Polyphase harmonic rejection mixer: AirSpy HF+
IntroductionCan you get excited about a new mixer, usually boring devices that haven't changed in decades? Yes, the new polyphase harmonic rejection mixer in the AirSpy HF+ is almost as revolutionary as SDRs and will have a major influence on their design.
The big advantage of a polyphase harmonic rejection mixer is that it acts as a RF filter for the selected signal, as well as suppressing harmonics and other aliases of the mixing process and local oscillator. It means that the mixer can virtually be connected to the antenna. Typically, a polyphase harmonic rejection mixer converts down to an ADC at base-band. It seems they can be used for both RX and TX.
The post covers how the AirSpy HF+ works, and gives references to what I have been able to find out about polyphase harmonic rejection mixers. They are new and still covered by recent patents. A link to a PowerPoint gives general technical details of the mixer.
AirSpy HF+The AirSpy HF+ is rather unique for modern SDRs as its main purpose is to cover the HF bands, although it does cover VHF as well, although it only covers 200 kHz. And costs just $199. Most new SDRs start at VHF and go to daylight, well 3 or 6 GHz! They are intended for wide band mobile phone type applications, with coverage up to 30 MHz. The new LimeSDR (and $99 mini) and transverter ($299) covers up to about 10 GHz, but has limited RF band-pass filtering.
The unassuming appearance of the HF+ is shown in Picture 1 and the basic architecture of the HF+ is shown in Picture 2, clipped from https://airspy.com/airspy-hf-plus/.
Picture 1 AirSpy HF+
Its maker's description: "Airspy HF+ achieves excellent HF performance by means of a low-loss preselection filter, high linearity LNA, high linearity tunable RF filter, a polyphase harmonic rejection (HR) mixer that rejects up to the 21st harmonic and multi-stage analog and digital IF filtering.
The 6 dB-stepped AGC gain is fully controlled by the software running in the DSP which optimizes the gain distribution in real time for optimal sensitivity and linearity. Harmonic rejection is a key issue in wide band HF receivers because of the large input signal bandwidth of the input signal. The output of the IF-filter is then digitalized by a high dynamic range sigma delta IF ADC for further signal processing in the digital domain."
Picture 2 The basic architecture of the HF+
Polyphase harmonic rejection mixerThe way the new mixer works is not simple, it uses multiple phases of the local oscillator to use phasing to reject its harmonics, but at the same time, and because it is to a 200 kHz base-band, it rejects everything else too.
The big advantage is not needing a large number of band pass filters like a direct sampling SDR; the IC-7300 has 15!
The best explanation I have found is a slide show; http://icd.ewi.utwente.nl/temp_files/158b39412cff88a4181bfec0f4449c24.pdf. It is also subject to patent; https://www.google.ch/patents/US20110298521?hl=de. One of the authors wrote the slide show.
Presumably the mixer is an analogue CMOS device, but I have not found one. And the RF cover on the HF+ is too hard to remove! The HF+ site cites ST Microelectronics as a collaborator, but I have searched their site with no success; http://www.st.com/content/st_com/en.html. It may still be in development or proprietary intellectual property.
The new mixer is not entirely new, as stated in the patent, it relies on existing harmonic rejection mixers and other patents.
Performance of AirSpy HF+The HF+ is still very new, I only received mine in the last couple of weeks. The HF+ gives some performance results. There have been a number of comparative reviews against other SDRs, such as the new $99 RSP1a, by radio amateurs and shortwave listeners. However, there has not been a full technical review by the ARRL or RSGB.
However, with the limited testing the HF+ seems to have a high dynamic range and superior ability with weak signals near large signals, as would be expected from the design.