HMC635LC4, I am using two of these amplifiers cascaded.

I currently have two setups with the same components and in one of the setups I am experiencing some type of oscillation since these amplifiers do not deliver the typical gain. The only difference between the setups is the layout, distance between components. I was wondering what could be causing oscillations, but also if you guys have a spise type model for the HMC635LC4 so that additional simulation can be done to prevent oscillation in the future.

My setup is using a doubler XX1000QT, HMC899LP4E filter, then two HMC635LC4 with a KAT+6 attenuator in between. It is a little strange that in one of the setups the amplifiers work well over the entire frequency band, but in the second setup the amplifiers oscillate (don't deliver the appropriate gain) from 33.5GHz to 37GHz.  We have tried to compare the differences between the setups, but the most obvious is the distance between the components.  We don't have additional DC capacitors around HMC635 since it is already built in.  Could it be some type of DC leakage into the amplifier?

  • 0
    •  Analog Employees 
    on Oct 26, 2021 12:52 AM


    Moving this thread to its proper community.


  • +1
    •  Analog Employees 
    on Oct 27, 2021 6:42 PM

    Hi Kavi,

    Unfortunately there is not enough information here to determine what your problem is. Having a lower gain response than expected does not necessarily mean the amp is oscillating. Do you have any small signal plots/data to share as well as bias conditions, additional set-up specifics, a schematic, etc.? This is something that most likely would require trouble shooting in a lab setting but I'll try to help out if I can.



  • Hi DanO,

    Thank you for your reply. We have investigated the layout more carefully between the two PCBs and we found out that the setup that is working the pads around the input and output are connected to coplanar ground, while pads on the setup that is not working are only connected to the ground pad under the component. Under the component we have vias going through all layers while complanar vias are between the two top layers (microwave layer)s.  This creates an inductance... not sure why the layouts have this discrepancy.  We are in the process of checking if this can be the cause that the amplifiers are not working.

    Thanks for your help,