REF_IN 50ohm resistor position in ADRF6650


I am trying to use ADRF6650 internal LO downconverter.

For the REF_IN(pin number 45), I put 10[MHz]

On the datasheet, I need to put 50[ohm] resister and AC couple capacitor close to the pin.

However, I put those resistor and capacitor with some distance, 5cm?, unfortunately.

I have trouble with the lock of PLL, this resistor and capacitor position can be the reason of trouble?

Any answer will be greatly appreciated.

Sangkyun Kim

  • 0
    •  Analog Employees 
    on Aug 26, 2021 8:01 PM

    HI Kim,

    Sorry to hear that you are having trouble with locking the internal PLL.

    For PLL lock condition, 50 Ω to ground is  required to be close to REF_IN pin.

    The REF_IN pin is internally biased and must be ac-coupled and terminated externally with a 50 Ω resistor. Place the ac coupling capacitor between the pin and the resistor.

    What ac-coupling value did you use on the EVB?.. Are the cap and 50ohm resistor away near the launch or SMA connector and are both the 50ohm resistor and ac coupling cap close to each other and away from from the REF_IN pin by 5cms.. that's  quite far away especially for the PLL lock condition, 50 ohms to ground is required to be close to REF_IN pin.

    As mentioned in the DS, The ADRF6650 incorporates a very low noise PLL/VCO, and care must be taken when designing the PCB routing around the PLL/VCO pins. It is required to place the decoupling capacitors for the supply pins as close as possible. If 0402 capacitors are
    used, placing all of the decoupling capacitors close to the pin becomes problematic. In such a case, place the smaller value decoupling capacitor as close as possible to the pin. It is a good practice to keep the first capacitor of the loop filter close to the
    CPOUT pin, and the last capacitor close to the VTUNE pin, as shown in Figure 72.

    Please confirm that you have followed these instructions.

    Can you please describe the trouble you are having with the PLL lock, any details would be helpful?

    I am assuming the EVAL board you are referring to is not ADI EVB.. right?.. Its your own EVAL board designed for the application.

    Warm Regards,


  • Hi Rachna

    Thank you for your kind answer and question.

    1. I used ac-coupling value using Evaluation board design of ADI EVB. 50ohm resistor and 100pF capacitor between REF_IN pin and the resistor.

    2. Unfortunately I put the ac-coupling capacitor and the 50ohm resistor near SMA connector. The SMA connector has 5cm distance from ADRF6650. Can you give me any advice abut this? If I have to do, I will redesign my PCB, but want to have advice as much as possible.

    3. I put bypass capacitors near the pins as much as possible. But, I will check my pattern design again.

    4. I have one more question. What is usual voltage signal for REF_IN? When I put 3.3[V] 10[MHz] signal(it means 0V to 3.3V clock), REF_IN has 1.3Vp-p ac signal(It means -0.65V to 0.65V ac signal) by ac-coupling capacitor. I used 3.3[V] clock signal, just DS said -0.3[V] is the minimum absolute potential for REF_IN. If it is OK, I am thinking to put 3.3Vp-p 10[MHz] clock to SMA connector.

    5. I want to make 2.3[GHz] LO using 10[MHz] REF_IN. If you give me any samples for register setting, it will be great help for me.

    Thank you for your help again.

    Best Regards

    Sangkyun Kim

  • 0
    •  Analog Employees 
    on Aug 31, 2021 2:06 PM in reply to skkim

    HI Kim,

    It is recommended to use a PFD of 30.72 but there is no Min limit on  PFD.

    From the absolute MAX rating table on the DS the minimum is -0.3V. So you want to make sure the REFIN signal meets these requirements.

    Is the 1.3V p-p  measured near the DUT  or right after the 100pf cap?

    Warm Regards,


  • 0
    •  Analog Employees 
    on Sep 14, 2021 1:11 PM in reply to

    Please let me know if you have any further question on this.


  • Hi Rachana

    Thank you for your reply.

    I am working on new board design to place 50ohm resistor near to REF_IN pin as mentioned on datasheet.

    I will send message or question after I test the new board.

    By the way, as I said before, evaluation board of ADRF6650-EVALZ is not recognized on my PC.

    Do you know any advice about this?

    I am thinking driver is not matched with Windows10, because datasheet of evaluation board has no mention of Windows10. It say "PC running Windows XP or Windows 7".

    Any advice will be great help for me.


    Sangkyun Kim