July 13, 2015

A list of 5 Free Virtual Audio Cable Software Programs

The dxzone.com website has uploaded a post that lists five free virtual audio cable software programs. Virtual audio cable’s allow you to “pipe” or pass the audio from one application to another. For example, in order to pass the audio of a digital P25 signal received in SDR# into a decoding program such as DSD+, you need to use a virtual audio cable. The most commonly recommended software is Virtual Audio Cable, but this software is not free. See the dxzone.com post for their list of five alternatives for Windows, Linux and MaxOS.

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July 10, 2015

Optimizing Reception of VLF frequencies on an RTL-SDR Dongle

Over on his web site, Martin (G8JNJ) has uploaded a new article discussing how to optimize reception of VLF (<10 kHz) signals on a direct sampling modified RTL-SDR. He writes that the main problem with the RTL-SDR receiving VLF frequencies is that there is significant overloading caused by broadcast stations operating on the LF and MF bands.

So in order to optimize VLF reception, Martin built an external frequency equalisation network which has the following components and functions:

2MHz Low pass Filter – to minimise alias signals originating at 30MHz

20dB Variable attenuator – to set the overall signal level fed into the dongle

Switched LF roll-off – to optimise the performance at frequencies around 10KHz in the presence of strong lightning surges

-10dB notch at 198KHz – to reduce level of BBC R4 broadcast station in the LF band

-10dB notch at 800KHz – to reduce level of local broadcast stations in the MF band

His screenshot results show that his filters work well and significantly reduce the effect of lightning pulse noise at 9 kHz. With the filters in place and properly optimized with the attenuator and various switches, he is able to receive Russian Alpha navigation signals at frequencies around 12 and 14 kHz and the 300 to 500 kHz aeronautical and maritime navigation bands.

G8JNJ's LF Band Equalizer Circuit — G8JNJ’s LF Band Equalizer Circuit

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July 10, 2015

Using a BladeRF or RTL-SDR to Gather Entropy for /dev/random in Linux

Last month we posted about an experimenter who showed us a tutorial on how to use an RTL-SDR and rtl_entropy to generate random passwords. Now another experimenter, Sean Cassidy has used a BladeRF to generate entropy and used it to seed /dev/random. In the post Sean explains what /dev/random is, and how important it is to provide a good entropy source in Linux, or risk having encryption keys discovered.

He writes that Linux usually gets entropy from activity such as mouse movements, network activity or even hardware random number generators that are available on some Intel CPUs. However, he mentions that hardware random number generators are likely to be back doored by the government for spying purposes and so cannot be trusted. To get around this Sean decided to use his BladeRF as a hardware random number generator, but he also writes that the RTL-SDR will also work.

The set up simply involves installing the software brf_entropy, or rtl_entropy for the RTL-SDR, and then using the “rngd” command to sample randomness for /dev/random from the BladeRF’s output.

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July 7, 2015

Measuring the input impedance of the RTL2832U direct sampling input pins

The direct sampling mod allows you to listen to the HF frequencies between 0 – 14 MHz on an RTL-SDR by simply connecting an antenna directly to the ADC pins on the RTL2832U chip. Until recently the impedance of these pins was unknown, but most people assumed that it was about 300 Ohms.

Now Martin (G8JNJ) has recently updated his webpage with some interesting results that he and another experimenter (Robert Symanek) obtained regarding the input imedpance. Robert found that using a Minicircuits T16-6T-KK81 transformer with 50 Ohms on the primary and 800 Ohms on the secondary gave a 16dB to 19dB higher signal than a simpler 200 Ohm transformer.

This inspired Martin to do a proper measurement of the input impedance. Martin’s measurements found that the differential input impedance of the RTL2832U is approximately 3,330 Ohms when the input is enabled, and this would require a 66:1 transformer. However, Martin writes that a wideband transformer like this probably does not exist, but that the T16-6T-KK81 with terminating resistors added is probably a good choice.

