Smith Chart

https://en.wikipedia.org/wiki/Smith_chart

by benbreen • 7 years ago

66 34 7 years ago

en.wikipedia.org

This brings back memories of the microwave lab in school which held the donated network analyzers ($$$) and was behind a keycard door within another lab that was behind its own keycard door.

Still amazed that someone was able to come up with this tool

paulirwin• 7 years ago

How times have changed. You can now get a two port (S11/S21) battery-powered color touchscreen NanoVNA vector network analyzer that can do Smith charts, SWR, and all kinds of other things for under $40. I use one to analyze ham radio antennas and it's not perfect, but works well enough for my needs and is an incredible value. The firmware for it is open source, too.

mNovak• 7 years ago

Above a couple GHz, VNAs are still $$$. With mm-wave being the hot topic now, you're still looking close to $100k

segfaultbuserr4• 7 years ago

Its performance is nowhere comparable to a real VNA, those VNAs made by Hewlett-Packard in the late 80s are still the golden standard for many labs.

But NanoVNA is great value for the money for anyone interested in radio electronics! For $50, almost nothing beats a NanoVNA, and it's much more effective than the traditional sweep generator setup. You get complex S11/S21 and 900 MHz bandwidth [0], good for measuring complex impedance, the insertion loss of the coax, the frequency response of filters, SWR of antennas, and even basic Time-Domain Reflectometry. The firmware is available under GPLv3 for hacking as well.

I recently used it to experiment different ferrite RF transformers for my homebrew radio receiver, and to characterize frequency response of inductors.

The only thing to beware - since it's a "almost" free hardware design (i.e. FOSS firmware + a block diagram for hardware, only the PCB layout is not available), there are a number of low-quality clones that use low quality components and don't include proper shielding. I recommend hugen79's version ("NanoVNA-H"), he is not the original designer (it was designed by edy555 [2]), but hugen79's version is currently the most common source with reasonable quality, see the picture for comparison. [1]

[0] > 600 MHz uses higher-order harmonics, and is less reliable, but still better than nothing.

[1] https://github.com/hugen79/NanoVNA-H/blob/master/doc/clone.j...

[2] https://github.com/ttrftech/NanoVNA

blackguardx• 7 years ago

Smith charts are really versatile. You can use them for all sorts of RF circuit design tasks, as a complex number calculator, and perhaps more importantly a way to visualize impedance. It is almost how people think in their native language, you can visualize how impedances will interact using the smith chart in your head.

Vysero2• 7 years ago

Ah yes the Smith Chart. The physics equivalent to programming punch cards.

jessaustin• 7 years ago

Is this comparison based solely on the idea that both of these tools are outdated? If you have something deeper in mind, could you fill in some details of this metaphor? I haven't seen Smith charts before, but a cursory read of TFA doesn't really evoke punch cards.

segfaultbuserr4• 7 years ago

Smith Chart is both a visualization tool (as a chart) and a calculation tool (as a nomogram), you can use a printed Smith Chart to do RF-related calculations (impedance matching, return loss, etc) with a ruler and compass on a graph paper, pretty much like using a slide rule to calculate logarithm and trigonometry. I think it's why the OP compared it to a punch card.

It's true that today it's rarely used as a calculation tool, but it's still useful as a visualization. I don't think it's obsolete like punch cards.

jjoonathan• 7 years ago

Punch cards are made thoroughly obsolete many orders of magnitude over by modern storage and IO technologies. What replaced Smith Charts? The >$100k only-a-few-years-old VNAs next room over still plot S11 and S22 on Smith Charts and I'd bet you an entire VNA that this will still be the case in 10 years.

Preemptive: No, 3D Smith Charts do not obsolete Smith Charts, just like 3D line charts don't obsolete 2D line charts.

kstrauser• 7 years ago

Punch cards are made thoroughly obsolete many orders of magnitude over by modern storage and IO technologies.

You weren't kidding. From https://en.wikipedia.org/wiki/Punched_card_input/output :

If all columns of an 80 column card encode information this translates to approximately 2,500 characters per second (CPS).

According to https://www.techradar.com/reviews/samsung-970-evo-plus a 970 EVO Plus SSD is about 1,400,000 times faster. Wow. I would love to go back a few years and blow young me's mind.

