Toolwh,

  Flexible balance inherits the default simulation setup from the
time domain, Shooting-Newton analysis. If you want to run pure
Harmonic Balance analysis, then set tstab=0. It may be useful
to review the discussion Harmonic Balance analysis section of
the SpectreRF Simulation Option Theory manual.

                                                            Best Regards,

                                                               Sheldon

Thanks Sheldon

I had not set a value for tstab but, as the guide specifies, it defaults to "one cycle of the signal period". Setting tstab to 0, as you pointed out, forces it to run a pure HB sim. I had read the relevent section before but missed the bit about tstab. I am still not clear what  it does with the transient when tstab is not set to 0. I assume it uses it as an intial guess for the HB solution, similar to what "Transient Assisted Harmonic Balance" (TAHB) does in RFDE and GoldenGate. However, in RDFE and GoldenGate TAHB, it runs the trasient only for the first value in a swept sim, not every swept value. I also don't like the fact that for a two tone HB PSS you still have the set the beat frequency, and, if the tone are close together, you have to set a large number of harmonics, which significantly slows things down. It appears that QPSS is the better sim to run, which, is what they suggest in the guide.

Bill

Thanks again Sheldon.

I am working with my local Cadence apps engineer on these issues but things can be slow with them. I also wanted to find out what and how real RF designers are using the tool and what their impressions are. My first impression is that the HB option in cadence coupled with QPSS is a significant improvement over the "old" PSS sims. Is the HB engine robust enough to handle large circuits for say a receiver chain (or a good portion of it) and how does it compare to RFDE or GoldenGate is another question. I need to bash away at it and see where the tool breaks. Let the fun begin.

Bill

Thanks Andrew.

It is encouraging to hear that the Cadence HB is robust enough to simulate a receiver chain. I am getting that sense as I continue to play with the simulator. Your comment about using HB for weakly nonlinear circuits is always the case no matter what HB simulator you use. I have had reasonable success using RFDE and GoldenGate for circuit well beyond 1dB compression point but you need a lot of harmonics and transient assist helps a fair bit. For those types of sims it is probably best to use the shooting method but it can be done in HB with some care. As you said, with Cadence, it is nice to have the choice to choose the appropriate engine.

As far as transient assist HB (TAHB) goes, I would agree that the initial tstab at low powers will not be valid at higher powers but most other HB simulators will use the tranient solution the first point only in the sweep and then use the previous HB solution for the next point and so on.  They usually have the option to run TAHB for each point but in 95% of any cases I simulated, it was not needed. It would be nice if Cadence had the option. I have asked my local Cadence Apps engineer about this and he is checking into it.

Thanks again.

Bill

Andrew

Perfect, it worked like a charm. Thanks for the info.

Bill

Nandy,

  Could a little more information about what you are trying to
do? I believe that the default tstab is controlled by the designer
when the set the fundamental period for PSS analysis. If the
transient is taking too long, it indicates that there is an issue
with the fundamental period you have specified.

                                                         Best Regards,

                                                            Sheldon

Hi

Im running pss as a forerunner to pnoise of a 10G LC oscillator I have designed. I notice from the stored transient response that the swing becomes uniform after only 10ns, but pss runs transient for 280ns (I set the additional stabilization time to be 0). I observed that the amount of time that pss intends to run is established as soon as the simulation starts, which means pss isnt doing any dynamic checks to see if response has settled. It decides upon the run time and sticks to it no matter what. I tried changing the methods by which tstab is set, like trap, traponly, euler etc. but all of them run for the same amount of time.