but it should be stable
Hi volker@muehlhaus, thank you for your reply. I checked again the SAW and it says in the data sheet that the SAW is matched to 50 Ohm at both input and output.
On the other hand, I don't agree with SLK001 that 20dB must be instable.
I said your matching circuit is not correct therefore you should review this circuit then you may run forward step by step.Yes the matching is not good, but it should be stable and there is no reason such kind of harmonics or oscillating should happen in my understanding.
Your LNA is oscillating due to the S21 being too high. There is NO WAY that you will be able to achieve a stable single device design with a gain greater than 90. This is a typical rookie mistake, thinking that if 10dB of gain is good, 20db must be better and a simulation will happily show you that the design is possible. However, reality often raises its ugly head and slaps the rookie upside the head. Any RF engineer can design an LNA with 10dB of gain. An expert RF engineer can design an LNA with 12.5 - 13dB of gain. No one can design a stable LNA with 20dB of gain.
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