The Pi Pico, An SDR Receiver Front End

Making a software defined radio (SDR) receiver is a relatively straightforward process, given the right radio front end electronics and analogue-to-digital converters. Two separate data streams are generated using clocks at a 90 degree phase shift, and these are passed to the software signal processing for demodulation. But what happens if you lack a pair of radio front ends and a suitable clock generator? Along comes [Mordae] with an SDR using only the hardware on a Raspberry Pi Pico. The result is a fascinating piece of lateral thinking, extracting something from the hardware that it was never designed to do.

The onboard RP2040 ADC is of course far too slow for the task, so instead an input is used, with a negative feedback arrangement from another GPIO to form a crude 1-bit ADC. A PIO peripheral is then used to perform the quadrature mixing, resulting in the requisite pair of data streams. At this point these are sent over USB to GNU Radio for demodulating, mainly for convenience rather than necessarily because the microcontroller lacks the power.

The result is a working SDR front end, demonstrated pulling in an FM broadcast station. The Pico has to be overclocked to reach that frequency and it’s more than a little noisy, but we’re extremely impressed with how much has been done with so little. Oddly it isn’t the first Pico SDR we’ve seen, but the previous one was a much more conventional and lower-frequency affair for the European Long Wave band.

DIY Passive Radar System Verifies ADS-B Transmissions

Like most waves in the electromagnetic spectrum, radio waves tend to bounce off of various objects. This can be frustrating to anyone trying to use something like a GMRS or LoRa radio in a dense city, for example, but these reflections can also be exploited for productive use as well, most famously by radar. Radar has plenty of applications such as weather forecasting and various military uses. With some software-defined radio tools, it’s also possible to use radar for tracking aircraft in real-time at home like this DIY radar system.

Unlike active radar systems which use a specific radio source to look for reflections, this system is a passive radar system that uses radio waves already present in the environment to track objects. A reference antenna is used to listen to the target frequency, and in this installation, a nine-element Yagi antenna is configured to listen for reflections. The radio waves that each antenna hears are sent through a computer program that compares the two to identify the reflections of the reference radio signal heard by the Yagi.

Even though a system like this doesn’t include any high-powered active elements, it still takes a considerable chunk of computing resources and some skill to identify the data presented by the software. [Nathan] aka [30hours] gives a fairly thorough overview of the system which can even recognize helicopters from other types of aircraft, and also uses the ADS-B monitoring system as a sanity check. Radar can be used to monitor other vehicles as well, like this 24 GHz radar module¬†found in some modern passenger vehicles.

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Breadboard SDR Doesn’t Need Much

[Grug Huhler] built a simple Tayloe mixer and detector on a breadboard. He decided to extend it a bit to be a full-blown software defined radio (SDR). He then used WSJT-X to monitor FT8 signals and found that he could pick up signals from all over the world with the little breadboard system.

A Raspberry Pi Pico generates a quadrature clock that acts as the local oscillator for the radio. All the processing of the input signal to a quadrature signal is done with a 74LV4052A, which is nothing more than an analog multiplexer. In principle, the device takes a binary number from zero to three and uses it to connect a common signal to one of four channels. There are two common lines and two sets of four channels. In this case, only half of the chip is in use.

An antenna network (two resistors and a capacitor) couples the antenna to one of the common pins, and the Pi generates two square waves, 90 degrees out of phase with each other. This produces select signals in binary of 00, 01, 11, and 10. An op amp and a handful of passive components couple the resulting signals to a PC soundcard, where the software processes the data. The Pi can create clocks up to about 15 or 20 MHz easily using the PIO.

The antenna is a 20-meter-long wire outside, and that accounts for some of the radio’s success. There are several programs than can work with soundcard input like this and [Grug] shows Quisk as a general-purpose receiver. If you missed the first video explaining the Tayloe mixer design, you can catch it below the first video.

This isn’t the first breadboard SDR we’ve seen, but they all use different parts. We’ve even seen a one-bit SDR with three components total (not including the microcontroller). Seriously.

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Pi 5 And SDR Team Up For A Digital Scanner You Can Actually Afford

Listening to police and fire calls used to be a pretty simple proposition: buy a scanner, punch in some frequencies — or if you’re old enough, buy the right crystals — and you’re off to the races. It was a pretty cheap and easy hobby, all things considered. But progress marches on, and with it came things like trunking radio and digital modulation, requiring ever more sophisticated scanners, often commanding eye-watering prices.

