EMI suppression at 2.4 GHz

[zappedagain]

I can find chokes (Fair-rite 61 material) that are good to 1000 MHz. What materials are good for attenuating frequencies above a GHz? I have a legacy amplifier in a system that we want to get a CE mark on. It is having immunity issues in the band from 100 MHz to 2400 MHz. I have seen good initial test results by adding chokes with 43 material (<250 MHz) on the amplifier pins, so I'm hoping to solve this by blocking the EMI path across the full band. The EMI source is predefined, and I'd rather not redesign the EMI receptor (amplifier).

Thanks,

Z

 

[Higgler]

Magram, get a free sample kit from Arc-Tech, Cuming Corp, emersoncumingmp.
Attenuates higher frequencies, but not as convenient as a ferrite coil.
To get good attenuation, you need current on the wire to be surrounded by Magram, then metal.
Picture coax., your problem wire is the center conductor, Magram is the Teflon, and metal on the outside. Most Magram is very flexible, you can make a loop.

 

[Comcokid]

Other thing might be a couple small ceramic capacitors of different values, 10pF, 100pF or 1000pf, with as absolutely little lead length as you can do on the power pins, in addition to the ferrite bead of 43 material (or other material). No doubt your active (amp) components already have a bypass of maybe 0.1uF, but if you calculate how much lead inductance it takes with a 0.1uF to go past resonance at a given frequency, you will find that it takes only the inductance of a few pico-Henry. A few pico-Henry is only equal to a few thousandths of an inch. The additional low pF caps bypassing added where they're not sharing the same leads (not sharing the same lead inductance) as the other bypass capacitors will help address this issue if your immunity problem is arising from the power leads.

 

[zappedagain]

Thanks for the ideas. So far it looks like I'm seeing similar improvements with the chokes on the power leads or the output pins.

 

[biff44]

you could slip some of this stuff over the wires. Probably good for another 10-15 dB.

http://www.arc-tech.com/cables.php

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[zappedagain]

Thanks. The Magram is interesting; I requested a sample kit so I can learn more.

So far I've found I can get about a 6-12 dB improvement at 2.4 GHz with 61-material chokes on my power and output leads of my amplifier.

I ran a few calculations and it looks like I'm only getting 12-14 dB of shielding off my enclosure so I may have better luck with improvements there. I'm starting to evaluate feedthrough caps at the entry into the main enclosure. Tusonix has some that are good to 10 GHz.

Z

 

[Higgler]

Magram is used on top covers, underneath to absorber. But salt water in a small ziplock does pretty well too. Or a wet sponge.
If you don't want to wait for the magram, try the cheap approach with salt water.

 

[zappedagain]

Thanks. I always like learning a new shortcut!

 

[zappedagain]

Well, I'm back from the test facility. I have a 1.5" circular opening in the enclosure that surprisingly is not a leakage path; I didn't have to screen that opening. I found out my existing aluminum cover with approx. 2" spacing between the screws was only providing about 9 dB of attenuation at 747 MHz. Adding Marian RF Gasket improved that to 86 dB. Adding Tusonix feed thru caps on the signals passing through the cover results in overall shielding at least 100 dB, as the interference is now below my noise floor. I'm impressed, I didn't think I'd get this far with only shielding.

NASA has a really good article online about shielding effectiveness - NASA Contractor report 4784 - Design Guidelines for Shielding Effectiveness, Current Carrying Capability, and the Enhancement of Conductivity of Composite Materials. It is available here:

http://www.twpinc.com/pdf/19970036055_1997067804.pdf

Z