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Why am I unable to repeat the phase alignment between RFin and REFin using the ADF4106?

I an using "Analog Devices Int-N PLL Software" with the USB interface to control an ADF4106.  Is it possible to achieve the same phase relationship between the RFin and REFin signals for a given set of device settings after power-cycling the ADF4106 and VCO?

I have tried the "Initialization Latch Method" and the "Counter Reset Method" as described in the ADF4106 data sheet.  It seems that for frequency settings where RFin is not a multiple of REFin the resulting phase relationship is always different after power-up.  I was expecting that the internal reset pulse or the counter reset operation would ensure that a consistent phase relationship would result.  It does not.  In fact, any counter reset that is executed results in a different phase relationship for the cases where RFin is not a multiple of REFin.  Am I missing something?

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  • The issue is that you're disabled counter reset at different times relative to the reference signal. To do what you want, the register write that disables counter reset must be synchronized with the reference signal; specifically, the rising-edge of LE on the register write must be synchronized with the reference signal.

    This isn't possible on the evaluation board. Two solutions:

    1. Put your LE signal and reference signal through a flip-flop to synchronize the two.
    2. Run your reference signal through your microcontroller/FPGA/DSP (the same one that's writing the SPI signals) and synchronize them that way.
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  • The issue is that you're disabled counter reset at different times relative to the reference signal. To do what you want, the register write that disables counter reset must be synchronized with the reference signal; specifically, the rising-edge of LE on the register write must be synchronized with the reference signal.

    This isn't possible on the evaluation board. Two solutions:

    1. Put your LE signal and reference signal through a flip-flop to synchronize the two.
    2. Run your reference signal through your microcontroller/FPGA/DSP (the same one that's writing the SPI signals) and synchronize them that way.
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