I commented on the value of twisted pair for power and loudspeaker wiring.
On Wed, 01 Aug 2007 22:15:57 +0100, David Woolley wrote: >there might be some benefit if there was a problem >from a co-sited transmitter, We're hams, right? We have co-sited transmitters! :) >but one would need a common mode choke at >the equipment end, and unless the power supply was well balanced or >differential mode filtered, at the power supply end. Differential filtering is ALWAYS a good idea on power leads. Unfortunately, not all equipment has that, and not all filtering is sufficient. The primary cost of paired cable is the copper. For a given wire size, twisting is essentially free. I LOVE free techniques that make things better! Common mode and differential mode susceptibility are additive. Using twisted pair ALWAYS reduces differential mode noise (and RFI) both TO and FROM a given circuit. It is VERY common for RFI to be reduced by 20 dB or more simply by replacing zip cord with twisted pair. >> standards worth changing. >Headphones might benefit, but you would need a common mode choke, or an >output transformer. Not necessarily -- ordinary headphone and loudspeaker output stages are well known to be susceptible to RFI coupled on their output wiring, and it is also well known that replacing zip cord with twisted pair reduces RFI a lot. And neither a common mode choke or output transformer must be present for this to happen. > Also, I suspect that headphone lead pickup tends to >get injected in common mode. All of this stuff is received common mode by antenna action, but can be converted to differential mode at the terminals of the equipment because the two conductors are not balanced with respect to the equipment signal reference or power supply earth AT RADIO FREQUENCIES. And an output transformer is NOT likely to be balanced at RF, thanks to the stray capacitance associated with its construction. These concepts are among those covered in my tutorial on RFI, which also includes a discussion of common mode chokes. http://audiosystemsgroup.com/RFI-Ham.pdf 73, Jim Brown K9YC _______________________________________________ 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
