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LC Oscillators startup

MTP3

Hi all!

I a trying to simulate a cross coupled LC oscíllator using the components from the pdk provider (ST microelectronics) in my case. Ths issue I am facing is kind of confusing me that is the oscillator does not start oscillating if I attach a series resistor with the caps(the resistor is to model the loses in the caps, which themselves are not ideal in this case). Even if I use a 1 Ohm series resitor the oscillator would not start oscillating.

 So far I have tried the following to get around the issue without success

1) Using the different errprest as suggested by Mr. Andrew Beckett does not help.

2) I have also tried forcing initial conditions on the circuit like tying the complementary nodes to vss and vdd during initial conditions but still no.

3) Adding noise contribution from mos devices and turing on the transient noise. 

4) Injecting an ideal sine-wave of the same frequency but smaller amplitude into the osillating nodes (complementary ofcourse) as suggested by Ken Kundert in one of documents but no help.

5) Ramping up the supplies isn't helpful either.

6) Setting the Tmax to 25 times the oscillation period as suggested in some documents doesn't help either.  

Can anyone please suggest some alternate ways or a work around this issue. The inductor has a Q of 13 where as the capaciotrs have a Q of around 80, So I guess adding a 1 ohm series resistance should be that disastrous.

I am using IC6.1.5-64b.72 with MMSIM10.1. 

 
Parents

  • Hi MTP3,

    If your oscillator isn’t even starting up…then that suggests one of two things.

    • There is a problem with the design
    • The oscillator hasn’t been properly started

    You may want to check The Designer’s Guide to Spice and Spectre by Ken Kundert. There is a good section on starting oscillators.

    Also, please take a look at this document: Virtuoso Spectre Circuit Simulator RF Analysis Theory -- Oscillators and Autonomous PSS Analysis

    http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs;q=/spectreRFTheory/spectreRFTheory11.1/chap3.html

    “When applying PSS analysis to oscillators, you need to start the oscillator, just as you would if you were simulating the turn-on transient of the oscillator using transient analysis. The Designer's Guide to Spice and Spectre [kundert95] describes in some depth techniques for starting oscillators.

    In summary, there are two techniques for starting oscillators, using initial conditions, or using a brief impulsive stimulus. Initial conditions would be provided for the components of the oscillator's resonator. If an impulsive stimulus is used, it should be applied so as to couple strongly into the oscillatory mode of the circuit, and poorly into any other long-lasting modes, such as those associated with bias circuitry.

    Either way, after the trigger is applied to start the oscillator, it is important to allow the oscillator to run for a while before the shooting methods are applied to compute the steady-state result. To do so, specify an additional stabilization interval using the tstab parameter. In practice, an additional stabilization interval often improves convergence, especially when simulating high-Q oscillators.”

    (Note: For your particular circuit, I recommend Harmonic Balance rather than the Shooting Newton engine.)

    Then it goes on to discuss convergence issues and methods to start your oscillator. There are some really important points here. Rather than copy verbatim to the Forum, please read that chapter, specifically the section on Convergence Issues with Autonomous PSS Analysis. It goes through different settings that will assist in getting the oscillator to start.

    Another item to look at:

    Virtuoso Spectre Circuit Simulator and Accelerated Parallel Simulator RF Analysis User Guide -- Frequency Domain Analyses: Harmonic Balance

    http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs;q=/spectreRF/spectreRF11.1/Chap3.html

    best regards,

    Tawna

Reply

  • Hi MTP3,

    If your oscillator isn’t even starting up…then that suggests one of two things.

    • There is a problem with the design
    • The oscillator hasn’t been properly started

    You may want to check The Designer’s Guide to Spice and Spectre by Ken Kundert. There is a good section on starting oscillators.

    Also, please take a look at this document: Virtuoso Spectre Circuit Simulator RF Analysis Theory -- Oscillators and Autonomous PSS Analysis

    http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs;q=/spectreRFTheory/spectreRFTheory11.1/chap3.html

    “When applying PSS analysis to oscillators, you need to start the oscillator, just as you would if you were simulating the turn-on transient of the oscillator using transient analysis. The Designer's Guide to Spice and Spectre [kundert95] describes in some depth techniques for starting oscillators.

    In summary, there are two techniques for starting oscillators, using initial conditions, or using a brief impulsive stimulus. Initial conditions would be provided for the components of the oscillator's resonator. If an impulsive stimulus is used, it should be applied so as to couple strongly into the oscillatory mode of the circuit, and poorly into any other long-lasting modes, such as those associated with bias circuitry.

    Either way, after the trigger is applied to start the oscillator, it is important to allow the oscillator to run for a while before the shooting methods are applied to compute the steady-state result. To do so, specify an additional stabilization interval using the tstab parameter. In practice, an additional stabilization interval often improves convergence, especially when simulating high-Q oscillators.”

    (Note: For your particular circuit, I recommend Harmonic Balance rather than the Shooting Newton engine.)

    Then it goes on to discuss convergence issues and methods to start your oscillator. There are some really important points here. Rather than copy verbatim to the Forum, please read that chapter, specifically the section on Convergence Issues with Autonomous PSS Analysis. It goes through different settings that will assist in getting the oscillator to start.

    Another item to look at:

    Virtuoso Spectre Circuit Simulator and Accelerated Parallel Simulator RF Analysis User Guide -- Frequency Domain Analyses: Harmonic Balance

    http://support.cadence.com/wps/mypoc/cos?uri=deeplinkmin:DocumentViewer;src=pubs;q=/spectreRF/spectreRF11.1/Chap3.html

    best regards,

    Tawna

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