Fixing a long active USB Cable for RTL-SDR Use

Active USB cables allow cable lengths to be stretched to much longer than the maximum length of 5m allowed by the USB specification. However, although the packet timing requirements are met by the repeaters used in the active cables, there is still a significant voltage drop which can affect devices like the RTL-SDR.

Over on YouTube Shaun Dobbie discovered that his RTL-SDR would not run properly on his long active USB cable, and he suspected low voltage. After opening the case on the USB cable head he discovered two pins which allowed for external power input. By simply connecting an external 5V supply from a battery to the 5V input of the active cable he was able to fix the low voltage problem. If you’ve ever found that a long active USB cable doesn’t work then this may be the problem you have experienced. An alternative to this home solution might be to use an external powered USB hub, or buy an active USB cable that already has an external power input like this or this one.

RTL SDR USB Extension Cable

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Updates on using an RTL-SDR for GPS on a High Powered Rocket

Back in April we posted about Philip Hahn and Paul Breed’s experiments to use an RTL-SDR for GPS logging on their high powered small rockets. As GPS is owned by the US military, a standard GPS module cannot be used on a rocket like this, as they are designed to fail if the GPS device breaches the COCOM limit, which is when it calculates that it is moving faster than 1,900 kmph/1,200 mph and/or higher than 18,000 m/59,000 ft. The idea is that this makes it harder for GPS to be used in non-USA or home made intercontinental missiles. As SDR GPS decoders are usually programmed in open source software, there is no need for the programmers to add in these artificial limits.

In their last tests they managed to gather lots of GPS data with an RTL-SDR, but were only able to decode a small amount of it with the GNSS-SDR software. In this post Philip discovers a flaw in the way the GNSS-SDR performs acquisition and retracking that GNSS-SDR decodes in such a way that makes it difficult to obtain a location solution with noisy high-acceleration data. By using a different GPS implementation coded in MATLAB, he was able to get decoded GPS data from almost the entire ascent up until the parachutes deploy. Once the parachutes deploy the GPS has a tough time keeping a lock as it sways around. His post clearly explains the differences in the way the code is implemented in GNSS-SDR and in the MATLAB solution and shows why the GNSS-SDR implementation may not be suitable for high powered rockets.

In addition, they write that while the flight was just under the artificial COCOM GPS fail limits for speed and height, the commercial GPS solution they also had on board failed to collect data for most of the flight too. With the raw GPS data from the RTL-SDR + some smart processing of it, they were able to decode GPS data where the commercial solution failed.

GPS data acquired from the RTL-SDR on the rocket.
GPS data acquired from the RTL-SDR on the rocket (blue line shows solution from MATLAB code, yellow shows GNSS-SDR solution, and red shows commercial GPS receiver solution).
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Big Airspy Sale for US Customers: $149 Airspy R2, $99 Airspy Mini, $39 Spyverter

To celebrate the fourth of July, the US distributor of Airspy is throwing a sale. The prices are the lowest we’ve ever seen for any Airspy product before. Currently the sale price for an Airspy R2 is $149 (vs $199), $99 for Airspy Mini (vs $114) and $39 for the Spyverter. Unfortunately the sale only appears to be occurring with the USA distributor, and is not available for international customers.

If you’re interested in these products see our previous review on the Airspy R2 vs HackRF vs SDRplay, review of the Airspy Mini, and our review of the SpyVerter. In short the consensus from our reviews is that the Airspy is an excellent product. The Spyverter is also the best upconverter we’ve tried, and is an excellent choice for an upconverter for any other non Airspy SDR, such as an RTL-SDR. And at this price it is even cheaper than most of the alternative options like the ham-it-up.

airspy_logo

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University Course on Digital Signal Processing with the RTL-SDR

Over the past few years the Electrical Engineering department of the University of California, Berkley has been using RTL-SDR’s in their EE123 Digital Signal Processing (DSP) course. We’d posted about this course years before when it first came out, but recently Micheal Lustig (KK6MRI), the Associate Professor of the course wrote in to let us know that the course has evolved and is now better than ever.

The course covers DSP essential material such as the Discrete Fourier Transform, Fast Fourier Transform, RF Filter design, as well as more complex subjects. All the course material is available in note and video form if you scroll down on the main page at https://inst.eecs.berkeley.edu/~ee123/sp16/index.html.

However, the professor writes that the best gem that they have developed in their labs which can be found at https://inst.eecs.berkeley.edu/~ee123/sp16/labs.html. The labs run on the web based Ipython/Jupyter Notebooks and guide you through the implementation of an ADS-B receiver, broadcast FM and subcarrier demodulation, frequency calibration with GSM, and a full python APRS transceiver using the baofeng radio and a custom audio interface. These labs are an excellent tutorial into the world of DSP.

The final project of the class is also very interesting. The students of the class were given the task to send images using a Baofeng UV-5R handheld radio and receive them with an RTL-SDR. On the day of the project demonstration they were given two images, and the challenge was to transmit the best quality image over 75 seconds. Videos of the presentation can be found at https://inst.eecs.berkeley.edu/~ee123/sp16/projectVideos.html. The winning team used a combination of five Baofeng’s for simultaneous transmission of a compressed image and an RTL-SDR for receiving.

Richard-Allan-James

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Receiving the SAQ VLF Signal with an Airspy + Spyverter and SDRplay RSP

Over on YouTube user Mile Kokotov has uploaded a video showing his reception of the SAQ very low frequency (VLF) signal. The SAQ transmitter is based in Grimeton, Sweden and transmits at 17.2 kHz, which is well below the frequency of most radio communications. SAQ only transmits its beacon on certain days, and last Sunday July 3rd 2016 the SAQ beacon activated to celebrate Alexanderson day, which is named after Swedish radio pioneer Ernst Frederick Werner Alexanderson.

