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LMX2594: Active PLL filter Type A/B and PFD polarity

Andrew Gorevoy
Prodigy 50 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2531

In PLLatinumSIM three active integrators are present. Two of them use common inverting opamp architecture. It looks like that but i see that inverting and non-inverting inputs are swapped. How can it work? If there is a mistake in opamp inputs assignment how can it work with positive PFD polarity? It must be negative. In the datasheet i can't find any information about PFD polarity switching.

  • 0
    TI__Expert 5370 points
    dear Andrew,

    thank you for your interest in LMX2594. We will answer your question on Monday.

    Regards,
    Simon.
  • 0
    TI__Mastermind 27661 points
    Andrew,

    The concepts used for PLLatinum Sim active filters are discussed in depth in my PLL book at (www.ti.com/.../pll_book).

    For PLLatinum Sim, indeed there is an error in the type A and type B filters and the + and - signs are swapped. I will fix this and it will be in our next update of PLLatinum Sim to the web.

    Both Type A and Type B filters use a negative phase detector polarity. It also turns out that all of our devices with integrated VCO (LMX2531/41/71/72/81/82/92/94/95) have a negative tuning coefficient VCO and therefore a negative polarity in charge pump. Some of them have an option for a positive polarity charge pump, but the LMX2594 PFD Polarity cannot be switched.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 50 points

    Dean,

    Well, it is logical to use static PFD polarity in PLL with integrated VCO. My application uses non-standard connection with LMX2594 to improve its phase noise performance significantly. For that i need to switch between both positive and negative PFD polarity. Maybe my design will be a bit complex. Here is the phase noise plot at maximum frequency divided by two measured in experimental setup.

    Thank you, Dean.