Dual charge pump PLL oscilating

[sainiparvesh]

Hi,
I am converting a single charge pump PLL having loop element as R, Cbig, Csmall and CP current as Icp, into dual charge pump PLL having loop element R(=1/gm, unity gain buffer o/p impedence), Cbig/2, Csmall/2 and proportional and integral charge pump current as Icp and Icp/2. As per MATLAB simulations it gives aprox. same phase margin and bandwidth but real implementation gives oscillations in control voltage and o/p freq. while origional PLL is having ~50degree PM. Is there any other parameter needs to be changed to reduce loop cap(Cbig) by 2. I have implemented R using unity gain buffer after Cbig.

 

[BradtheRad]

implementation gives oscillations in control voltage and o/p freq. while origional PLL is having ~50degree PM.

1.

I suppose you wish to eliminate the unwanted oscillations?

These are likely to happen when resistor values are low, allowing excessive current flow. That is, if the components will provide it.

If they won't allow it, then it may pull down volt levels on certain components.

In turn this changes some other behavior, which the oscillator tries to compensate for, etc...

The result is unwanted oscillations.

2.

The large phase modulation (I think that is what you mean by PM) occurs on a capacitor in combination with a low resistance.

To solve these problems, it might help to increase some or all resistor values, and reduce capacitor values. It is possible to do this while maintaining a desired frequency of oscillation.

 

[sainiparvesh]

Hello Brad,
Thanks for your suggestions, it is quite informative for me and further add into my knowledge about CPPLL. Actually I solved this problem. There was the issue with big loop cap, there was a connection problem due to which actual big loop cap was not coming into picture and loop was oscillating. Anyway by PM I mean phase margin.

Thanks for your nice explanation once again.

Regards,
Pravesh

1.

I suppose you wish to eliminate the unwanted oscillations?

These are likely to happen when resistor values are low, allowing excessive current flow. That is, if the components will provide it.

If they won't allow it, then it may pull down volt levels on certain components.

In turn this changes some other behavior, which the oscillator tries to compensate for, etc...

The result is unwanted oscillations.

2.

The large phase modulation (I think that is what you mean by PM) occurs on a capacitor in combination with a low resistance.

To solve these problems, it might help to increase some or all resistor values, and reduce capacitor values. It is possible to do this while maintaining a desired frequency of oscillation.