In the video both the Airspy + Spyverter and the SDRplay RSP appear to receive the SAQ VLF signal equally well. In the video description Mile writes:

“SAQ”- Radio Station at Grimeton is a VLF transmission facility at Grimeton, Sweden. It has the only working Alexanderson alternator rotating armature radio transmitter in the world and is classified as a World Heritage Site.

The transmitter was built in 1922 to 1924 to operate at 17.2 kHz. The antenna is a 1.9 km wire aerial consisting of eight horizontal wires suspended on six 127-metre high freestanding steel pylons in a line, that function as a capacitive top-load to feed energy to six grounded vertical wire radiating elements.

Until the 1950s, the Grimeton VLF transmitter was used for transatlantic radio telegraphy to Radio Central in Long Island, New York, USA. From the 1960s until 1996 it transmitted orders to submarines in the Swedish Navy.

The Alexanderson transmitter became obsolete in 1996 and went out of service. However, because it was still in good condition it was declared a national monument and can be visited during the summer.

On July 2, 2004, the Grimeton VLF transmitter was declared a World Cultural Heritage site by UNESCO. It continues to be used on special occasions such as Alexanderson Day to transmit Morse messages on 17.2 kHz. Its call sign is SAQ.

Recent transmissions from SAQ on 17.2 kHz with Alexaderson 200 kW alternator, was on Alexanderson day (Sunday, July 3rd 2016) at 09:00 UTC.

Distance between SAQ transmitter in Grimeton, Sweeden and Macedonia where the signal was received is about 1850 km.

Receiving with:
1. AIRSPY R2 – SDR + Spyverter and SDRsharp software.
2. SDRplay RSP1 and SDRuno software.

Both SDR receivers settings were previously set for maximum S/N ratio.

Antenna is Mini-Whip 10cm homemade active antenna on 6.5 meter plastic pole.

The LPF filter (fc=535 kHz) is used also.

SAQ VLF Receiving with Airspy+Spyverter and SDRplay

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Making use of the Infrared LED on RTL-SDR Dongles

The infrared (IR) LED on most RTL-SDR dongles is a vestigial from the days when it was actually used for its original purpose as an DVB-T HDTV receiver. It was used to read a remote control that allowed you to change TV channels. For SDR use, the IR has little to no purpose and in many new dongles that come in metal cases (like ours) the IR LED is no longer even included on the PCB.

However, not one to waste a perfectly good interface, RTL-SDR experimenter R. X Seger created a new tool called rtl_ir which allows users to read IR data from any remote control with the RTL-SDR IR LED. Seger tested his program with the TV remote that comes included with some RTL-SDR dongles and was able to decode the scancode for power on/off as well as all the other buttons. He also tested an Apple and Siri Remote, and found that he was able to decode their scancodes too.

R. X Segers post goes over in detail what the IR spectrum is, how the IR driver works, and how to use the rtl_ir program and run it simultaneously with other RTL-SDR programs. He also shows an example on how it can be used to remotely power off a Raspberry Pi.

IR data received with rtl_ir.
IR data received with rtl_ir.
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LuaRadio: New Flowgraph Based Digital Signal Processing Framework for SDR

LuaRadio is a new Digital Signal Processing (DSP) framework for software defined radios such as the RTL-SDR. It is similar to GNU Radio in that the flowgraph is composed of graphical blocks that can be visually connected to one another in an editor. However compared to GNURadio it aims to be very lightweight in terms of disk space used (1 MB footprint) and the number of dependencies required (zero dependencies required unless you need real time highly optimized libraries). It is also written purely in the Lua programming language. The authors of LuaRadio write “LuaRadio is more inclined towards scripting and prototyping than GNU Radio, and emphasizes fast block development.”

On their website there are already several example application flowgraphs uploaded, such as decoders for WBFM Mono/Stereo, NBFM, AX.25, POCSAG, RDS, AM and SSB. Looking and building such flowgraphs is extremely helpful for learning DSP, and DSP languages like this are excellent for prototyping new signal decoders. In addition, if you are new to SDR they also have a very useful page that explains basic SDR and radio concepts.

A LuaRadio based POCSAG decoder flowgraph.
A LuaRadio based POCSAG decoder flowgraph.
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Building an ESP8266 Based Plane Spotter with an RTL-SDR Feeder

Living near Zurich airport, Daniel Eichorn wanted an easy way to show his house guests what planes are flying near him. Usually he opens up his Flightradar24 app on his phone, but he wanted a more permanent always on display. To do this Daniel has built an ESP8266 based OLED display which automatically displays the ADS-B flight information of aircraft outside his window. The ESP8266 is a very cheap and highly popular WiFi module which can give a microcontroller access to WiFi networks.

Daniel feeds his locally received ADS-B data to adsbexchange.com using a Raspberry Pi and RTL-SDR. While actually feeding ADS-B data with an RTL-SDR is not required to make the ESP8266 module work, this step ensures that he has good local coverage of his area. The ESP8266 module then queries the adsbexchange.com database via WiFi for information about planes in his area and displays the information on the OLED screen.

In previous posts we also showed how the ESP8266 could be used to transmit data like NTSC TV in a similar way to Rpitx.

ESP8266 + OLED screen displaying ADS-B data.
ESP8266 + OLED screen displaying ADS-B data.
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