HMC3716 loop filter question

Hi everyone, I have some questions about the loop filter as shown in the page 9 of the HMC3716  datasheet:

- What is the level of NU and ND at locked state? 3V or 5V?

- Since NU and ND can swing between 3 and 5V, so the Vtune would have the same swing?

- My VCO needs Vtune between 2 and 3.5V. How can I realize the loop filter with minimum opamp?

Thank you,

Anh

  • 0
    •  Analog Employees 
    on Sep 7, 2016 8:50 PM

    Hi Anh,

    Great question. When the PFD is locked NU and ND will be at 5.0Vdc and very narrow pulses will be present as it continues to adjust Vtune ever so slightly to retain phase lock.

    Not sure what you mean by a minimum op amp. Please refer to the applications circuit in the HMC3716 datasheet. Depending on which ADI op amp that you end up using and the available headroom, you should be able to set V+ to 5.0Vdc. Your "Vtune between 2 and 3.5V" should include additional Vtune margin so that process and temperature variation are included.

    Best Regards,

    Marty

  • Dear Marty,

    Many thanks for your input. The 2-3.5V range was actually my mistake, Vtune should be lower than 6V to protect the VCO varactor so 5Vdc is perfectly fine. Can you help to clarify other questions:

    - What are the pros and cons of having PFD chip (HMC3716) + active loop filter, compared to having PFD + chage pump chip (like HMC703) + loop filter (also active)? 

    - My PLL should have very wide bandwidth (say 50 MHz with 500 MHz reference and VCO). The loop filter can be synthesized in Hittite software, but have you ever tried HMC3716 for such wide BW? Is there any limit you observed? And have you some recommendations for choosing the op amp?

    - The ND and NU waveforms shown on page 6 of HMC3716 datasheet look strange to me. Why the NU have triangular shape and not a pulse shape? And why is there pulses on LD, even though the loop is not locked?

    Thank you a lot for your time,

    BR Anh

  • 0
    •  Analog Employees 
    on Sep 15, 2016 4:27 PM

    Hi Anh,

    The advantage of the HMC3716 is the capability to use a high PFD frequency (up to 1300MHz) to lower 'N' which improves the in-band phase noise by 20*LOG(N). The PFD parts that output voltage pulses (fixed 2V swing) are simpler devices from a circuit perspective so perhaps there's a reliability advantage but I can't say I've seen this. The charge pump driven parts generally allow Icp to be varied which allows the user to adjust the loop BW as needed to manage spurs, settling time, reduce component size, etc.

    Best Regards,

    Marty

  • Hi Anh

    What op amp did you use for that high loop bandwidth (50MHz)?

    John

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:34 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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