Tagged: sdrplay

Our Review of the Airspy HF+: Compared against ColibriNANO, Airspy Mini, RSP2

Over the last few months we’ve been posting and getting excited about the Airspy HF+, an upcoming high dynamic range HF/VHF receiver designed for DXing. The Airspy team were kind enough to supply us with an early pre-production unit for review.

Long story short, the Airspy HF+ is probably one of the best low cost SDRs we’ve seen for DXing or weak signal reception out there. So far few details on the availability of the HF+ have been released, but we’re aware that preorders are due to start soon, and the target price is expected to be $149 USD from iTead Studio in China. 

What follows is the full review and comparisons against other similarly priced SDRs. The Airspy team want us and readers to understand that our review unit is a pre-production model, and apparently already the matching and thus SNR has already been improved by about 2-4 dBs, so the sound samples we provide in the review below should sound even better with the newer revision.

Disclaimer: We received the HF+ for free in exchange for an honest review, but are not affiliated with Airspy. We’ve been in contact with the Airspy team who have helped clarify some points about the architecture and technology used in the design.


The Airspy HF+ is designed to be a HF/VHF specialist receiver with a frequency range of DC to 31 MHz, and then 60 to 260 MHz. It has a maximum bandwidth of 768 kHz. So the question is then, why would you consider buying this over something like the regular Airspy R2/Mini or an SDRplay RSP2 which both have larger frequency ranges and bandwidths? You would buy the Airspy HF+ because has been designed with DXing and weak signal reception in mind. Basically the main idea behind the HF+ is to design it so that it will never overload when in the presence of really strong signals. Combined with it’s high sensitivity, weak or DX signals should come in much clearer than on the other radios especially if you have strong blocking signals like broadcast AM/FM around.

Aside: What is overloading, intermodulation and dynamic range?

Basically strong signals can cause weak signals to be drowned out, making them not receivable, even though they’re there at your antenna. This is called overloading or saturation. Intermodulation occurs when the SDR overloads and results in images of unwanted signals showing up all over the spectrum.

A simple analogy is to think about what happens when you are trying to drive, but there is sunstrike. The road is very hard to see because the sun is so bright and right in your eyes. The human eye does not have enough “dynamic range” to handle the situation of sunstrike. Dynamic range is a measure of how well a radio (eye) can handle strong (bright) and weak (dark) signals at the same time. The same analogy applies to radios which can struggle to ‘see’ weak signals if there is a very strong signal nearby on the frequency spectrum. There are a few ways to solve this:

  • Filtering: Block the strong signals that you don’t want using LC filters.
    • Eye analogy: using your sun visor to block the sun.
  • Attenuation: Reduce the strength of all signals.
    • Eye analogy: using sunglasses or squint.
  • Increase dynamic range: Get a better SDR with better design/technology and more bits in the ADC.
    • Eye analogy: upgrade your eyes.

Technology and Architecture

The HF+ uses a typical Filter->Tuner ->ADC architecture. So it is not a direct sampling receiver like most of the more expensive SDRs. Direct sampling receivers directly sample the analogue spectrum, without the need for a tuner so they avoid losses and the intermodulation problems that usually come from the mixing stages. But there are some major cutting edge technology differences in the HF+ architecture that should make its performance even better than direct sampling receivers.

Tuner: The tuner on the HF+ is one of the first to use a “Polyphase Harmonic Rejection” architecture. Essentially this means that harmonics produced in the mixing stages are naturally rejected, making the front end filtering requirements much more relaxed. So unlike the tuners used in other SDRs, this one is extremely unlikely overload in the mixing stage.

An additional benefit to this architecture is that the mixer is very low loss, so the LNA in the tuner only needs to use low gain, giving it a very high IIP3 value. So the first LNA which is typically another point of saturation and imermodulation, is very unlikely to saturate in the HF+ design. Most of the amplification only occurs after the mixing stage with the filtered narrowband output of the tuner.

Analogue to Digital Converter (ADC): The ADC is 16-bits and uses a “Sigma Delta” (ΣΔ) design. Basically a Sigma Delta ADC has a natural filtering ability due to its narrowband nature. Instead of seeing say a 30 MHz signal, it only sees 1 – 2 MHz, thus increasing dynamic range and reducing the likelihood of out of band overload.

Digital Down-Converter (DDC): Then after the ADC is a DDC which decimates the output from the ADC, increasing the effective number of bits. The more bits the larger the resolution of the digitized RF signal, so weak signals are less likely to be lost when converted from analogue to digital.

