Transient Assisted Harmonic Balance

This topic describes the automated transient assisted harmonic balance simulation (TAHB). For a harmonic balance simulation, an initial guess of the solution is needed. For circuits that are highly nonlinear and contain sharp-edged waveforms (such as dividers), a transient simulation often provides a good initial guess for the starting point of harmonic balance. In a TAHB simulation, a transient is performed to generate the initial guess for the harmonic balance. Then an HB simulation is performed using the transient initial guess. The simulator automatically generates the transient initial guess when the Enable check box is selected.

It is strongly recommended to use this simulation technique for divider circuits and others with sharp edged or pulse-like waveforms. We suggest that you use the TAHB Auto mode, which is the default setting, for optimal performance. The simulator will turn on TAHB automatically if the circuit involves a divider. The user can choose the TAHB On mode if the circuit does not involve a divider, yet sharp edged or pulse-like waveforms are expected.

If you are not familiar with the harmonic balance simulator, see Harmonic Balance Basics.

The following topics describe details about automated transient assisted harmonic balance:

• Setting Additional Transient Parameters describes how to set parameters for automated TAHB.
• Using a One-Tone Transient for a Multi-Tone Harmonic Balance describes how to instruct the transient simulator to perform a one-tone simulation.
• Using Sweeps and Optimization Simulations describes how to use sweeps and optimization simulations with automated TAHB.
• Outputting the Transient Data to the Dataset describes how to control transient data output to the dataset.

For details about each individual TAHB parameter, see Setting Up the Initial Guess.

Setting Additional Transient Parameters

The transient simulator uses intelligent defaults and determines a steady state solution for the initial guess in harmonic balance. For the automated TAHB, you are not required to set any transient parameters. However, you may set the transient parameters by choosing the TAHB On mode if desired.

The first parameter to set is StopTime. The default for StopTime is 100 cycles of the commensurate frequency. In a single tone analysis, this is simply the value of Freq [1]. This parameter determines how long transient should run when generating the initial guess. If the transient simulator detects steady state before it reaches the StopTime, then it will end at that point (earlier than the StopTime ), then proceed with the harmonic balance simulation. When the transient does not find a steady state solution, then Harmonic Balance uses the last transient data point as an initial guess.

The next parameter to set is MaxTimeStep. The default for MaxTimeStep is 1/(2*2*Maximum Frequency). In a single tone transient analysis, the maximum frequency is Freq[1]*Order[1]. Adjust this parameter if a smaller or larger time step is required.

The simulator displays the values that it determined for StopTime and MaxTimeStep in the status window.

The parameter Min Detect Steady State Time sets the earliest point in time that the transient simulator starts checking for steady state conditions. The default is two periods of the fundamental frequency for autonomous circuits, and 10 periods of the fundamental frequency for non-autonomous circuits such as an oscillator. The units for this parameter are in seconds. If your circuit exhibits a large amount of over/undershoot, then this needs to be larger than the default so that the detector will begin to check for steady state after some of the initial transients have settled.

The parameter IV_RelTol is the transient current and voltage relative tolerance. The default value is 1e-3. This is a relative tolerance for transient only. If relative tolerances are set (on Options controller) and the IV_RelTol is given, then it will override the relative tolerances for the Simulator Option's relative tolerances for the transient simulation only. This parameter simply allows for having two sets of relative tolerances, one for transient and the other for harmonic balance.

The Transient Other parameter makes it possible to set transient parameters that are not found in the TAHB tab. For example, you can set the following transient convolution parameter ImpMaxFreq =10 GHz using the Transient Other parameter. To see all of the transient parameters, please see the Transient/Convolution Simulation documentation.

When TAHB_Enable is set to Auto, a user-supplied initial guess will be honored and TAHB will be turned off by the simulator. When TAHB is set to On, you will not be able to supply an initial guess file. For more information, please see Transient Assisted Harmonic Balance - TAHB in the Guide to Harmonic Balance Simulation in ADS.

Using a One-Tone Transient for a Multi-Tone Harmonic Balance

The Use only Freq[1] for transient parameter instructs the transient simulator to perform a one-tone simulation, and to use only the value of Freq[1] parameter set for the Harmonic Balance simulation for determining the StopTime and MaxTimeStep. This parameter is enabled by default. If there are multiple frequencies set for the HB simulation and this parameter is enabled, then sources in the circuit at the other frequencies will be turned off for the transient portion only. It is recommended to perform a one-tone transient simulation for a multi-tone HB simulation. Also, Freq[1] should be set to the frequency of the most nonlinear tone.

Using Sweeps and Optimization Simulations

Statistical simulations such as optimization, yield, Monte Carlo, DOE, and yield-optimization are not supported with automated TAHB. The simulation will terminate prematurely if such simulation types have TAHB set to On. If TAHB is set to Auto, the simulator will proceed without using TAHB.

Swept simulations are supported with automated TAHB. In ADS, there are two ways to sweep a parameter - using the Sweep tab on the Harmonic Balance controller or using the Parameter Sweep controller. When sweeping a parameter on the Sweep tab, a transient simulation is done for the harmonic balance simulation of the first sweep point only. When sweeping a parameter on the Parameter Sweep controller, a transient simulation is done for the first sweep point only, unless the Restart parameter is enabled on the Harmonic Balance controller's Initial Guess tab. When Restart is enabled, a transient simulation is done at each sweep point which generates a new transient initial guess for each sweep point of the harmonic balance simulation.

Outputting the Transient Data to the Dataset

The data from the transient simulation can be output to the dataset by enabling the Output transient data to dataset parameter. By default, this parameter is disabled and the transient simulation results are not in the dataset. Recall that the transient was used only to generate the initial guess for harmonic balance. If the transient results are desired in the dataset, then be sure to enable this parameter by checking the box. Proceed with caution for very large circuits since the dataset can grow to be very large if both transient and harmonic balance results are included.