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Why is RF circuit shielded

J

Jack// ani

Jan 1, 1970
0
Hi all,

Why is RF circuit shielded inside a metal case...like a cell phone,
where whole of the pcb and its components are shielded.

Thanks
 
H

Howard Long

Jan 1, 1970
0
Jack// ani said:
Hi all,

Why is RF circuit shielded inside a metal case...like a cell phone,
where whole of the pcb and its components are 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
 
B

Bob Agnew

Jan 1, 1970
0
one more that I can think of :

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

Joerg

Jan 1, 1970
0
Hello Bob,
In order to comply with FCC regulations Part 15 and other regualtions.

That is usually the reason.

Regards, Joerg
 
J

Jack// ani

Jan 1, 1970
0
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
 
H

Howard Long

Jan 1, 1970
0
Joerg said:
Hello Bob,


That is usually the reason.

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
 
J

Jack// ani

Jan 1, 1970
0
I had contemplated citing various regulatory requirements but I
thought the OP was after practical reasons.

Hi Howard,

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

Joerg

Jan 1, 1970
0
Hello Jack,
Yep, you are very true...here. I did wanted to know the practical
reasons.

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
 
R

Rich Grise

Jan 1, 1970
0
Hi all,

Why is RF circuit shielded inside a metal case...like a cell phone,
where whole of the pcb and its components are shielded.

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
 
A

Andy

Jan 1, 1970
0
Andy writes:

Let me add #6 to Howard"s post


6. 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
 
H

Howard Long

Jan 1, 1970
0
Hi Andy
6. To minimize interference which external radiation sources might
cause to the circuit inside the shield can.......

That's what I was aluding to in:

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

Cheers, Howard
 
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