Airspy HF+ Discovery & Spurious Signals – Much Ado about Nothing

Worst case: Those very few and low-level spurious signals of the “Discovery” disappear completely in the atmospheric noise after you connect an antenna – leaving even faintest stations in the clear.

There had been some discussion about the “real” performance of the brand new Airspy HF+ “Discovery”. It is not only my experience that this great little SDR is a perfect performer at a ridiculous low price.

The discussion focuses on “spurious signals”. I measured them with the Winradio SIGMA as spectrum analyzer, compared it to the two past Airspy HF+ models, plus Discovery, and did some work on how this effect might touch reception.

The result is clear: this discussion in the Ivory Tower is “Much Ado about Nothing”. You may read more about method & findings, with many diagrams, in this PDF.


  • Pingback: Testing for Spurious Signals on the Airspy HF+ Discovery

  • Pingback: Testing for Spurious Signals on the Airspy HF+ Discovery

  • Hi Nils, great article as usual. I suppose that band noise adds nature’s own dithering to any sdr, perhaps why some sdrs perform as well as they do on hf, better than their specs might suggest. Will have to look at the sdrs here and see how they fare as far as spurs.

  • Hi, Nils.

    I found your article very interesting and wanted to do something similar. I bought a Discovery recently, and it looks nice in SDR#, but won’t run with SpectrumSpy or RTL_Power. The SDR# is the only software that it runs with and that was a real pain.

    I pretty much only want to run scans over the full range of different SDRs. And to compare specific regions of the electromagnetic spectrum from multiple sensors. Is it too much to ask for a utility that dumps ALL the data to disk as is comes in? Or send it to my program to process? Oh well.

    Would you like another one? I can send it to you. It is worthless to me. $185 down the drain. Maybe you can put it to good use.


    • Hi, Richard – if I understood you right, you blame a software, not the Discovery, not being able to do some things (scanning) which neither the software, nor the hardware promised to do. Hmmmh. Software SDRC V3 by Simon Brown might be the multi-sensor, multi-instance software you have in mind. I use it successfully with many SDRs, among them also the Discovery. Alas, I don‘t know if it is already supporting the old fashioned art of scanning, because I don’t use this technique.
      73 Nils, DK8OK

      • Nils,

        No, I am blaming the disorganized state of the SDR community. People are wasting huge amounts of time just getting these things installed and working, let alone used for any purpose. I cannot condemn a group for how they relate socially or their inefficiency, but it seems a shame to have such intelligent and creative people spending weeks and months to do simple things.

        As for promises, the materials posted on the web to “sell” the device list software that you can use. They even say they are writing their own and making progress. But “Oh, this does not work in this case”, or “We don’t think it is very good at that, so did not bother”. So it is a false promise and misleading.

        I am just very tired. If it is any consolation, NASA and similar organizations are in worse shape. The Internet as a whole is in terrible shape because large groups cannot work together efficiently.

        It would be nice if all the software I see recommended would work with all the devices. I really don’t want to go back to writing device drivers and basic mathematics. I guess “Software Defined Radio ” still has a few decades to learn how to organize an industry.


  • Richard – if there is any market for a specific, universal SDR software, it would be there. In my view, free (!) SDRC V3 by Simon Brown comes very close to it. Everyone seeing any business model in developing and selling a more specific flavor or adding even more individual features may (let) write some such a software. The estimated investment will be at least 250‘000 US$, I presume. Even no one of the once big players like Icom et al. took this opportunity. Even more: they are far lagging behind real SDR technology and real understanding of its potential. This is perfectly matched by “the media” which fails to substantially report on the advantages of SDRs.
    SO, we have to use what is out there. And this is much more of which we ever dreamed of: a combination of hard and soft, outperforming more or less any professional equipment of the last decades. We simply have to use these stunning tools, and sometimes even have to develop some new applications for this. Everyone can be part of this game! 73 Nils, DK8OK

    • Nils, DK80K,

      Thank you for your insights and experience. My specific needs are to be able to pick up ANY of these devices, and new devices that have not been tried yet, and add them to routine scanning tasks.

      I can compare different devices across the entire frequency range for each device, where they overlap. I am currently buying two each of the devices and comparing them to determine manufacturing variations. I am not interested in writing software to sell. But I will share what I find about using these as plug and play devices.

      Since I wrote the note before, I am learning more about the software development, and finding more devices.

      My ultimate goal is to develop a global network of 3 axis, high sampling rate (Gsps digital or GHz analog) gravimeters, to be used as a single instrument for imaging the earth and sun, and for full sky low microHertz and milliHertz surveys. To get started, I am using low frequeny electromagnetic surveys as a proxy, since almost all the infrastructure, software, hardware, data sharing, correlation nodes, are essentiall identical past the initial sensor.

      Most people are not familar with gravimeters. They measure the gradient of the gravitational potential, the acceleration. There are about a dozen basic approaches, and about 40 main research groups. MEMS gravimeters, Bose Einstein condensate gravimeters, electrochemicall gravimeters, colt atom gravimeters, atom interferometer gravimeters. An accelerometer graduates to being a “gravimeter” when it can track the sun and moon through their vector tidal gravitational accelerastion signal, which is quite precise and can be used for initial calibration on a global scale.

      Besides these “gradient” or “acceleration” instruments, there is another class of what I call “direct instruments” which measure properties of the gravitational potential itself. These are mostly derivatives ot time dilation methods used to correct for general relativistic (velocity and gravitational potential changes in the rates of nuclear and atomic processes – atomic clocks). I consider LIGO to be a direct instrument since they are looking at changes in the potential. You need both kinds of insruments, The gravimeters are the SDRs of the gravitational industry.

