Passing 25W DC while rejecting 4W 4-20GHz - RF Cafe Forums

 Post subject: Passing 25W DC while rejecting 4W 4-20GHz

Posted: Sun Sep 16, 2012 9:35 pm 

I'm attempting to power some equipment in an enclosure that sets in an EMC test chamber.
My difficulty is that the customer tests up to 20 GHz for RFI and has some customers that want 20 V/m or more of signal strength, and I'm concerned that my shielded DC power cable will leak enough to interfere with my own electronics.

My first thought was to use a bias tee, but none have met the need (including cost).

My next thought was to bring power in through a back to back bulkhead connector, into a shielded chamber, and run the signal through about 10 feet of RG-174 before exiting through a bulkhead filter into the equipment chamber.

Any ideas?

Thanks,

Mike

 Post subject: Re: Passing 25W DC while rejecting 4W 4-20GHz

Posted: Tue Sep 25, 2012 10:45 pm 

That would work, assuming the lowpass filter held in there all the way up to 20 GHz.

I would be tempted to put some round ferrite toroids along the outside of the coax cable--just to keep any standing waves down. If there is a big standing wave, then you could get leakage thru the ground braid at the E field maximum points. The ferrite toroids would de-Q the outside braid so the standing wave would be small.

If you still had leakage, and needed to add another lowpass filter outside of the chamber, that is where it gets dicey. Since the lowpass filters are totally reflective, if you have two of them at either ends of the cable, you can make a resonant circuit out of the whole mess and lose your rejection. A standard fix is to add a few dB of attenuator pad at the outside LPF, but that would screw up your passing of DC.

_________________
Rich
Maguffin Microwave Consulting
www.MaguffinMicrowave.com