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ADF4002 Phase Detector

Hi! I'm trying to use the ADF4002 chip strictly as a phase detector and expected the output of the CP to be linear across the range from 0-359°. Instead, the voltage output had two separate linear ranges of vastly different slopes (180° to ~230° and ~230° to 179°, wrapping around to 0° after 359°). An example of this can be seen below when the PFD was tested with a negative phase polarity. The voltage output of the loop filter starts at 5V at 180° and had a large drop before smoothing into a much smaller drop as it wraps around towards 179°. This is in contrast to what I expected of it peaking at 180° and linearly scaling downwards as it wraps around at 360° and approaches 179°.

I have tried various setups, but the two whose outputs resulted in the figure above are described below. All testing was done using the EVAL-ADF4002 board and the corresponding software provided with the board.


For all tests, the reference and RF frequencies were set to 100MHz and the phase of the of the reference was stable, while the RF signal's phase was varied.
For Icp=625 uA:

  • R=1
  • N=1
  • Charge Pump Setting 1: 0.625mA
  • Charge Pump Setting 2: 0.625mA
  • Charge Pump Gain: 0
  • Charge Pump to Tri-state: disabled
  • FastLock: disabled
  • Timeout:3
  • Phase Detector Polarity: negative
  • Counter Reset: disabled
  • Lock Detect Precision: 5 cycles
  • Power down: Normal Operations
  • ABPW: 6.0 ns
  • Loop filter values:
    • C1=1.3 pF
    • C2=6.3 pF
    • R1=6.1 kohm

For Icp=5mA:

  • R=1
  • N=1
  • Charge Pump Setting 1: 5mA
  • Charge Pump Setting 2: 5mA
  • Charge Pump Gain: 0
  • Charge Pump to Tri-state: disabled
  • FastLock: disabled
  • Timeout:3
  • Phase Detector Polarity: negative
  • Counter Reset: disabled
  • Lock Detect Precision: 5 cycles
  • Power down: Normal Operations
  • ABPW: 6.0 ns
  • Loop filter values:
    • C1=10.5 pF
    • C2=51 pF
    • R1=750 ohm

Am I mistaken in thinking that the result should be linearly scaling for the full range of phase differences applied? If not, is there some setting/filter value that I should change to adjust the output to be more linear?

  • Hi,

    Firstly, the ADF4002 was not characterised for this use case. It was intended to be used in closed loop synthesizer applications, so operation in this manner would not be supported.

    I think what you are observing is partially due to the fact the ADF4002 does not use a phase detector, it uses a phase frequency detector with a charge pump output. I have also heard complaints when trying to use other standalone PFDs (like HMC439) as a simple phase detector.

    How are you measuring the charge pump output? 

    I wondering if what you are observing is in anyway related to the PFD deadzone. Did you experiment with other ABPW settings?

    Rather than one of our PLL PFDs used standalone, we would recommend using a mixer for phase comparison application like this. What about the LTC5510 or LTC5562? In particular see page 23 of LTC5562 datasheet for an example.