Category: LimeSDR

CalibrateSDR: Calibrating your SDR Frequency Offset with DAB+

Thanks to Andreas Hornig who has recently released a new program called "CalibrateSDR" (GitHub code) which is designed to accurately determine the frequency offset of an SDR via an IQ recording of a DAB+ station.

Cheaper RTL-SDR and SDRs use a low quality crystal oscillator which usually has a large offset from the ideal frequency. Furthermore, that frequency offset will change as the dongle warms up or as the ambient temperature changes. The end result is that any signals received will not be at the correct frequency, and they will drift as the temperature changes. Higher end SDRs and improved RTL-SDRs like our RTL-SDR Blog V3 use a temperature compensated oscillator (TCXO) which has a very small frequency offset and very little temperature drift.

CalibrateSDR can be used with almost any SDR to determine the frequency offset. Andreas notes that CalibrateSDR uses the synchronization channel symbols from DAB+ digital audio stations to determine the offset. His post contains a great explanation of how this works. If you don't have DAB+ in your area, an alternative is Kalibrate-RTL which uses GSM cellphone signals to calibrate.

His results were as expected, showing that the generic RTL-SDRs have large frequency offsets, and his RTL-SDR Blog V3 and LimeSDR have much better precision.

The null symbol (lower amplitude portion) and phase reference (Orange) in a DAB+ signal

TechMinds: Extending the Range of Transmit Capable SDRs with Amplifier

Over on his YouTube channel TechMinds has uploaded a new video showing how to use RF amplifiers to extend the transmit range of transmit capable SDRs like the LimeSDR, HackRF and PlutoSDR. Whilst they are transmit capable, most low cost SDRs like those mentioned above can only transmit at very low power levels typically much less than 30 mW. In the video TechMinds tests a wideband SPF5189Z and filtered 2.4 - 2.5 GHZ CN0417 based amplifier, and shows the output power obtained using an inline power meter.

He also notes that these wideband amplifier will also amplify harmonics so filtering is recommended. At the same time we note that you should only transmit if you are licenced to do so (for example with a ham radio licence), especially if you are amplifying the output.

Extend SDR Transmit Range - LimeSDR - HackRF - Adalm Pluto Amplifier

RadioSlate: A Tablet with Built in LimeSDR or HackRF

A new project called "RadioSlate" has recently been announced by Yian IT, a Chinese IoT company. RadioSlate will be an SDR-enabled tablet designed to be used with a HackRF or LimeSDR software defined radio that will be mounted internally behind the screen under some metal shielding. The tablet uses a 1024 x 600 touchscreen and runs an Intel M3 8100Y 1.1 to 3.4 GHz dual core CPU with 8GB of RAM, 64GB of storage and it supports both Linux and Windows. Batteries will not be included, but it supports batteries in the standard 18650 form factor which can be purchased anywhere.

The project is due to be crowdfunded on CrowdSupply in the near future, and you can currently sign up to receive updates and be notified when the project launches. They write:

RadioSlate is a sturdy aluminum tablet with an industry-favorite software-defined radio (SDR) board—your choice of HackRF or LimeSDR—tucked away behind its touchscreen. Whether you’re a Ham radio operator, a network engineer, a mobile base station designer, a security auditor, or some other variety of SDR enthusiast, RadioSlate lets you do your thing, even if that thing requires you to go outside and walk around, get unusually close to transmitters and receivers, keep one hand free for other tasks, or manage all of the above without drawing undue attention to yourself.

Explore the spectrum, while on the go, without having to drag along your laptop, an SDR board, and cables.

The RadioSlate: An SDR-enabled Tablet
The RadioSlate: An SDR-enabled Tablet

A Comprehensive Lab Comparison between Multiple Software Defined Radios

Librespace, who are the people behind the open hardware/source SatNOGS satellite ground station project have recently released a comprehensive paper (pdf) that compares multiple software defined radios available on the market in a realistic laboratory based signal environment. The testing was performed by Alexandru Csete (@csete) who is the programmer behind GQRX and Gpredict and Sheila Christiansen (@astro_sheila) who is a Space Systems Engineer at Alexandru's company AC Satcom. Their goal was to evaluate multiple SDRs for use in SatNOGS ground stations and other satellite receiving applications. 

The SDRs tested include the RTL-SDR Blog V3, Airspy Mini, SDRplay RSPduo, LimeSDR Mini, BladeRF 2.0 Micro, Ettus USRP B210 and the PlutoSDR. In their tests they measure the noise figure, dynamic range, RX/TX spectral purity, TX power output and transmitter modulation error ratio of each SDR in various satellite bands from VHF to C-band.

The paper is an excellent read, however the results are summarized below. In terms of noise figure, the SDRplay RSPduo with it's built in LNA performed the best, with all other SDRs apart from the LimeSDR being similar. The LimeSDR had the worst noise figure by a large margin.

In terms of dynamic range, the graphs below show the maximum input power of a blocking signal that the receivers can tolerate vs. different noise figures at 437 MHz. They write that this gives a good indication of which devices have the highest dynamic range at any given noise figure. The results show that when the blocking signal is at the smallest 5 kHz spacing the RSPduo has poorest dynamic range by a significant margin, but improves significantly at the 100 kHz and 1 MHz spacings. The other SDRs all varied in performance between the different blocking signal separation spacings.

