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. The use of two HC inverters produces a fairly symmetrical 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 1/2 VCC, and slightly less than 2 V, respectively. If the oscillator is to operate above 10 MHz, the resistor is replaced with a small inductor, as shown in Fig.68-4B.
The output frequency of the circuit in Fig. 68-4A is given as about 1/1.8rc, and can be made variable by connecting a 100-KΩ potentiometer in series with R. The solution adopted for the oscillator in Fig. 68-4B is even simpler: C is a 50-pF trimmer capacitor.
Required Field 
