Learning About VNAs

We live in a good time to be an electronics geek. It used to be only the richest or shrewdest among us had a really good oscilloscope, while these days it is entirely feasible to have a scope that would have cost a fortune a few decades ago, a logic analyzer, arbitrary waveform generator, and what would have once been a supercomputer and still not be in debt. One of the cooler pieces of gear for people working on RF electronics is a vector network analyzer (VNA) which used to be exotic, but now can be bought for very little. But what do you do with it? [W2AEW] has the answer.

We always look forward to a video from [W2AEW]. Even if we know about the subject he covers, we usually pick up something new or interesting. Like all of his videos, this one is intensely practical. Not a lot of drawing but plenty of scope shots and experimenting.

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Fail Of The Week: Ambitious Vector Network Analyzer Fails To Deliver

If you’re going to fail, you might as well fail ambitiously. A complex project with a lot of subsystems has a greater chance of at least partial success, as well as providing valuable lessons in what not to do next time. At least that’s the lemonade [Josh Johnson] made from his lemon of a low-cost vector network analyzer.

For the uninitiated, a VNA is a versatile test instrument for RF work that allows you to measure both the amplitude and the phase of a signal, and it can be used for everything from antenna and filter design to characterizing transmission lines. [Josh] decided to port a lot of functionality for his low-cost VNA to a host computer and concentrate on the various RF stages of the design. Unfortunately, [Josh] found the performance of the completed VNA to be wanting, especially in the phase measurement department. He has a complete analysis of the failure modes in his thesis, but the short story is poor filtering of harmonics from the local oscillator, unexpected behavior by the AD8302 chip at the heart of his design, and calibration issues. Confounding these issues was the time constraint; [Josh] might well have gotten the issues sorted out had the clock not run out on the school year.

After reading through [Josh]’s description of his project, which was a final-year project and part of his thesis, we feel like his rating of the build as a failure is a bit harsh. Ambitious, perhaps, but with a spate of low-cost VNAs coming on the market, we can see where he got the inspiration. We understand [Josh]’s disappointment, but there were a lot of wins here, from the excellent build quality to the top-notch documentation.

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Hackaday Links: September 1, 2019

The sun may be spotless, but that doesn’t mean it isn’t doing interesting things. A geomagnetic storm is predicted for this weekend, potentially giving those at latitudes where the Northern Lights are not common a chance to see a cosmic light show. According to SpaceWeather.com, a coronal hole, a gap in the sun’s atmosphere that can let the solar wind escape, is about to line up with Earth. The last time this hole was on the Earth-facing side of the sun, the resultant storm gave aurora as far south as Colorado. So if you’re in any of the northern tier states, you might want to find somewhere with dark skies and a good view to the north this weekend.

It’s not only space weather that’s in the news, but weather-weather too. Hurricane Dorian will probably make landfall as a Category 4 storm, probably along Florida’s Atlantic coast, and probably in the middle of the night on Monday. That’s a lot of uncertainty, but one thing’s for sure: amateur radio operators will be getting into the action. The Hurricane Watch Net will activate their net for Dorian on Saturday afternoon at 5:00 PM Eastern time, ready to take reports from stations in the affected area. Not a ham? You can still listen to the live feed once the net activates.

Hams aren’t the only ones getting geared up for Dorian, though. Weather satellite enthusiasts are pointing their SDRs at the sky and grabbing some terrifyingly beautiful pictures of Dorian as it winds up. Some of the downloaded images are spectacular, and if you’ve got an SDR dongle and a couple of pieces of coat hanger wire, you too can spy on Dorian from any number of satellites.

Speaking of which, over on r/RTLSDR, someone has done a little data mining and shown that NOAA 15 is still very much alive. u/amdorj plotted the scan motor current draw and found that it steadily decreased over time, possibly indicating that the bearings aren’t as worn as previously thought. We recently covered the story of the plucky satellite that’s almost two decades past its best-by date; here’s hoping our report on its death was greatly exaggerated.

In one of the weirder bits of marketing we’ve seen lately, NASA decided to name a rock on Mars after septuagenarian rockers The Rolling Stones. The golf ball size rock was blasted about a meter across the Martian landscape when the Mars InSight lander touched down in 2018, leaving a small scar in the dust. The stone had obviously rolled, so phone calls were made and one thing led to another, and before you know it, Robert Downey Jr. is making the announcement before a Stones concert at the Rose Bowl, right in JPL’s backyard. There’s even a cute animation to go along with it. It’s a nice piece of marketing, but it’s not the first time the Stones have been somewhat awkwardly linked to the technology world. We dare you not to cringe.

We’ll finish up today with something not related to space. As Al Williams recently covered, for about fifty bucks you can now score a vector network analyzer (VNA) that will do all sorts of neat RF tricks. The NanoVNA sounds like a great buy for anyone doing RF work, but its low price point and open-source nature mean people are finding all kinds of nifty uses for it. One is measuring the length of coax cables with time-domain reflectometry, or TDR. Phasing antenna arrays? the NanoVNA sounds like the perfect tool for the job.

NanoVNA Is A $50 Vector Network Analyzer

There was a time when oscilloscopes were big and expensive. Now you can get scopes of various sizes and capabilities on nearly any budget. Vector network analyzers — VNAs — haven’t had quite the same proliferation, but NanoVNA may change that. [IMSAI Guy] bought one for about $50 and made a series of videos about it. Spoiler alert: he likes it. You can see one of the several videos he’s posted, below.

NanoVNA is tiny but sweeps from 50 kHz to 900 MHz and has a touch screen. The device uses a rechargeable battery if you need to haul it up to an antenna tower, for example. Just as a quick test, you can see early in the video the analysis of a rubber duck antenna. The device shows return loss as a plot and you can use a cursor to precisely measure the values. It also shows a Smith chart of the reactance.

