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Harmonic Balance -LC Oscillator

dtzlou

Hi all,

I am facing a weird issue. When I am simulating a LC Oscillator in HB, the Estimated oscillating frequency from Tstab Tran differs around 1GHz from the calculated fundamental frequency. The oscillator aims to oscillate in 11.5Ghz.

When I am saying "Estimated oscillating frequency from Tstab", I am referring to the frequency that SpectreRF simulator calculates after the simulation is done. It's written in the output log file.

I have followed all the recommended settings that MMSIM 14.1 Spectre RF document proposes, such as:

A] Transient-Aided Options: 1) Run Transient -->either YES  or Decide automatically

                                                   2) Detect Steady State -->YES

                     3) tstab= 30n

B] Fundamental Freq =12G

    Number of Harmonics =12

    Over-sample Factor = 12

C] Accuracy: conservative

D] Options button:

1)Integration method tstab: I tried both traponly and trap. I am getting the same freq. deviation

2)maxstep=1psec,350f --> I am getting the same freq. deviation

The testbench is consisted  of:  1) the extracted view of the capacitor bank and the GM circuit

                                                         2) N-Port S -parameter file for the inductor and the tracks (extracted from EMX tool)

From my previous experience, the estimated osc. frequency from tstab tran is a bit close to the fundamental freq. 

My questions are: 1) Could anybody tell me what is the reason behind this big deviation? Did anyone has experienced similar HB/tstab behavior?

   2) Is it safe as a verification check to run the same testbench in Transient analysis and set the stop time around 200nsec and then measure the frequency using the "freq" function from the calculator    menu?

Thanks in advance

  • Without seeing the data and the circuit (I would suggest you contact customer support) what I'm saying here is guesswork, but it might be that the frequency hasn't really converged yet. The HB or PSS simulation could potentially settle on a different final frequency than the relatively short transient in the tstab might suggest. If that's the case, then running the verification in transient may not give the right frequency either.

    If you are simulating an LC oscillator, I doubt you'd need oversampling=12 - that seems high even for a square wave oscillator in most cases. This is why seeing more detail is going to be important, which is why I'd prefer you to contact customer support if possible.

    Regards,

    Andrew.

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