Why is RF circuit shielded

Five reasons I can think of:

1. To stop unwanted radiation exiting the device, possibly interfering with other devices (eg, local oscillators, unwanted mixing products, harmonics);
2. To prevent unwanted radiation from external devices affecting the device (eg mixing products and image rejection, and for devices using external antennas almost any digital device, and some analogue ones, in your domicile);
3. To stop subsytems within the device interfering with each other (eg, self-oscillation, unwanted mixing products).
4. Increased surface area decreasing the resistance due to skin effect.
5. Maintaining correct impedance in transmission lines.

Cheers, Howard

one more that I can think of :

In order to comply with FCC regulations Part 15 and other regualtions.

Hello Bob,

That is usually the reason.

Regards, Joerg

Thanks all. I got it, but why only RF circuit, why don't they do the same at low frequency? Do HF devices produces more noise compared to LF devices, thats why or something else??

Thanks

Hi Howard,

Yep, you are very true...here. I did wanted to know the practical reasons.

I think you'll find that Part 15 is only a US regulation. The rest of the world operate under different regulations. My previous answer (1) essentially covers this, but there was no indication that the OP was in the US. FWIW I had contemplated citing various regulatory requirements but I thought the OP was after practical reasons.

Kind Regards, Howard

So that it doesn't radiate extraneous signals and interfere with other equipment. At line and audio frequencies, this is not so much of a problem because of the physical characteristics of the radiated electromagnetic field.

Hope This Helps! Rich

Hello Jack,

In other countries you have regulations similar to what the FCC does in the US. They are just called differently, for example in Germany the agency's name would be RegTP. EMI limits aren't very different. That's the practical reason.

As for your question why not LF: LF circuits usually do not radiate higher frequency harmonics. The ones over 30MHz are the main concern here. Even a regular digital circuit can generate enough trouble up there to require at least some shielding.

Regards, Joerg

Let me add #6 to Howard"s post

1. To minimize interference which external radiation sources might cause to the circuit inside the shield can.......

You gotta worry about bad stuff getting in, as well as good stuff getting out......

Think of an RF shield as a metallic condom..... Lots of bad stuff out there that can mess up your life......

Andy

Thanks all.

Andy

hehe...thats very funny.

Hi Andy

That's what I was aluding to in:

1. To prevent unwanted radiation from external devices affecting the device

Cheers, Howard