vco gain

I would appreciate if you guys could comment on the statement below in detail. Thanks in advance

Statement:
A high VCO gain results in high sensitivity to the noise of VCO tuning voltage, which increases the spur level and degrades the phase noise.

Rgds

VCO gain is stated in MHz/volt. So if the VCO changes 10 MHz for 2 volt change on the tuning line, the gain is 5MHz/V. The higher this gain, the more sensitive the tuning line is to any interference. Interference on the tuning line translates to unwanted FM on the VCO output.