Well technically the RTL-SDR and Airspy do have a really nice internal tracking filter (well probably a whole range of them switched in for various frequency bands) inside the tuner chip. Which I always assume worked something like the below diagram (using varicap/varactor diodes), but there is no public information about how the Rafael Micro tracking filters actually works. All I know for sure is that there are two external inductors (~8n2H to 180nH according to the leaked datasheet each directly connected to two pins on the chip), and everything else to do with this function is internal to the tuner chip.
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