Hi guys, and thanks for the good questions. Add me to the list of those who strongly believe in connecting the shields of BALANCED interconnecting cables at the sending end only at low frequencies, and through a capacitor at the receiving end if the cable is a significant fraction of a wavelength at the frequency of an interfering signal. But it's important to understand WHY we need to do that -- the pin 1 problem!
The key word here is BALANCED. Serial connections are UN-balanced, so any voltage drop on the return is added to the signal. So one thing we've learned in the audio world is to use as much beef as we can in the return of unbalanced circuits. If you reduce the resistance of the shield by half, you reduce the hum/buzz by 6 dB. Reduce the resistance by a factor of 5 and you reduce hum/buzz by 14 dB. Bill Whitlock and I both recommend using video coax (8241, 8281) that has a beefy copper braid for unbalanced audio connections, and this is one reason why. Another point, that I think we talked about in Dayton, Don, and that I've talked about on the reflector, is that the pin 1 problem is a major reason that we NEED to prevent that shield current. If you can eliminate the pin 1 problem, you've eliminated much of the coupling of that shield current into the equipment. The IR (or IX) drop is still there, but reducing the shield resistance helps that. Reminder of what the pin 1 problem is: A cable shield SHOULD go to the shielding enclosure of the equipment at each end. In the old days, that was easy -- the connector shell was bonded to the chassis by virtue of its mounting hardware. Today, in the interest of making things easier and cheaper to build, connectors are terminated to the circuit board, and a circuit trace connects the shell to the chassis eventually. But that circuit trace has both R and L, and other circuitry is also connected to that trace. So any shield current (like RF current induced on that cable shield, and any power-related hum/buzz that results from voltage differences between the connected equipment) flows on that trace and there's a drop across the trace. That drop gets injected into the circuitry based on the whim of the circuit layout specialist, and we've got RF in the shack (and buzz in the audio). Now, if you were using SHIELDED CAT5 (which is hard to find and I do have some), a case could be made for connecting the shield at only one end, and using the pairs as I've described, and with their returns tied to pin 5. But that shield doesn't do much below about 20 MHz -- I've tested it with the K2/100 feeding that long wire at full power all the way up to 10 meters, and it only improves stability slightly on 15m and more on 10m. And that's with the wire running within a foot or so of the RS232 cable! I haven't done any testing of the long wire interface at high power (KW amp) above 80 meters, because the wire doesn't work well enough as an antenna above 80m for me to want to use it. :) All of the RS232 cables I've built for my rigs have the cable shields tied ONLY to the DB9 shell on both ends. So far, I've built interfaces for a TS850 (also works on the 791A VHF/UHF radio), Omni V, Icom 746 (which I use on 6m and 2m), and my K2's. On the radio end of the 850, the shield goes only to the shell of the DIN. BTW -- the second reason why you want a beefy shield on coax that encounters noise below 1 MHz or so is that the shield cutoff frequency is reduced in direct proportion to that resistance! The shield cutoff frequency is the low frequency limit of where the shield starts working to cancel magnetic coupling to the center conductor -- below that frequency, it provides only electric field shielding. For a really beefy double copper braid, the cutoff frequency is on the order 1 kHz. For a typical foil/drain shield, it's more like 50 kHz. Also, shielding effectiveness above the cutoff frequency is inversely proportional to that resistance, so dividing the shield resistance by 10 improves the magnetic field shielding at 200 kHz by 20 dB. There's a nice derivation of this in Henry Ott's book. Another point re: connecting or not connecting at both ends. For the coax shield to work to cancel magnetic coupling, it must be connected at each end at the frequency of the interference. If the interference is at 200 kHz, you would need a capacitor that looks like a very low impedance at 200 kHz. No problem, you just need to choose the right value of you have that kind of interference. 73, Jim Brown K9YC On Thu, 7 Jul 2005 14:49:55 -0400, W3FPR - Don Wilhelm wrote: >Adding to that grounding 'magical stuff', what I learned in large system >computer design and testing experience was that shields are only DC grounded >at one end, and that end is the 'driving' device or in the RS-232 world, >that would be the box defined as the DCE - in the DTE devices, the shield >would be grounded through a capacitor, or alternately left open. This was >accomplished in the device wiring itself, the cable shield was always >connected to pin 1 at both ends. _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
