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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Prop speed is ~ 1/2 C. posted by rick_m on April 27, 2007 at 20:22:38:
Thanks for your reply. My concern with the propagation speed of traces on F4 pcb material stems from an observation some years ago by John Algaier, then a digital engr. with Sun Systems, regarding the speed of data in digital circuitry : The data lines must provide not only for the speed of the data being sent, but also for their EDGE SPEED (rise time) which can be as high as several mega hertz or more. This is also borne out in a book by James Buchanan (Advisory Engr., westinghouse Electric Corp.) "CMOS/TTL Digital Systems design": If a digital data line cannot achieve the necessary speed of the data being sent it will result in false toggling (too soon or too late) which has the same effect as "jitter". Buchanan states that the ideal impedence of data lines is approximately 60 ohms and that data lines' impedence should be matched to 80% of that figure i.e. 50 ohms. It would seem logical that if the data line cannot provide for the edge speed of the data being transmitted that too could result in false toggling. Buchanan also points out that most of the radiation (crosstalk) from traces is from their edges and therefore data lines should be "guarded" by a ground trace close by each side of the data line, or the data should be transmitted by twisted pair wires with one of the pairs being grounded at each end. In the event, it may well be desireable to send critical data via twisted pair wires since no CD player I have seen has data lines guarded by ground traces on either side. I have thought that such data might better be sent using Cat5e twisted pairs (rated for 350 MHtz transmission speed)). However, in that case the problem of available space rears it ugly head. I'm convinced that no one yet has even an inkling of the sonics that CD players are capable of. Oh yes, they work (they make noise) but nothing near what they are truly capable of producing. Tarzan was right : "It's a jungle out there !"
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Follow Ups
- Re: Prop speed is ~ 1/2 C. - pkell44 18:21:38 04/29/07 (3)
- Re: Prop speed is ~ 1/2 C. - rick_m 00:23:01 04/30/07 (2)
- Re: Prop speed is ~ 1/2 C. - pkell44 19:09:34 04/30/07 (1)
- Re: Prop speed is ~ 1/2 C. - rick_m 20:38:29 04/30/07 (0)
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