Electronics > RF, Microwave, Ham Radio

Is this EMI shielding missing on purpose?

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PeterFW:
Hello!
I got this new WiFi device today, it emits a fairly loud buzz/hum so i opened it to have a look were to put some nail varnish in case the offending part is accessible.

I expexted a tin can to cover most of the PCB but i was a bit surprised when i found this:
http://imgur.com/a/jAg1K

There are traces for the EMI shield on the board but it is not populated.
This can not be right, right?

I thougth RF stuff has to be shieldet for various reasons.

Greetings,
Peter

ataradov:
If you can pass regulations without it, then shield is not needed.

The reason designers always put footprint for the shield is that some markets (US included) will require a shield even if it is not needed technically and does absolutely nothing.

T3sl4co1l:
Is that actually a requirement?  Do you have the standards that say so? ???

Tim

ataradov:

--- Quote from: T3sl4co1l on June 21, 2016, 10:53:08 pm ---Is that actually a requirement?  Do you have the standards that say so? ???
--- End quote ---
It is a very strict requirement for modules and modular approval. Here is a document, for example https://apps.fcc.gov/eas/comments/GetPublishedDocument.html?id=50&tn=916170 It has following text:
--- Quote ---The radio elements must have the radio frequency circuitry shielded. Physical/discrete and tuning capacitors may be located external to the shield but must be on the module assembly.
--- End quote ---
.

FCC is a bit vague on device approval, but in general certification labs like to see shields around RF components, but it is possible to do without a shield.

T3sl4co1l:
Hmm, interesting.

The horse's mouth is here:
http://www.ecfr.gov/cgi-bin/text-idx?SID=65383194e02b51c7035811b81823af69&mc=true&node=pt47.1.15&rgn=div5#se47.1.15_1212
It's not at all clear to what length "shielding" constitutes, or what a "crystal" is and why it's implicitly required.  Or "buffering", though I assume they mean to constrain the input voltage range and bandwidth (or data rate, or anything that gets turned into radio modulation, in any case).  Not that it should just have an op-amp or logic buffer, which would do little...

But go figure, the FCC is underfunded these days...

In any case, the pictured device at least doesn't appear to meet the definition of a modular transmitter -- it doesn't have a self-contained antenna, and doesn't appear to use a "non-standard" connector (not that that's a point in favor of its legality in the US..).

Tim

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