The HF+ Block Diagram
The HF+ Block Diagram

So the block diagram flow goes like this:

A weakly filtered signal enters the tuner, is weakly amplified by the tuner LNA, mixed down to baseband and filtered to 1-2 MHz. It is then amplified and sampled with the sigma delta ADC into 16-bits. The DDC decimates the output into 18-bits which is then sent to the microcontroller and PC via USB.

The Airspy team also compiled this comparison chart for us to understand the differences in architecture between the current SDRs on the market (click to enlarge). This shows that the HF+ is a different type of design compared to other SDRs. Generally the best SDRs out the market right now are direct sampling receivers with many filter banks. The HF+ approaches the problem in a different way, and according to the specs seems to match or better the performance of heavily filtered direct sampling receivers.

Performance from the Airspy HF+ product page is stated as:

  • -141.0 dBm (0.02 µV / 50 ohms) MDS Typ. at 500Hz bandwidth in HF
  • -141.5 dBm MDS Typ. at 500Hz bandwidth in FM Broadcast Band (60 – 108 MHz)
  • -139.5 dBm MDS Typ. at 500Hz bandwidth in VHF Aviation Band (118 – 136 MHz)
  • -139 dBm MDS Typ. at 500Hz bandwidth in VHF Commercial Band (136 – 174 MHz)
  • -138 dBm MDS Typ. at 500Hz bandwidth in the upper VHF Band (> 174 MHz)
  • +26 dBm IIP3 on HF at maximum gain
  • +13 dBm IIP3 on VHF at maximum gain
  • 110 dB blocking dynamic range in HF
  • 95 dB blocking dynamic range in VHF

Continue reading

Listening to July’s Arecibo Observatory Ionospheric Heating Campaign

During July 24-31 the large Arecibo Radio Observatory in Puerto Rico (the big dish antenna that you may be familiar with from the movie ‘Contact’) ran an Ionospheric heating experiment which involves transmitting 600kW of net power up into the Ionosphere. This type of experiment is used for researching plasma turbulence in the ionosphere and upper atmosphere.

“The new Arecibo ionosphere HF heater nominally transmits 600 kW net power and has a unique Cassegrain dual-array antenna design that increases gain of three crossed dipoles for each band, using the signature 1000-foot spherical dish reflector,” explained Chris Fallen, KL3WX, a researcher at the University of Alaska-Fairbanks HAARP facility. He has reported that Arecibo would use 5.125 or 8.175 MHz, depending upon ionospheric conditions, but emphasized that these are estimates and frequencies may be adjusted slightly. On July 25, Arecibo was transmitting on 5.095 MHz.

Over on YouTube Mike L. used his SDRplay RSP1 together with our BCAM HPF to record some transmissions from the observatory.

SDRplay RSP1 Price Reduced to $99.95 USD

SDRPlay have just announced that their RSP1 unit has just been reduced in price to $99.95 USD. Their press release reads:

SDRplay are pleased to announce a price reduction for their entry-level SDR receiver, the RSP1 to $99.95 USD making it the most competitive mid-range SDR to include reception down to low frequencies without the need for an upconverter. The RSP1 provides general coverage receiver and panadapter capability from 10 kHz to 2 GHz. As well as providing SDRuno SDR software, support for popular 3rd party packages like HDSDR, SDR-Console and Cubic SDR is provided. Recent availability of an SD Card image makes for easy set up on a Raspberry Pi.

Over time we’ve seen the RSP1 reduce in price originally from $299 USD, to half price at $149 USD in March 2015 and then to $129 USD in September 2016, and now finally down to $99 USD. The newer RSP2 remains at a price of $169.95 USD.

The SDRplay RSP1
The SDRplay RSP1

Receiving Jupiter Noise Bursts with an SDRplay RSP1

Over on YouTube user MaskitolSAE has uploaded a video showing him receiving some noise bursts from Jupiter with his SDRplay RSP1. The planet Jupiter is known to emit bursts of noise via natural ‘radio lasers’ powered partly by the planets interaction with the electrically conductive gases emitted by Io, one of the the planets moons. When Jupiter is high in the sky and the Earth passes through one of these radio lasers the noise bursts can be received on Earth quite easily with an appropriate antenna 

In his video MaskitolSAE shows the 10 MHz of waterfall and audio from some Jupiter noise bursts received with his SDRplay RSP1 at 22119 kHz. According to the YouTube description, it appears that he is using the UTR-2 radio telescope which is a large Ukrainian radio telescope installation that consists of an array of 2040 dipoles. A professional radio telescope installation is not required to receive the Jupiter bursts (a backyard dipole tuned to ~20 MHz will work), but the professional radio telescope does get some really nice strong bursts as seen in the video.

