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#1
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Roy:
[snip] There are a lot of opportunities for typos in a derivation like this, especially when restricted to plain ASCII characters. I'd appreciate very much if anyone finding an error, either in concept, fact, assumption, or just typo, to call it to my attention so it can be corrected. Roy Lewallen, W7EL [snip] Wow! You said it Roy. BTW... thanks for all of your nice work. But for my taste it's far too detailed and seems filled with gratuitously long symbols for ASCII text consumers. My eyes glazed over and I nearly fell asleep and had to stop following after a couple of screens of what seemed to turn into gibberish before my eyes. Not your fault mind you, it's mine. On the other hand, ASCII NewsGroup postings are hardly the media for sharing such detailed algebraic/numeric developments! I just don't get the point of all of your wonderful efforts! Thoughts, comments, -- Peter |
#2
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Peter O. Brackett wrote:
My eyes glazed over and I nearly fell asleep and had to stop following after a couple of screens of what seemed to turn into gibberish before my eyes. Not your fault mind you, it's mine. What helps for me is to print it out. I evolved looking at printed pages, not computer screens. I can scribble the correct math operators down so I don't have to remember what e** means. I just don't get the point of all of your wonderful efforts! Roy may have explained Richard's data. In any case, it's good to know that we cannot use the simplified wave reflection model on very lossy lines. It appears that a lossy line doesn't yield a smooth spiral on a Smith Chart. -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
#3
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Cecil:
[snip] In any case, it's good to know that we cannot use the simplified wave reflection model on very lossy lines. It appears that a lossy line doesn't yield a smooth spiral on a Smith Chart. -- 73, Cecil http://www.qsl.net/w5dxp [snip] Amen brother and... heh, heh... especially for broad band signals. Smith Charts are for mono-chomatic signals. Most tough transmission problems are broad band and the Smith Chart yeilds no useful insight in those problems. A widely applied practical example is the transmission of bi-directional broad band digital subscriber loop (DSL) signals over telephone twisted pair. Telephone twisted pair is very lossy... at the "standard" 18,000 foot length you can barely tell what is connected on the other end, short or open. In fact it might just as well be "semi-infinite"! The longest spans we have built chips for were up to 47,000 feet of #24 AWG full duplex data transmission at the basic rate with digital echo cancellation on both ends using trellis coded pulse amplitude modulation. I can assure you that 47,000 feetof #24 AWG definitely has a complex and lossy Zo! I had quite a few big "spools" of such cable in my lab for the beta tests! The big problem with such designs is not maximum power transfer, rather it is hearing the remote end in the presence of the local transmitter blasting away on the same pair as the receiver [talker echo] and so one needs to "image match" the transmitter to eliminate as much talker echo as possible and just take whatever power reaches the receiver at the other end. Of course you have some control over the spectrum of the power that reaches the other end by "pre-coding" at the transmitter, still the optimum strategy at the transmitter is to get an "image match". i.e. make the generator internal impedance as close to the complex Zo as you can make it! And... in those problems you need to differentiate two forward waves and two reflected waves. Heh, heh... hard to do that using just the two symbols Vfwd and Vref or V_+ and V_-, you need symbols for at least two each... Say Vfwd_1 and Vfwd_2 and Vref_1 and Vref_2, etc... messy to say the least! For this reason I much prefer the Scattering Formalism symbols "a" for incident and "b" for reflected, a1 for indicdent on port 1 and a2 for incident on port 2, then b1, b2, etc... Sometime, when I get some free time from my current consulting gig, I'll prepare a short example for the group of the problems inherent in full duplex signalling over complex Zo lines in situations where the "best" Engineering solution is "image match" not "conjugate match". ;-) -- Peter K1PO Indialantic By-the-Sea, FL. |
#4
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"Peter O. Brackett" wrote in message link.net... Sometime, when I get some free time from my current consulting gig, I'll prepare a short example for the group of the problems inherent in full duplex signalling over complex Zo lines in situations where the "best" Engineering solution is "image match" not "conjugate match". ;-) -- Peter K1PO Indialantic By-the-Sea, FL. Peter, You have just answered the question that people have been arguing about the last few weeks. I hope you have time to describe this in more detail. Tam/WB2TT |
#5
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Peter O. Brackett wrote: A widely applied practical example is the transmission of bi-directional broad band digital subscriber loop (DSL) signals over telephone twisted pair. Peter K1PO Indialantic By-the-Sea, FL. As Peter notes, telephone transmission line impedance is always complex. The parameters R, L, G & C per unit length (series resistance, series inductance, shunt conductance, shunt capacitance) are NOT CONSTANT with frequency, or temperature. So the cable impedance is not constant either. Signal spectra extend from nearly DC (a few kHz) up to 12MHz or more - many octaves. Even over voiceband, 400Hz to 2800 Hz, the cable impedance changes a _lot_. Lengths vary from several feet to 10s of kft. There are often/usually open-ended shunt cable sections, a.k.a. bridged taps, along the cable. Other things, like series lumped loading coils (inductors), may appear if not removed from longer cables. Signals at the DSL receiver ends are umpteen dB below the transmitter signal levels on the same pair of wires, and can be in the same band if separate to-the-customer and to-the-network bands are not used. Smith Charts, as much as I like them for ham purposes, are of no help. This subject is addressed in T1.417-2001, Issue 1 "Spectrum Management For Loop Transmission Systems" January, 2001 Developed by Sub-Committee T1E1.4 which develops the xDSL standards (DSL, HDSL, ADSL, VDSL,...) for North America. Annex B of T11.417 deals with the modeling of cables for such cases: formulas, RLCG vs. freq. and other data for common AWG and metric cables... Software packages are available offline and online. http://net3.argreenhouse.com:8080/dsl-test/index.htm (A free registration is needed.) (The other 200+ pages are left for the reader.) The latest working draft of Issue 2 is available free* as document T1E1.4/2003-002R3 from http://www.t1.org/filemgr/filesearch.taf Do a "Simple Search" for filename 3e140023 When the "Results of Simple Search" page appears, click on the blue full name T1E1.4/2003-002R3 under the Contributions column. When the next page appears, click on the blue 3e140023 after "File Prefix" to finally download the document. (2.1MB) I just tried this procedure to be sure it works. * The official Issue 1 is over US$300. In 1995 I was the first editor and wrote the first draft of what became T1.417. Much of what I wrote is intact word-for-word as the first half of Annex B (to my amazment) - the general descriptive part before the nitty-gritty models and numbers. Have fun. There will be an exam. Cheers, 73, Ron McConnell Retired Secretary T1E1.4 N 40º 46' 57.9" W 74º 41' 21.9" FN20ps77GU46 [FN20ps77GV75] http://home.earthlink.net/~rcmcc |
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