ADF4371 with HMC1031/ADF4002, 20dB Phase Noise hit at 100Hz Foffset

Hi ADI,

I have an eval for the ADF4371 and I'm using an HMC1031 eval to lock the 100MHz ref to a precision 10MHz OCXO.  I am generating an 18GHz LO.  As seen here:

The problem here is measured PN at 100Hz offset.  With the on-board crystal reference PN @ 100Hz = -87dBc/Hz.  With HMC1031 PN @ 100Hz = -67dBc/Hz.

Notable design parameters:

- HMC1031 Loop BW set to 10Hz as outlined in the spec sheet page 12.  www.analog.com/.../hmc1031.pdf

- 100MHz VCXO, CVHD-950, CMOS output

- The measure slew rate from the HMC1031/CVHD-950 better than on board 100MHz reference.

Thanks in advance!

Chris



Added other parts to title
[edited by: aandrews at 4:14 PM (GMT -4) on 24 Sep 2021]
Parents
  • +1
    •  Analog Employees 
    on Oct 23, 2020 12:06 PM

    Hi Chris,

    The Flicker FOM of HMC1031 is -252dBc/Hz. This gives -112dBc/Hz @ 100Hz offset @ 100MHz output.
    PNFLICK = Flicker FOM + 20log(fVCXO) − 10log(fOFFSET)

    If this is transferred to 18GHz;
    -112 -20log(18G/100M) = -67dBc/Hz.

    If I did not make any mistake in calculations, math says that your measurement is very accurate. The contributions from other sources (OCXO, VCXO, PLL floor FOM, loop filter) are not significant.

    When using CVHD in open loop, I would also expect to see something close to -87dBc/Hz. But it is possible that the instrument cannot track the signal as you advised. Perhaps you can try to force Vtune to a mid-level instead of connecting it to GND. Min and max levels might be problematic.

    Regards,
    Kazim

Reply
  • +1
    •  Analog Employees 
    on Oct 23, 2020 12:06 PM

    Hi Chris,

    The Flicker FOM of HMC1031 is -252dBc/Hz. This gives -112dBc/Hz @ 100Hz offset @ 100MHz output.
    PNFLICK = Flicker FOM + 20log(fVCXO) − 10log(fOFFSET)

    If this is transferred to 18GHz;
    -112 -20log(18G/100M) = -67dBc/Hz.

    If I did not make any mistake in calculations, math says that your measurement is very accurate. The contributions from other sources (OCXO, VCXO, PLL floor FOM, loop filter) are not significant.

    When using CVHD in open loop, I would also expect to see something close to -87dBc/Hz. But it is possible that the instrument cannot track the signal as you advised. Perhaps you can try to force Vtune to a mid-level instead of connecting it to GND. Min and max levels might be problematic.

    Regards,
    Kazim

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