This blog was inspired by this one by Shabaz. It would be worth reading that one first, if you haven't already done so.
I decided to have a go at making a simple balun with galvanic isolation between the two sides - single-ended in, balanced out. This configuration dates back to the 1950s. Apparently, there's a treatment of it in Millman and Taub (I don't have a copy), but I got this from Hamilton's much later book An Analog Electronics Companion.
Having made a reasonably accurate 50R transmission line from twisted 32 SWG enamelled wire, I took that, cut it in half, and wound the two lines around an EMC ferrite. They're connected like this.
I've only shown one twist of each twisted pair as it makes it clearer what connects to what at the ends. The input is 50 Ohm from the Arduino. The output is 100 Ohms. The dashed line represents the toroid - the toroid doesn't really have a twist in it instead the windings are wound in opposite senses.
Here it is on the bench.
Here's what I got probing the output. Blue is the output at the UNO, yellow is the output. (This absolutely needs good probing - the waveforms will be a mess if you use the flying ground lead.) By chance I got it inverted.
Here it is again with the output probe connections swapped over.
The rise time is good and very little different to the input waveform. For about 10ns, the pulse has a nice flat top, but once we're beyond the propagation time along the line it starts to discharge. Here it is on a longer time scale.
Unfortunately, the UNO, wonderful that it is, can't manage a 10ns pulse width [it's only by writing directly to the port that I can get it even as short as the trace shows], but you can see such a pulse would pass nicely intact through the balun.
1. Pulse, Digital, and Switching Circuits. Millman J. and Taub H. McGraw-Hill, 1965
2. An Analog Electronics Companion. Hamilton S. Cambridge University Press, 2003
3. Circuits for Electronic Instrumentation. O'Dell T. H. Cambridge University Press, 1991