You could buy one. If you are unfamiliar with synthesizer design, that phase noise may be elusive, since any noise source in your design could violate the spec.
If you want design help, we will need to know the exact frequency range, the frequency step size, the level of spurious outputs desired in dBc, and the settling time required.
If you already have a reference clock (crystal oscillator) that you want to use, we will need to know its frequency and phase noise.