Monday, 15 January 2018

Polyphase harmonic rejection mixer: AirSpy HF+

Polyphase harmonic rejection mixer: AirSpy HF+

Introduction

Can 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 mixer

The way the new mixer works is not simple, it uses multiple phases (16?) 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.

The mixer is an analogue CMOS device, STA709 from ST Microsystems, but the full datasheet is currently only available under NDA (non-disclosure agreement). So, no point taking RF cover off the HF+, too hard to remove anyway!

The new mixer is not entirely new, as stated in the patent, it relies on existing harmonic rejection mixers and other patents.

From AirSpy group: "You can see it as a "super Tayloe mixer". The problem with the original Tayloe Mixer is the harmonic responses at multiples of the LO frequency. The fix is to mathematically suppress these responses by adding more phases. The LO will no longer look like a square wave, but rather like a quantized sine wave. Basically, the more phases you add, the more harmonics you cancel.
This method is combined with narrow band filtering at the mixer itself. There is a switched-capacitor N-Path filter built into the mixer that is tuned using the same LO phases, which provides additional selectivity.
When you see it, all the ingredients required to implement this architecture can be implemented using CMOS silicon, and have a very good "horizontal" and "vertical" scalability: Horizontal with more phases (hence, less harmonics); Vertical with better fab processes (better linearity and NF).
The icing on the cake: This same technology can also work for TX."

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.

Conclusion

The polyphase harmonic rejection mixer of the Airspy HF+ is a significant development in radio design and is likely to rival other technologies over the coming years.

Appendix 1

Summary incorporating comments from AirSpy IO Group
Hi All

I asked the Airspy group about the workings of the HF+, and have summarized my own findings and comments from the group:

The HF+ uses a very modern and novel architecture, primarily a polyphase harmonic rejection mixer. See https://airspy.com/airspy-hf-plus/

As best I can work out, when converting to base-band, it is an effective filter for the desired signal and rejects even strong signals close by with virtually no filtering ahead of the mixer.
 
It uses multiple (16?) 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. A bit like the old phasing SSB modulators, that used two phases.
 
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;  icd.ewi.utwente.nl/publications/get_file.php?pub_id=563. It is also subject to patent; https://www.google.ch/patents/US20110298521?hl=de. One of the authors wrote the slide show.

From AirSpy group: "You can see it as a "super Tayloe mixer". The problem with the original Tayloe Mixer is the harmonic responses at multiples of the LO frequency. The fix is to mathematically suppress these responses by adding more phases. The LO will no longer look like a square wave, but rather like a quantized sine wave. Basically, the more phases you add, the more harmonics you cancel.
This method is combined with narrow band filtering at the mixer itself. There is a switched-capacitor N-Path filter built into the mixer that is tuned using the same LO phases, which provides additional selectivity.
When you see it, all the ingredients required to implement this architecture can be implemented using CMOS silicon, and have a very good "horizontal" and "vertical" scalability: Horizontal with more phases (hence, less harmonics); Vertical with better fab processes (better linearity and NF).
The icing on the cake: This same technology can also work for TX."
 
Apparently the mixer is a CMOS device, STA709, but the full datasheet is currently only available under NDA. So, no point taking RF cover off the HF+, too hard to remove anyway!
 
Regards Drew VK4ZXI




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