Home > Community > Forums > Custom IC Design > do stb analysis on five stage invter ring osc

Email

* Required Fields

Recipients email * (separate multiple addresses with commas)

Your name *

Your email *

Message *

Contact Us

* Required Fields
First Name *

Last Name *

Email *

Company / Institution *

Comments: *

 do stb analysis on five stage invter ring osc 

Last post Thu, Jun 5 2014 7:17 AM by xianweng. 3 replies.
Started by xianweng 04 Jun 2014 07:37 AM. Topic has 3 replies and 1809 views
Page 1 of 1 (4 items)
Sort Posts:
  • Wed, Jun 4 2014 7:37 AM

    • xianweng
    • Top 500 Contributor
    • Joined on Sun, Jan 1 2012
    • Posts 20
    • Points 280
    do stb analysis on five stage invter ring osc Reply

     

     hi,everyone

           i designed a five stage ring osc,and each stage is made up of an inverter. I perform DC analysis first,and I found output voltage of each stage is at the half of VDD. Then I  do  stb analysis ,founding that when the phase shift is 180,the gain is below the 0dB. Which means that it can not oscsillate.But when I do tran ,it oscillate at 1.3GHz,which is very weir.

        My cadence version is IC615,MMSIM is 13.0

     

         Does anyone could helpe me figure it out?Thanks any help in advance
    Filed under:
    • Post Points: 20
  • Wed, Jun 4 2014 9:07 AM

    • smlogan
    • Top 75 Contributor
    • Joined on Tue, Jun 10 2014
    • Posts 105
    • Points 1,800
    Re: do stb analysis on five stage invter ring osc Reply

     Dear Xianweng,

     

    Your small signal stability simulation does not capture the physical delay of your inverter stages and hence the actual "phase" of the feedback from the last stage of your 5 stage ring to the input of its first stage. The propagation delays are a large signal effect. Hence, the stability is not accurately predicted by the stb analysis. The situation is analogous to predicting the stability of a sampled data frequencny synthesizer (i.e, one that uses a PFD as the phase-frequency detector). The sampled nature of the feedback and its delay is not predicted well when one uses a linear approximation to its transfer function.

     

    Shawn

    • Post Points: 35
  • Wed, Jun 4 2014 9:12 AM

    Re: do stb analysis on five stage inverter ring osc Reply

    Using pstb might be a better option. Of course, to use this on an oscillator, it would have to actually oscillate, but it can be used to give you the time-averaged loop gain over the period and from this give you an idea of the stability of the loop as the oscillator is actually operating.

    Regards,

    Andrew.

    • Post Points: 5
  • Thu, Jun 5 2014 7:17 AM

    • xianweng
    • Top 500 Contributor
    • Joined on Sun, Jan 1 2012
    • Posts 20
    • Points 280
    Re: do stb analysis on five stage invter ring osc Reply

     Thank you for you quick reply ,I understand now

    • Post Points: 5
Page 1 of 1 (4 items)
Sort Posts:
Started by xianweng at 04 Jun 2014 07:37 AM. Topic has 3 replies.