Hello Everyone,
I am using the code below for VCO, and it is pretty standard and straightforward. I want to make sure that the noise transfer function from control voltage to phase noise is correct, as I am going to use it in my PLL. Now, when I put a purely thermal noise using a resistor on the control voltage, the output phase noise slope is -40dB/dec using PSS and Pnoise!!!!  Just to clarify, I do not want to model VCO PN itself but I wanna make sure that the transfer function of noise from its control voltage to output is correct.
I would appreciate any comment if you see it as helping. I am attaching both schematic and simulation results below.


module vco_va(vin, voutp, voutn);
input vin;
output voutp , voutn;
electrical vin, voutp, voutn;
parameter real amp = 0.5;
parameter real center_freq = 20G;
parameter real vco_gain = 1G;
parameter integer steps_per_period = 32;
parameter real DC_val = 1;


  real  phase;
  real inst_freq ;


  analog begin


           
            inst_freq = center_freq + vco_gain * (V(vin)-1.25);
           
           phase = idtmod(inst_freq, 0,1);


     $bound_step (1.0 / (steps_per_period*inst_freq));
           
           V(voutp) <+ amp * sin (2*`M_PI*phase) + DC_val;
           V(voutn) <+ -1*amp * sin (2*`M_PI*phase) + DC_val;
  end
endmodule