Why the Sinad is low when solder the shieldcan?

[John_li]

low sinad

Hi all,
We used a FM Rf system.Now we met a problem that if we soldered the shieldcan ,the sinad is very low.on the contrary,the sinad is ok without shieldcan.I can not find the root cause?Anybody kindly tell me what happened?
Another hand ,PCB is FR4(H=1.0mm).
Thank you in advanced,

 

[madengr]

If you have switching digital circuitry it generates harmonics and noise (simultaneous switching noise). Even with a 10 MHz clock you will have noise past several GHz with spurs around -100 dBm which can wreak havoc on a sensitive receiver. Hook a coxial loop probe to an LNA, hold it over your circuit, and you will see all sorts of spurs and noise on the spectrum analyzer. The shield can will worsen these over a certain band since it forms a resonant cavity. You can also have ground currents flowing over your shield can which is a bad thing. So if you have a microcontroller enclosed in the same can as your RF circuitry you are asking for trouble. I have built shield cans that did absolutely nothing, and one that made all the difference in passing FCC emissions test.

 

[John_li]

Thank Madengr for your reply ,and i will follow your suggestion to study tommorrow.Another hand,our guy asked the PCB vendor to reduce the height of the PCB copper,and make a trial run for 100 units,all of them can pass the test.My question is make the copper's height thick(more 7~8um---we use the different vendors's PCB) can cause the problem?

 

[madengr]

I don't think the copper thickness directly affected it however higher resistance in a high-speed digital line will dampen ringing and hamonics. Howver, thinner copper also means the the internal planes may now be more closely spaced which raises their resonant frequency, moving a resonance (or attenuating a spur) out of your receiver bandwidth. Yes, the power planes on the PCB can resonant too. Placement of bypass caps at certain points can break these resonances into higher modes. Anosft has a good picture of this and makes software to address these sorts of issues. However 2.5D MOM simulator such as Sonnet Lite can give youe quick insite into how a power plane will resonant. Use the current density viewer then place bypasss caps at the hot-spots to break it into higher modes. A neat trick I have not tried is to place series RC at the edges of the planes to dampen resonances.

**broken link removed**

 

[John_li]

Dear Madengr,
Sorry for the delay.And you are right.As your judgement,we added the bypass caps at the power plane can fix the problem.
I want to simulate the case visibly using Ansoft,can you share me some design guideline about how to use ansoft to simulate the PCB EMC/EMI?
Thanks for your help.

 

[madengr]

See attached PDF file. I don't have Ansoft 2.5D, rather Microwave Office and Sonnet. I suppose you can use HFSS but it will take longer and I don't think the 3D simulators have the dynamic range that the 2.5D simulators provide to simulate coupling.

 

[John_li]

Thanks a lot