Dual Supplies for Low Offset Phase Noise Analyzer
On 7/14/22 4:49 PM, Chris Caudle via time-nuts wrote:
On Thu, July 14, 2022 2:15 pm, Lux, Jim via time-nuts wrote:
I like the rectangular boxes made from an extrusion, with covers.
How do those boxes work for low and mid frequency shielding? Does the
aluminum oxide that grows on bare aluminum keep the side extrusions and
plates you add to make a box from making good electrical contact?
They are iridited or otherwise chemical conversion processed (aka
chem-film) so the surface is conductive.
https://www.cpiaero.com/s/compac_catalog_2018.pdf which seems to map to
this:
"... finish chemical film per MIL C 5541, Class 3, conformance to MIL
45208A "
I assume that at GHz frequencies the oxide is thin enough that you get
capacitive coupling, but that doesn't seem reliable at 10MHz, and
especially not for power line protective earth connection if you put a
power supply inside the box.
I like aluminum because it is easy to drill, but I've always been nervous
about relying on it for electrical connections because of that oxide
coating.
chem film is your friend. Inexpensive, standard process, etc.
True, the inside of your drilled hole is bare aluminum and instantly
oxidizes.
Hi
It’s a tradeoff, with the extruded boxes, the end plates are the “obvious” place to mount
things. That end plate is (typically) not very well connected to the rest of the structure. With
the die cast, you can mount everything on the “top cover” and have a solid ground. Like the
end plate on the extrusion, it’s connection to the rest of the box is questionable. You can
instead mount on the sloping sides and put up with it (which is my preference).
If you want to go insane, most of the die cast products are set up for a watertight gasket.
You can put a conductive gasket in there and get a pretty good lid to case connection. Any
time things have been that nutty, a CNC “aluminum block” made more sense. Yes, that was
in a “work” environment rather than DIY basement stuff.
With any sort of die cast / extruded / aluminum block enclosure, getting things grounded
to the enclosure without strange inter-metallic issues coming up is “fun”. Still, unless your
copper board is truly massive, the box wall will win in terms of conductivity. If bonding (per
the earlier post) is the objective, then low conductivity is a really good thing.
Never a perfect solution …..
Bob
On Jul 14, 2022, at 11:15 AM, Lux, Jim via time-nuts time-nuts@lists.febo.com wrote:
On 7/14/22 10:38 AM, Bob kb8tq via time-nuts wrote:
Hi
Die cast boxes are your friend when building this sort of thing. Hammond
makes them and so do a lot of other folks. They are reasonably cheap and
pretty easy to work with. I have accumulated a pretty good pile of them “in
inventory” in a dusty box in a back corner of the basement :).
Except, the sides are tapered on most die cast boxes.
I like the rectangular boxes made from an extrusion, with covers. That way, the sides are at right angles to each other. COMPAC is one brand. You can get them with machined grooves, gaskets, etc. too (for a price <grin>)
There are a variety of sellers of extruded aluminum housings with slots to hold a PCB, etc. and end caps. I'm not so wild about those for tinkering, because I like being able to take the big lid off and peer in and probe. But for a product that's being sold, they're wonderful.
I'll make a pitch here for a company that makes CNC front panels, which I've used as RF breadboards - As long as what you want can be machined out of a flat sheet of aluminum, they can make weird cutouts, they've got just about every connector outline known, and they can do engraved labels, etc. But they can also put 4-40 tapped holes on a regular grid, for instance, if you're bolting down minicircuits flanged parts.
https://www.frontpanelexpress.com/
There are also companies that do cases as well as places that do quick turn run of the mill CNC machining (e.g. https://protolabs.com). The trick then is that you need some design tools and expertise.
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IMO End plate extruded boxes might as well not be shielded at all.
Especially those with slide in lids. I DO use them for commercial RF
product with internal shielding tins as required.
You can get an 'EMC' gasketed version with stiff end plates but they are
pain to work with.
A diecast zinc-aluminium box is my 100% go-to for anything RF or
sensitive...
Hammond have a great range. Digikey usually stock most of them.
On 15/07/2022 11:06 am, Bob kb8tq via time-nuts wrote:
Hi
It’s a tradeoff, with the extruded boxes, the end plates are the “obvious” place to mount
things. That end plate is (typically) not very well connected to the rest of the structure. With
the die cast, you can mount everything on the “top cover” and have a solid ground. Like the
end plate on the extrusion, it’s connection to the rest of the box is questionable. You can
instead mount on the sloping sides and put up with it (which is my preference).
If you want to go insane, most of the die