Category: RTL-SDR

A New Software Automatic Gain Control Algorithm C Library for RTL-SDR and other SDRs

Thank you to Chris Gianakopoulos for writing in and sharing with us the release of his open-source software-based Automatic Gain Control (AGC) library written in C. The library is hardware agnostic and designed to make it easy for programmers to implement an AGC algorithm into their programs. The AGC library can help automatically optimize the signal-to-noise-ratio (SNR) on SDRs with variable-gain amplifiers (VGA). Chris explains:

I converted my software AGC to C code with the following enhancements:

1. It is radio-agnostic.
2. Itis written in C so that both, C and C++ apps can use it.
3. The app provides two callback functions: one to provide the current amplifier gain setting and one to set the amplifier gain.
4. A signal magnitude is provided as input to the AGC algorithm
5. Among other things,the number of bits to represent the signal magnitude, at init time.

For more details, the repo is located at,

https://github.com/wizardyesterday/AutomaticGainControl

I have successfully integrated this AGC into a test version of radio diags, located at

https://github.com/wizardyesterday/RtlSdrWork_agc

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An Automatic Gain Control (AGC) is a feeback system that adjusts the gain of a variable-gain amplifier (VGA) to maintain an operating point such as a voltage magnitude level, current magnitude level, or in the case of a digital radio, the magnitude of signal samples presented to the AGC. Typically, an average magnitude of a block of data is used to perform a smoothing action to the input provided to the AGC.

An attempt was made to make an accurate implementation of what was described in the paper (by Fred Harris and Gregory Smith) in the doc/papers/ directory. For details on design and implementation, refer to that paper.

He goes on to mention why a software AGC is useful:

My motivation for creating an AGC was to give people the ability to run SDR software on radios which conain A/D converters that produce 8-bit output samples. With a 48dB (theoretically, ignoring implementation loss), you don't have much to work with in a radio environment with radically different signal strengths.

With an amplifier, whose output drives an A/D converter, on the rtl-sdr, when I listened to aircraft frequencies, I would hear strong tones when a strong signal would be received. The solution was to reduce the LNA and mixer gains.

I asked myself, why would I want to reduce front-end sensitivity when signal overload was not occuring at the variable gain amplifer input? It was A/D converter overload!

With an AGC, the user can establish a safe operating point that allows enough headroom to avoid overload when a strong signal arrives. When the signal goes away, the gain is increased so that you can hear weak signals.

Hopefully, developers of SDR software will see this and implement it into their software!

Our Review of the Vivid Unit GPSDR: A Portable Single Board Computer with Screen and RTL-SDR Expansion Board

As mentioned in a previous post last week, UUGear have recently released their VU GPSDR expansion board for their Vivid Unit single board computer with touchscreen. Together, this combination results in a handheld Linux system, with built-in RTL-SDR and upconverter.

The VU GPSDR has some interesting features, including:

  • GPS-assisted 24 MHz clock for improved frequency accuracy and stability
  • An integrated 108 MHz up-converter for HF (under 30 MHz) reception
  • Dual programmable rotary encoders for tactile control
  • A software-controlled frequency output port for experiments
  • Software features, including OpenStreetMap integration and ADS-B aircraft tracking 
Vivid Unit with VU Extender and VU GPSDR
Vivid Unit with VU Extender and VU GPSDR

Assembly

We won't repeat the assembly steps as the instructions show everything clearly, but we can say that the assembly steps were clear, and the assembly itself was easy. It was simply a case of plugging in a few jumper wires between the Vivid Unit and VU Extender board, screwing down the extender board, and then slotting in the VU GPSDR into the Extender boards mini-PCIe slot, before finally screwing down the GPSDR. Assembly took less than 10 minutes.

Physical Design Review

The system is put together like a sandwich. You have the screen and Vivid Unit on the top, then the Extender board, and finally the VU GPSDR on the bottom.

The Vivid Unit and GPSDR are essentially bare PCBs that connect to one another via the PCIe slot on the Vivid Extender board. This means that there is no enclosure, and you are essentially handling PCB parts in their raw form. In the future, we would like to see an optional enclosure to protect the unit better.

The exposed design results in some flaws that we have to point out. The shielding cans on the VU GPSSDR unit sit on the rear of the system, and during operation, they get very hot to the touch. So much so that handling the unit requires a bit of care to avoid the hot spots. Most of the heat appears to be coming from the AMS1117 LDO on the rear, which gets up to 80 °C, so be careful not to touch it accidentally. From the photos you can see that the RTL2832U and R860 are heatsunk to the shield. This is a good idea to keep the chips cool, but it also means that the metal gets quite hot to the touch. So handling the unit only from the edges is recommended. 

