Charge pump PLL

Discussion in 'General Electronics Chat' started by lkgan, Dec 29, 2009.

  1. lkgan

    Thread Starter Member

    Dec 18, 2009
    117
    0
    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?
     
  2. lkgan

    Thread Starter Member

    Dec 18, 2009
    117
    0
    The attached picture is here, forgot to attach in previous post.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    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
     
    Last edited: Dec 30, 2009
  5. lkgan

    Thread Starter Member

    Dec 18, 2009
    117
    0
    Hi,

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