The crystal is the frequency reference of the VCXO and the overall performance of the circuit depends on the characteristics of it. It is important that the crystal meets all required specifications if the VCXO is expected to work reliably. ICS works with crystal vendors to define, build, and certify crystals that meet these requirements, and the crystal vendors maintain an inventory of these devices in stock. Please see our web site for recommended part numbers. Using a packaged clock oscillator to drive these VCXO products will not work correctly. The clock will be generated, but the frequency cannot be pulled because the VCXO circuit has no control over the oscillator frequency.