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  • Author Author: jc2048
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Experimenting with a Transmission-Line Transformer

jc2048

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.

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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.

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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.

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Here it is again with the output probe connections swapped over.

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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.

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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.

References.

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

  • in reply to jc2048

    I have the O'Dell book, but didn't like that one (exactly for the misleading reason you mention! : )  i.e. felt cheated : ) 

  • in reply to shabaz

    I've added them as a proper reference at the end.

    There were several later editions of the Millman and Taub book.

    Apparently, back in the 1950s, these kind of devices were used as part of very high-speed sampling systems, before transistor switching speeds were up to the job. They could generate very fast edges with step recovery diodes and subsequent narrow pulses by feeding the result to a length of shorted transmission line where the length would define the pulse width. A balun would then give simultaneous complementary pulses to drive a diode bridge to do the actual sampling.

    For a while, these pulse applications were important, so that's why there's this literature separate from the rf applications - electronics can be very 'bitty' like this at times, with similar things cropping up in different contexts.

    I've also added O'Dell's book to the list. The title is a bit misleading. It's not a book that's intended as a cookbook of instrumentation circuits, instead it's a teaching text with experiments based on the kind of circuits once found in test equipment. One of those experiments involves sampling and he outlines a pulse generator using a varactor in place of a proper step diode (the internal structure is similar, though the varactor is slower) and goes on with a transmission line balun driving a diode bridge.

  • Hi Jon,

    Nice results! Those books are new to me, but sound interesting, I'll have to see if I can get used copies on Amazon.