[time-nuts] potential low-RFI power supply

[ed breya]

Taka,

You can set up a mini conducted-EMC lab with a few additional items that 
can be purchased, and some that you can make yourself, if you have the 
right parts.

The most important item is a wideband AC current probe or transformer. A 
commercial clip-on type is most convenient, and ideally it should have 
jaws big enough to clip around fairly large conductors, like an entire 
power cord. I have some nice Tek P6021s, but they are too small for 
large conductors - they only go up to about 3-4 mm diameter wire. I 
don't know what's available out there that would work for bigger stuff, 
but there may be some. It's not that you need a high current capability, 
but that it's very convenient to go around large cable assemblies or 
even components. In the old days, EMC labs would use nice big shielded 
split-toroid HF current transformers to clamp around the big stuff - I 
would imagine it's the same nowadays. When I need such measurements, I 
just wind up a transformer on a ferrite core. What you don't want is one 
of those big clamp-on amp meter type things - they are for measuring 
high currents at line frequency, and can't reach SMPS type frequencies. 
About 20-50 MHz or so BW is plenty, and you don't need to worry about 
the low end response like at line frequency, although that would be nice 
too.

I'm guessing you won't find such a probe with all these characteristics 
commercially, unless you get a high precision, professional type one 
made for EMC or SMPS design use, and likely very spendy. So, it's an 
opportunity to make your own. I'll explain later how to make some, but 
at this point it's necessary to know what you have available, 
ferrite-wise. Many people have nice collections of magnetic cores and 
parts, new or salvaged, so already may have plenty of pieces to play 
with. For SMPS/EMC use, it's good to have some nice ferrite power 
toroids - the bigger (like 2-3" ID) the better - for winding 
transformers with a hole big enough for a one-turn pass-through loop of 
big stuff, like a power cord, or even its plug too. For clamp-ons, you 
can use any un-gapped two-piece core like E-I, C-I, C-C, etc, again, the 
bigger the better, for putting around cords and such. If you have a 
bunch, that's great. Also, if you have some junker equipment with SMPS 
stuff in it, you can likely salvage many pieces that you can use.

Remember, all this current probe/transformer stuff is about measuring 
high speed common-mode currents, not operating loop currents in 
circuits. You'll want to first explore the time domain with a scope, 
then later an SA.

The next category of ferrites is more of the same stuff, but for 
steering the currents away from where they're not wanted, to where they 
don't bother anything, or can be dissipated. Lots of smaller cores are 
handy for this too, especially the split-toroid (or box) types that are 
commonly used and encountered on cords and internal wiring for EMC noise 
suppression. These are commonly called "prayer beads," for good reason.

I'll have more to say about all this next time.

Regarding your SA, you won't really need it to make any measurements on 
the DC output, until maybe later. The best use will be to look at the CM 
currents via the probe/transformer, which is an isolated AC measurement, 
and will have protection (I'll explain if necessary) against excessive 
signal power.

If you already know about thoroughly protecting your 50 ohm gear when 
making any measurements with DC present, then good. Just in case, I've 
included a couple of links to recent pertinent discussions in the hp and 
tek groups. This subject comes up every so often, and I always have 
something to say about it. Please read both threads all the way up and 
down, for context, and you'll have a pretty good idea of what's involved.

https://groups.io/g/HP-Agilent-Keysight-e quipment/message/106348

https://groups.io/g/TekScopes/message/166344

That's all for now. Good luck in finding lots of good ferrites, and 
maybe even a nice current probe.

Ed