the frequency response versus control voltage characterization of a vco is performed with or without its output buffer ? I wonder this because while decreasing the control voltage the sinusoid amplitude of vco decreases so there is a value where this signal cannot excite the output buffer, so it'd loss frequency work range if take in account the buffer. Hope my question is clear.
the buffer is to avoi the load effect.
Are you conecting a load to the output?
Usually the most important factor of the VCO is frecuency and not amplitude.