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Instability of PA experienced at 38.0 ... 38.5 GHz when RF output is connected to a diplexer filter

Category: Hardware
Product Number: HMC7229LS6

  Hello,

in our latest RF design we are experiencing instability of HMC7229LS6  when the RF output is connected to a diplexer filter in a FDD microwave Radio Unit for point-to point applications.  Oscillations and instability are more likely to happen when temperature is descrased from +25°C down to  -10°... -20°C  but are visible with different HMC7229LS6 samples in different RF boards.  Idq  current is set to 1200 mA. 

NOTE:  every time we remore the diplexer filter and connect the output of RF module directly to a waveguide adapter, the instability disappears.  

We also observed that the reduction of Idq from 1200mA to 800mA  helps   (even though this action has some side effects, such as degradation in linearity and peak power).

Question:  any idea about the reason for this problem ?   Looking at S-parameters on the datasheet,  the HMC7229LS6  should be unconditionally stable,

                 but in the real case it looks like some out-of-band impedances shown by diplexer filter make the HMC7229LS6 to become unstable.

  Many thanks in advance for your support

   Regards - Sergio Fumagalli

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