Noise Analysis for Modulated Noise Using Verilog-A Modelling

Hi Community,

I am doing experiments about modulated flicker noise and I aim to stick to Verilog-A behavioural modelling.

Below is the test bench I brought up and the Verilog-A modules that I wrote seperately:

// VerilogA for lab_simu, noise_source, veriloga

`include "constants.vams"
`include "disciplines.vams"

module noise_source (op);

output op;
voltage op;

analog begin
V(op) <+ flicker_noise (1000,1,"Flicker") ;
end

endmodule

// VerilogA for lab_simu, Oscillator_fixed_freq, veriloga

`include "constants.vams"
`include "disciplines.vams"

module Oscillator_fixed_freq (out);

output out;
voltage out; // output signal

parameter real freq = 1e9 from (0:inf); // output frequency
parameter real vl= 0; // high output voltage
parameter real vh=1; // low output voltage
parameter real tt=0.01/ (1e9) from (0:inf); // transition time of output

integer n;
real next;

analog begin
@(initial_step) begin
next = 0.5/freq + $abstime;
end
@(timer(next)) begin
n = !n;
next = next + 0.5/freq;
end
V(out) <+ transition(n ? vh : vl, 0, tt);
end

endmodule

// VerilogA for lab_simu, mixer, veriloga

`include "constants.vams"
`include "disciplines.vams"

module mixer (vin1, vin2, vout);
input vin1, vin2;
output vout;
voltage vin1, vin2, vout;
parameter real gain = 1;

analog
V(vout) <+ gain * V(vin1)*V(vin2);

endmodule

Nothing terribly bizarre.

The question is, I would love to extract the Noise Power Spectral Density measured in V^2/Hz for both 'noise' and 'output':

when I set the noise analysis as below:

I get the plot expected (green curve is noise PSD and red line is that taken 10dB - a straight line!):

However, when I set the 'positive output node' in the noise analysis to be the modulated noise output, I got nothing in the plot (0V^2/Hz for the whole spectrum):

Could somebody explain why and suggest any solution to it? I want to plot the noise PSD of the output modulated noise.

In case noise analysis could not handle this task. Could somebody suggest in which analysis I can extract the same nice noise PSD plot as in the first case and how?

Thank you so much!!!