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I've designed an electronic load for testing power supplies (circuit and photo in this post: Designing a *linear* MOSFET driver stage).

The load can be controlled (programmed) using an arbitrary waveform from a function generator; a typical example would be using a 100mV square wave to test the small-signal step response of the power supply, although large steps, ramps and other wave forms are interesting too.

I used a panel-mount BNC connector to provide the function generator interface, wiring it to the circuit board with a few inches of 24 AWG twisted pair taken from a CAT-5 cable. I connect it to the function generator with a standard Pomona 2249-C-36 RG 58 C/U coax cable.

The external programming seems to work just fine, but I undertook the project for the sake of motivating my learning, so I want to proceed now to study how this aspect would be done by a practicing engineer and learn about the general realm of cable interface design or whatever it's called.

And that brings us to my question: "How do I find the learning resources I need to study this topic?"

A good start would be knowing what the topic is called :) I found some questions that talked about transmission theory, but searches on that term seem to be focused on high-voltage power transmission. I also consulted my trusty copy of The Art of Electronics, but this topic doesn't seem to be included.

How can I bootstrap myself on this? I vaguely expect I should be adding a 50Ω resistor in there to match impedance, but wonder if maybe running coax with high impedance on both sides is fine too, since my oscilloscope and function generator both seem to do that by default and they use BNC-terminated coax. Also I wonder if twisted pair is appropriate or if maybe I'm introducing some possible quirks in the signal (although I don't think I can see any).

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scanny
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  • Questions on learning resources are usually closed because answers recommending this book or that book become out of data quickly AND recommendations are a matter of opinion than hard fact. – Andy aka Sep 01 '15 at 07:42

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Have you read Wikipedia: Impedance matching and particularly the part about transmission lines ?

An example of a transmission line is the RG58 coaxial cable you mention. The confusion start with the fact that this cable is a transmission line with a characteristic impedance of 50 ohms. Yet when you measure it with your multimeter, there's no 50 ohms to be measured ! It either open (between core and shield) or a near short (couple of ohms, depending on how long the cable is) between both ends of the core and both ends of the shield. Where's that 50 ohms ???

Well, if you would like to transport signals over this cable this is where the 50 ohms comes in. Only if you apply the signal from a source with a 50 ohms source impedance and at the other end terminate the cable with 50 ohms to ground, will you be able to transfer the signal properly ! If you omit either of the 50 ohms resistors the signal will reflect and disturb itself. The value of 50 ohms is a property of the cable, an RG 58 cable needs 50 ohms. There are also cables that need 75 ohms or 600 ohms.

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Transmission lines is a subject that is not so much a part of "transistor electronics" so that's why it's not mentioned in The Art of Electronics. If you search for books on RF-design then for sure you will find books on this subject. Note that many RF-design books are on RF-IC design, most of these do not include information on transmission lines (except for on-chip transmission lines maybe).

Bimpelrekkie
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