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Loop filter for HMC3716/HMC394

Category: Hardware
Product Number: HMC3716

Hi  , et.al,

I have follow up questions to the post at Unable to get HMC440 working - Q&A - RF and Microwave - EngineerZone (analog.com).
Specifically concerning what to expect as the output of the loop filter placed after the PFD HMC3716.

I refer to the datasheet of the PFD, that shows what the typical NU/ND signals will look like. Attached below:

I deduce that NU is 2Vp-p with 2V offset sawtooth signal and ND is 5V DC signal. And in my test using the copy of the evaluation board of the HMC3716 that I procured, I could see these signals on my oscilloscope.

But I am not clear on the implementation of loop filter and its output that is meant to drive the VCO. The VCO of choice here is the HMC6380 requiring Vtune up to 23V. And the schematic of the loop filter follows from the ADISimPLL as follows:

I developed the schematic of the loop filter on a custom PCB, and 'built' up the PLL/VCO. Measured outputs on the spectrum analyser is far from what I'd expect from a properly functional/locked PLL. I ascertain that all subparts are working as expected, but not so confident that the loop filter is.

The schematic of the loop filter transferred to the layout is shown:

Measured output on the spectrum analyzer for the developed PLL/VCO looks as follows:

Screenshot of the spectrum analyzer above suggests a constantly swept VCO across a wide range of Vtune.

I therefore isolated the loop filter for testing, and created the test signals from an AWG following NU/ND as will be obtainable from the PFD: NU is sawtooth signal of 1kHz at 2Vp-p and +2V DC offset; and ND is +5VDC. The green trace in the image below is the NU signal:

In following image below, the output of the loop filter is the yellow trace from an input NU signal which is the green trace:

Can you tell if this is the normally expected output of the loop filter? I would have expected DC type of signals that will drive the VCO and not this rail-to-rail type I recorded on my scope. Did I made some errors in implementing the loop filter? 

Kindly advise on how to proceed.

regards!

Parents
  • Hi Obabrin,

    Your active loop-filter should output a DC voltage to drive your VCO. From your last screenshot, the yellow VTUNE voltage (Loop-filter output) does not tally with what is expected, although the Green waveform is correct. Also, it looks like you are operating out of range for you Op-AMP. I saw at room temperature +/-15V, is there a reason for this?

    Some recommendations when designing your loop filter:
    1. Ensure your loop bandwidth is less than 1/10th of your PFD frequency
    2. Also ensure that your gain-bandwidth of 10x greater than the loop bandwidth

    On your V- of your Op amp can be grounded to 0v while your V+ is kept at 15v room temperature. Make sure your VCO can handle your VTUNE range.

    Regards

  • Hi  ,

    I could not find your reply for the notification I received. I hope you have resolved this now.

    Regards,

    Jude

  • Hi  ,


    The reply was deleted as I am now being stressed by this minor circuit. Which is why you did not see it. I do not have an explanation for why the loop filter behaves as reported.

    I thought I had blown the op-amp circuit I was using in my setup, thinking I provided it with excess Vs but a fresh copy of the loop filter PCB that I made performs exactly the same way. And on this fresh copy, I ensured that the +Vs is +15V and -VS is GND.

    I was also contemplating on making new loop filter PCBs, but that may not be reasonable as I have not figured out what is wrong and what corrections I am trying to implement.

    Are there any guidelines on implementing the loop filter on a PCB? I was using the FR4 substrate, and I ensured that my traces are 50-Ohm. Any other suggestions? Also, is/are there any checks to perform on a loop filter to check its functionality?

  • Hi ,

    I would assume that you had simulated your configuration, which is usually the first suggestion we give, to make sure you have the right components for setup. 

    Are there any guidelines on implementing the loop filter on a PCB?

    The few guidelines when designing your loop filter PCB are:
    1. Try to stick to high quality capacitors like C0G/NP0 capacitors to maintain performance.
    2. You should only use low noise power supply to your loop filter.
    3. The capacitors connected directly to your NU/ND output and to your VTUNE signals should be as close as possible to minimize trace length, reduce noise pickups, and prevent degradation of your signals.

    4. For grounding, insert vias in ground pads of shunt caps so you are optimizing your ground connections.

    5. Ensure you have proper thermal dissipation for your active components, as high temperatures can lead to drift in the values of your components. 

Reply
  • Hi ,

    I would assume that you had simulated your configuration, which is usually the first suggestion we give, to make sure you have the right components for setup. 

    Are there any guidelines on implementing the loop filter on a PCB?

    The few guidelines when designing your loop filter PCB are:
    1. Try to stick to high quality capacitors like C0G/NP0 capacitors to maintain performance.
    2. You should only use low noise power supply to your loop filter.
    3. The capacitors connected directly to your NU/ND output and to your VTUNE signals should be as close as possible to minimize trace length, reduce noise pickups, and prevent degradation of your signals.

    4. For grounding, insert vias in ground pads of shunt caps so you are optimizing your ground connections.

    5. Ensure you have proper thermal dissipation for your active components, as high temperatures can lead to drift in the values of your components. 

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