It's just coincidence that you happened to find some papers that used oscillators in the 26 MHz range. You can phase lock any oscillator, not just a crystal oscillator, that has a voltage control input. Your choice of frequency is determined by the input frequency, the desired output frequency, and the ratios between them. I was recently experimenting with phase locking a yig oscillator at 6.8 GHz to a synthesized generator at 680 MHz via a harmonic mixer. It worked, but my crude circuit was too noisy for my application.