FET oscillator - HELP!

[simsi]

Hello everybody,

i am trying to build a FET oscillator and i have a strange problem with the bias. I bias the FET independently at gate and at the drain. At some point, i cannot change my gate bias anymore (to a more positive value, say at around -0.6V i cannot go to -0.5 anymore) unless i decreasy my drain bias. What can this mean?

In addition, my oscillator oscillates at the wrong frequency but this frequency does not come from the circuit i built but apparently from the instability in the transistor even if i did not measure any instability when i identified the S-parameters. What can the problem be?

Thank you very much! I am not very experienced with active circuits, so i appreciate every every answer....

simsale

 

[IanP]

It means that you may have problem with FET itself ..
A good idea would be if you posted diagram of your circuit ..
Regards,
IanP

 

[simsi]

Hello,

here's an "idea" of my circuit. The oscillation occurs because the transmission line between the gates transforms the input reactance of one transistor to its negative value. Like this there is a point of virtual ground in the middle of that line and so at the gates and at the drains the oscillations are out of phase. The source feedback causes negative resistance at the gates with a pF-capacitance.
The output filter selects the instable point in the smith chart, and is a short circuit at LO (sounds strange but i don't need the oscillation at the output, just at the gates).
Bias is Vg at -0.6V and Vd at 2V (the device is a NE34018)

Instead of 1.8 GHz it oscillated at around 3.3 GHz and this seems not to come from the circuit because it is in phase and does not even change when i remove the source capacitance... (only slightly)

Thank you very much for your help!