Vysero2• 7 years ago

Well in my EMAG class in college we were introduced to Smith Charts. We studied them for about a week or two and used software to check our work. So software has replaced them. There is nothing a Smith Chart can do that a simple software application can't manage in 1/16'th the time.

CamperBob22• 7 years ago

A severely misguided analogy. Every VNA made to this day supports a Smith chart display, and every credible course on RF engineering covers them.

Meanwhile, nobody who's not involved in running a computer museum uses punch cards for anything, and there's little or no pedagogical value in continuing to teach IT or CS students about them.

LandoCalrissian• 7 years ago

I still you the smith chart on my NanoVNA for ham radio.

willis936• 7 years ago

I've found two uses for smith charts: causing students pain and impedance matching. They're good learning tools but imo it's less of a headache to do impedance matching in code.

For actual measurements: there is much less ambiguity in a standard magnitude/phase response, since you actually can see the values of your independent variable (frequency).

falcrist• 7 years ago

They're quite similar to slide rules in that respect. Learning how to use one gives you an intuitive grasp of the operations you might otherwise miss (or not see as clearly), but there are far better tools for doing the actual computation.

madengr3• 7 years ago

On a couple of tests, we actually had to analytically draw our smith charts on graph paper, then use it for then next problem.

Say you want 15 dB return loss at some frequency, well there are two impedances points for that. One with the impedance locus outside Z0, and another with the locus encompassing Z0. The latter has broader bandwidth, but that intuition is lost without a Smith or polar plot.

segfaultbuserr4• 7 years ago

Alan Wolke (W2AEW) has a good video tutorial (along with his other good radio electronics tutorials) on YouTube about the basic applications of Smith Chart, understandable to most radio hobbyists without an electrical engineering background.

https://www.youtube.com/watch?v=TsXd6GktlYQ

person_of_color• 7 years ago

RF/Analog people are wired differently!

GeorgeTirebiter2• 7 years ago

I'm pretty sure RF people have no wires .... (ducking into a Faraday Shield...)

madengr3• 7 years ago

Plumbed with waveguide

Merrill2• 7 years ago

"Remembering Phillip H. Smith on his 100th Birthday" - https://faculty.up.edu/ainan/aps2005inanMarch05.pdf

Smith approached a number of technical magazines for publication of his transmission line diagram; however the acceptance process was slow. Finally, after two years, Smith’s article describing his chart was published in the January 1939 issue of Electronics magazine. In a second article, published in the January 1944 issue of Electronics, Smith incorporated further improvements into his chart, including its usage alternatively as an impedance chart or an admittance chart.

Merrill2• 7 years ago

In the late '60s, the introduction of PL/I, which had complex numbers as a primitive type and a math library of functions of complex numbers, made transmission line calculations much easier.

GeorgeTirebiter2• 7 years ago

Fortran 66 had a COMPLEX data type.

madengr3• 7 years ago

No way is the Smith Chart outdated. I use it daily in simulation software and on the VNA. If a Smith Chart is not shown in a meeting, then that meeting is a waste of time.

I have a collection of various RF slide calculation tools. The prized one is rotary sliding Smith Chart.

krapht• 7 years ago

This is so funny to me. At my workplace we preferred magnitude phase plots and tables. People had to be reminded how to interpret Smith charts

etimberg2• 7 years ago

If anyone wants to see what an implementation of one looks like, I have an old one I wrote as an extension to Chart.js

https://github.com/chartjs/chartjs-chart-smith

CamperBob22• 7 years ago

Nice work, but it seems there are no examples of the output images?

etimberg2• 7 years ago

You're right, I thought I had added those a while ago. Will add some.

BrianHenryIE• 7 years ago

We had fondness for Smith Charts when we were in college. We came across cakes and tattoos online:

http://math-fail.com/wp-content/uploads/smith-chart-tattooo....

https://www.microwaves101.com/uploads/SmithCake-NITT.jpg

foxes• 7 years ago

Looks like a Mobius transformation which is rational function f : C -> C, on the complex plane, of the form

f(z) = (a z + b) / (c z + d).

The key feature is that this is a conformal transformation which means it preserves angles (between curves).

Reading it a bit more I guess thats clear.

Smith Chart

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