All of the types of oscillators that have been discussed thus far have one inherent problem—their operational frequencies will vary with temperature changes. For digital circuits requiring high-stability oscillators, a crystal-based oscillator is usually employed. Crystals are “piezoelectric” elements with vibrational characteristics that are highly immune to temperature changes. Figure 13-11 is a crystal timebase oscillator.

Depending on the crystal frequency chosen, start with a 10-Mohm resistor for R1, and increase its value as needed. The NTE4011B is a CMOS device, so take precautions in assembly, as detailed previously. Crystal oscillators operate at very high frequencies. To lower the output frequency, an appropriate modulo flip-flop divider circuit is required.