Vivid Unit with the shielding cans removed.
Vivid Unit with the shielding cans removed.
VU GPSDR Thermals
VU GPSDR Thermals

Secondly, because the Vivid Unit does not have a built-in battery, you need to power it separately via its USB-C port on the side. This makes the ergonomics of handling the unit a little trickier as you also have a cable sticking out. UUGear has noted that they are working on an 18650 battery pack, so this issue may be resolved in the future.

Finally, the "GPS" in the GPSDR comes from the fact that there is a GPSDO with a built-in GPS patch antenna on board. When active, a GPSDO provides excellent frequency stability, meaning that signals will be on frequency and will not drift.

But because of how the system is designed, the GPS patch antenna faces the ground when you look at the screen, even though it should face upward to get a clear view of the sky for satellite signals. However, despite this, we were happy to see that even while upside down, the patch antenna was able to receive several GNSS satellites with sufficient strength in order to obtain a fix when used outdoors.

Indoors, of course, no GPS fix is possible. But the uBlox NEO-M8N GPS module used in the GPSDR also has a fallback TCXO, so even without any GPS fix, the frequency accuracy of the system is good. UUGear also noted that the GPSDO automatically activates once a GPS fix is achieved, so no action is needed when you take the unit outdoors.

Realistically, the design issue with the GPS patch doesn't really matter anyway. For most use cases in handheld operation, the built-in TCXO will be sufficient. Any use case requiring extreme GPSDO precision will probably involve the device being mounted upside down and used remotely. 

The screen is clear and bright, the two encoder wheels are non-indented and are in a good spot, and so is the SMA antenna port, although the VU Extender's USB-C plug can block the antenna SMA port if a really fat plug is used (normal-sized USB-C plugs fit OK). The screen is large and has a high resolution, making it possible to use the onscreen keyboard. However, it is still a little fiddly for typing and clicking, so we ended up plugging in a small wireless keyboard.

Continue reading →

VU GPSDR: An RTL-SDR Expansion Board for the Vivid Single Board Computer

Thank you to Shawn from UUGear for writing in and sharing with us news about the release of "VU GPSDR" an RTL-SDR expansion board for the "Vivid Unit" single board computer. The Vivid single-board computer (SBC) is based on the RK3399 CPU (same chip used on the Orange Pi 4), and comes with a built-in 5.5" touchscreen.

The VU GPSDR is an expansion board for the Vivid Unit that, when combined with a VU Extender board, sits on the back of the Vivid Unit, transforming the computing platform into a portable SDR capable of running software like their fork of SDR++ or an OpenStreetMap display of live ADS-B aircraft positions.

The VU GPSDR also integrates a GPSDO (GPS disciplined oscillator), with a built-in GPS patch antenna, for precise frequency tuning. It also comes with a built-in HF upconverter and two encoder wheels, presumably customizable, but primarily for tuning and volume control. Finally, they have also integrated a software-controlled frequency output port for experimenters.

The Vivid Unit costs €99.00 (approximately US$115) on its own. The Vivid Unit Extender, which is required for using expansion boards, costs €25.00 (approximately US$29). The VU GPSDR itself costs €65.00 (approximately US$75). So, in total, for a complete RTL-SDR system, you are looking at a total cost of €189 (US$220).

Shawn has also recently sent us a review sample, which we will be testing out and writing a review for within the next week, so stay tuned for that!

The VU GPSDR Expansion Board for the Vivid Unit Single Board Computer
The VU GPSDR Expansion Board for the Vivid Unit Single Board Computer

Amateur Radio Direction Finding With a KrakenSDR: AREG Presentation

Over on YouTube, we've seen a talk by Mark Jessop that may be interesting to some readers, as it covers Amateur Radio Direction Finding / Fox Hunting with the KrakenSDR, as well as various other radio tools. If you are unaware, KrakenSDR is our 5-channel coherent RTL-SDR based software defined radio system, designed for coherent applications like radio direction finding.

In the talk, Mark explains the amateur radio fox hunting sport, which involves the organizer hiding a transmitter somewhere in a defined area and having participants search for it using just its radio emissions. He goes on to show the different types of antennas, radio systems and vehicle setups participants used.

Mark further explains that on his particular vehicle, he uses a KrakenSDR as the primary receive system. He explains how the KrakenSDR works, how he integrated it into this vehicle and the custom software and LED display that he is using with it. 

