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LMX2571: External VCO Setting issue

Arking Chiu
Intellectual 790 points

Part Number: LMX2571

Hi Team,

    I have some problem when using the LMX2571 in external vco mode.(TX mode use EXTVCO,RX mode use internal VCO )

    1. I can't change the CPOUT voltage from 0.3V~2.7V; I have changed the EXTVCO_CP_IDN/EXTVCO_CP_IUP/EXTVCO_GAIN of Register 42/Register 41.

    a)When the EXTVCO_CP_GAIN was set as 1x or 1.5x ,whatever the EXTVCO_CP_IDN/EXTVCO_CP_IUP(2 to 30) is ,the CPOUT always output near 0.08V. EXTVCO_CP_IDN = EXTVCO_CP_IUP

    b.When the EXTVCO_CP_GAIN was set as 2x ,whatever the EXTVCO_CP_IDN/EXTVCO_CP_IUP(2 to 30) is ,the CPOUT always output near 2.68V. EXTVCO_CP_IDN = EXTVCO_CP_IUP

    c.When the EXTVCO_CP_GAIN was set as 2x ,whatever the EXTVCO_CP_IDN/EXTVCO_CP_IUP(2 to 30) is ,the CPOUT always output near 2.78V. EXTVCO_CP_IDN = EXTVCO_CP_IUP

 

    2.We exchange the RFoutRX port and RFoutTX port in the hardware design.So we set the RFoutRX port to output the TX frequency,but I can't catch the TX frequency in RFoutRX port(pin 16).

 

    I mail my source code to u,the source code have two mode of both internal vco and external vco. In internal vco mode,the LMX2571 can out TX frequency in RFoutRX port(pin 16).  But when enable the macro "EXTVCO" ,it can't work fine in external vco mode.

    Plz help me check whether the register setting is right?

  • 0
    TI__Mastermind 28031 points

    Arking,

    It seems that the charge pump is working as you can change the voltage.  You might also check toggling the EXTVCO_CP_POL bit to make sure you can make it slam to both rails.  Also realize that for external VCO mode, ExtCPout pin is used, not CPout.

    If that's not it, then you might want to make sure that there are no issues at the Fin pin, where the external VCO is fed in.  If the device is counting a harmonic or there is a sensitivity issue, it could make flaky performance. 

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 10 points

    Dear Dean

        When I change to the ExtCPout pin,it works well now.

        Thank you very much!

        And thanks Arking also.

    Best Regards,

    BruceLai