Question about a simple VCO phase noise model

I see a phd paper form fudan university, the author give a simple VCO phase noise mode like following:
                   Svco,n=A(f1/f)^3 + B(f2/f)^2 + C;
A is phase noise at offset f1(1/f^3 region), unit dBc/Hz;
B is phase noise at offset f2(1/f^2 region), unit dBc/Hz;
C is the VCO noise floor, unit dBc/Hz;
who can explain why the vco phase noise model can like this? And the f1 and f2 no need special value in the seperate region?

Dear 123:
A,B,C is fitting parameter by measurment.
its a simple piece-wise-linear model.
u can write A*f1^3=A1,B*f2^2=B2
then get an easier equation...

who know it?

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