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CDCE949: Looking for explanation of the P Q R parameters

Part Number: CDCE949

I am looking for explanation of the P Q R parameters for CDCE949.

I can choose N & M parameters and I can calculate the P Q R parameters.  
Though not specifically stated, it seems that results with R = 0 may be be desired above R > 0 results.
But I don't UNDERSTAND what R signifies, except that it is a remainder.

I notice that several combinations of N/M produce multipliers that result in R = 0.

,16,17,18,19,20,21,22,23,24,25,26,27,28  (there are more, I just stopped this list at 28)

I realize that P is inversely proportional to the size of the multiplier, for example:  
        P = 4 for a multiplier of 1.00,    P = 0 for a multiplier of 16 or above.

What is the purpose of multiplying the original numerator "N" by 2^P ?

Q = int( (N * (2^P)) / M) but again what is the significance?

  • Hi Lee,

    Please see this thread: 


  • Hao,

    Your reply did not answer my questions. This does not resolve my issue.

    Perhaps I should have said that NO current answer in this forum meet my needs.

    I KNOW how to do the calculations, I NEED an explanation of the parameters P Q R and "N"

    Since my desired output frequency may involve a non "R=0" result, I need to know how to

    calculate the frequencies that will be averaged to my desired output.

    Please provide an explanation of the P Q R parameters. What do they do?



    Btw, for you and future readers

    In the thread link you gave: Gabe Ayala refers to a book, no longer at that link, but it is archived here:

    In that same thread, Julian Hagedorn wrote

    "the CDCE9xx is a fractional n PLL, but it does not use N internally. N gets translated to Q and R, where R controls the fractional logic."

    But he also wrote "N,P,Q,R needs to get written into the registers to get the right configuration."

    So he contradicted himself.

  • Hi Lee,

    More details below:

    I'm not sure how this will help with the actual application though. These are just frequency synthesis parameters. The optimization will be to aim at higher N and M, according to the referred thread.

    Julian did not contradict himself. The N in the first quote is different than the N in the second quote. We typically use (N + num/den) for the fractional feedback divider between the VCO and PFD. In other words, f_VCO = fPFD * (N + num / den). CDCE9xx, however, is off the standard. Instead of using the (N + num/den) that we typically do, the feedback N divider is translated into Q and R parameters. And again, this "N" in the N divider is not the same "N" in the "N, P, Q, R" parameters.


  • Thank you for the picture diagram of the internal working pieces. It will take a day to review it.

    I do notice that internal parameters V and W seem to be cut off at the bottom of the page.

    Can you provide a better image that shows those calculations?

    Concerning my statement about Julian contradicted himself, this new info may show that I am incorrect.

    It seems I interpreted "but it does not use N internally" to mean the value "is not stored", which is incorrect.

  • No problem. It was the full view. The bottom line just has a different color. It's UU = RR - U.