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his is a very simple square wave generator circuit built with IC 74LS00 that can generate square signals with frequencies between 20 Hz and 1 MHz. Its stability is good enough for most applications. The output freq. is dimensioned through the RC components and time delay of the 3 inverter gates. The time delay of a logic gate is the elapsed time between a change of input state and the resulting change of the output state.

To prevent the temperature and voltage supply from affecting the frequency of the square wave generator, the oscillator freq. fo must be smaller than 1/2tpn where:

tp = average delay time of each gate
n = the number of gates
The above described oscillator has a tp of 10ns and n-3. The freq. is therefore:

fo << 1/2tpn = 1/2*10ns*3 = 16.6MHz
The voltage at the gate inputs varies from about _6V to -4V. The oscillation freq. can be made variable by using a 2.2KΩ potentiometer for R.