18. Why don't HC49S and surface mount crystals pull as much as HC49U crystals?
The HC49U crystal utilizes a circular AT-cut crystal blank. Due to their small size, most surface mount crystals use a rectangular AT-strip cut crystal blank. ("Strip" refers to the rectangular shape of the blank.) While both are AT-cut blanks, several differences exist due to their different geometries. In general, given the same frequency and overtone, an AT-strip cut will have a lower Shunt Capacitance (CO) and Motional Capacitance (C1) than the AT-cut. Because pullability is a function of the capacitance ratio CO/C1, AT-strip crystals have less pullability.