Electronic – Generator voltage dipping at specific loads with observed increase in frequency

You can use several methods:

1) PLL - for example ADF4002 works up to 400 MHz. The resolution is limited, because it works by multiplying reference frequency. This chip will work with your VCO.

2) Integrated PLL synthesizer - ADF4350 can generated frequencies within 137-4400 MHz range with fairly good resolution. However, within your range the result will be more square wave rather than sine.

3) DDS - direct digital synthesis - AD9910 can generate frequencies up to 400 MHz with resolution under 1 Hz.

Trying to tune wideband VCO without any stabilization and measuring the result will not provide stable frequency.

Here is a list of PLL chips and PLL synthesizers made by ADI. Other vendors have some too, but usually working within GHz range.

It depends on whether the weak link is the generator itself or the prime mover.

If the construction of the generator limits its power output — the resistance of the wire, the strength of the magnetic field, etc. — then the voltage will sag, even while the torque on the shaft remains within the capability of the prime mover, so the frequency will not change appreciably.

On the other hand, if the prime mover is undersized with respect to the torque that the generator can produce, then it will slow down, and both the frequency and the voltage will drop.