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.

Continue reading “Learning About VNAs”

Portable MRI Machine Comes To The Patient

To say that the process of installing a magnetic resonance imager in a hospital is a complex task is a serious understatement. Once the approval of regulators is obtained, a process that could take years, architects and engineers have to figure out where the massive machine can be installed. An MRI suite requires a sizable electrical service to be installed, reinforced floors to handle the massive weight of the magnet, and special shielding in the walls and ceiling. And once the millions have been spent and the whole thing is up and running, there are ongoing safety concerns when working around a gigantic magnet that can suck ferromagnetic objects into it at any time.

MRI studies can reveal details of diseases and injuries that no other imaging modality can match, which justifies the massive capital investments hospitals make to obtain them. But what if MRI scanners could be miniaturized? Is there something inherent in the technology that makes them so massive and so expensive that many institutions are priced out of the market? Or has technology advanced far enough that a truly portable MRI?

It turns out that yes, an inexpensive MRI scanner is not only possible, but can be made portable enough to wheel into a patient care room. It’s not without compromise, but such a device could make a huge impact on diagnostic medicine and extend MRI technologies into places far beyond the traditional hospital setting.

Continue reading “Portable MRI Machine Comes To The Patient”

GPU Turned Into Radio Transmitter To Defeat Air-Gapped PC

Another week, another exploit against an air-gapped computer. And this time, the attack is particularly clever and pernicious: turning a GPU into a radio transmitter.

The first part of [Mikhail Davidov] and [Baron Oldenburg]’s article is a review of some of the basics of exploring the RF emissions of computers using software-defined radio (SDR) dongles. Most readers can safely skip ahead a bit to section 9, which gets into the process they used to sniff for potentially compromising RF leaks from an air-gapped test computer. After finding a few weak signals in the gigahertz range and dismissing them as attack vectors due to their limited penetration potential, they settled in on the GPU card, a Radeon Pro WX3100, and specifically on the power management features of its ATI chipset.

With a GPU benchmarking program running, they switched the graphics card shader clock between its two lowest power settings, which produced a strong signal on the SDR waterfall at 428 MHz. They were able to receive this signal up to 50 feet (15 meters) away, perhaps to the annoyance of nearby hams as this is plunk in the middle of the 70-cm band. This is theoretically enough to exfiltrate data, but at a painfully low bitrate. So they improved the exploit by forcing the CPU driver to vary the shader clock frequency in one megahertz steps, allowing them to implement higher throughput encoding schemes. You can hear the change in signal caused by different graphics being displayed in the video below; one doesn’t need much imagination to see how malware could leverage this to exfiltrate pretty much anything on the computer.

It’s a fascinating hack, and hats off to [Davidov] and [Oldenburg] for revealing this weakness. We’ll have to throw this on the pile with all the other side-channel attacks [Samy Kamkar] covered in his 2019 Supercon talk.

Continue reading “GPU Turned Into Radio Transmitter To Defeat Air-Gapped PC”

So. You Bought A VNA. Now What?

It’s never too late in life for new experiences, but there’s a new experience I had a few weeks ago that I wasn’t expecting. I probably received my first piece of test equipment – a multimeter –  in the early 1980s, and since then every time I’ve received a new one, whether an oscilloscope, logic analyser, spectrum analyser or signal generator, I’ve been able to figure out how to use it. I have a good idea what it does, and I can figure out whatever its interface may be to make it do what I want it to. My new experience came when I bought a piece of test equipment, and for the first time in my life didn’t have a clue how to use it.

That instrument is a Vector Network Analyser, or VNA, and it’s worth spending a while going through the basics in case anyone else is in the same position. My VNA is not a superlative piece of high-end instrumentation that cost the GDP of a small country, it’s the popular $50 NanoVNA that has a fairly modest frequency range and performance, but is still a functional VNA that can take useful measurements. But I’m a VNA newbie, what does a VNA do? Continue reading “So. You Bought A VNA. Now What?”

