PLL Loop filter Design : Cap considerations

pll loop filter design

Hi,
In PLLs with charge pump , there are generally two caps before the VCO
The first cap is in series with the resistor and the second one is just connected b/w control voltage and groun

Can you explain the physical significance in deciding the sizes of these caps and resistor?

thanks

pll loop filter

Dear auj :

please use matlab to check the PLL transfer function is stable or not to
chose the cap and R !!



mpig

loop filter design

If you are talking active filters with op-amps then generally if you make the feedback capacitor larger then the input resistors can be smaller. This is important as the larger the input resistors are, the more noise they generate. Typically most input resistors are set to be a few hundred ohms and kept to less than 1K, resulting in a larger Cc. Go for a phase margin of between 65 & 75 degrees.

pll loop filter calculator

the second cap. is mainly for spur suppression while the first along with the resistor define the loop pole and zero which contribute in the BW and PM , damping factor , and .......
usually the 2nd cap is chosen such that it is less than the first one to make the mathmatics of the loop calculation easier ,(if it is smaller it can be neglected) , while the first along with the resistor sets the above mentioned parameters and their values has some consideration to acheive some required performance as settling time , BW ,...
also the loop BW must be smaller than the ref. freq. to make the continous time approximation valid.
regards.

loop filter transfer function

the small cap has little affect on system response

pll loop filter design national

safwatonline said:

the second cap. is mainly for spur suppression while the first along with the resistor define the loop pole and zero which contribute in the BW and PM , damping factor , and .......
usually the 2nd cap is chosen such that it is less than the first one to make the mathmatics of the loop calculation easier ,(if it is smaller it can be neglected) , while the first along with the resistor sets the above mentioned parameters and their values has some consideration to acheive some required performance as settling time , BW ,...
also the loop BW must be smaller than the ref. freq. to make the continous time approximation valid.
regards.

Click to expand...

2nd cap is usually chosen to be 1/10 of the 1st cap or less.

pll loop back filter

it depentd on the order and loop transfer function of PLL

loop filter pole location pll

May I ask another question to everybody....
PLL Loop filter Design : OP-amp considerations
It's related to the reference freq. of PLL or something.....?

loop filter design in pll

i think it just should have a BW bigger than the loop BW.

caps considerations

The second pole shoul be much larger than the loop BW.I think 10 times is OK. The OP should be a low noise OP because it would be mixed to the VCO output.[/quote]

pll loop filter design tool

where can i find "the matlab to check the PLL transfer function" program with many input parameters?

pll loop filter calculation

Instead of pulling capacitor values out of thin air, with rules of thumb that may or may not apply, why don't you actually calculate the loop filter values (Resistor and Capacitor) that actually mean something? Loop filter component values can be calculated from the desired loop filter bandwidth, VCO Kv, charge pump current, and divide ratio.
Lance Lascari has an excellent MathCAD worksheet available at his web site tools.rfdude.com. If you don't have MathCAD, there are pdf's in the archive to reproduce his calculations in another tool.
I've written an Excel worksheet that reproduces some of these features, it's available at http://www.keystoneradio.com/?n=Main.PLLDesign . It's not as full featured at Lances, but it's a starting point

Furthermore, you can go to National Semiconductor's web site and download an excellent application note/book on PLL's. **broken link removed** is the link.


Dave
www.keystoneradio.com

national.com filter calculator

auj said:

Hi,
In PLLs with charge pump , there are generally two caps before the VCO
The first cap is in series with the resistor and the second one is just connected b/w control voltage and groun

Can you explain the physical significance in deciding the sizes of these caps and resistor?

thanks

Click to expand...

the capacitor and resistor determine the PLL bancwidth, and the large cap should 10 times larger than the small one.

charge pump pll loop filter design

auj said:

Hi,
In PLLs with charge pump , there are generally two caps before the VCO
The first cap is in series with the resistor and the second one is just connected b/w control voltage and groun

Can you explain the physical significance in deciding the sizes of these caps and resistor?

thanks

Click to expand...

Which is very improtant for PLL stablity.