A Single-Resistor Radio Transmitter, Thanks To The Power Of Noise

January 27, 2023 by Dan Maloney 36 Comments

One of the great things about the Hackaday community is how quickly you find out what you don’t know. That’s not a bad thing, of course; after all, everyone is here to get smarter, right? So let’s work together to get our heads around this paper (PDF) by [Zerina Kapetanovic], [Miguel Morales], and [Joshua R. Smith] from the University of Washington, which purports to construct a low-throughput RF transmitter from little more than a resistor.

This witchcraft is made possible thanks to Johnson noise, also known as Johnson-Nyquist noise, which is the white noise generated by charge carriers in a conductor. In effect, the movement of electrons in a material thanks to thermal energy produces noise across the spectrum. Reducing interference from Johnson noise is why telescopes often have their sensors cooled to cryogenic temperatures. Rather than trying to eliminate Johnson noise, these experiments use it to build an RF transmitter, and with easily available and relatively cheap equipment. Continue reading “A Single-Resistor Radio Transmitter, Thanks To The Power Of Noise” →

Take A Deep Dive Into A Commodity Automotive Radar Chip

January 17, 2023 by Dan Maloney 24 Comments

When the automobile industry really began to take off in the 1930s, radar was barely in its infancy, and there was no reason to think something that complicated would ever make its way into the typical car. Yet here we stand less than 100 years later, and radar has been perfected and streamlined so much that an entire radar set can be built on a single chip, and commodity radar modules can be sprinkled all around the average vehicle.

Looking inside these modules is always fascinating, especially when your tour guide is [Shahriar Shahramian] of The Signal Path, as it is for this deep dive into an Infineon 24-GHz automotive radar module. The interesting bit here is the BGT24LTR11 Doppler radar ASIC that Infineon uses in the module, because, well, there’s really not much else on the board. The degree of integration is astonishing here, and [Shahriar]’s walk-through of the datasheet is excellent, as always.

Things get interesting once he gets the module under the microscope and into the X-ray machine, but really interesting once the RF ASIC is uncapped, at the 15:18 mark. The die shots of the silicon germanium chip are impressively clear, and the analysis of all the main circuit blocks — voltage-controlled oscillator, power amps, mixer, LNAs — is clear and understandable. For our money, though, the best part is the look at the VCO circuit, which appears to use a bank of fuses to tune the tank inductor and keep the radar within a tight 250-Mz bandwidth, for regulatory reasons. We’d love to know more about the process used in the factory to do that bit.

This isn’t [Shahriar]’s first foray into automotive radar, of course — he looked at a 77-GHz FMCW car radar a while back. That one was bizarrely complicated, though, so there’s something more approachable about a commodity product like this.

Continue reading “Take A Deep Dive Into A Commodity Automotive Radar Chip” →

Homebrew Radio Telescope Bags Pulsar

May 25, 2022 by Dan Maloney 11 Comments

When one mulls the possibility of detecting pulsars, to the degree that one does, thoughts turn to large dish antennas and rack upon rack of sensitive receivers, filters, and digital signal processors. But there’s more than one way to catch the regular radio bursts from these celestial beacons, and if you know what you’re doing, a small satellite dish and an RTL-SDR dongle will suffice.

Granted, [Job Geheniau] has had a lot of experience exploring the radio universe. His website has a long list of observations and accomplishments achieved using his “JRT”, or “Job’s Radio Telescope.” The instrument looks like a homebrewer’s dream, with a 1.9-m satellite TV dish and precision azimuth-elevation rotator. Behind the feedhorn are a pair of low-noise amplifiers and bandpass filters to massage the 1,420 MHz signal that’s commonly used for radio astronomy, plus a Nooelec Smart SDR dongle and an Airspy Mini. Everything is run via remote control, as the interference is much lower with the antenna situated at his family’s farm, 50 km distant from his home in The Hague.

As for the pulsar, bloodlessly named PSR B0329+54, it’s a 5-million-year-old neutron star located in the constellation of Camelopardalis, about 3,500 light-years away. It’s a well-characterized pulsar and pulses at a regular 0.71452 seconds, but it’s generally observed with much, much larger antennas. [Job]’s write-up of the observation contains a lot of detail on the methods and software he used, and while the data is far from clear to the casual observer, it sure seems like he bagged it.

We’ve seen quite a few DIY radio astronomy projects before, both large and small, but this one really impresses with what it accomplished.

[via RTL-SDR.com]

Probe The Galaxy On A Shoestring With This DIY Hydrogen-Line Telescope

September 29, 2019 by Dan Maloney 6 Comments

Foil-lined foam insulation board, scraps of lumber, and a paint-thinner can hardly sound like the tools of a radio astronomer. But when coupled with an SDR, a couple of amplifiers, and a fair amount of trial-and-error tweaking, it’s possible to build your own hydrogen-line radio telescope and use it to image the galaxy.