The UTR-2 Radio Telescope. Photo Attr. Oleksii Tovpyha (Link)

Running an SDRplay RSP2 on a Raspberry Pi 3 with CubicSDR

Over on YouTube user Kevin Loughin has uploaded a video demonstrating his SDRplay RSP2 running on a Raspberry Pi 3. The software he uses is CubicSDR which is a multiplatform program that is similar to software like SDRUno, SDR#, SDR-Console, HDSDR etc. The video shows CubicSDR running, but the interface is quite slow and laggy, although the audio is at least not choppy.

In a previous post we showed one of Kevin’s earlier videos where he does a tutorial and some scripts that help to actually set up the SDRplay drivers and CubicSDR in Linux. In the new video he first goes over a specific hack that needs to be done in Raspbian to fix the PulseAudio server. Then he explains that you can run the Linux build script mentioned in his previous tutorial video and it should work on the Raspberry Pi 3 just fine. Finally he mentions that CubicSDR and the SDRplay use a high amount of CPU processing on the pi3 so some sort of cooling mechanism is required or the pi3 may throttle down its CPU.

Video Tutorial on Setting up the SDRplay RSP2 in Linux

Over on YouTube user Kevin Loughin has recently uploaded a video that shows a step by step guide on how to set up an SDRplay RSP2 in Linux. Setting up the RSP2 in Linux is not a simple task, but Kevin’s video walks us through the entire process step by step. At the end of the process you’ll be set up with the SoapySDR framework which is the glue software that sits between the hardware driver and SDR software. You’ll also have the CubicSDR software installed which is what you use for general browsing and listening. CubicSDR is similar to SDRuno, SDR#, HDSDR etc.

Over on his blog he’s also posted the steps in text form, and uploaded some of the scripts that he’s created to simplify the install process.

Decoding the LilacSat-1 FM to Digital Voice Transponder

LilacSat-1 is an educational CubeSat built by students from the Harbin Institute of Technology (HIT) in China. It was recently launched from the ISS on 25 May 2017 as part of the QB50 science experiment to explore the lower thermosphere, and it is expected to stay in orbit for about 3 months. Apart from BPSK telemetry at 145.935 MHz, LilacSat-1 is interesting because it contains on board an FM to Codec2-BPSK digital voice amateur radio transponder at 145/436 MHz (uplink/downlink). It is probably the first amateur radio satellite to contain an FM to digital voice transponder.

To decode LilacSat-1 digital voice and telemetry you can use a Linux live CD provided by HIT, or download the GNU Radio decoder directly from the LilacSat-1 information page on the HIT website. The GNU Radio program can be used with any GNU Radio compatible SDR, such as an RTL-SDR.

Over on his blog, destevez has also created a lower latency digital voice decoder for LilacSat-1 that can found in the gr-satellites GNU Radio package, which contains decoders for multiple satellites as well. Destevez has also written about the Codec2 implementation used in LilacSat-1 in one of his previous posts.

An example of LilacSat-1 being decoded has also been uploaded by YouTube by Scott Chapman. In his test he used an RTL-SDR to work the pass live, but in the video shows an offline decoding received by his SDRplay which was also monitoring the same pass.

Mike’s SDRuno Tutorial Series

Mike Ladd, one of the top volunteer contributors of the SDRplay community was recently hired by SDRplay officially and has now been working on a fairly comprehensive SDRuno tutorial series over on the SDRplay YouTube channel. SDRuno is the official software for the SDRplay line of SDRs and is a slightly modified version of the ‘Studio1’ software which was previously acquired by SDRplay. SDRuno also supports the RTL-SDR.

SDRuno is a complex piece of software with many features and settings, so it’s great to see a comprehensive video tutorial like this. Mike’s tutorial series currently has 10 episodes, and discusses things like the basic layout and settings of SDRuno, using Virtual Audio Cable (VAC), noise reduction, memories, calibration, DSD, notch filters and FM broadcast with RDS. More videos are probably still on the way.