Martin's Direct Sampling + Diplexer Mod — Martin’s Direct Sampling + Diplexer Mod

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July 7, 2015

A Review of the SDRPlay Software Defined Radio

In the May 2015 issue of The Spectrum Monitor magazine, the SDRPlay was reviewed in an article by Thomas. Now the article has been released for free over on the swling.com blog. The SDRPlay is a $149 USD software defined radio with a tuning range of 100 kHz – 380 MHz and 430 MHz – 2 GHz, a 12-bit ADC, and up to 8 MHz of bandwidth. We consider it as a decent upgrade to those wishing for something better than the RTL-SDR and a good competitor to the Airspy SDR.

In the review Thomas unboxes the unit, installs the software and reviews the HF DX performance comparing it to the more expensive Elad-FDM S2. While the performance isn’t as good as the Elad, Thomas writes that for the price it performs very impressively. In his conclusion he writes:

[The] SDRplay RSP really impressed me from the beginning with its low noise floor, variable IF bandwidth options, and relative ease of installation. Since the RSP only requires one USB cable for both data and power, it’s also an ideal portable SDR.

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July 6, 2015

New RTL-SDR software: rtl_ais

Over on cruisersforum.com we’ve seen news of a user who has worked to combine rtl_fm and aisdecoder into a single command line program called rtl_ais. AIS stands for Automatic Identification System, and is used in the marine industry to broadcast vessel GPS coordinates to one another to work as a collision avoidance radar system. With the correct software and an RTL-SDR, nearby boat AIS broadcasts can be received and the boat GPS coordinates plotted on a map.

Until recently, to decode AIS you had to pipe the AIS audio from software like rtl_fm or SDR# into a decoder. rtl_ais is a decoder which allows you to directly connect to the RTL-SDR and decode AIS without the need to pipe audio. The software is compatible on Linux and Windows and the current source code and Windows binary release is available at https://github.com/dgiardini/rtl-sdr-misc/releases/tag/v0.1.

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July 5, 2015

Hamspirit.de Airspy Review

Over on the hamspirit.de blog, author January has posted his review of the Airspy software defined radio (in German, use Google translate). The Airspy is a software defined radio with specifications that include a 12-bit ADC, 24 MHz to 1800 MHz tuning range and up to 10 MHz or bandwidth. It currently sells for $199 USD shipped from China or the local US distributor, or for £160 GBP from the new Airspy UK distributor.

Januaries review discusses the Airspy specifications, the unboxing, ease of use and reception results. The review is generally positive and he writes that in the coming weeks he hopes to do some comparisons between the RTL-SDR and Airspy.

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June 30, 2015

HackRF Portapack Now Shipping to Kickstarter Backers

The HackRF PortaPack is a portable LCD screen with control interface and processor that connects to a HackRF software defined radio. The PortaPack’s goal is to allow for portable RF spectrum visualization, tuning and eventually demodulation of many modes. It has been in development from around the time of the August 2013 HackRF kickstarter and is now almost ready to be shipped out to the initial backers. For more information about the PortaPack see this Hak5 segment that we previously posted about where Jared Boone the inventor of the PortaPack is interviewed.

In his post Jared writes:

Getting the PortaPack H1 ready for shipping was a long slog. And as is my way, I took a lot of detours along the way. I incorporated a lightweight operating system (ChibiOS) into the firmware. I built a simple UI framework that would support arrow-key navigation, with touch as an option where appropriate. I developed a sophisticated test jig (based on this) to ensure the units I ship work correctly. I designed a milled aluminum case that I’ll offer as an option. And I finished and tested all the units myself, including doing failure analysis on a bunch of PortaPacks. I learned a great deal, and hope that my next product development cycle will be much easier and faster.

Because of all the manufacturing effort, work on the firmware hasn’t advanced very far. At this point, the PortaPack is mostly useful as a basic narrowband AM/FM receiver. But there’s still a lot of capability to be tapped in the HackRF ARM processors! I’m eager to get back to firmware, and implement more signal analysis and capture functionality, along with some digital modes demodulation and decoding support.

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