Using two gates from a CMOS 4011 NAND chip, a simple Squarewave oscillator can br made. Alternatively a CMOS 4001 chip can also be used, or a TTL equivalent. In this circuit the mark space ratio can also be independently controlled by varying the value of the resistors. The rise and fall times of the output pulses depend on the operating voltage o
f the IC and type of IC, but will be typically in the order on tens of nanoseconds.