Why RF shielding?

[francois]

Hi All,

I keep wondering why STs (and Amigas just the same) have this elaborate metallic RF shielding all around their electronics.

Well I understand the concept of RF shielding, but:

- Why do older 8-bit machines (running at 1 MHz) not have it?
- Why do modern PCs not have it? (I am thinking about all these "gamer PC" cases that have glass planels and front panels made entirely of plastic and fans...)
- Also, even the first 386/486 PCs seemed to not care that much about shielding. Sure they had metallic cases but, as far as I remember, there wasn't great care taken in electrical continuity of the opening panel and/or shielding of unused drive bays. Anyways PCs run at anywhere between 33 MHz to 5 GHz...

So, is there something special about the 8 to 16 MHz range or were there different regulations around in 1990 than before and after?

PS: and why is it that the Mega STE used brown (supposedly conductive) paint (maybe the TT also) but the Falcon switched back to metal?

[Steve]

It's all to do with countries RF regulations which changed significantly over time, you've gotta realise how much random electronics was being produced in the 60s/70s/80s using various different 'new' technology that could interfere with other electronics. If certain machines from a certain era don't/ do have more or less RF shielding it's purely down to regulations at the time and what they required to pass the RF tests.

These days electronics are designed in such a way that they don't give off crazy amounts of RF, they're also a lot more immune to RF.

Here's a story my Grandad told me: during the 1960's all the houses in the street couldn't get radio or TV reception, when people came out to inspect, they found an electronic device in a factory at the end of the street causing the RF interference blackout, which was produced before any shielding regs were out.

So you can see why old electronics needed a lot of shielding.

[francois]

Makes sense but now I wonder: how do modern PC motherboards manage to give off less RF without any shielding?

[Steve]

Makes sense but now I wonder: how do modern PC motherboards manage to give off less RF without any shielding?

There's probably more to it than I understand, but I think the gist is:

1. Modern components, internally designed to a better standard.
2. Modern PCB design with specific requirements in how the ground is implemented to reduce RF.

Oh and answer to you're PS about the Falcon ... Simply because the Falcon used the cheaper ST casing. Simply a cost reason afaik.

[terriblefire]

Makes sense but now I wonder: how do modern PC motherboards manage to give off less RF without any shielding?

PC and office equipment only need to pass the test in a case of the manufacturers choosing. Not any case they might be shipped in. So they basically cheat the tests a bit too.

[peters]

I read somewhere, not too long ago, that the shielding was to prevent interference with a, now obsolete, comms system.

[rubber_jonnie]

If you look at the older Atari 8 bit machines, so 400, 800 2600 heavy sixer, they all used very heavy cast aluminium shields, and in fact removing those shields wasn't possible simply because they were a structural part of the machine.

Later on we got the thin sheet metal finger shredders, though the VIC-20 managed with aluminised cardboard. As I understand this was down to very strict FCC regulations in the US designed to prevent interference. From what I've read the FCC RFI requirements became stricter in 1981. Early US computers like the OSI Superboard didn't have shielding, but then came the FCC...

Looking at it, most of the machines we see with shielding were from the US - Atari, Commodore, Texas. Apple didn't fit much in the way of shielding, I think there was some at the back that often got removed, but they tended to use a spray on metalised coating rather than a physical shield, which would have got in the way of expansion slots.

Looking at machines in the UK, you just didn't see the shielding. There was none in the ZX80, 81, Spectrum or Ql and I don't recall any in the likes of Dragons, BBC/Acorns, Grundy's, Jupiter Aces etc at all. In fact I don't think my MSX machine has any shielding either.

I guess we were much less concerned with it here in the UK :)

[stephen_usher]

Indeed, Sinclair machines were very good at locally blocking FM radio with hiss. Around the 100 frequency on the dial you could tune into the data bus to hear what the machine was doing. A bit like the hiss on the BBC speaker. :-)

[francois]

That all explains at lot, but still:

- Did the FCC later lower the restrictions or are the rules from 1981 still in place?

- Why were (presumably) single digit MHz Sinclair computers generate interference specifically at 100 MHz on the radio? (I could think "harmonics" or "base clock" faster than bus clock... but sill I'd like to have a better understanding of this... ;)

Also: it suddenly makes sense to me why Apple has been using Aluminum bodies for their computers for the last 15 years. But if you remember the colorful transparent plastic e-Macs from the early 2000s: you could see the components inside... no shielding as far as I can tell...

[rubber_jonnie]

That all explains at lot, but still:

- Did the FCC later lower the restrictions or are the rules from 1981 still in place?

Good question, it wouldn't surprise me if the answer was a 'no'.

- Why were (presumably) single digit MHz Sinclair computers generate interference specifically at 100 MHz on the radio? (I could think "harmonics" or "base clock" faster than bus clock... but sill I'd like to have a better understanding of this... ;)

Also a good question, but one I'm not technical enough to answer, though harmonics seems reasonable.

Also: it suddenly makes sense to me why Apple has been using Aluminum bodies for their computers for the last 15 years. But if you remember the colorful transparent plastic e-Macs from the early 2000s: you could see the components inside... no shielding as far as I can tell...

Well there is that, plus the metal bodies give a premium feel and are hard wearing, but I have seen the inside of a few earlier Apple machines and some certainly used metal shielding, and some used metalised coatings on the plastic. The e-Macs though, not machines I've looked inside but maybe they had localised shielding?