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You are here: Home » Blog » Transmission Lines

Twisted pair cable termination

November 16, 2013 by Rolf Ostergaard

A recent question on the Electrical Engineering Stack Exchange website, which is the closest we get to a hardware version of the hugely popular software engineering resource Stackoverflow, got me thinking of twisted pair cable termination and how bad things can really go.

The user Andy sent an 80 MBps signal through a 50-meter cable to a differential receiver and found the thing working only when he used one termination scheme (#2) and not the other (#1).

Twisted pair cable termination styles

When you compare the two termination schemes with an ideal differential signal, they are identical from an AC point of view.

You may want to use a different termination voltage, use a cap or 3 resistors in order to center the received signal around the optimum working point for the receiver. But that is more of a DC consideration.

Common Mode

The interesting thing here is when you look at the two schemes with a common-mode signal. Meaning both signals of the pair are identical. In this case, termination #1 is very high impedance resulting in 100% reflection or voltage doubling at the receiver. Termination #2, on the other hand, has a termination for a common mode signal as well. This common-mode termination may or may not be perfectly matched depending on the specific cable used, but it’s much closer than in scheme #1.

Sometimes you see odd mode referring to the ideal differential and even mode referring to the ideal common-mode signal reflection.

Mode Conversion

So how come a perfectly symmetric output from the drivers suddenly have a significant common-mode component at the receiver?

This can happen in multiple ways, which we commonly refer to as ”differential to common mode conversion”.

In this case, you have a fairly long cable (50 meters) which means a number of things that affect the propagation delay in the cable may be very different between the two wires in a pair. This could be the physical length, average dielectric properties, etc. In addition to this, no drivers are perfectly symmetric and this does usually not get any better when mounted on a real board with differences in stray capacitance, etc.

Long story short: Some of the signal energy will arrive at the receiver as common mode energy. No matter what you do.

Simulation

But just how much of a problem is this? Let’s do a quick simulation to find out.

A differential driver with an 80 MBps signal is driving two ideal transmission lines with a 2ns difference in propagation delay is used for the simulation. The results show how much effect termination of the common-mode signal has.

blog_diff_plot1

Termination scheme #1 (differential mode termination only). Notice how this seriously affects the differntial mode signal.

 blog_diff_plot2

Termination scheme #2 (both common-mode and differential mode termination). Notice how the differential mode signal is still as nice as it can be given the massive skew.

As can be seen, the effects of not terminating the common-mode signal in the receiver end are quite visible on the signal quality. This would probably not be enough, in this case, to render a system unusable, but notice that we have not accounted for loss and other impairments in this idealized simulation setups.

There is a whole lot more to say about common mode and differential mode signals that we can get back to some other time. Just remember that common-mode signals on cables are a likely source for EMI problems as well.

Update

So why is scheme #1 often recommended in application notes for LVDS termination on a board? Well, if you compare the amount of skew the happens across a pair of traces on a PCB to the rise/fall time of the signals, you will see that the skew introduced is usually less than the duration of the edge. In that situation, the common-mode signal resulting from the mode conversion is much less severe.

Simulation setups

For completeness, here are the simulation setups used for the two twisted pair cable termination schemes.

Twisted pair cable termination - case 1

Simulation setup for termination scheme #1.

 Twisted pair cable termination - case 2

Simulation setup for termination scheme #2 (update: both resistors are 100R in the simulation shown).

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Filed Under: Transmission Lines Tagged With: Termination, Twisted Pair

About Rolf Ostergaard

Rolf V. Ostergaard, M.Sc.EE. has worked with signal integrity in many different projects since working for 3Com in 1998 as a colleague to Lee Ritchey in Silicon Valley. While building a consulting business focused on advanced electronics and embedded software in Denmark, Rolf has been helping numerous companies with signal integrity and power integrity both as design, simulations, coaching, measurements, and troubleshooting. He started conducting training in SI in 2004 and has trained hundreds of engineers, which lead to founding EE-Training to further expand this.
You can hire Rolf to do signal integrity training and consulting worldwide and remote.

Comments

  1. Andy aka says

    December 3, 2013 at 2:40 pm

    For scheme 1 the two resistors terminating the cable should be 50 ohm each not 25 ohm each – 25 ohms would imbalance the basic data received differentially

    Reply
  2. Rolf Ostergaard says

    December 4, 2013 at 10:07 am

    You are so right. Thanks for spotting the error. It’s corrected in the above post now – and as you can see, the difference is not that big. The conclusion still holds.

    Reply
  3. Muhammad Ahamd says

    November 15, 2018 at 10:38 pm

    Strange. Using a 2 50 ohm TL rather than a 100 ohm diff TL. Does not make sense for LVDS. LVDS uses 100 ohm diff traces and cables.

    Thanks

    Reply
    • Rolf Ostergaard says

      March 9, 2019 at 6:18 pm

      Muhammad,
      As a model, two 50R transmission lines behaves the same as an ideal 100R differential transmission line 🙂

      Reply
  4. Terry Wade says

    March 29, 2021 at 5:56 pm

    I came across a receiver that used your scheme #2 for a RS-422 setup. We are experiencing RS-422 transmitter failures with 60ohm to grounds. We try to isolate our grounds. Data is at 2Mb/s or less.

    Why do the designer if the transceivers just show a termination like your Scheme #1?

    Reply
    • Rolf Ostergaard says

      May 12, 2021 at 6:31 am

      Good question. Ask that to the designer – remember, not all designers are as smart as you are 🙂

      Reply

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