Having had enough of that, [Top DNG] decided to roll his own digital trunking scanner on the cheap. The first video below is a brief intro to the receiver based on the combination of an RTL-SDR dongle and a Raspberry Pi 5. The Pi is set up in headless mode and runs sdrtrunk, which monitors the control channels and frequency channels of trunking radio systems, as well as decoding the P25 digital modulation — as long as it’s not encrypted; don’t even get us started on that pet peeve. The receiver also sports a small HDMI touchscreen display, and everything can be powered over USB, so it should be pretty portable. The best part? Everything can be had for about $250, considerably cheaper than the $600 or so needed to get into a purpose-built digital trunking scanner — we’re looking at our Bearcat BCD996P2 right now and shedding a few tears.

The second video below has complete details and a walkthrough of a build, from start to finish. [Top DNG] notes that sdrtrunk runs the Pi pretty hard, so a heat sink and fan are a must. We’d probably go with an enclosure too, just to keep the SBC safe. A better antenna is a good idea, too, although it seems like [Top DNG] is in the thick of things in Los Angeles, where LAPD radio towers abound. The setup could probably support multiple SDR dongles, opening up a host of possibilities. It might even be nice to team this up with a Boondock Echo. We’ve had deep dives into trunking before if you want more details.

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Skip The Radio With This Software-Defined Ultrasound Data Link

We know what you’re thinking: with so many wireless modules available for just pennies, trying to create a physical data link using ultrasonic transducers like [Damian Bonicatto] did for a short-range, low-bitrate remote monitoring setup seems like a waste of time. And granted, there are a ton of simple RF protocols you can just throw at a job like this. Something like this could be done and dusted for a couple of bucks, right?

Luckily, [Damian] wanted something a little different for his wireless link to a small off-grid solar array, which is why he started playing with ultrasound in an SDR framework. The design for his “Software-Defined Ultrasonics” system, detailed in Part 1, has a pair of links, each with two ultrasonic transducers, one for receiving and one for transmitting. Both connect to audio amplifiers with bandpass filters; the received signal is digitized by the ADC built into an Arduino Nano, while the transmitted signal is converted to analog by an outboard DAC.

The transducers are affixed to 3D printed parabolic reflectors, which are aimed at each other over a path length of about 150′ (46 m). Part 2 of the series details the firmware needed to make all this work. A lot of the firmware design is dictated by the constraints introduced by using Arduinos and the 40-kHz ultrasonic carrier, meaning that the link can only do about 250 baud. That may sound slow, but it’s more than enough for [Damian]’s application.

Perhaps most importantly, this is one of those times where going slower helps you to go faster; pretty much everything about the firmware on this system applies to SDRs, so if you can grok one, the other should be a breeze. But if you still need a little help minding your Is and Qs, check out [Jenny]’s SDR primer.

Roll Your Own SDR

If you have software-defined radio hardware and you are only using someone elses’ software, you are missing out on half of the fun. [Tech Minds] shows you how easy it can be to roll your own software using GNU Radio Companion in a recent video.

GNU Radio usually uses Python, but with the companion software you rarely need to know any actual Python. Instead, you simply drag blocks around to represent filters, DSP processing, and other functions you need to create the processing for your application.

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Junk Bin Cyberdish Turns You Into The Satellite Tracker

The good thing about listening in on satellites is that they tend to beam down all kinds of juicy information from their lofty perches. The bad thing about satellites is that to stay in those orbits, they’ve got to be moving really fast, and that means that you’ve got to track them if you want to keep a nice consistent signal during a pass. And that can lead to all sorts of complexity, with motorized two-axis mounts and fancy tracking software.

Or does it? Not if you’re willing to act as the antenna mount, which is the boat [Gabe] from the saveitforparts channel on YouTube recently found himself in when searching for L-band signals from the GOES satellite. His GOES setup uses a 30″ (0.8 m) dish repurposed from a long-range wireless networking rig. Unfortunately, the old security camera pan-tilt unit it was mounted on wasn’t quite up to satellite tracking duty, so [Gabe] pulled the dish off and converted it to manual tracking.

With a freshly wound helical antenna and a SAWbird LNA at the focal point, the dish proved to be pretty easy to keep on track manually, while providing quite the isometric workout. Aiming was aided by an app called Stellarium which uses augmented reality to point out objects in the night sky, and a cheap tablet computer was tasked with running SDR++ and capturing data. Sadly, neither of these additions brought much to the party, with the latter quickly breaking and the former geared more toward stargazing than satellite snooping. But with some patience — and some upper-body strength — [Gabe] was able to track GOES well enough with the all-in-one “cyberdish” to get some usable images. The whole saga is documented in the video after the break.

Kudos to [Gabe] for showing us what can be accomplished with a little bit of junk and a lot of sticktoitiveness. He promises that a legit two-axis mount is in the works, so we’ll be on the lookout for that. We’ve seen a few of those before, and [Chris Lott] did a great overview of satellite tracking gear a while back, too.

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