analysing an already made PLL design

[actra]

deansbook3

from an already made pll design i want to analyse it to improve its BW to cover more range...

so whats the steps to find the loopfilters paramters like BW, phase noise and other parameters ?? from the capacitors and resistances combinations that forms the loop ?

 

[flatulent]

You need to do an open loop gain calculation and do a Bode plot. From this you can determine what changes you can make in the gains and pole-zero frequencies.

 

[actra]

this is the loop filter as shown ,

Q \ whats the advantage of using C1 and R1 as shunt to the output from phase detector ? they have no effect on the transfer function ???

Q\ whats the ordr of this loop filter ?

 

[mr_chip]

Does your PLL has charge-pump or it's a linear PLL?

 

[actra]

it has a charge-pump

 

[Sergiu_Q]

actra...your filter is a 2-order filter...is the worst filter you can use
why you don't try some 3-order active filter?

 

[actra]

Sergiu_q ... why its the worst filter ??

 

[Sergiu_Q]

If you will use a 2-order you will have a lot of problem with jitter ...and the jitter is one of the big problems of the PLL's ....for decrease jitter you will need a more high order filter...and an active filter will offer you and some gain

 

[mr_chip]

2 actra - I'd recommend you first to read a basic book about electrical circuits, passive filters etc and then to return back to PLL.

The 2nd order filter has smaller spurs and out-band phase noise attenuation than higher-order filters. But it has a faster lock time. You need to design a filter according to you phase noise & lock time spec.

There're plenty of references about the open-loop PLL equations - check for example **broken link removed**

Especially the book
**broken link removed**

 

[actra]

how do they determine the loop filter order ? is it from the open loop transfer function or the total filter impedance ?

 

[mr_chip]

Well, usually people say about the loop filter order, but sometimes about the PLL open loop transfer fucntion (which has an additional pole because of VCO), so you need to be careful with that.