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ADF4360-8 driving AD9268 problem

We use an ADF4360-8 at 125MHz with a successive AD9513 to source 2 AD9268 125MHz/16bit ADCs and a FPGA.

The Lext is 180nH, the reference frequency 24MHz, PDF=1MHz, loop filter 3rd order passive, 105kHz and 44.4deg phase margin.

Almost the recommended values of ADIsimPLL.

The ADC input is driven by an ADA4938-2 differential amplifier. Measuring a grounded input signal we sometimes get the expected good noise data with some 9.5 LSBs FWHM distribution. But then, we as often get additional 282kHz (and harmonics) that distroy the good ADC data.

Fig 1: GND noise and its FFT when good

Fig 2: GND noise and its FFT when bad

We found that this seems to come from the PLL circuitry as a finger on the 2 inductors can fairly influence and improve this behaviour.

Any ideas where this really comes from and how to avoid this?

The inductors are at a 90° angle as recommended.

  • Hello Michael,

    Can you take a spectral plot of the RF output of the ADF4360-8...1) narrow band , and 2) wide band to show what spurs are present .



  • Hi Brigid,

    thank you for the quick response.

    Sorry, I do not have a spectrum analyzer at hand. Only a 200MHz scope with FFT. But then, the frequency resolution around 125MHz is not good enough.

    I only can provide ADC measurements with the onboard AD9268 ADC like I have posted.

    I've got a feeling as if the two 180nH inductors are the most sensitive points. While I have placed them at a 90° angle can still mutual coupling occur? And, could this lead to such a behaviour?



  • Hi Michael,

    Can you send me your schematic and register writes to the ADF4360-8 and I can simulate it and check the loop stability.

    Yes, the external inductors form part of the VCO tank circuit, placing them at right angles as close as possible to the pins should give you expected performance, I will take a look at your circuit and get back to you, regards,


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