ARDF at Mt Gambier - by Mark VK5QI and Grant VK5GR - AREG September 2025 Presentation

A Small 11.2 GHz Motorized Radio Telescope with TV Dish and RTL-SDR

Thank you to Kaustav Bhattacharjee for writing in and submitting to us his project, where he created a small 11.2 GHz motorized radio telescope with a TV dish and an RTL-SDR. A full description of Kaustav's work can be found in a white paper he wrote on behalf of the Department of Physics at the Indian Institute of Technology Roorkee. In summary he writes:

Briefly put, the hardware Setup comprises a 66 cm parabolic dish, a standard Ku-band LNB with bias tee power injection as the frontend, an RTL-SDR V3 tuned to 1.45 GHz (due to downconversion) as the receiver and a Raspberry Pi 5 handling SDR data (via GNU radio) and stepper motor control (using GPIO pins). A heatmap of the southern sky at 0.9° resolution, showing a belt of geostationary satellites, is the primary result of interest!

We also want to point out that his rotor setup involves several 3D printed gears driven by two NEMA17 stepper motors. However, Kaustav notes that the long term resolution is limited due to cumulative backlash errors from the open-loop control scheme.

Kaustav's 11.2 GHz RTL-SDR Radio Telescope
Kaustav's 11.2 GHz RTL-SDR Radio Telescope
Geostationary satellites visualized with the radio telescope
Geostationary satellites visualized with the radio telescope

Creating a Spectrum Analyzer with Zoom Capability with an RTL-SDR in GNU Radio

Thank you to Paul Maine "The SDR Guy" for submitting his latest video showing how to create a simple spectrum analyzer with zoom capability, using an RTL-SDR and GNU Radio. Paul writes:

Zoom capabilities are discussed in the 3rd edition of Richard G Lyons “Understanding Digital Signal Processing” book. This is a novel approach when compared to other YouTube videos about creating a Simple Spectrum Analyzer with an RTL-SDR.

Additionally, in the video, Paul explains what a spectrum analyzer is and what it's used for, as well as explaining the use of attenuators and discone antennas. In the video, Paul uses an RTL-SDR Blog V4, but has mentioned that an RTL-SDR Blog V3 would work well too.

E19 Create a Spectrum Analyzer with Zoom Capabilities

Reverse Engineering the RF Communications on a 27 MHz RC Toy with an RTL-SDR and GNU Radio

On his blog, Jacob has recently uploaded an interesting post showing how he used an RTL-SDR to reverse-engineer the 27 MHz RF communications protocol used by his kids' RC toy truck.

To reverse engineer the protocol, Jacob used GNU Radio to visualize and demodulate the signal. He discovered that it was modulated via Amplitude Shift Keying (ASK), and viewing the waveform in a time-domain plot confirmed the on-off nature of the signal. Next, using symbol sync and thresholding blocks, he generated a bit pattern, which was then processed using Python.

Reverse Engineering the RC Toy Truck 27 MHz Signal
Reverse Engineering the RC Toy Truck 27 MHz Signal

rtl_tcp_echo: Record and Replay IQ Streams with a Transparent rtl_tcp Proxy

Thank you to Sarah Rose Giddings for submitting news about the release of one of her latest software programs called "rtl_tcp_echo". This “man-in-the-middle” application enables you to simultaneously monitor and record signals of interest, then replay the captured IQ data at a later time using software compatible with rtl_tcp.

The software is compatible with Linux, and Sarah notes that a Windows build will be available soon.

RTL_TCP_ECHO is a Go application that acts as a proxy between an rtl_tcp server and its client. It transparently passes control commands (such as frequency, gain, and sample rate), forwards IQ data, and records the IQ stream to a file. Later, you can run the application in playback mode, serving the recorded IQ data as a fake rtl_tcp server—allowing SDR software to connect and replay the IQ stream.

Features

  • Proxy Mode:
    Forwards all rtl_tcp commands (including frequency and gain) and IQ data between client and server. Simultaneously records IQ data to a file.

  • Playback Mode:
    Serves a previously recorded IQ file as an rtl_tcp-compatible server for SDR software to connect and decode.

  • Transparent Command Handling:
    All client commands (frequency, gain, sample rate, etc.) are passed through with optional logging.

  • Simple Configuration:
    Easily specify listen/forward addresses and recording/playback file paths via command-line flags.
rtl_tcp_echo usage
rtl_tcp_echo usage