Oscillator using ADS need help.

[ericyeoh]

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

 

[BradtheRad]

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.