Hi Jim, The equation that Ted supplied to you to pass on to us on the emc-pstc turned invisible, at least for me. Would you be kind enough to type the equation out and resend it so that those of us with the invisible equation can know what Ted was referring to? Thanks Best regards, Ron Pickard [email protected]
______________________________ Reply Separator _________________________________ Subject: FW: Twisted Shielded Pair Author: "Knighten Jim" <[email protected]> at INTERNET List-Post: [email protected] Date: 7/31/98 9:26 AM Jeff, I pass this along in hopes that it helps you with your question. Jim Knighten ------------------------------------------------------------------------ --------------------------- Dr. Jim Knighten NCR 17095 Via del Campo San Diego, CA 92127 Telephone: 619-485-2537 Fax: 619-485-3788 e-mail: [email protected] ---------- From: DiBene, Ted Sent: Friday, July 31, 1998 9:11 AM To: Knighten, Jim Subject: RE: Twisted Shielded Pair Jim; I have an answer but I am not sure it is the answer he is looking for. I believe he is really talking about the reduction in overall attenuation from increasing the braid coverage to reduce return loss. In going from 85% to 90% shield coverage the change in return loss is not significant. Neither is it the dominant source of conductive losses. Even if this was a single-ended communication link with low impedance (say 50 ohms) the return loss would still be only a fraction of the overall conduction loss. Since we are now talking about a twisted pair and a differential communication path, if the signals are well balanced then the signal and return paths are intrinisically the primaries. However, if there is strong even mode coupling in structure then there will be currents on the shield that will contribute to the overall conduction losses. This, of course is very much dependent upon this modal impedance which is construction dependent. If the signals are imbalanced then, as you know, the common-mode current return path is the shield and the overall losses may be increased slightly due to signal energy loss. Basically, this is a very difficult question to answer accurately since it involves so many variables relating to construction and signal propagation. However, all the theory aside, there is a simple formula he can use that is really for approximations to coaxial line return loss. The differential mode will be much different and thus the loss contribution will be different however if he is looking for a swag, here it is: Assuming that your friend knows how to determine the attenuation constant as function of resistive losses one can use the simple formula below; where rb is the radius of the inner shield and ra is the radius of the primary conductor. If he takes the first part of the formula and ratios this with second he can get a feel for the percentage of loss differences. Again, this is assuming the frequencies are high enough to consider the dominant losses to be due to skin effect. The DC resistance calculation is much more difficult with a twisted wire and braided shield construction. The formula above is a very crude estimate and will almost always be worst case but I think it is a good start. Ted. ---------- From: Knighten, Jim Sent: Thursday, July 30, 1998 5:33 PM To: DiBene, Ted Subject: FW: Twisted Shielded Pair Importance: High Ted, You may have an answer to this guy's question. If so, send it to me and I will post it. Jim K. ------------------------------------------------------------------------ --------------------------- Dr. Jim Knighten NCR 17095 Via del Campo San Diego, CA 92127 Telephone: 619-485-2537 Fax: 619-485-3788 e-mail: [email protected] ---------- From: [email protected] [SMTP:[email protected]] Sent: Thursday, July 30, 1998 3:13 PM To: [email protected] Subject: Twisted Shielded Pair I am looking for a good rule of thumb regarding attenuation versus shield coverage for a twisted shielded pair cable. Specifically what level of attenuation change will be expected in going from a 85% coverage to 90% coverage. Thanks in advance Jeff Duvall
