Hi. Just a quick and interesting follow-up video here to my one, uh, where I measured the noise of the microcurrent with the Opa-189 Op-amp using my Hp Dynamic Signal Analyzer using the power spectral, uh, density. uh to get the Uh micro volts per root hertz noise from zero to a hundred kilohertz here, and I thought I'd just show you an interesting aspect to do with shielding. Now I've got the microcurrent inside here.

It's got the Opa-189 Op-amp We're measuring noise from zero to oh well, 256 hertz up to Uh. one kilohertz here because it's only got 400 points so you can't go to exactly uh, zero at the start. Anyway, Um, look, we're getting some interesting little spikes in there. I've got a hundred averages on and I've got the cursor right there and you'll notice that it's actually around about 25 kilohertz there for this spike.

What is this spike here? Aha, is that the switching frequency? Because the Opa 189 is a chopper amplifier? No? well. As I explained in a previous video, that's way off. Uh, it's up in like the 200 to 300 kilohertz region up here, which is beyond the measurement capability of this dynamic signal analyzer. It's only, and this is only designed for Dc to 100 kilohertz.

So what is this? 25 kilohertz spike here? And look, there's two other spikes here and if I go over, I've got to use the fast mode because it's otherwise it's going through 400 points. Aha, 50 Kilohertz. And what's this one over here? Well, you guessed it. 75 kilohertz? So 25 50 and 75 Kilohertz? Uh-huh These might be some uh, you know, sub-modulation frequency of the Uh of the switching frequency of the Op-amp But no, that's not it.

Watch what happens if we let's actually, um, start that measurement again. But let's actually remove the lid. Tada. Look, they're going up.

Look at that. A significant difference. So it's not fundamentally the switching frequency of the chopper amplifier of the Auto Zero amplifier here. So what is this? Aha? For those playing along at home, you might have guessed.

Old-school Crt Cathode Radius Ray Oscilloscope. These are electromagnetic devices. They have big thump and coils in them which generate large magnetic fields, and a common switching frequency happens to be Kilohertz. So that's actually what we're measuring here is the switching frequency of the Uh Crt oscilloscope here.

So switching it's the horizontal scan rate of the Uh Crt here. And by taking off the top lid on here, we're easily seeing those spikes and I won't actually physically take it out and all that, and we could get higher. It doesn't matter, but you know, if we put the lid on, let's start that again. There we go.

If we put the lid on, it's going down. but it's not going to go down to zero and this has to do with an excellent video I've done. If I may say myself, I don't have the original, but this is just A. this is just a tribute, um, er, to a number 1273 Linkedin down below at the end.

If you haven't seen it where I explain near field versus far field uh Emc and how there are both Um H field which is a magnetic field and the E field which is the electric field and this is a distance from the source in terms of uh wavelength of the Uh frequency and when you're what's called near field when your device is physically near to your source like this, the electric fields and the magnetic fields actually differ. They actually separate like this. It's only when you get uh, basically a wavelength pi on two, uh, distance away do they start to combine to form the Em field or the electromagnetic radiation that you're more familiar with. The electric and magnetic fields combine in the far field I.e further away you go to give you an electromagnetic field and that's what you used to do with.

you know, like Rf shielding and all that sort of stuff is, uh, you're talking typically uh, far field shielding, but when you're very close like this to a magnetic source, in this case, we're going to get the electric uh field noise as you know from various things around as well as the magnetic uh field coming from the Crt which is you know behind there it's like a you know, a foot away or something. I don't know how deep this Crt is, but it's pretty darn close and we're able to pick up the magnetic field even through the shielded box. And even when I have the input grounded like this so it's gone to mains earth ground. So this box is completely shielded, right? But it's a die cast alloy box, and both copper and aluminium and die cast alloys like this aren't particularly good at shielding down at, you know, Dc to low frequencies.

in terms of what's called the near field, they can shield electro electric fields, but they can't shield magnetic fields The magnetic fields well. they're not perfectly go through, but they're attenuated a little bit, but they will. They can penetrate completely shielded boxes like this, and you know, typically as a rule of thumb, you might say yeah, Copper is, you know, pretty much only good for kind of like, you know, the Kilohertz range and above anything, sort of like below that is going to, uh, like anything you know, really low frequency stuff. Uh, below say you know roughly a Kilohertz or so is not going to be shielded by copper.

so you can have a completely, you know, completely welded copper or aluminium uh box that's completely shielded, but magnetic fields will still get through. So let's actually do a little experiment here. I've got because this is 25 kilohertz, this is actually quite. You know, relatively high frequency copper should actually work.