As the wonderfully named [ArtichokeHeartAttack] explains in the remarkably thorough project documentation, the characteristic 1420.4-MHz signal emitted when the spins of a hydrogen atom’s proton and electron flip relative to each other is a vital tool for exploring the universe. It lets you see not only where the hydrogen is, but which way it’s moving if you analyze the Doppler shift of the signal.

But to do any of this, you need a receiver, and that starts with a horn antenna to collect the weak signal. In collaboration with a former student, high school teacher [ArtichokeHeartAttack] built a pyramidal horn antenna of insulation board and foil tape. This collects RF signals and directs them to the waveguide, built from a rectangular paint thinner can with a quarter-wavelength stub antenna protruding into it. The signal from the antenna is then piped into an inexpensive low-noise amplifier (LNA) specifically designed for the hydrogen line, some preamps, a bandpass filter, and finally into an AirSpy SDR. GNURadio was used to build the spectrometer needed to determine the galactic rotation curve, or the speed of rotation of the Milky Way galaxy relative to distance from its center.

We’ve seen other budget H-line SDR receiver builds before, but this one sets itself apart by the quality of the documentation alone, not to mention the infectious spirit that it captures. Here’s hoping that it finds its way into a STEM lesson plan and shows some students what’s possible on a limited budget.

SDR Is At The Heart Of This Soup-Can Doppler Radar Set

November 29, 2018 by Dan Maloney 6 Comments

Want to explore the world of radar but feel daunted by the mysteries of radio frequency electronics? Be daunted no more and abstract the RF complexities away with this tutorial on software-defined radar by [Luigi Cruz].

Taking inspiration from our own [Gregory L. Charvat], whose many radar projects have graced our pages before, this plunge into radar is spare on the budgetary side but rich in learning opportunities. The front end of the radar set is almost entirely contained in a LimeSDR Mini, a software-defined radio that can both transmit and receive. The only additional components are a pair of soup can antennas and a cheap LNA for the receive side. The rest of the system runs on GNU Radio Companion running on a Raspberry Pi; the whole thing is powered by a USB battery pack and lives in a plastic tote. [Luigi] has the radar set up for the 2.4-GHz ISM band, and the video below shows it being calibrated with vehicles passing by at known speeds.

True, the LimeSDR isn’t exactly cheap, but it does a lot for the price and lowers a major barrier to getting into the radar field. And [Luigi] did a great job of documenting his work and making his code available, which will help too. Continue reading “SDR Is At The Heart Of This Soup-Can Doppler Radar Set” →

Everything You Didn’t Know You Were Missing About Bias Tees

June 26, 2018 by Dan Maloney 23 Comments

Do you need a bias tee? If you want to put a DC voltage on top of an RF signal, chances are that you do. But what exactly are bias tees, and how do they work?

If that’s your question, [W2AEW] has an answer for you with this informative video on the basics of bias tees. A bias tee allows a DC bias to be laid over an RF signal, and while that sounds like a simple job, theory and practice often deviate in the RF world. The simplest bias tee would have a capacitor in series with the RF input and output to pass AC but block DC from getting out the input, and a DC input with a series inductance to prevent RF from getting into the DC circuit. Practical circuits are slightly more complicated, and [W2AEW] covers all you need to know about how real-world bias tees are engineered. He also gives some use cases for bias tees, from sending DC signals up a feed line to control an antenna tuner or rotator to adding a DC bias to a high-speed serial line.

It’s an interesting circuit, and we learned a lot, which is par for the course with [W2AEW]’s videos. Check out some of his other offerings, like a practical guide to the mysteries of Smith charts, or his visualization of how standing waves work.

Continue reading “Everything You Didn’t Know You Were Missing About Bias Tees” →

Fallen Radiosonde Reborn As Active L-band Antenna

January 5, 2018 by Dan Maloney 14 Comments

If your hobby is chasing radiosondes across vast stretches of open country, and if you get good enough at it, you’ll eventually end up with a collection of the telemetry packages that once went up on weather balloons to record the conditions aloft. Once you’ve torn one or two down though, the novelty must wear off, which is where this radiosonde conversion to an active L-band antenna comes from.

As it happens, we recently discussed the details of radiosondes, so if you need a primer on these devices, check that out. But as Australian ham [Mark (VK5QI)] explains, radiosondes are a suite of weather instruments crammed into a lightweight package with a GPS receiver and a small transmitter. Lofted beneath a weather balloon into the stratosphere, a radiosonde transmits a wealth of data back to the ground before returning on a parachute after the balloon bursts. [Mark] had his eyes on the nice quadrifilar helical antenna used by the Vaisla R92 radiosonde’s GPS receiver, with the aim of repurposing them. He had a lot of components to remove while still retaining the low-noise amplifier (LNA), but in the end managed to get a working antenna with 40 dB gain in the L-band, and with the help of an RTL-SDR dongle he picked up solid signals from Iridium satellites.

Want to score your own radiosonde to play with? First, you have to know how to listen in so you can find them. Or, you know – there’s always eBay.

[via RTL-SDR.com]