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CDCE913: CDCE913 VCXO PULLING RANGE

hiroshi
Prodigy 80 points
Part Number: CDCE913

I'm trying to use CDCE913 in +/- 150ppm range with 27MHz OSC to 27MHz LVCMOS 3.3V. but I can only adjust the frequency about +/-50ppm in my board.

(I used DSX321SH(27MHz/10pF/10ppm as XO source, the spec meets the requirement)

I think Changing the CL onchip can get +/- 150ppm, but  I don't think this usage is correct?

In the datasheet, it's saying the pulling range is "min +/-120ppm, Nom. +/-150ppm."
Is there any limitation?


Also I checked document  scaa085a.pdf.  can anyone explain what Figure 4 and 6. mean?

I'm confused about them.

FIgure 4 is saying the pulling range is adjustable just by VCTRL.

Figure 6 is saying  it's need to use CLonchip

Any advice or information you could provide would be greatly appreciated.

  • 0
    TI__Expert 8865 points

    Hello,

    Frequency pulling using the Load capacitance depends on the Xtal parameters. Please refer to the sections 2.2 & 3.1 of the "VCXO Application Guideline" document (scaa085a.pdf) on calculating the pullability of the Xtal.

    +/-150ppm pulling specs refers to the Xtals which meet the pullability requirement.

    Frequency of the Xtal oscillator depends on the Load capacitance.

    If you are using VXCO and use the Vctrl to pull the xtal, then you should set the CLon-chip to the desired value.The variation of the control voltage in the range between 0V and 1.8 V allows a corresponding sweep of the varactor’s value from 1.3 × CLOn-Chip (30% higher than starting value) up to 0.3 × CLOn-Chip (70% lower than starting value). (Figure 4)

    If you not using the Vctrl, then you can change the CLon-chip to change the frequency. (Figure 6)

    Don't hesitate to contact if you have further questions.

    Best regards

    Puneet

     

  • 0 in reply to Puneet Sareen
    Prodigy 80 points

    Hi, Puneet

    Thank you for the reply.

    I'm using Vctrl and use the Vctrl, but the measured pulling range is +/-50ppm max. not +/- 150ppm.

    we'd like to have +/-150ppm pulling range.

    I'm using OSC DSX321SH below.

    >+/-150ppm pulling specs refers to the Xtals which meet the pullability requirement.

    I think DSX321SH meets the pullability requirement.

    I'd like to know if thee is any point I should check.

    DSX321SH(27MHz/10pF/10ppm) / KDS

    DSX321SH[SMD03225C4]-2.pdf

    Best regards,

    hiroshi

  • 0 in reply to hiroshi
    TI__Expert 8865 points
    Hello Hiroshi,

    I cannot find the xtal parameters in the attached spec document. Can you send me the motional capacitance & shunt capacitance specs of the xtal you are using?

    Regards
    Puneet
  • 0 in reply to Puneet Sareen
    Prodigy 80 points

    Hi, Puneet.

    Thank you for your cooperation.

    The shunt capacitance(C0) is 2.0 pF max.
     I'm checking motional capacitance (C1) now.

    img-Z19123412.pdf

    Best regards,

    Aoyama

  • 0 in reply to hiroshi
    Prodigy 80 points

    Hi, Puneet

    I get spec from our supplier and motional capacitance (C1) of DSX321SH is 3.746fF(typ.)

    I used SMD49 below for a test and pulling range was about +/- 100pm.

    SMD49/KDS CODE D270G3C ( 27MHz, +/-30ppm, DL=10uW, CL=12pF, C0 = 7.0pF, Rr=40ohms, C1 = no data)

    (SMD-49 is recommended on table 1 on Application report SCAA085A)


    The spec (C1, C0) seems to be met to the requirement, but DSX321SH can't have this range.

    I'd like to know if you have any reference Crystal to have +/- 150ppm pulling range?


    Best regards,
    hiroshi

  • 0 in reply to hiroshi
    TI__Expert 8865 points
    Hello Hiroshi,

    I used following parameters of the xtal for DSX321SH :
    C1=3.746fF
    C0=2pF
    CL=10pF
    F=27MHz
    I come up with about +/-50ppm.

    We have used Quartzcom UM-1 MJ 27MHz xtal for +/-150ppm frequency pulling.
    I would recommend to use the TI Pro-Clock software to display the frequency pulling curve. The chip parasitics are taken into consideration for estimating the pulling curves. Please let me know if you need support on that.
    If your frequency pulling is still less than the one predicted by this software, then one reason could be that there is high parasitic capacitance on board.

    Thanks & regards
    Puneet
  • 0 in reply to Puneet Sareen
    Prodigy 80 points

    Hi Puneet,

    Thank you very much for your help.

    we'll check the spec with TI Pro-Clock.

    One question on this software, I'd like to know what the value of Crystal [F] mean on Pulling Range Plot?

    Best regards,

    hiroshi

  • 0 in reply to hiroshi
    TI__Expert 8865 points

    Hello Hiroshi-san,

    Please keep the value of F default and just enter the other Xtal parameters.

    Regards

    Puneet

    hiroshi

  • 0 in reply to Puneet Sareen
    Prodigy 80 points

    Dear Puneet san

    I finally got DSX530GA with below spec.

    [spec]
    F = 27MHz +/-20ppm
    C1 = 11.5fF typ. (+/-20%)
    C0 = 3pF typ. (+/-20%)
    R1 = 20 ohms typ.
    L1 = 3mH typ. (+/-20%)
    CL = 8pF

    the pulling range is supposed to be near +/-150ppm according to Pulling Rang Plot of TI ClockRro
    but the result is +82ppm to -128ppm.
    I will give up to use this with +/-150ppm pulling range.
    but I'd like to know why the pulling range is not equal between negative and positive.

    [measured value with CL=7pF(resistor 0x05[7:3]]
    27.002,212MHz (+82ppm)
     to
    26.996,536MHz (-128ppm)

    * Fcenter (VCC/2) = 26.999,986MHz (-0.5ppm)

    Let me have any advice, very helpful.

    Best regards,
    hiroshi