100Hz half-wave sinusoids are voltage divided and shaped into a square wave by schmitt trigger.
Schmitt trigger output is fed to XOR gate (exclusive or gate).
Additionally the same pulse is fed to an RC timing circuit.
The charging voltage on capacitor is "shaped" by schmitt trigger and fed to other input of XOR gate.
Characteristic of XOR gate is O/P is true when one only of inputs is true.
Therefore what happens is o/p goes high with rising edge of first input only to go low when RC network delayed other input goes high.
This cycle then repeats.
This is effectivley frequency doubling and output can be seen and measured at 200 Hz.
A further cascaded stage then doubled again to 400 Hz - output.
The Time Constant of the RC network (TC = R x C) must be slightly less than half the period of the incoming frequency to be "doubled"
Note this is a "quirky" and inelegant way to double a frequency, but in the above example seems to work and may be useful in an emergency.
I feel it would only work for vey low frequencies 100 ~ 400 Hz, higher frequencies or more than a "quadrupling" probably wouldn't work !
I have used a quad nand gate type 4011 configured as XOR (requires all 4 gates in package to make one XOR gate - see diagram)