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 about a 66:1 transformer. However, Martin writes that a wideband transformer like 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

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 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.

SDR Play Radio Spectrum Processor
The SDR Play Radio Spectrum Processor

New RTL-SDR software: rtl_ais

Over on 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

The RTL_AIS help screen
The RTL_AIS help screen Airspy Review

Over on the 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.

The Airspy receiving TETRA
The Airspy receiving TETRA

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.

The HackRF Portapack
The HackRF Portapack

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

Recently Dr. Daniel Kaminski wrote into 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 Passive Radar plugin by Dr. Kaminski in SDR#.
The Passive Radar window.
The Passive Radar window.

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 IF Average plugin by Dr. Kaminski.
The IF Average plugin by Dr. Kaminski.

The plugins require the installation the XNA Framework Redistributable 3.1.

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.

RTL-SDR Fan Cooling
RTL-SDR Fan Cooling
RTL-SDR Heatsink Cooling
RTL-SDR Heatsink Cooling

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

Recently 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

Pocket HF SDR:

Pocket HF SDR Cookbook:


HF Direct Sampling:

AM Images:

FM Images:

SW Images:

Generic RTL:

Mirics Chipset:

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