I'm working with the ADF4351+ADL5801 project, in which the ADF4351 provides a 1.7 GHz LO for the mixer ADL5801. I referred to the Circuit Note CN-0239 which is attached with the message. In this circuit note, the two bias inductors L2 and L3 on the evaluation board are replaced with two-50 Ohm pull-up resistors. So, it presents a nominal differential impedance of 100 Ohm, but the differential input impedance of the LO port  of the ADL5801 is 50 Ohm. Why not use two-25 Ohm resistors to replace L2 and L3 to get a 50 Ohm differential impedance?

Besides, in Table. 2 of this circute note, T2/T4/T7 on the ADL5801-EVALZ is repalced with two-0 Ohm resistors, so what's the impedance of the LO port of ADL5801?


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    •  Analog Employees 
    on Dec 6, 2013 1:43 AM

    The input impedance of the LO port of the ADL5801 is 50-ohms differential. T2/T4/T7 was populated with 0-ohm resistors to utilize the differential output of the ADF4351.

    Based on experimental results, we determined that 50-ohm pull up resistors provided a better broadband match compared to other resistive matching configurations. Please refer to the 'Output Matching' section for ADF4351 datasheet for more information about matching the output port of the ADF4351. As indicated in the circuit note, the mismatch on the LO path will not degrade performance of the mixer. We recommend keeping the length of the traces connecting the mixer and the PLL to be short as possible.