Hc-Based Oscillators

EE JULY/AUGUST 1987 Two inverters, one resistor and one Capacitor are all that is required to make a HC(T)- based oscillator that gives reliable operation up to about 10 MHz. This sort of circuit is well-known, and appears in Fig. la. The use of two HC inverters gives fairly good symmetry of the rectangular output signal. In the same circuit, HCT inverters give a duty factor of about 25%, rather than about 50%, since the toggle point of an HC and an HCT inverter is 1/2Vcc, and slightly less than 2 V, respect- ively. When the supply voltage for the oscillator is switched on, C initially has no charge, and the output of Ni and N2 are at the same logic level. Capacitor C is then charged via R, until it has acquired a charge voltage that corresponds to the toggle volt- age, Us, of Ni. Assuming the output of N2 initially to be logic low, the waveform of the signal at the input of Ni is essentially as shown in Fig. 2. When C is la C)50pF charged up to level 1, the out- put of Ni toggles...