A Practical Look At Chokes For EMI Control

Radio frequency electronics can seem like a black art even to those who intentionally delve into the field. But woe betide the poor soul who only incidentally has to deal with it, such as when seeking to minimize electromagnetic interference. This primer on how RF chokes work to reduce EMI is a great way to get explain the theory from a practical, results-oriented standpoint.

As a hobby machinist and builder of machine tools, [James Clough] has come across plenty of cases where EMI has reared its ugly head. Variable frequency drives are one place where EMI can cause problems, and chokes on the motor phase outputs are generally prescribed. He used an expensive choke marketed as specific for VFD applications on one of his machines, but wondered if a cheap ferrite core would do the job just as well, and set to find out.

A sweep of some ferrite cores with a borrowed vector network analyzer proved unsatisfying, so [James] set up a simple experiment with a function generator and an oscilloscope. His demo shows how the impedance of a choke increases with the frequency of the test signal, which is exactly the behavior that you’d want in a VFD – pass the relatively low-frequency phase signals while blocking the high-frequency EMI. For good measure, he throws a capacitor in parallel to the choke and shows how much better a low-pass filter that makes.

We love demos like this that don’t just scratch an intellectual itch but also have a practical goal. [James] not only showed that (at least in some cases) a $13 ferrite can do the same job as a $130 VFD choke, but he showed how they work. It’s basic stuff, but it’s what you need to know to move on to more advanced RF filter designs.

Continue reading “A Practical Look At Chokes For EMI Control”

A Patch Antenna Is Just A Rectangle, It Should Be Easy To Design, Right?

If a grizzled RF engineer who bears the soldering-iron scars of a thousand projects could offer any advice, it would be that microwave antennas are not a field to be entered into lightly. Much heartache is to be saved by using an off-the-shelf design, and only the foolhardy venture willingly down the stripline into the underworld of complex microwave resonances.

But every would-be microwave designer has to start somewhere, and for [Adam Gulyas] that start came with a 2.4 GHz patch antenna. His write-up is a fascinating tale of the challenges and pitfalls of creating something which is deceptively simple at first sight but which becomes significantly more complex as he characterizes his design made real as a PCB.

The process started with a set of calculations to derive the patch dimensions and a bit of PCB work adding a stripline feed. This was produced on a PCB, a normal 1.6mm thick FR4 fiberglass board. When hooked up to a VNA its impedance was all wrong. Further, it had a resonance at the required frequency but also unexpected ones at 3.7 and 4.6 GHz. Simulation of the design also yielded a different resonance from the one calculated, and discussing it with others yielded the conclusion that the feed might be at fault. He ended up using an inset feed, with a co-axial cable emerging away from the edge of the patch, and was able to achieve a far better result.

We can all learn something from [Adam]’s write-up, and we salute him for staying the course to get the design to a usable point. It would be interesting to see the same antenna produced from a more consistent dielectric material than generic FR4. Meanwhile, if you are interested in microwave RF design, take a look at Michael Ossmann’s primer on the subject.

RF Modulation: Crash Course For Hackers

When you’re looking to add some wireless functionality to a project, there are no shortage of options. You really don’t need to know much of the technical details to make use of the more well-documented modules, especially if you just need to get something working quickly. On the other hand, maybe you’ve gotten to the point where you want to know how these things actually work, or maybe you’re curious about that cheap RF module on AliExpress. Especially in the frequency bands below 1 GHz, you might find yourself interfacing with a module at really low level, where you might be tuning modulation parameters. The following overview should give you enough of an understanding about the basics of RF modulation to select the appropriate hardware for your next project.

Three of the most common digital modulation schemes you’ll see in specifications are Frequency Shift Keying (FSK), Amplitude Shift Keying (ASK), and LoRa (Long Range). To wrap my mechanically inclined brain around some concepts, I found that thinking of RF modulation in terms of pitches produced by a musical instrument made it more intuitive.

And lots of pretty graphs don’t hurt either. Signals from two different RF dev boards were captured and turned into waterfall and FFT plots using a $20 RTL-SDR dongle. Although not needed for wireless experimentation, the RTL-SDR is an extremely handy debugging tool, even to just check if a module is actually transmitting. Continue reading “RF Modulation: Crash Course For Hackers”