The NE602 balanced mixer allows AM radio to receive SW by down conversion
Example from popular electronics shows 8.5Mhz Crystal and a 10.7 IF transformer.
The 8.5MHz mixes with incoming such as 7.5MHz the difference frequency is 1 MHz
which is center frequency of an AM radio. The incoming signal's amplitude can be adjusted manually
by R1 potentiometer. Some preamps and local station's amplitude are strong compared to weak signals/
Pins 4,5 are audio output, pins 1,2 are signal input. you are not limited by crystal, a set of crystals or dds
an adjustable oscillator, a PLL. Could use a 455kHz IF transformer, VLF 160meter to 10 meter oscillator to pins 6,7,8
Using The NE602 (lb3hc.net)

 

A simpler scheme will be what is called a DIRECT CONVERSION Receiver, one that mixes a steady frequency identical to the one to be sensed with the incoming signal. I am not going to elaborate, there is a lot of published literature, (not on you tube) and a fair number of detailed construction projects published.