Airspy HF+ is a low cost SDR for HF and VHF bands.
One of the features of the receiver, they claim, is that a polyphase harmonic rejection mixer is employed.
So what exactly is that mixer? The followings are what I have gathered so far.
First, the WIPO publication WO 2010/089700 A1.
The document discloses a two-stage polyphase harmonic rejection mixer.
The set of the coefficients {2, 3, 2} and {5, 7, 5} generates the amplitude ratio of 41/29 (=1.4138) which approximates the desired sqrt(2) quite closely.
The effect is the cancellation of 3rd and 5th order harmonics.
Next, the dissertation by Rafi.
The figure shows why the LO harmonics must be suppressed if one wants to avoid using (tunable) RF filters.
This is Fig. 2.7, and shows the basic concept of harmonic rejection mixers.
The above two figures are from [6];
Finally, yet another nice figure showing the effect of LO harmonics from the paper, A Sine-LO Square-Law Harmonic-Rejection Mixer—Theory, Implementation, and Application.