can you explain me your formula please: -174+10*Log10(15e9)+45+1.8 = -25.4dBm.
-174 it is the noise floor ?The
-174 is noise floor dBm/Hz (KTB)
the formula
KTB +10*log10(bandwidth Hz) + Gain dB + Noise figure dB = O/P noise power dBm
To detect a -80dBm signal you will have to include some filtering before the detector as pointed out by Biff44. The input refered noise over the bandwidth of the amplifer is about -70dBm, KTB +10*log10(bandwidth Hz), assuming 15GHz BW.
An input filter may also be needed, not knowing your application I can't say for certain.
The problem is most likely electro-magnetic feed back from input to output. It is a 3 dimensional EM problem and difficult to simulate even with a good EM simulator.
Good screening and absorber should cure it. Be aware that a filter at this frequency will radiate and needs to be screened. A gap of 10mm or less if there is dilectric involved will radiate like an antenna at 15GHz i.e. the sceening will be virtually non-existant.
Place a screen between stage 1 & 2 or stage 3 & 3, ideally screen each stage but that's over kill.
The power distribution looks to me as though it would cause probelms. Appart from the capactors there is no isolation between the stages. the track is just a low impedance transmission line. You need some high impedance in series with each feed line; a thin track should do.
it might seem at first glance to be easy to string a few ampliers together, but as the others can no doubt confirm there is a lot that can go wrong at these frequencies and even those who have many years of experience can have problems. At this frequency every part of the design has to be right or something will come out and bite.