Calculating the loop parameters of a FM PLL

[actra]

if i used the vco in a pll system as a FM or ASK modulator...

when adding the modulating data before the vco...is it going to change the loop filter paramters of this pll system ?

imean if i designed a pll with certain loop filters parameters then added an external data source after the loop filter but before the VCO is it going to change the loop paramters that i designed at ? if so ? how can i caluclate the new paramters?

 

[flatulent]

Re: FM PLL..

The loop will try to remove the FM. Therefore the loop bandwidth should be lower than the lowest modulating frequency. Add your audio signal to the VCO frequency control pin.

 

[actra]

FM PLL..

for the modulating part if added on the control voltage of the VCO ..does it has a resistve or capactive load on the loop filter ? is it going to change the poles and zeros locations ??

 

[flatulent]

Re: FM PLL..

If you do it cleverly, you will only change the magnitude of the loop gain. The best way is to add in another op amp with an inverting gain of 1 just before the VCO control pin. You then need to swap the two inputs on the phase detector to get another inversion of the loop gain. Then putting your modulating signal to the first mentioned above amplifier summing junction through a resistor will not affect the loop gain.

The method you are probably thinking of using two resistors to the VCO summing junction will cause extra problems and should be avoided if you are not familiar and comfortable calculating loop gains and plotting Bode diagrams.