No HB convergence, even with TAHB

Question asked by nicolasc on Jul 13, 2005
Latest reply on Jul 27, 2005 by wrivas
Subject:   why no HB convergence, even with  good Transient_HB solution guess file (TAHB approach) ?

My application is a power amplifier with strongly non-linear elements. I am using a 2_Tone excitation: a pure sinusoidal carrier signal at frequency Frf = 1.88 GHz, and a purely sinusoidal modulating signal at frequency Fmod = 2 MHz, combined to generate a simple AM envelope signal as input, to be amplified by the amplifier.  I have not been able to get Harmonic Balance to converge by itself, even though I believe I have tried all hints & suggestions I found in ADS documentation.

I am trying the TAHB (Transient followed by HB) approach.  I managed to get a Transient analysis to converge to what I believe is a stable and steady state solution. I used Start/Stop to capture the 4 last cycles of the low frequency 2MHz envelope.  The key parameters I used for Transient simulation are:
- TimeStepControl = Fixed
- MaxTimeStep = 1/(10*10*Frf) â€¦ i.e. 10 points for the 10th harmonic of highest frequency
- Start= 200 ns, Stop =2200 ns, â€¦i.e. to capture 4 cycles of the 2MHz  (previous Transient showed that steady state is reached before 200 ns)

I also specified a HB solution with these parameters:
- MaxOrder = 6; Order of Frf: Freq[1]= 10; Order of Fmod: Freq[2]= 7;      For the Transient simulation to converge (and to avoid ringing & divergence) I had necessarily to use: Integration_Gear_order=1, TimeStepControl=Fixed .  And setting Local_Trunc_error_factor = 70 (10 times the default), and reducing Charge_accuracy to 1e-12 and specifying Connect_nodes_to_GND_via_GMIN has probably helped Transient convergence as well.

The Transient plot shows a perfect time domain solution, with the 4 distorted envelope peaks at equal amplitude (indicating steady state), and all voltages across the circuit very close to the values where they should be.  So I performed HB simulation using this HB solution as an initial guess file, without any change in circuit topology & values, with exactly the same frequency definition of sources & amplitude values, same MaxOrder/Orders (6_10_7 as above), hence all conditions absolutely identical as in Transient, but HB still does not converge. The messages still points to suggestions like changing preconditioner, doubling fundamental oversampling, etcâ€¦ until try TAHB, so it looks like the Transient_HB solution is not helping at all in my case ?

I would appreciate a lot any suggestion that might not be in the documentation, or not obvious, about what more can be done to help HB converge, when an apparently good Transient_HB solution is used as an initial guess. Thanks a lot.