      The earliest application might be earthquake early warning using the direct gravimeter signals from earthquakes and their seismic displacement generated changes to the gravitational potenial that diffuse to the gravimeters at the speed of light and gravity. That is long so I put notes at GravityNotes.Org. One diversion is that the low frequency electromagnetic networks (I include magnetometer arrays, VLF, and electric field measurements as part of the low frequeny electromagnetic field and go down to microHertz and millHertz) are picking up early electromagnetic precursors of earthquakes, but cannot confirm. So one subnote to monitoring the global low frequency electromagnetic field is that it can help calibrate these types of measurements. They certainly would benefit from using SDR thinking and methods. And working together globally in a tight knit network.

      Standardizing SDR devices and software across the entire SDR “industry” or “SDR Community” will help these other networks.

      I was the first Director of the Famine Early Warning System ( and know very keenly how small groups and low cost computer tools and methods can save lives and economies. So making money and solving technical problems are fun, but ultimately it is society that should benefit.

      One thing I did not mention, but I am working on, is sound card, audio interface, and oscilloscope technologies. These are also expanding quickly because SDR is being applied to audio frequency (and lower) electromagnetics and audio. They use the same types of linear amplifiers, ADCs, data gathering and handling, sharing and global cooperation. My starting point is global networks of people working on common topics. That is changing in every field. It is my personal interest in gravitation and electromagnetism that leads me to help some before others. I think I am gently saying, “SDR, get your act together. Things are happening.”

      Richard Collins, The Internet Foundation

  • “colt” atom –> “cold” atom. I do not see an edit button.

  • Hello Nils,

    I am very appreciative of your article,it resolves the small signal concerns. From my viewpoint, the dynamic range and the large signal properties is a top priority though.The dBc single tone spurious test, according to application notes from Analog Devices is a promising avenue forward, but the requirement is that people leave the old concept of “intercept point” in the dust bin, once and for all. The reason is that there is no such thing as “gain compression” for large signals in an SDR. A single tone test is a repeatable and objective method that provides a dBc value as a function of signal strength and test frequency. The sensitivity is slightly lower than a two-tone test, but not dramatically so. The purity of the test signal needs to be confirmed independently during the duration of the test, that is the only complication.

    Best regards,
    Anders H.

    • … thanks, Anders, and we certainly agree in many aspects. We also have to keep in mind that an SDR is a device mainly to listen to and not primarily a box for purely technical measurements. This point of view is important for the application, as this takes place in a system, comprised by antenna, man-made noise of the surrounding and also experience and task of the user. I am convinced that 16 bit SDRs as we have them now show a real high plateau of all which can be achieved in receiver design, at least for the hobby user. This one might be happy with even a 14 bit design. The future will provide us with even wider bandwidths – Winradio’s Sigma with 64 MHz recording and processing bandwidth is an even materialized tool, and in future we will also see new entries with 4 x 16 bit, thus an enhancement of 6 dB dynamic range, additionally. So, listeners now only have to use this range … 73 Nils, DK8OK

      • Nils,

        My main focus is on the radio frequencies, so I rarely listen, and only look to see what the signals are, to use them for calibrations. I have limited resources right now, so I am happy to use the 3 Msps ADC-based SDRs that are readily available. I am looking at ALL the ADCs and related tools, but I build a base of experience at one level, then move on to the next.

        I have looked at the audio interface devices that are at 24 bit and 192 ksps for the most part. But the 32 bit and 384 ksps are coming out at lower cost. That covers more all the low electromagnetic signals, but it requires me to brush up on my near field methods. The audiop devices, too, are extending upward in sampling rate and in bit size. I am looking at THz and optical methods that can be down sampled. ALL of these are being worked on by many groups and individuals.

        It might not be apparent, but the optical interferometry and optical communications industries are now well into the Hz and sub Hz resolution for many “optical” frequencies. I am reviewing the Mossbauer methods that go from gamma ray frequencies down to essentiall audio frequencies. The whole of the ‘phonon” and photo acoustic research communities deal with optical/acoustic mixing. And they use some methods and tools that overlap, but have many of their own. All the super resolution methods can be adapted. Much is available, and I try to keep up with it all.

        But, I am trying to concentrate on the microHertz to kiloHertz global electromagnetic field. I want to check that against the magnetometer data, and some gravimeter and seismometer results. But, the slow (millisecond to one minute) variations in the transmission characteristics of electromagnetic transmissions cannot be easily separated from other sources of magnetic variation. I will keep at all the pieces until I get an answer that satisfies me.

        The ham radio network is very active in global mapping. I spent most of the last 18 hours checking their datasets to see if that constrains my other data. I have learned to use all data in a consistent framework, so I will just keep plugging away at it until I am done, or die of exhaustion. Some of them are using satellite monitoring to improve their data. Some of the GPS research on transmission channel variations overlaps. Some of the astronomical networks and radio telescope and communications industries overlap. I can only keep a general sense of all that is going on.

        Pardon me for interrupting. I write down what I am doing to keep myself on track, and in the vague hope someone will get interested enough to help. LOL!

        Richard Collins, The Internet Foundation

  • Airspy HF+ Discovery & Spurious Signals – Much Ado about Nothing

    Well, I don’t fully agree here. If the noise level of an active antenne is 40 dB above the noise level of the receiver, as in your example, then you are loosing the same amount of dynamic range. The supplier should rather work to get rid of those 20 dB spurious signals.


    Werner Karn

    • Werner – I had shown the worst case on one discrete frequency. There is no practical impact. As said before: so far, no one has found any degradation when using this 169 $ receiver as as a receiver (!) in praxi. 73 Nils, DK8OK

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s