 07, 2018, 01:37:08 am
content: That's tomorrow's project!I bought a 20 W dummy load from QRP-Labs which I'm going to install in a similar tin and fill with mineral oil so it can take a larger load. I'm soldering a valve stem to the top to fill it. I just love the side cutting can openers because you get a lid for the can when you open it. The lid went MIA while I was waiting for the BNCs and meter test probe sockets to arrive, but I've opened another tin and am all set. At least once I finish my chores.