[SI-LIST] Re: Adapting the Metallic Transmission (UTP modelling) Part 3 (mistake in part 2)
- From: "Dr. Howard Johnson" <howie03@xxxxxxxxxx>
- To: "Chris Chalmers" <cchalmers@xxxxxxxxxxx>, "Si-List@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 20 Aug 2004 12:26:48 -0700
Hi Chris,
Sorry for not getting back to you sooner. I'm a little short on time this
week, as we just finished filming a double-feature movie for a
transceiver-chip client and now (finally) I've got a moment to look at your
mail.
Here's the scoop.
(1) Use the R0 value (1.253 ohm/meter ) from the optimized table of values,
p. 463. The values in table 8.3 (p. 463) are optimized to match the
worst-case EIA specification. If you try to calculate R0 from first
principles you will undoubtedly get a better answer because you will
probably be using the NOMINAL PARAMETER VALUES (like AWG24) rather than the
worst case values. Real cables aren't actually exactly 24-gauge.
(2) The Z0 value can vary from 85 to 115 ohms. You get the worst performace
at 85. Use that.
(3) Those two changes alone bring your AC resistance calculation to
4.34*(1.253/85)*sqrt(100/10) = 0.202 dB/m
(4) Now add dielectric loss, using the optimized values from p. 463 --
theta=.001153
w = 2*pi*100E+06
v0 = 0.6*(2.998E+08) (where 2.998E+08 is the speed of light is a
vacuum, m/s)
4.34*((theta*w)/v0) = 0.017 dB/m
(5) adding these values and multiplying times 100m produces
(0.202+0.017)*100m = 21.9 dB/m, pretty much spot-on with the spec value (22
dB/m) and with the graph on page 464.
About the calculation of resistive loss, because we are well above the onset
of the skin effect I can get away with ignoring the DC losses -- don't add
them back in. This issue is discussed starting on page 70.
About the calculation of dielectric loss, because we have a pretty low-loss
dielectric material here, and because we aren't extrapolating very far from
the AC specification frequency omega-zero, you can ignore the exponential
correction term (this issue is discussed at the top of page 209).
As you can see both the skin and dielectric loss were important in this
calculation. If you want accuracy, that's usually the case.
Best regards,
Dr. Howard Johnson, Signal Consulting Inc.,
tel +1 509-997-0505, howie03@xxxxxxxxxx
http:\\sigcon.com -- High-Speed Digital Design seminars, books, and
articles
-----Original Message-----
From: Chris Chalmers [mailto:cchalmers@xxxxxxxxxxx]
Sent: Wednesday, August 11, 2004 12:01 AM
To: howie03@xxxxxxxxxx
Subject: RE: Adapting the Metallic Transmission (UTP modelling) Part 3
(mistake in part 2)
Dr Johnson,
Sorry I made a mistake for the wo value in the equation below.
After the correction its still not right though. See below for the
corrections.
Chris
-----Original Message-----
From: Chris Chalmers [mailto:cchalmers@xxxxxxxxxxx]
Sent: 09 August 2004 17:09
To: howie03@xxxxxxxxxx
Subject: Adapting the Metallic Transmission (UTP modelling) Part 2
Dr Johnson,
Sorry to bother you (yet again). This message is a related
question to an email I sent earlier about the quantity wo.
I have tried to take the example in the HSSP book to
calulate the attenuation (skin effect region) at 100MHz for Cat 5E
cable.
taking Ro = 1.189ohms/m
and wo = 10MHz
attenuation from page 192
attenuation db/m = 4.34 (Ro/Zo) * SQRT(w/wo)
= 4.34 (1.189/100) * SQRT ( (2x PI x
100000000)/ 2 x PI 10000000)
= 4.34 (0.01189) * 3.1623
= 0.163 db/m
From the graph on page 464 for cat 5E, the attenuation looks like
22db/100m
not 16.3 db/100m like I have calculated.
I can't see how from the terms in the equation how you can change it to
cat 5e
since the AWG is the same yet there would clearly be a difference
between
Cat 3 and cat 5E. From this attenuation I would just draw a graph for
the attenuation
against frequency on a log scale between the skin effect onset
frequency and
the frequency when the dielectric loss becomes important as this is the
frequency
range I am intereseted in.
I am possibly being very silly here but any help you can provide me with
would help loads.
Best Regards
Chris
-------------------------------------------------------
Chris Chalmers
Development Engineer
CRL Opto Ltd
1 St David's Drive
St David's Business Park
Dalgety Bay
Fife
KY11 9PF
Scotland
Email: cchalmers@xxxxxxxxxxx
Direct dial: +44 1383 828828
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