A question about super-heterodyne receivers
Yeah the incoming RF frequency is fixed, but every station has a different fixed frequency. That makes tuning the sort of filter you need for decent reception really tedious! Especially since early radios often had a separate tuning knob for each tuned circuit.

Brilliant new innovation : the Osram Music Magnet Four has THREE tuned circuits, with all the tuning capacitors controlled by the same knob! In about 1932... But it's still not enough, and component variations mean the three circuits don't stay in tune right across the band.

So by 1934 it was normal to build one really good filter, with lots of tuned circuits, tune it ONCE, and leave it... And convert all your favourite stations to that frequency with a mixer stage...