You need to share more info on the circuit. Frequency/capacitance range, biasing of the VCO and FET, etc. Like dick_freebird said, biasing matters if the signal amplitude is large, and the switch resistance may be a factor.
I have looked at different things that mentioned in this thread. The most important of all is the how the MOSFET works as a switch in large signal case. As it is known that if it is properly biased, the working state will not vary in the small signal analysis like applications in amplifier. But in the VCO, the D and S ends are in the large signal situation. It cannot work steadily in one region.
In order to understand this case in more detail. I have attached the schematic at the end and the main parameters are described as follows:
My target Frequency is 5GHz. L = 1.33nH. The total C vary from 30pF +/- 600fF depending on the introduction of different paths which has parallel connection with Capacitance. The MOSFETs as switch work at this part and thus the D and S ends are connected one at the Vout_negative and the other is connected with the capacitance (or the capacitor device). After passing through the capacitor the Vout_positive is connected to. Several path can be introduced in parallel. The total voltage is supplied by a Vpulse, 0V @ 0ns, 1.2V after 1ns. The total I_bias is 2.2mA.
I have noticed that the AC current drop on the ideal switch is almost 0, but on the MOSFET device, Vds_AC drops a lot. This makes fewer AC voltage drop on desired capacitance I wanted to introduce on a specific path. The AC voltage between the Vout_N and Vout_P is around 3V to 550mV in the ideal case for the 5G desired target frequency. But in the MOSFET_switch case, if this voltage is applied into the path (one MOSFET (under 1.2 V supply or bias) + one capacitor), the MOSEFT cannot only working in a stable region.
This probably the biggest problem not appearing in the low frequency domain or small signal application.
Therefore I would think of somebody shed light on how to use MOSFET as a switch in the large signal case providing not over supplying 1.2V for 90nm technology.
Or any direction on which knowledge is suitable for desiring such a large signal MOSFET switch which is also Resistance and Capacitance sensitive one?
Switch capacitor might be a good topic but I do not think the circuit need extra knowledge on how to clock it.
Attached is the circuit schematic.