Category: RTL-SDR

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.

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

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.

June 25, 2015

Two New SDR# Plugins for Passive Radar and IF Signal Averaging

March 2019 Update: Website appears to be down now. But the DropBox download links are still active.

Passive Radar Plugin: https://www.dropbox.com/sh/tqjycu9nxdfhk0u/AAA9KSE6-mRUwV10s0F9v7Jpa?dl=0

IF Average Plugin: https://www.dropbox.com/sh/tqjycu9nxdfhk0u/AAA9KSE6-mRUwV10s0F9v7Jpa?dl=0

Recently Dr. Daniel Kaminski wrote into RTL-SDR.com to let us know about two very interesting new SDR# plugins that he has developed to use with the RTL-SDR dongle. The first plugin is called "Passive Radar". Passive Radar allows you to use an existing strong transmitter such as an FM station to detect reflections from things like aircraft and meteors. Dr. Kaminski writes about his plugin:

The first one is Passive Radar which bases on the signal from only one dongle. The ambiguity function is the same as in advanced projects with the difference that I implemented self-correlate function instead of cross-correlate one which is used in 2 dongles projects. Such solution theoretically should works as can be found in internet. It should be noticed that for proper work of such passive radar the direct signal should be comparable in strength to the reflected one. This plugin is still under development.

In the future he hopes to be able to support two dongle passive radar as well.

The Passive Radar plugin by Dr. Kaminski in SDR#.

The second plugin is called "IF Average". This plugin allows the IF signal (the entire active bandwidth is what he seems to be referring to) to be averaged which is useful for many applications including radio astronomy projects such as detecting the Hydrogen line. He writes:

The second plugin which is finished is for IF signal averaging. It is important in case of radio-astronomical observations. It allows to cumulate signals (up to 10000 samples in real time), present them in friendly way and save for further work.

The plugins require the installation the XNA Framework Redistributable 3.1.

June 25, 2015

Cooling the RTL-SDR for Improved Sensitivity

Over on his blog, Nobu an RTL-SDR experimenter has made a post about cooling RTL-SDR dongles (post is in Japanese use Google translate if needed). Nobu writes that another RTL-SDR experimenter, Toshi, has previously pointed out that an increase in heat appears to reduce the sensitivity of the RTL-SDR.

From his measurements, Nobu found that the internal temperature of the RTL-SDR can reach up to 70 degrees Celsius. So in order to cool the RTL-SDR Nobu has tried two methods. One involving using small cooling fans, and the other involving adding heat sinks to all heat producing components. It seems from the translation that he writes that the improved heat dissipation has extended his ADS-B reception slightly.

If you are interested, Nobu also has a previous post where he also mentions cooling of the RTL-SDR.

June 23, 2015

Several PDF Files Related to RTL-SDR’s and HF/MW Reception

Recently RTL-SDR.com reader Dr. Phil wrote in to let us know about some PDF notes that he has created about the RTL-SDR dongle. There is some good information in his documents and the notes mainly focus on using the RTL-SDR with the direct sampling mod to receive HF.

His other documents also explain concepts such as imaging, interference and gain, how to reduce interference, input impedance of the Q sampling pads, intermediate frequency, and sample rate. In addition he has also uploaded some documents where he has calculated for various AM, FM and SW stations at what frequencies images will show up. His final document also discusses the Mirics SDR chipsets which are used in the SDRPlay.

He writes that any corrections are welcome. Dr. Phil’s main website can be found at http://home.comcast.net/~phils_radio_designs/.

Pocket HF SDR: http://home.comcast.net/%7Ephils_radio_designs/Pocket_SDR.pdf

Pocket HF SDR Cookbook: http://home.comcast.net/%7Ephils_radio_designs/SDR_Cookbook.pdf

RTL2832U: http://home.comcast.net/%7Ephils_radio_designs/RTL2832U.pdf

HF Direct Sampling: http://home.comcast.net/%7Ephils_radio_designs/Direct_Sample.pdf

AM Images: http://home.comcast.net/%7Ephils_radio_designs/AM_Images.pdf

FM Images: http://home.comcast.net/%7Ephils_radio_designs/FM_Images.pdf

SW Images: http://home.comcast.net/%7Ephils_radio_designs/SW_Images.pdf

Generic RTL: http://home.comcast.net/%7Ephils_radio_designs/GenericRTL.pdf

Mirics Chipset: http://home.comcast.net/%7Ephils_radio_designs/Mirics.pdf

A photo of the direct sampling mod with a wire antenna.

June 23, 2015

RTL-SDR Bias Tee Hack

Over on his blog Elias has uploaded a post that shows how he modified his RTL-SDR dongle to provide remote DC power through a bias tee. A bias tee allows you to inject DC voltage into the coax cable to power active antennas, or devices that need to be near the antenna such as LNA’s. The bias tee prevents DC power from entering the RTL-SDR front end (which would fry it) via a blocking capacitor, and also prevents RF energy leaking into the power supply by using a blocking inductor.

In his post he writes how he made a simple hardware hack to the RTL-SDR PCB to enable 3.3V from the USB power supply to be used to power his active GPS antenna. To do this he removes the static protection diode and connects a nearby 3.3V pad to the antenna output through an inductor. The RTL-SDR already has a DC blocking capacitor in place.

He writes that this mod unfortunately requires the static protection diode to be removed, so the RTL-SDR is no longer protected from static discharge.

Previously in June of last year we posted about a similar bias tee hack by Fabio, where he used the 5V rail to power an inline LNA.

June 21, 2015

Using the RTL-SDR as a Transmitter

Back in July of last year we posted about a video from oh2ftg where he showed how he was able to get his RTL-SDR to act as a crude transmitter by using the RTL-SDR’s leaky oscillator.

Now another RTL-SDR experimenter, Oscar Steila (IK1XPV) has had a similar idea to use the RTL-SDR as a transmitter, and has taken the idea further than OH2FTG did.

Oscar decided to take a standard RTL-SDR dongle and modify it so that it outputs a signal from the mixer output of the R820T tuner chip. To do this he removes some unneeded components from the PCB, and wires pin 5 of the R820T to the MCX antenna port through a 100pF capacitor. Pin 5 is connected to the mixer output from inside the R820T chip.

After performing the hack the RTL-SDR is able to output a signal anywhere between ~~500 MHz to 1500 MHz~~ 1.8 GHz to 3 GHz (see why). To control the output frequency you simply need to tune to the frequency you want to transmit at in SDR# (after setting an offset to account for the R820T’s IF offset). This tunes the mixer in the R820T and causes the output frequency to change.

In the future Oscar hopes to take this idea further by creating a specific tuning application for the generator and finding a way to possibly FM modulate the output.

Using SDR# to tune the TX RTL-SDR, and using another instance of SDR# and RTL-SDR to receive the 1GHz signal. — Using SDR# to tune the TX RTL-SDR to 1 GHz, and using another instance of SDR# and another RTL-SDR to receive the transmitted 1 GHz signal.

Update: Oscar has revised the frequency range from 500 – 1500 MHz to 1.8 GHz – 3 GHz. More information about his new tests can be found at http://www.steila.com/SDR/RFgenmod/index.html.