455 KHz Double tuned IFTS for Wideband MW AM reception
Besides Tony Stewart's inductive bottom coupling, you can also couple two single-tuned resonators with a small capacitor:






simulate this circuit &ndash; Schematic created using CircuitLab
 R1 and R2 are wire resistance estimates of 10mm cup-core tunable inductors - these are not real resistors. Source and load resistance of about 90K ohm give about 18 kHz bandwidth. C2 is the top-coupling capacitor. A smaller value decreases coupling, a larger value over-couples giving double-hump frequency response.
This network has superior attenuation for frequencies less than 455 kHz. Tony's coil-coupling network has superior attenuation for frequencies higher than 455 kHz.
These networks can include more resonators, each coupled to the next. It gets very tricky to tune more than four, but there are methods (see Dishal method).