What type of signal source should I use for clocking a high-speed converter?
It really does not matter what type of clock signal is used. The major considerations for the clock signal are rise/fall time (slew rate), jitter or phase noise. A good start point in selecting a clock source would be to look at the jitter specification for the ADC. These days most ADCs specify an aperture jitter (tj) of <0.1ps. So a low noise crystal oscillator should be able to meet this jitter specification in order to achieve datasheet performance.
For additional information, please refer to the attached paper "Analog-to-Digital Converter Clock Optimization: A Test Engineering Perspective" by Rob Reeder, Wayne Green, and Robert Shillito, Analog Dialogue 42-02, February (2008).
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