Is there a spice model I can use for simulation?
Also, pls answer the specific question
why the switch will attenuate the sine-wave of the TCXO?
Good talking to you on the phone.
To confirm, it looks like your off channel should not be a problem here. For the on switch, it will be a low resistance path from your VCO to the PLL . Just note that a DC bias through the switch will start to effect Ron, see figure 1 in the following app note to see the magnitude of this effect. http://www.analog.com/static/imported-files/application_notes/AN_952.pdf
Unfortunatly I don't have a SPICE model for this part right now but as mentioned I have s-parameters. These can be used in simualtion software like ADS, Ansoft Designer, AWR etc to look at the frequency response of the switch. They might be helpful.