HMC703 circuit


I am using HMC703 in a design and I have two questions:

Q1: In the evaluation board schematics, there are several 1 ohm resistors on power supply pins between decoupling capacitors. Are these resistors really necessary? (I have already ferrite beads between HMC860 LDO and HMC703)

Q2: There are four 4.7uF capacitors around THS4031CD (loop filter Opamp) in parallel with 100nF capacitors. Isn't that too big? Isn't it better to put 10pF parallel with 100nF?



  • 0
    •  Analog Employees 
    on Jun 2, 2016 4:55 PM over 4 years ago

    A ferrite bead will probably work fine.  Just consider the  eval board power decoupling has been verified many times to meet the datasheet performance specifications.  Changing these circuits introduces a degree of uncertainty especially if spur levels are a concern.

    Use 4.7uF capacitors as shown in the eval schematic for filtering the noise on the non-inverting node of the THS4031 opamp.  Any noise on this input is passed directly through the loop filter which will show up as an increase in output phase noise.  You can simulate this with either ADISimPLL or HittPLL.