So what I've got here is I've got a large, uh, copper clad. It's the old school. Uh, this is a positive photoresist coating. That's why it looks, uh, green hasn't been exposed.

but you know, big one ounce copper sheet like this? We should. At 25 kilohertz, we should see this arm actually go away. So we'll start that again. Actually, let's take the lid off and then we'll sorry you won't be able to see it.

but well, I'll put that in there like that. Let's start and let's wait a bit and we'll be able to see that. This copper sheets probably going to do the business. Um, whereas the aluminium the diecast box.

Let's let's have a look ready. take it away it might. it's still averaging. well it's still there.

you can still see it. And when I took it away, it was quite low, but it was still there. So having that one big copper sheet there was actually better than both than the uh, diecast box here. and if we use both, we'll probably find that it goes away.

We can pretty much make make that go away completely. I know this is not the best example. If this was down at like you know, a hundred hertz or something make magnetic field, this copper sheet pretty much wouldn't do it. attenuated a little bit, but pretty much wouldn't do.

uh, jackal. So yeah, let's there we go. Yep, yep, it's basically completely gone and you'll see it start moving up again there. And let's actually try to do this same thing with a an aluminium sheet like this.

Just add some more. Although, aluminium is not perfect, but we'll pretty much see that go away, I suspect. Yeah, pretty much pretty much Gonski, but you'll notice that although the copper clad worked reasonably well if I just put some copper perth board like that, Well, that's not. That's not doing too terrific, is it? It's uh, you know, once again, it's it's the copper.

But it's not a continuous sheet and it's not a large sheet. So the magnetic fields, uh, getting through that? No problems whatsoever. And if we put the aluminium lid through like that, no, that's not going to do the business you really need to. You really need to mostly shield that so it can drop down, but it's not going to be perfect.

You'll notice that that sheet is not on its own without the lid is not going to do much. It has has attenuated it a little bit, see it goes back up. but you need the combination. You need to really play whack-a-mole here.

I don't know if you can know you can't see that I can see the I can just see the waveforms through there. There we go and she's gone ski and she's back. So to really get down to the lower uh, shield in lower frequency stuff like you know, down in hundreds of Hertz you know, the sub Kilohertz that range right down to Dc you need You need to start getting um, what's called a high permeability uh steel. So you need like a new metal it's called there's you know specific like brands and types of new metal.

they have a high permeability so they don't saturate uh as much and you know, pretty much like a steel uh will do the job. But a a low carbon steel would do better than a high carbon steel. For example, because the saturation point is higher so it can you know absorb more of that magnetic field before it saturates and then well, it can't you know, do the job anymore and shield that. So if we put a steel sheet between these, let's try that.

I don't know what type of I just got this from one of my instruments. so let's give it a burl and we should find that steel should work pretty darn well. especially at 25 kilohertz with uh, no shielding on the top of the box there and Ta-da completely gone and she goes back up. Look at that.

Beautiful. That's the high permeability of steel doing the business on the H-field So there you go. I just thought that is a interesting little example of how we can pick that up. And of course, if we move this away, if we move it away, we'll probably find that it's not going to pick it up any.

Not going to pick it up as well anymore. There you go. It's only when we get closer, closer closer. Unfortunately, you've got to sort of like restart the averaging because it's too noisy if I don't have the averaging on.

But yeah, we can get closer and there you go. Just really goes to town there. So there you go. Hope you found that interesting useful.

If you did, please give it a big thumbs up. Catch you next time you.