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linear oscillator analysis - will it oscillate ?

Discussion in 'Electronic Design' started by cubeagree, Apr 10, 2007.

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  1. cubeagree

    cubeagree Guest


    I am trying to design a 24GHz transistor oscillator (feedback with
    transmission line resonator). I have only s-parameter files from the
    transistor available - so I can do only linear analysis to evaluate
    whether the oscillator will be unstable. I am using ADS for simulation
    - with the modul "osctest" it is possible to evaluate the closed loop
    gain. According to Nyquist criterium the loop gain should be greater
    than 1 when the phase is zero.
    All the examples I have seen are using real transistor models , i.e.
    they are starting with the linear analysis and afterwards a harmonic
    balance simulation to confirm the oscillation.
    My loop gain is only "a little bit" greater than 1, actually 1.006 ! I
    have tried to change feedback characteristics and I have taken other
    transitor s-parameter, but the result is the same = only 1.006 of loop
    What are your experience with the loop gain when using only s-
    parameters ? Does it make sense to build this oscillator on the basis
    of this simulation ?

    Thanks for your feedback !

  2. Wimpie

    Wimpie Guest

    Hi Peter,

    It comes very strange to me that two different transistors (probably
    with different Ft) have same loop gain under same load conditions. I
    think there is something wrong in the result.

    If I really had a circuit with a (power) loop gain of 1.006 in a
    linear simulation, I would not build it. Normal deviation in component
    characteristics would result in a very unreliable circuit.

    When I have to design an oscillator, I also start with the small
    signal model. It doesn't matter in what type of parameter system you
    do the analysis, as long as you know what you are doing. If I only
    have S-parameter data, I would use negative resistance method.

    Did you try to simulate an amplifier based on the S-parameter file? If
    so, were the results as expected (for example did they matched the
    published data)?.

    Did you consider the negative resistance (|input reflection
    coefficient| > 1) method for your circuit?

    My experience with ADS dates back to the first introduction and I
    never used an "osctest" option.

    When you could find package data valid up to 24 GHz, in combination
    with a spice model, you could use a SPICE simulator. Of course, you
    have to convert every structure to transmission line components and or
    C's and L's. If you are designing a common emitter oscillator,
    parasitic emitter induction has large influence on whether or not your
    transistor will oscillate.

    Best Regards,

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