> > All that said, the real hazard with transformers is that people tend to use > > them to drive unbalanced coax cables with balanced signals. This turns the > > coax shield into an antenna, at which point you may end up with with more > > noise and spurs than you had before. > > Could you explain this a little bit more? Because this would be exactly > what i would like to do.
I often find that when I use coaxial baluns to cut down on ground loop noise, I end up with more noise and interference than I started with. Not always, but often enough that I'm leery of them. Due to skin effect, most signal propagation in a coaxial cable takes place between the outer surface of the center conductor and the inner surface of the braid. Ideally, the outer surface of the braid just underneath the jacket will act like an equipotential shield to keep external EMI away from the signal path inside the cable. But that's really only true when the cable connects two devices in well-shielded enclosures that are themselves at a similar ground potential. When you "lift the ground" with a coaxial balun such as an FTB-1-1+, you can no longer pretend that the coax braid is at ground potential along its length. From an RF perspective the braid is floating at one end, which makes it an antenna. Put another way, a balun will reject common-mode signals in favor of differential signals. That's fine if you're using it with a twisted pair or other balanced line (you're probably aware that this is how UTP Ethernet cables work). RF interference in such a line is picked up equally by both conductors and rejected by the balun. But a length of coax cable is as far from a balanced line as you can get. One conductor is well-shielded, the other has its outer surface flapping in the breeze. The balun can't tell the difference between desired signals on the inside surface of the braid and undesired signals on its outside surface. They both look like differential-mode signals, relative to the inner conductor. The same thing happens with instruments that allow you to lift the ground at their input jacks. Apart from the unwanted-antenna effect, this is almost always a bad idea because it's very hard to properly ground the jack's outer shell to the chassis. Few things in EMC are more important than ground integrity at the point of entry to an enclosure. When fighting ground loops, a good first step is to minimize the loop area if you can. Try plugging your DUT, reference, instrumentation, and computer into a single power strip. That will take care of most of your power-line interference problems. Baluns can help too, but don't be surprised if they don't. -- john, KE5FX Miles Design LLC _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
