Long PCB Shows Effects of Ludicrous Speed

Transmission lines can seem like magic. When you make use of them it seems strange that a piece of wire can block or pass certain frequencies. It is less common to use transmission lines with pulses and typically your circuit’s transmission line behavior isn’t all that significant. That is, until you have to move a signal a relatively long distance. [Robert Baruch] has been using a long PCB to test pulse behavior on a bus he’s working on. He actually has a few videos in this series that are worth watching.

What makes it interesting is that [Robert] has enough distance on the board to where light-speed effects show up. By using a very nice DPO7104 oscilloscope and a signal generator, he shows how the signal reflects on the line at various points, adding and subtracting from it. The measurements matched theory fairly closely. You shouldn’t expect them to match exactly because of small effects that occur randomly throughout the system.

Before you decide you want to run out and buy the same oscilloscope, you better check your bank account. The cheapest used one we could find was about $5,100 and most were double that (possibly due to installed options; we lost interest in looking). A new one is quite a bit more.

It would be really interesting to build a board like this for showing how a half wave or quarter wave stub shorted at one end can act like a filter. In fact, it isn’t so magic if you think about how currents flow in an antenna. Transmission lines can be on a PCB like [Robert’s] but you can also make antennas out of PCB traces.

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Thoughts or comments?