Re: Wanted: CAT 5 cable simulator

From: Allan Herriman (allan.herriman.hates.spam_at_ctam.com.au.invalid)
Date: 08/13/04 

Date: Fri, 13 Aug 2004 14:21:00 +1000

On Thu, 12 Aug 2004 18:51:05 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

>I read in sci.electronics.design that Greg Neff <greg@microsym.com>
>wrote (in <d98nh01h4idbr7b6olksre8bo3rngj1mbt@4ax.com>) about 'Wanted:
>CAT 5 cable simulator', on Thu, 12 Aug 2004:
>>I have been googling to no avail. Does anyone know where we might be
>>able to purchase CAT 5 cable simulators? This would be use as part of a
>>test bench for Ethernet (10/100Base-T) equipment. Something to simulate
>>100 meters of cable would be ideal.
>
>You may not be able to buy them, but it is possible to make them.
>Depending on how precise you need your simulation to be, you need either
>a large number of components or a LARGE number. But 100 m should be
>practicable.

The OP specified 100Base-T over Cat 5, which implies a bandwidth of
~100MHz. (Well, 802.3 specifies the return loss, etc. performance up
to 100MHz.)

The number of sections needed in the simulator is proportional to the
length multiplied by the bandwidth. In this case, it is a LARGE
number (I calculate about 500 sections x 4 pairs => 18000 components,
if we model the frequency dependence of G and R giving 9 components
per section), so cable drums are a better idea.

100Base-T also uses four pairs (two each way), and crosstalk
cancellation between the pairs is an important part of the transceiver
design. Crosstalk "comes for free" in a real cable, but is a bit
harder in a simulator.

Cable drums can have other problems though: I once (back in the '90s)
used some for DSL testing, and the tech (who was new to this sort of
testing) wired up both ends into the same patch panel. There was
enough crosstalk in the patch panel to upset the high frequency
response and degrade the modem performance. In the real world, the
ends of the cable would be km apart (or 100m apart in the OP's world)
and there would be no crosstalk between the ends.

Regards,
Allan.