and some various others from salvaging crystals
Various crystals use different load capacitor values. It is a function of the crystal cut
Load capacitance is one of the most overlooked parameters when specifying crystals. In the popular Pierce oscillator circuit, which has a capacitor to ground on either side of the crystal, the load capacitance is equal to the series combination of the two capacitors plus Cstray. (Cstray is the sum of capacitances that are contributed to the circuit by the layout, board material, and the input and output impedance of the active device.)
A good rule of thumb for Cstray is 5pF. If one of the capacitors is replaced with a varactor, the frequency can be "pulled" by applying a tuning voltage on the varactor. This configuration can be the basis for a simple VCXO. If the tuning voltage is derived from a thermistor network, the frequency can be adjusted to null out the effects of temperature. This configuration is an approach for a TCXO. A combination of both features is known as a TCVCXO.
A pullability specification for an HC49U crystal used in a fundamental mode VCXO might have the following form:
CL = 20 to 45pF, pullability = -100ppm max, CL = 20 to 10pF, pullability = +100ppm min.
Smaller crystals have about half the pullability of the HC49U. The pullability of overtone crystals is reduced by 1/n2, where n is the overtone mode (i.e. 1, 3, 5, etc.).