PLL&Matlab question

[AdvaRes]

Hi members,
I calculated the transfert function of the PLL in order to simulate it. My goal is to plot the Vtuning signal. I verified by the Routh creterion that the system is stable by calculatin the closed loop transfert function of the PLL. I also calculated the transfert function of Vtun Unfortunately I obtained a strange representation of Vtuning versus time.
Can someone helpe me to resolve this problem ?

note in the relations that F represents the filter's transfert function.

Open loop transfert function: Gol=Kcp*F*Fvco/N
Vtun transfert function: Vtun=Kcp*F/(1+Gol)

 

[LvW]

Sorry, but I don´t understand your problem.
Vtune is a voltage which controls the VCO. It is not a "transfer function" or something like that.
In any case, since Vtune is a voltage which is a measure of the PLL offset (phase/frequency difference) it is mainly a function of this difference multiplied with some constant parametrs.

 

[Element_115]

It might be because at start up t=0+ there is no Fo at the VCO so the
phase detector see Fo=0 and ramps up the Vtune to make the VCO go
higher in frequency.

 

[chody]

Use "LTIVIEW" of matlab.
You will see the step response or impulse response of PLL.