Overall the PlutoSDR seems to perform quite well, with the LimeSDR performing rather poorly in most tests among other problems like the NF being sensitive to touching the enclosure, and the matching network suspected as being broken on both their test units. The owner of Airspy noted that performance may look poor in these tests as the testers used non-optimized Linux drivers, instead of the optimized Windows drivers and software, so there is no oversampling, HDR or IF Filtering enabled. The RSPduo performs very well in most tests, but very poorly in the 5 kHz spacing test.

The rest of the paper covers the TX parameters, and we highly recommend going through and comparing the individual result graphs from each SDR test if you want more information and results from tests at different frequencies. The code and recorded data can also be found on the projects Gitlab page at

DragonOS: Debian Linux with Preinstalled Open Source SDR Software

Thank you to Aaron for submitting news about his latest project called "DragonOS" which he's been working on while in COVID-19 lock down. DragonOS is a Debian Linux based operating system which comes with many open source software defined radio programs pre-installed. It supports SDRs like the RTL-SDR, HackRF and LimeSDR.

Aaron's video below shows how to set up DragonOS in a VirtualBox, and he has two other videos on his channel showing how to set up ADS-B reception with Kismet, and how to run GR-RDS in GNURadio. He aims to continue with more tutorial videos that make use of the software installed on DragonOS in the near future.

DragonOS 10 Installer (download in description)

Screenshot of the GR-RDS Tutorial

PiSDR Updated to Version 3.0: Now Supports the Airspy HF+

PiSDR is a Raspberry Pi distribution that is pre-loaded with multiple programs for various software defined radios. It currently supports RTL-SDR, LimeSDR, PlutoSDR, Airspy, and as of the most recent update the Airspy HF+. The currently pre-installed software packages include SDR Angel, Soapy Remote, GQRX, GNURadio, LimeUtil, and LimeVNA.

Recently version 3.0 was released, and this new version adds a few new features like Desktop shortcuts, Raspberry Pi 4 support, Airspy HF+ support and documentation.

The latest image can be downloaded from the PiSDR website at It can be burned to an SD card in the same way that you would with a standard Raspbian installation. This is a great image to start from if you're experimenting with RTL-SDRs on a Raspberry Pi, as it means that you don't need to go through all the steps of installing the drivers and software like GQRX and GNU Radio which can take a long time to install.

PiSDR Running a SDRAngel with a LimeSDR
PiSDR Running a SDRAngel with a LimeSDR

Using a LimeSDR and RTL-SDR to Transfer a Text File Over the Air

Over on his blog nuclearrambo has been working on a project that uses a LimeSDR and RTL-SDR to transfer a small CSV text file over the air.

The transmitting side consists of a GNU Radio flowchart that encodes the text file into a binary string, modulates that binary string with Binary Phase Shift Keying (BPSK), and then transmits it using the LimeSDR.

The receiving side uses an RTL-SDR, and is based on another GNU Radio flowgraph that uses a polyphase clock sync block to synchronize the sampling time, a costas loop for fine frequency correction, an LMS DD equalizer block to compensate for multipath effects, and finally demodulation blocks that recover the bits and text file from the BPSK signal.

His results showed that he can almost recover the entire file except for the first few bytes of data which is always lost since it takes time for the clock sync and costas loop block to converge. The post goes into further detail about what each of the blocks do and some of the signal theory math behind everything. The GNU Radio GRC file is also provided if you want to try it out yourself.

LimeSDR Transmitting a CSV file to a RTL-SDR with BPSK modulation.
LimeSDR Transmitting a CSV file to a RTL-SDR with BPSK modulation.

XYNC: A Massive MIMO SDR with up to 32×32 TX/RX Channels

Back in 2017 we posted about the crowd funding of the Fairwaves XTRX, a small PCIe based TX/RX capable software defined radio that back then cost US$199 (now only the XTRX Pro is available for US$599). The XTRX is based on the same RF chips that are used in the LimeSDR and each unit has 2 x 2 MIMO (multi-input, multi-output), 120 MSPS SISO / 90 MSPS MIMO, 30 MHz to 3.7 GHz tuning range and comes with an on board GPSDO.

Recently Fairwaves have begun crowdfunding a new software defined radio called the XYNC. The XYNC is essentially a motherboard for connecting up to 16 XTRX boards together which results in an SDR with 32 TX and 32 RX channels.

If you’re working on a massive MIMO system or have a large swath of spectrum you need to monitor, XYNC (pronounced iks-sync) is right for you. XYNC builds on the success of the Octopack SDR we offered during the XTRX campaign and takes into account feedback from the original Octopack users.

You can connect two XYNC boards, either to increase the number of RX/TX channels (e.g., two XYNC Octos give you 32 TX and 32 RX channels) or to increase throughput per channel (e.g., two XYNC Quadros give you twice the throughput of a single XYNC Octo). Connecting more than two XYNC boards is also possible, but requires an external clock and 1 pps signal distribution circuitry, neither of which is provided as part of this campaign.

While advertised as low cost, the pricing is probably out of reach for most hobbyists, with the quad 8x8 unit coming in at US$4500 and the top 16 board 32x32 unit priced at US$13,000. Still, these prices are very good for a massively MIMO SDR and pricing is set to rise once the crowdfunding campaign ends in 39 days.

The XSYNC Massively MIMO SDR with up to 32x32 TX/RX Channels
The XSYNC Massively MIMO SDR with up to 32x32 TX/RX Channels