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Ethernet Termination Resistors

Hi,

Can anyone tell me why the transmit Ethernet lines have 50 ohm termination resistors fitted at the source? My understanding is that you only need to have a termination at the end of a signal line. There should be a 50 source impedance, a 50 ohm characteristic impedance of the transmission line, and a 50 ohm termination resistance. Is it because Ethernet is a differential pair you need a bi-directional 100 ohm differential termination at both ends (2x 50 ohms).

Thanks in advance
 

rjenkinsgb

Well-Known Member
Most Helpful Member
There are two different approaches to driving an ethernet cable.
See the schematics for the different concepts.

A transmission line needs impedance matched termination at both ends to avoid reflections and distortion.

Current mode using two open collector / open drain devices with current limiting and power fed via the centre tap of the isolating transformer.
That needs termination across the line as one or the other end of the transformer winding is otherwise floating and a mismatch.

Voltage mode, using two push-pull drivers. They always have low impedance so series resistors are used to match the line impedance.

 
Ok, so are you saying that the reference schematics I've been looking at are for a current-mode line drivers and the 50 ohm RC termination seen in the left schematic is the equivalent of the series termination.

Many thanks
Ed
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You did not post any schematics or links, so I'm not sure what you have been looking at..

If you men the 2 x 50 Ohm part, that is to help maintain balance and minimise EMI on the cable, whilst matching 100 Ohms.
 

kubeek

Well-Known Member
Most Helpful Member

kubeek

Well-Known Member
Most Helpful Member
Also edofbrighton are you looking at 100Mbit or 1Gbit connection? Gbit is bidirectional so will have different termination than 100Mbit.
 

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