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Transmission Lines and Cabling: Everything You Need to Know About Sending Bits Down a Wire

Back in October, I posted a link to an excellent article by Bertho about decoupling capacitors. It was one of the best articles I’d ever seen on that topic (including chapters in textbooks, appnotes, and around the web). So you can imagine how thrilled I was when he told me about his newest article, titled “Cable Connected, Signal Lost“, about transmission lines. He writes:

It is one thing to put together LEDs, a few logic chips and a CPU to make something interesting. But, it is a completely different story to connect cables to your creation and try to run various low- and high-frequency signals through them. I’m saying this not to denigrate, but the laws of physics are relentless and good cabling is an area of expertise all for and by itself.

My first encounter with the practicalities of cabling came some 20 years ago when I tried to run a 8MHz digital CGA video signal through 15..25m cables. As you can imagine, my first try did not succeed. I had learned about transmission lines almost 10 years earlier, but never had used it in practice. So, I had to read up on it and understand the problem.

The theory behind transmission lines is well understood, but can be daunting if not versed in the world of analogue electronics. Just know, there is no way you can wiggle your way out of a transmission line’s path. You will encounter them at some stage in your hacking and then you need to solve your problem.

Luckily, there are a few relatively simple things you can do to save yourself from defeat. The problems that transmission lines introduce can be visualized reasonably easy, and, with a lot of pictures, I will try to show you what happens to signals of various frequency and with different transmission line connections.

Check out the article and bookmark it, because there’s a lot of information there worth referring to. Thanks again to Bertho for sending this along to me!



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4 Comments

  1. Am I the only one who gets a timeout every time I try to access http://www.vagrearg.org?

  2. Great article! While reading it, I suddenly understood how antennas worked: they’re basically a transmission line where the length of the line is calculated so that the reflections coming back are in phase with the signal going in, allowing them to add up and to maximise the power radiated.

  3. @Matthew: he’s had a spike in traffic. It might be affecting his hosting a bit. It worked for me just now, so you might want to try again.

    @Amr: Awesome! 🙂

  4. Thanks much for sharing, John. I loved the article on decoupling caps. It helped me better explain the basic concepts to my coworkers. This latest article is just as enlightening. Bertho’s doing some great instructional stuff here!

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