Found in a 1924 book by Gernsback, this circuit uses a double variable capacitor, a component not very common at the time. Listening was done on a loudspeaker at the time.
The circuit shown is from the National Semiconductor's Linear Applications Handbook. The CI is not...
This circuit is from the Linear Circuits Application of Texas Instruments, using an operational amplifier...
Found in an RF Design from the 90's this circuit is intended for crystals of less than 1 MHz....