The Pierce-gate oscillator of Figure 1 is well recognized by most designers, but few understand how to specify the crystal correctly. The crystal used in the topology of
Figure 1 can be either a fundamental AT-CUT or BT-CUT. A BT-CUT crystal has poor
frequency stability over temperature compared to an AT-CUT. This topology uses a
parallel crystal and not a series crystal. When a parallel crystal is specified, the crystal
manufacturer will also require that you specify a load capacitance.
To understand load capacitance, think of a series LC circuit where the crystal is the L and the load capacitance is the C. The resonance frequency of the LC circuit will vary as a function of L and C. But in the crystal case, the L is fixed (temperatur