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LMX2430/LMX2433/LMX2470 , VCO tuning voltage range (VCPout range)

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: LMX2430, LMX2470

Hello,

Could you please let me know the VCO tuning voltage range(VCPout range)  of LMX2430/2433/2470?
We would like to these device with frequency range of 1636.2～1671.6MHz with 300kHz step. Ref in is 19.2MHz.

Best Regards,
Ryuji Asaka

  • 0
    TI__Mastermind 27581 points

    Ryuji,

    The simple answer is thatl, you can get the charge pump within 0.5 V of the supply rails.  So if you are at 2.75 V, then this means that you have a usable tuning range of 0.5 to 2.25 V.

    In more detail, if you go outside of this range to a voltage of 0.3 V or 2.5V, the consequence is that the charge pump will work, but the mismatch will be worse, causing a longer on time that leads to higher spurs and phase noise.

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Guru 20090 points
    Hell Dean san,

    Thank you. I understood that the range for LMX2430/2433.
    Can I use the range of 0.5V to Vcc-0.5V for LMX2470 ?

    I checked the data sheet in reference to your reply.
    Then, I think that the tuning range is 0.4V to Vcc-0.4V for LMX2470.
    Is my understanding correct?

    Best Regards,
    Ryuji Asaka
  • 0 in reply to Ryuji
    TI__Mastermind 27581 points
    Ryuji,
    The full tuning range is 0V to Vcc, however if you get too close to 0V or Vcc, spurs and phase noise suffer.
    If you use a narrower range closer to Vcc/2, then you get better performance.
    For the LMX2430, it was the opinion of the person who wrote the datasheet that you could go to 0.5 V within a rail. On the LMX2470 we test to 0.4 V within the rail and the performance is not too bad. If you look a the charge pump curves, you will see a "knee" and going past this is when the performance starts to suffer.
    Because 0.5 to Vcc-0.5 is contained within the range 0.4 to Vcc-0.4, you can certainly use this for the LMX2470.
    These devices have a lower charge pump voltage, so sometimes people like to use an op amp and center at Vcc/2 for optimal perfomance and to get a higher tuning voltaeg. If
    Regards.Dean