Hi everyone,

From the attaached diagram and in Figure 15.34(a), it's showing loop gain characteristics of simple charge-pump PLL which is not stable. In order to stabalize the system, we must modify the phase characteristics such that the phase shift is less than 180° at the gain crossover. As shown in Fig 15.34(b), this is accomplished by introducing a zero in the loop gain, i.e., by adding a resistor (which is circled in red) in series with the loop filter capacitor (Figure 15.35).

My questions is, why by adding a series resistor will not add a pole there instead of zero since RC is a time constant?

 

The attached picture is here, forgot to attach in previous post.

 

Hello,

Take a look at this PDF:
http://www.minicircuits.com/pages/pdfs/vco15-10.pdf

This link comes from the EDUCYPEDIA page on PLL's:
http://www.educypedia.be/electronics/pll.htm

Greetings,
Bertus

 

The attached picture is here, forgot to attach in previous post.

Here is a quick analysis to show the effect of the added series resistor. It is clear from the comparison between the two cases (with and without the resistor) that the resistor introduces a zero into the transfer function.

hgmjr

 

Hi,

Thanks for your effort in writing out the analysis and contribution to this post. Have a blessed new year......