The circuit shown in the figure below is from the 1993 Linear Technology Applications. The operational ones used are uncommon and the use of equivalents can be analyzed.
The L3 coil is what basically determines the resonant frequency, around 1 MHz, in this oscillator found...
The filter shown in the figure, suggested by National Semiconductor, only allows frequencies above...
This circuit uses unijunction transistors and drives a small speaker through an output...