Category: RTL-SDR

Building a 520 kHz High Pass Filter for the RTL-SDR

Over on YouTube user kugellagers has uploaded a video showing how he designs and builds a 520 kHz high pass filter for his RTL-SDR dongle + upconverter. In the video he explains how to design the filter with the free Elsie software which is an electrical filter design and analysis program. He then shows how he builds and selects the filter inductors and capacitors and how he assembles the components on a PCB. Finally he demonstrates how his 520 kHz high pass filter is useful for filtering out atmospheric noise from lightning strikes.

Previously we posted about kugellagers’s other video in which he demonstrates his FM bandstop filter and 1.8 MHz high pass filter.

520 kHz High Pass Filter Construction

Retesting Nobu’s 14 MHz Low Pass Filter on a Direct Sampling Modified RTL-SDR

Back in May we did a review of Japanese RTL-SDR experimenter Nobu’s products, which were his HF Upconverter, Galvanic Isolator and 14 MHz Low Pass Filter. The low pass filter was designed to be used with a direct sampling modified RTL-SDR receiver, but unfortunately we didn’t have one of those on hand at the time.

Nobu was kind enough to send us one of his direct sampling modified RTL-SDR dongles that he also has on sale on his Japanese Amazon page. This is a nice little unit that has an upgraded 10 ppm oscillator, and an additional MCX port connected to the direct sampling pins of the RTL2832U chip through an impedance transformer. With this unit we were able to give the low pass filter a better test.

The image below shows the AM broadcast band with the filter in place. Mouse over the image to see the effect of removing the low pass filter. (If on mobile click inside the image, and outside the image to toggle the mouse over effect). We can see that there is some insertion loss from the filter, however with the LPF not connected there is severe interference from the broadcast FM band and some AM signals are completely unusable.  

We repeated the same test at 9 MHz. Again, mouse over the image to see the effect of removing the low pass filter. Once more we see that without the LPF there is severe interference from the broadcast FM band, as well as in this case what looks to be a DAB signal.

Similar interference is found all through the 0 – 14 MHz frequencies without the low pass filter in place and most weak signals cannot be listened to without the filter connected. It is clear that without a low pass filter the direct sampling modification is almost useless in the presence of strong interfering signals, such as those from the FM broadcast band. 

Nobu’s products are made in Japan, and at the moment can only be bought from the Japanese Amazon store [Direct Sampling Dongle – $~48 USD] [HF Upconverter – $~56 USD] [Upconverter Case ~$25 USD] [Galvanic Isolator – $23 USD] [Low Pass Filter – $~23 USD].

To purchase from outside of Japan you can use a third party shopping service available at http://agent.jzool.com/, which will buy and ship the product to you from Japan.

RTL-SDR as a Hardware Random Number Generator with rtl_entropy

Over on his blog, Aaron Toponce has posted a tutorial that shows how to use the RTL-SDR app rtl_entropy.  This app uses the RTL-SDR to create random numbers from the atmospheric noise that it receives from the antenna. Aaron writes:

The theory behind the RNG is by taking advantage of atmospheric noise, which is caused by natural occurrences, such as weak galactic radiation from the center of our Milky Way Galaxy to the stronger local and remote lightning strikes. It’s estimated that roughly 40 lightning strikes are hitting the Earth every second, which equates to about 3.5 million strikes per 24 hour period. Interestingly enough, this provides a great deal of entropy for a random number generator.

In the post Aaron also shows how to put the rtl_entropy generated data through some standardized randomness tests, how to visualize the random output and also shows how to use rtl_entropy to generate 80-bit entropy passwords.

Visualizing the random noise output of rtl_entropy.
Visualizing the random noise output of rtl_entropy.

New SDR# Plugin: File Player

A new plugin for SDR# has been released by Vasilli over on rtl-sdr.ru. The new plugin is called File Player and replaces the default SDR# IQ file source player (page is in Russian, use Google translate if necessary). The new features include:

  • The ability to play 32-bit WAV files up to 4GB.
  • The ability to play very large 64-bit WAV files.
  • Adds a new display that shows a compressed image of the entire waterfall and shows where in time the playback is up to.
  • Allows you to modify the waterfall play time position with the mouse.
  • Adds a stop and pause button.

Note that to install this plugin you do not add the magicline to the plugins.xml file. Instead you need to add it to the <frontendPlugins> section of the SDRSharp.exe.Config text file.

