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Old 27th January 2014, 03:59 PM   #21
DF96 is offline DF96  England
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The ratio of C and L fixes characteristic impedance. The product of C and L fixes velocity ratio. You need the former fixed to avoid signal reflections. You need the latter fixed so all signals arrive at the far end at the same time. Therefore C and L are fixed.
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Old 27th January 2014, 04:01 PM   #22
AndrewT is offline AndrewT  Scotland
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what you show applies to one cable, usually a cable pair or a coax pair (core+screen)

but when you parallel multi strands, the capacitance must change.

Looks like that must be my first experiment. Confirm that a twisted pair is different from a star quad.
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Last edited by AndrewT; 27th January 2014 at 04:08 PM.
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Old 27th January 2014, 04:02 PM   #23
AuroraB is offline AuroraB  Norway
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Characteristic impedanse has no value at audio freq's- only valid for RF and higher speed pulse trains, say,- from 1MHz and upwards...
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Old 27th January 2014, 04:04 PM   #24
AndrewT is offline AndrewT  Scotland
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The proposal (post18) I made was about measuring capacitance of multistrand CAT5 speaker cables.

What has >1MHz got to do with that?

Are you expecting me to use a test frequency of > 1MHz?
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Old 27th January 2014, 04:14 PM   #25
AuroraB is offline AuroraB  Norway
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Characteristic impedance calculations demands the use of at least fractions of a wavelength, which for 20kHz is in the order of 10-15 km, ( choose your own Er ).
For lengths in the order of a handful of meters, only the discrete value of capacitance is of importance.... maybe also some inductaance, but this value will be rather smallish...
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Old 27th January 2014, 04:19 PM   #26
guangui is offline guangui  Puerto Rico
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Quote:
Originally Posted by DF96 View Post
Silver + teflon = triboelectricity?
Not likely as both produce negative charges with teflon producing very little negative charge.
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Old 27th January 2014, 04:24 PM   #27
DF96 is offline DF96  England
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The RF characteristic impedance sets the C and L, which remain valid for audio frequencies even though the characteristic impedance changes due to the dominance of copper R. The RF characteristic impedance usually holds true from some kHz upwards - exactly where depends mainly on cable resistance.

As I said, if the cable has the same defined impedance for each pair and the same velocity for each pair then the C and L must be the same for each pair. This of course assumes normal use, where the pairs carry independent signals. It may be that the velocity varies somewhat over a short length, but averages to the same figure over a long length.
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Old 27th January 2014, 05:51 PM   #28
AuroraB is offline AuroraB  Norway
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You are right, of course, in the sense that f does not play part in the equations for char. impedance, hence the classical definition of 600 ohms audio lines, but on short lengths like this, it has no meanig as such. 1/10th of a wavelength used to be the classical limit. Common mode conditions and all that is of course still valid.
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Old 27th January 2014, 07:38 PM   #29
marce is offline marce  United Kingdom
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Cat 5 some specs:
DC RESISTANCE
The resistance of any conductor shall not exceed 28.6 Ohms per 1000 ft at or corrected to
a temperature of 200 C.
DC RESISTANCE UNABALANCE
The resistance unbalance between the two conductors of any pair shall not exceed 5%
when measured at or corrected to a temperature of 200 C.
MUTAL CAPACITANCE
The mutual capacitance of any pair at 1 kHz, measured in accordance with ASTM D 4566 is
not to exceed 5.6 nf per 100 m at or corrected to a temperature of 200 C.
CHARACTERISTIC IMPEDANCE*
The cable shall have a characteristic impedance of 100 Ohms? 15% for frequencies from 1
MHz to 100 MHz.
NEAR END CROSSTALK (NEXT)
Maximum values permitted for NEXT (rounded to the nearest dB) are listed in table 4 :

* PCB traces to the transformer and then to the connector should be laid out with a differential impedance of 100Ohms.
Should read 20 deg C didn't copy very well.

Last edited by marce; 27th January 2014 at 07:41 PM.
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Old 27th January 2014, 07:48 PM   #30
SY is offline SY  United States
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Quote:
Originally Posted by DF96 View Post
Silver + teflon = triboelectricity?
At those impedances and signal levels, probably not an issue.

Quote:
Originally Posted by guangui View Post
Not likely as both produce negative charges with teflon producing very little negative charge.
Actually, the combo is a classic case of things from the opposite ends of the triboelectric series creating charge.
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