Oscillator using ADS need help.

Guys,

I'm facing problem in the oscillator design by using the ADS. I'm trying to design an fix frequency oscillator (colpitts) working at 90Mhz.
In in the initial stage , I set my biasing current at 10m and Vc at 5V for the transistor 2n3904. Then i try to make the amplifier to perform at unstable region which having S(1,1) >1 and S(2,2) >1 (thats the requirement for oscillator design) by selecting suitable load resistance. Pls look at attachments "S-result".

So the Z1 now is having negative resistance = -8.638 + 0.551j (normalised) . So i use thumb rule method where |R1|/3 = Rs And Xsj = - X1j.
Zs = 143.9 - 27.5j.
)
The problem i facing is when i measured at output, the oscillator was not sustain oscillating. (result on attachments)

Can anyone tell me which part i was wrong? Pls help me

Assuming your circuit is set up correctly...

Oscillators are finicky beasts to get going. The transistor bias has to receive a very narrow volt/current range in order to sustain oscillations.

The bias is determined by R7 in your schematic. I suggest you try various values. The idea is to push it just barely to the threshold where it begins operating. After that, the bias is influenced by swings in voltage/current, coming from your control network.

I don't know what formula to use to predict what value will work. It will depend on what action is taking place at each leg of the transistor.

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