File Player plugin for SDR#.
File Player plugin for SDR#.

Modifying an RTL-SDR by adding a Diplexer to receive HF and VHF/UHF

The lowest frequency that a standard RTL-SDR dongle can receive is about 24 MHz. However, by applying a hardware hack called the direct sampling mod, it is possible to use the RTL-SDR to listen to the HF frequencies.

Usually the direct sampling mod requires that you add a separate antenna port to the dongle, but Martin G8JNJ decided to take another route and instead use a diplexer to be able to use the same antenna port for both HF and VHF/UHF. A diplexer allows both HF and VHF/UHF signals to coexist on the same input port without causing interference to one another.

Along with the diplexer Martin added an impedance transformer, added additional coupling capacitors to the power rails and removed the IR LED components to make space for the transformer. Martin writes that the final modded RTL-SDR allows for tuning between 15 kHz to 1.8 GHz.

The finished diplexer RTL-SDR mod.
The finished diplexer RTL-SDR mod.

Trunking with the Latest DSD+ 1.08t Fast Lane Version

DSD+ stands for Digital Speech Decoder Plus and is a software program that can allow you to decode digital voice signals such as P25 and MotoTRBO/DMR. DSD+ is under continual development, and in their last public update they began offering early access to the latest DSD+ features in development through their fast lane subscription. The fast lane subscription costs $10 USD for one year and $25 for unlimited early access. Information about joining the fast lane service can be found in the readme file of the latest DSD+ 1.074 public release.

Over on YouTube user John Miller has been testing the latest early access version DSD+ 1.08t. This new version adds trunking support which allows you to follow conversations. Previously other software like Unitrunker was required to follow the trunking signal. On YouTube John has uploaded a video first showing trunking in action, and a second video showing how to set up DSD+ 1.08t for trunking.

DSDplus 1.08t trunking

DSDplus Trunking Setup 1.08t

Some new RF filters from Adam 9A4QV

Adam 9A4QAV is mostly known as the manufacturer of the popular LNA4ALL, a low cost low noise amplifier which is often used together with the RTL-SDR to improve reception of weak signals. He also sells an ADS-B bandpass filter and an ADS-B antenna, the latter of which we reviewed in a previous post.

Now Adam has come out with two new RF bandpass filters which are for sale. RF filters are used to block unwanted interference from other strong signals which can cause trouble, especially with low cost receivers such as the RTL-SDR. 

The first new filter that he has developed is for FLARM (FLight Alarm System). FLARM broadcasts at 868 MHz and is a protocol similar to ADS-B. It is used by Gliders and some Helicopters for collision avoidance. It is possible to decode FLARM with an RTL-SDR which allows you to track gliders on a map, as discussed in one of our previous posts.

Characteristics of Adam's FLARM Filter.
Characteristics of Adam’s FLARM Filter.

The second filter is for amateur radio astronomers who wish to detect the Hydrogen Line at 1420 MHz. Hydrogen molecules in space occasionally emit a photon at 1420 MHz. A single emission can’t be easily detected, but space and the galaxy is full of Hydrogen and the net result is an observable RF power spike at 1420 MHz. This can be detected with a high gain antenna, LNA, RF filter and radio like the RTL-SDR. The Hydrogen line can be used to measure things like the rotation and number of arms in our galaxy. Filters are very important for radio astronomy work as man made interference can easily drown out the relatively weak cosmic signals.

Characteristics of Adam's Hydrogen Line Filter.
Characteristics of Adam’s Hydrogen Line Filter.

Adam sells all his fully assembled filters for 20 euros, plus 5 euros worldwide shipping.

One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.
One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.

Sniffing “Crazyradio” NRF24 Signals with a HackRF Blue

Thanks to DangerousPrototypes.com we’ve heard about this project in which experimenter Arnuad has been using his new HackRF Blue to sniff and debug the communications protocol from the Crazyradio which is used on the Crazyflie quadcopter. The Crazyradio is a 2.4 GHz radio transceiver dongle that uses the nRF24 chip. It is designed to be used with the Crazyflie quadcopter.

By using a Python script to make the Crazyradio constantly transmit, and then by using GNU Radio, Arnuad was able to sniff and demodulate the GFSK signal from the nRF24 based Crazyradio and pipe the demodulated signal into a nRF24 decoder. 

Decoded NRF24 Packets from the Crazyradio.
Decoded NRF24 Packets from the Crazyradio.