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LMX2485E: choosing RF_A,R_B,RF_C for N counter

KANJI SHIBATANI
Mastermind 6205 points
Part Number: LMX2485E

Hello

My customer chosen LMX2485E for 2021 model.

Would you please how to choose RF_A to RF_C? ( any app note?)  I have 2 question in the following.

1) condition

・PLL Ref OSC =10.35MHz

・3rd(FM[1:0]=3)　

・CSR[1:0]=0 (disabled)

・RF_P[1]=1 (Prescaler=16)


2) Goal

1st local = 405.05～707.725 MHz 

set frequency within +/- 1Hz  ( ex. 421.590000x)

3) Question

<Q1>

Customer ( and I ) don't know how to set the value of  RF_A, RF_B, RF_C .

Would you please let us know how to set RF_A to RF_C? ( Do you have any app note?)

We know a following criteria * in LMX2485E d/s 8.6.1.2 RF_N[10:0]—RF N Counter Value, but we didnot find other

criteria/discription in order to choose RF_A ~ RF_C.


*
N ＝ RF_P ・RF_C ＋ 4 ・RF_B ＋ RF_A
( N － 2FM － 1…N ＋ 2FM ; where RF_C ≧ Max{RF_A, RF_B} )


<Q2>

Customer set N counter value with cut & try using Coadloader RF PLL tab.

So, customer just follow the value in the Register tab to have within +/- 1Hz ( ex 421.59000043),but

don't know whether this is okay or not.

(In fact, they had tested those in their board, and test result was fine (C/N=66dB))

I attached customer's  xx.mac file for Coadloader.

Is there anything mistake or anything violation for choosing RF_A ~ RF_C ?

Thank you for your support.

Best Regards

( please change .rar  to .mac)

LMX2485E_for_middle_model.mac.rar

  • 0
    TI__Guru** 107200 points

    Hi Shibatani-san,

    N = P*C + 4*B + A, where

    N = PLL N counter value.

    P = 8 if R1[22] = 0; P = 16 if R1[22] = 1.

    C = the integer value of N divided by P. For example, if N = 35 and P = 8, then N/P = 4.375, so C = 4.

    A = the remainder of N divided by 4. For example, if N = 35, then N = 8*4 + 3, so A = 3.

    B = (N - P*C)/4. 

    For example, given N = 81 and P = 16,

    C = 5; A = 1; B = 0

    The frequency resolution is equal to fpd / DEN, where fpd = phase detector freq; DEN = fractional denominator. For example, if fpd = 1MHz and DEN = 1000000, then the freq resolution is 1Hz.

    For example, 

    Output frequency will change in exactly 1Hz. 

  • 0 in reply to Noel Fung
    Mastermind 6205 points

    Dear Noel-san

    Thank you for your answer so quickly.

    My customer has 2 questions for your answer.

    Would you please advise again?

    Q1


    When customer had changed denominator of fraction ( 3450000 in your capture), the result

    is reflected in R1　RF_FD[11:0] ?


    Q2

    If customer hoped frequency resolution in the same fpd ( phase detector frequency),

    customer should increase DEN ( denomination of fraction  3450000 in your capture ) ... is this

    undestanding correct?

    Best  Regards

  • 0 in reply to KANJI SHIBATANI
    TI__Guru** 107200 points

    Hi Shibatani-san,

    Correct for both questions.

  • 0 in reply to Noel Fung
    Mastermind 6205 points

    Dear Noel san

    Thank you very much!