ADS with Oscillator design need help.

Discussion in 'The Projects Forum' started by ericyeoh, Nov 14, 2011.

  1. ericyeoh

    Thread Starter Active Member

    Aug 2, 2009

    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
  2. Audioguru

    New Member

    Dec 20, 2007
    You do not understand "stray capacitance".
    Almost everything has 10pF or more between them but your circuit has only 1pF which is the capacitance of two bees that are 1m apart.