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Hi dearThe main difference is that YIG oscillators are tuned via a large current in a big magnetic coil. VCO's are tuned with a tuning voltage (almost no current).
Depending on your application, this might make no difference at all, or it might mean a great deal.
You usually make some sort of YIG coil driver, that linearly converts voltage to current. Then you put a standard PLL in front of that.
Where you get into trouble is that you are still driving a big magnetic coil. That coil has inductance, which means there will be a "lowpass" pole in the dynamic tuning curve. As you may be aware, PLL's do not like low frequency additional poles in the control loop filter design. You may have to add a control loop zero, or more fancy phase compensation (lead-lag) network in the PLL to compensate for the tuning coil pole. Otherwise you might have ringing, or even control loop instability.
You also need to inject a DC current into the coil to get the oscillator "centered" in the desired frequency band, so the coil driver has to be able to provide this. Sometimes, you can buy a YIG oscillator with a permanent magnet in it to accomplish the same thing without wasting DC current (but watch out for tuning hysteresis).
I have seen some synthesizers use YIGs with two tuning coils. A DAC drives the coarse tuning coil to get you close to the desired frequency, and the PLL drives a (lower inductance) fine tuning coil. This allows faster lock-up time, and wider control loop bandwidths.
Rich