Is metal case essential for GSM RF?

[Tuppe]

Hello, I'd like to ask a quick question that I couldn't find a exact answer to.

I'm designing a GSM/GPRS unit. Is it essential to have metal(such as aluminium) casing, with external antennas?
GSM obviously uses high-energy RF, which can induce to the board, but the GSM chip itself(Simcom SIM900 is factory shielded)

My project seems to work just fine without any casing on the table, so is aluminium case worth the double price of plastic casing? What kind of benefit could I get with the metal casing, over plastic?
Or is there a cheap way incorporate the proper shielding in a plastic enclosure?

Thanks!

 

[jiripolivka]

Shielding need generally depends on conditions under which your RF block operates.
GSM RF block is sensitive to external signals as well as it can generate stray signals itself.
If you tested your RF block under "real" operating conditions without problems, then you can use no shield. If you can assume any possible RF interference, using a metal enclosure is a good solution.

Instead of expensive metal enclosure, there are solutions to metallize plastic enclosures with good results. Only remember to connect a good contact from metallized layer to PCB ground. Using good RF connectors and DC filter is important, too.

 

[Tuppe]

Thanks for the insight!

I'm not certain about the terminology you used about the RF block. I understood that it's inside the shielded chipset:
**broken link removed**

So if I have metal casing, I also need to ground it to my PCB? The device is for vehicle use, would there be any benefit of grounding the device to the car body?
Just curious about the details as I can't seem find solid information about this anywhere.

 

[jiripolivka]

Thanks for the insight!

I'm not certain about the terminology you used about the RF block. I understood that it's inside the shielded chipset:
**broken link removed**

So if I have metal casing, I also need to ground it to my PCB? The device is for vehicle use, would there be any benefit of grounding the device to the car body?
Just curious about the details as I can't seem find solid information about this anywhere.

Details ca be found in EMC textbooks. The purpose of screening enclosure is to contain all RF-sensitive electronics inside and reject all external interference. The only way for RF is via the RF "antenna" connector where RF filters allow only wanted signals to pass.
DC lines and other than RF paths should be low-pass filtered, this is why DC feedthrus (capacitor-inductor) filters are used.
The common ground is essential in all designs where high- and low-frequency signals are processed in one system. Including digital controls but some use balanced structures which are treated otherwise.

 

[D.A.(Tony)Stewart]

Fine wire Tinned Copper gauze is also used for shielding. Look at product design tear-downs for ideas.

We used to use custom tinned brass folded shields soldered to PCB. You can buy custom in volume or use std sizes. But if not , protoypes can be made using photo tools to define etching with dotted lines on one side for 50% etch-thru for folds. which can be fabricated by any 2 layer PCB shop using thin metal lamination panels. We used this for 1GHz LO, LNA,Rx/Tx ceramic hybrids over Lexan PCB with special soldering jigs / process. Design can be done in any PCB/CAD tools to step the patterns with breakaway tabs.

 

[Mattylad]

You can get your plastic enclosure sprayed with metallic pint that also protects from RFI which is what jiri was referring to.

 

[D.A.(Tony)Stewart]

1.5 mil thick nickel paint gets medium attenuation. 50+/-25 dB **broken link removed**

But perimeter ground attachment is critical otherwise , it becomes a patch antenna.