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LMX2430: About LMX2430 typical Application

WU YISYUAN
Expert 2165 points
Part Number: LMX2430
Other Parts Discussed in Thread: LM6211,

Hi,

    I have one question about typical application.

   PIN13 connect LM6211 for voltage amplification, if remove LM6211 connect to VCO circuit direct, is there any problem?

   why use LM6211?

   btw, if I don't use VCO(U2), is there any recommended circuit??

thanks.

  • 0
    TI__Mastermind 27831 points

    Wu,

    The op-amp is on the LMX2430 EVM because the charge pump supply is only 2.75V max, and if you stay about 0.5V from the rail, this is 2.25V max.  Most VCOs like higher tuning voltage.  So you can bypass the op-amp, but the frequency range of the VCO will be reduced.

    If you do bypass the op-amp, don't forget to program the phase polarity bit on the LMX2430 from negative to positive.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Expert 2165 points
    Hi Dean,
    Many thanks for your feedback. I have another questions.
    If use 2.75V, frequency range of the VCO will be reduced. What range of this frequency?
    And if Idon't use component U2, is there a recommended circuit?

    thanks.
  • 0 in reply to WU YISYUAN
    TI__Mastermind 27831 points
    Wu,
    I would expect that you could tune the VCO all the way to the lower end of the frequency, but it would only go about half way towards the max frequency.
    This is really more of a question about the VCO than the LMX2430.

    For U2, this is a standard footprint that accomodates many different VCOs. However, you can also try using a discrete VCO circuit using transistors. In my book "PLL Performance, Simulation, and Design, 5th", available for free download at ti.com, there is a chapter on VCOs and it has an example circuit, although it is an older design.

    Regards,
    Dean