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A Sneak Peek At Anechoic Chamber Testing

[Mathieu Stephan] has something new in the works, and while he isn’t ready to take the wraps off of it yet, he was kind enough to document his experience putting the mysterious new gadget through its paces inside an anechoic chamber. Considering the majority of us will never get inside of one of these rooms, much less have the opportunity to test our own hardware in one, he figured it was the least he could do.

If you’re not familiar with an anechoic chamber, don’t feel bad. It’s not exactly the sort of thing you’ll have at the local makerspace. Put simply it’s a room designed to not only to remove echos on the inside, but also be completely isolated from the outside. But we aren’t just talking about sound deadening, the principle can also be adapted to work for electromagnetic waves. So not only is in the inside of the anechoic chamber audibly silent, it can also be radio silent.

This is important if you want to test the performance of things like antennas, as it allows you to remove outside interference. As [Mathieu] explains, both the receiver and transmitter can be placed in the chamber and connected to a vector network analyzer (VNA). The device is able to quantify how much energy is being transferred between the two devices, but the results will only be accurate if that’s the only thing the VNA sees on its input port.

[Mathieu] can’t reveal images of the hardware or the results of the analysis because that would give too much away at this point, but he does provide the cleverly edited video after the break as well as some generic information on antenna analysis and the type of results one receives from this sort of testing. Our very own [Jenny List] has a bit more information on the subject if you’d like to continue to live vicariously through the accounts of others. For the rest of us, we’ll just have to settle for some chicken wire and a wooden crate.

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Hackaday Journal Completes First Review Process, Seeks More Submissions

Congratulations and thank you go to Theodore Yapo for authoring the first paper to complete the peer review process for the Hackaday Journal. You can read the standalone paper here; it will be included in the first volume of the Hackaday Journal officially released later this year.

The Hackaday Journal is an open access, peer reviewed journal that seeks to ensure hard-won domain knowledge is preserved and made available for the benefit of all. Before jumping into Ted’s topic, please take a moment to consider submitting your own paper for the journal.

Paper Submissions Wanted

We have other submissions in the pipleline now but we still need more papers to round out the first volume of the Hackaday Journal. Please consider authoring a paper on any creative research, engineering, or entertaining discovery in the areas of interest to the Hackaday community. The full name of our journal is the Hackaday Journal of What You Don’t Know — it will be a tome of infinite appeal to any who seek to broaden their minds in the engineering space. But for that to happen we need you to share your knowledge.

We are in an age of unparalleled opportunity for individuals and small teams to make interesting discovery. You should not need to be working on a degree to have your findings published, but of course students and faculty are encouraged to submit their papers. Do not hesitate to get in touch with us about topics you want to write about.

Scalar Network Analyzer Leakage Correction by Theodore Yapo

Low-pass filter being tested by a Rigol DSA-815 using the phase shifting correction technique

The title of Ted’s paper is a mouthful and the subject material wades into radio frequency knowledge with gusto. We applaud him, and the peer reviewers, for the attention to detail while moving toward publication.

In his work, Ted finds an interesting opportunity to get more performance out of relatively inexpensive bench equipment used to characterize RF components. This task is often reserved for Vector Network Analyzers (VNA) but with a heafty price tag these tools aren’t available to everyone. Spectrum Analyzers with Tracking Generators (SA/TG) have come onto the market, but especially with early versions, there is a leakage problem that causes inaccuracy. Ted found a simple technique that can correct for the leakage.

The solution is based on phase shifting the measurement. Starting with a properly calibrated machine, Ted uses a small board he built to electronically shift the phase of the Tracking Generator where the leakage is a problem. The signal is first measured, then measured again with a phase shift of 180 degrees. This effectively cancels out the error while preserving the signal being measured.

This paper goes into great technical detail in the RF domain. It is worth noting that the Hackaday Journal is open to discovery on multiple topics and levels of complexity. Don’t let what you think is a simple, useful idea go unpublished. We’re interested in a wide range of the simple, the obscure, and the frighteningly technical as long as the ideas of both novel and well supported.

This is Your Journal

You, yes you reading this right now, embody a movement of inventive and curious people working both inside and outside of formal academic structures. This is our time to contribute to the knowledge base of humanity. Pour yourself a refreshing beverage, saddle up your headphones, crack those knuckles, and let the writing process begin. Let us now what we don’t know. Submit your paper now.

Vector Network Analyzer Uses SoC FPGA

If you are working with AC circuits a vector network analyzer (VNA) is quite handy. As an entry to the InnovateFPGA competition for students, [Evgenii Vostrikov], [Danila Nikiforovskii], and [Daniil Smirnov] created a VNA using a DE10-Nano, high-speed analog to digital and digital to analog converters, and a circulator. Most of the details are in the video below, and on the project’s GitHub page.

The DE10-Nano has a dual-core ARM processor and an Altera FPGA in one package. That allows you to use the CPUs where that makes sense and still leverage the FPGA where you need high performance.

The circulator uses an op-amp to allow the test signal to route to the device under test, while steering any reflected signal back to the device for measurement. The design also uses a lock-in amplifier, something we’ve talked about a few times recently. This allows less expensive converters to generate magnitude and phase information.

Judging by the fan in the video, we suspect the setup gets a little toasty. The GitHub page has a lot of Russian on it, so we aren’t sure how much we could puzzle out since our Russian skills were mostly from watching the Adventures of Moose and Squirrel.

If you are interested in a VNA, they aren’t as expensive as they used to be. Particularly, if you roll your own and already have some things in your junk box.

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