Wall wart spurs

Buy your wall-warts from the ASPCA?

Depending on your switcher frequency, making the 'electrostatic' shield thicker, by many skin depths, gives you a magnetic shield benefit. Seamless (cast or deep-drawn) is best, for obvious reasons.

I've seen very low magnetic leakage from cup-and-lid inductors, it may be possible to reduce size (and therefore dipole field) and save money by replacing the toroid.

Are these wall warts Powering your receiver or just nearby and powering something else?

The switcher fundamental is down below 400 fA RTI, which makes it invisible over the Johnson noise of the feedback resistor in a 100 Hz bandwidth. I'm seeing something at the fourth harmonic, near 570 kHz, but it's only 10 dB above the Johnson noise in the same bandwidth. It would be convenient if I could dither the switcher a bit, but unfortunately the 2594 has an internal oscillator that I can't get at.

No other harmonics are visible.

The inductor is a nonprogressive toroid, and the only place the shield seems to help is wrapped around that. It's made of 4-mil copper tape, which is couple of skin depths at 600 kHz.

It's a bit inconvenient to chase, because those numbers only apply when it's in the box with at least one mounting screw installed.

I bodged in a small loop of wire-wrap wire in series with the inductor, so that I could investigate magnetic effects, but there wasn't much difference regardless of how I moved it.

The switcher takes +16V and makes -5 for the op amps and -23 for the photodiode bias. The switching waveform is beautiful--nice slow edges, zero ringing or overshoot.

The proto has been through a lot, including soldering and desoldering about 30 JFETs to do noise comparisons. It's getting flaky, so I'll ask BLH to stuff another one and we'll try again.

Cheers

Phil Hobbs

Powering the photoreceiver. It turns out it's internal to my box--what fooled me is that I'm only seeing the fourth harmonic (600 kHz), so I thought it was the computer wart. There's nothing with a switching supply turned on nearby, and it does the same thing when powered from an old school 60-Hz transformer + rectifier supply.

The board lives in an uber-fancy Hammond RF shielded box (1457L1201EBK), which drops the switcher stuff down almost but not quite into the noise. There are four mounting screws in the edges of the box, so the grounds are pretty good for ~1 MHz use.

Cheers

Phil Hobbs

If your equipment has to run in the presence of laptops that are plugged in there is only one way to make that work: Complete isolation of power into your box. The barrier (usually a toroid of pot core) sometimes even has to be driven sinusoidally. Also, the supply wiring on your side should be 100% symmetrical so it has the least chance of coupling with the outside world which is how such stuff gets around CM chokes.

Most wall warts and laptop power supplies have issues with grounding and all their designers really care about is passing EMC. That is radiated and conducted, but conducted only towards the power line, not towards the load.

Cover the whole switcher because the traces to the inductor and the leadframe in the IC are radiating as well. I normally use a suitably sized off-the-shelf shield can from Laird and make sure there is a full ground plane on the other side of any switching-noise carrying traces as well. IOW the whole switcher runs inside a Faraday cage.

If you run a switcher far below its rated output load, as would be the case if you're using a laptop supply to run a low-power gadget, it may either not regulate properly at all, or it may go into a cyclical operating mode with duty cycles and transition times that are fast enough to cause EMI problems. To catch this sort of problem reliably, the various regulatory agencies would have to require that the supply be tested with countless different loads.

So the best strategy is to either specify a linear wall wart, or a switcher whose rated capacity is a good match for your device. Failing that you'll just have to test them individually until you find one or more candidates that you can live with.

-- john, KE5FX

I know the 2 wire AC input switchers are pretty noisy, I have been itching to try some 3 prong ones that at least have a earth shield and a proper common mode input filter. Output filtering and chokes are a good idea.

Cheers

The linear wall warts are TERRIBLE at 60 Hz for magnetic field leakage, and that's harder to shield and filter than higher frequency. The field leakage gets better when you load them, actually: at light output current loads, they're the worst low-frequency emitters.

I use an isolated USB hub to connect all my equipment because of ground problems. The biggest problems are sound and serial dongles.

Huh, well that's what I did with CUI dc switcher's.. But with copper tape hats, tacked to the ground plane. (low volume.)

Phil was asking about wall warts, I thought. And I think conductive spurs.

Besides spurs, I see 60/120 Hz AC line stuff when I load a wall wart down.

Phil, If it's conducted doesn't it have to be a filter, L/C, R/C, you know I've never quite gotten what a common mode choke is working against... Should I think of that as an L/R filter. (I've wrapped usb cables a few times through one of those ferrite brick things, and cut down the "crud", so I've no doubt they work.)

George H.

Some wall-warts somehow make the box make spurs, or some wall-warts make spurs that appear to come from the box?

Solutions for the former may not be the same as solutions for the latter.

Can't say as how I've ever had problems with magnetic field leakage from a wall wart several feet away. I guess you wouldn't want to use them to power your unshielded SQUIDs.

But interwinding capacitance can definitely bring in a lot of hash from noisy switchers on the same AC circuit. That's the nasty thing about those supplies, they don't just interfere with the device they're powering.

-- john, KE5FX

AFAIU he has now narrowed down the hunt to the switcher inside his box.

Yup. We have to spin the board anyway, so I'll go ahead and put one of those Laird shields on it--the 44 mm square x 10 mm ones will work. Hopefully that'll get rid of the 600 kHz.

Cheers

Phil Hobbs

Make sure the offending traces are also going to be inside the Faraday cage. That sometimes requires the ground plane not to be the usual 2nd layer unless you are able to route all switcher power traces (to the inductor and to the input and output caps) on the top layer.

If you want to be extra good provide a pad at the switch node to which a non-stranded short air wire can be soldered if needed. When that is bent "just so" over the inductor it can at least partially cancel an E-field. Probably not needed but the pad is free.

OK... (I seem to be having trouble with reading/understanding lately.)

You could hack a little shield out of copper tape... see if it works. (or copper clad?)

George H.

Yup. We'll put the additional traces on L3 and another ground pour on L4.

Convenient that Laird makes 10 mm tall shields with removable covers. They've sure gotten pricey lately though! $3.50 per half in qty 10, sheesh. A good candidate for a custom part, once we get going.

Thanks

Phil Hobbs