Using Wireless Test Bench Models

Introduction

Wireless test bench simulation is used to perform complex wireless system measurements using wireless test bench models. TDSCDMA, WCDMA3G, and WLAN Wireless Design libraries provide pre-configured models as well as templates.

WTB simulation uses Circuit Envelope analysis along with the ADS Ptolemy Data Flow (DF) controller to perform Analog/RF and system cosimulation. Each wireless test bench model is packaged such that the RF designer avoids working with DF analysis parameters.

Working with WTB Template

WTB Template is the starting point of the WTB simulation.

To insert a WTB template:

1. In an Analog/RF schematic window select Insert > Template.
2. In the popup Insert Template dialog box, choose one of the installed WTB templates, for example 3GPPFDD_BS_TX_test, click OK.
3. Click left to place the template in the schematic window.

You are now ready to replace the DUT with your own RF design, select measurements, and set parameters. See Setting Circuit Envelope Analysis Parameters, and Setting up a Wireless Test Bench Model.

Setting up a Wireless Test Bench Model

Replace the Amplifier2 DUT with your own RF DUT, set the required parameters, and set optional parameters as necessary. The required parameters include setting simulation frequencies and selecting measurements; at least one measurement must be enabled for successful simulation. Optional parameters provide additional control of the selected measurements. These topics are discussed in the following sections:

• Setting Required Parameters

• Setting Optional Model Parameters

The following table describes the various types of parameter data entry fields in parameter dialog boxes.

Parameter Field Type	Description
String data entry field	Used to enter integer, real, complex, or string type of scalar data. Array and string type data must be enclosed in quotes.
String data entry field with multiple choices	This gives you multiple choices for the value of the parameter to select from. The pull down list field also enables you to type in the data directly. The last choice in the list specifies what the type-in value can be, such as an option index, a real value, etc. Use this mode to assign a variable to this type of parameter; then use the variable to either sweep or optimize the parameter.
Information fields	This non-editable information field provides information on a certain aspect of the WTB model.
Instrument selection field	This is a string data entry field followed by a button to launch the instrument-selection user interface. This type of parameter is used only when you enable ESG download. You can enter the instrument address directly or use the button to launch the instrument-selection user interface to interactively select the instrument and update the address.

Setting Required Parameters

After connecting the WTB model and DUT ports, double click on the WTB component to set the Required Parameters in the popup parameter dialog box. The number of parameters varies between different models. These parameters control the simulation and identify the selected measurements. They must have valid values and cannot be left blank. For detailed information about setting these parameters, click Help in the parameter dialog box.

Note You must set the required frequencies and select at least one measurement for successful simulations.

Setting Circuit Envelope Analysis Parameters

The WTB analysis is a cosimulation between an Env (Circuit Envelope) analysis and a DataFlow analysis to verify an RF design's system performance. The DataFlow analysis is pre-configured with the WTB model.

The RF designer must configure the Circuit Envelope analysis to ensure proper simulation. For details about setting the Circuit Envelope parameters, see Envelope Simulation Parameters.

Simulation stop time is controlled by the selected measurements and their parameter values. The measurements and their parameters are located after the Required Parameters. If necessary for your measurement requirements, set the optional parameters for the WTB model. See Setting Optional Model Parameters.

Setting Fast Cosimulation Parameters

When the Circuit Envelope controller dialog box is open, choose the Fast Cosim tab. Select Enable Fast Cosim to gain acces to and set its parameters. Fast Cosim is also known as Automatic Verification Modeling (AVM).

Fast Cosim is a user-selected mode of operation, available for co-simulation that can significantly speed up Circuit Envelope simulations of Analog/RF circuits. When this mode is enabled, the analog subcircuit is characterized using a variety of Harmonic Balance simulations at the start of each wireless test bench simulation. During the actual wireless test bench simulation, this characterization data is then used to predict the response of the subcircuit instead of performing the full circuit simulation at each time point.

Since this characterization is normally done at the start of each WTB analysis sweep based on the full circuit level schematic, the overall capability is basically the same as if the actual circuit level representation is used throughout the Circuit Envelope simulation. For example, optimization of circuit level parameters, or swept parameters including bias, temperature or swept carrier frequency, will continue to operate as expected. These capabilities do not exist when the circuit is manually replaced with behavioral models.

This ability to predict the modulated response based on the Harmonic Balance characterization relies on the fact that many circuits, when used in relatively narrow band modulated applications, can have their nonlinearity represented as a static nonlinearity that is strictly a function of the instantaneous amplitude of the carrier. Many of these circuits (amplifiers and mixers) have little or no frequency response over the modulation bandwidth of interest; any frequency response effects that do exist can often be represented as a linear filter on the input or output of the nonlinearity.

Each output of the Analog/RF subcircuit is then characterized by the equation:

The P _k ( mag ) functions are determined by the swept amplitude Harmonic Balance simulation. The HarmGain is the harmonic gain determined from the harmonic indices of the input and output frequencies.

If phase characterization has been enabled (setting the Num. of phase pts parameter to a non-zero value) each output is then characterized by this modified equation:

In this case, P _k ( mag, phase ) functions are determined by a 2D swept amplitude and swept phase Harmonic Balance simulation.

If the subcircuit nonlinearities are a function of the input phase, as in a nonlinear IQ demodulator, then the amplitude-only characterization is not accurate and the 2-dimensional amplitude and phase characterization must be used.

Note that the magnitude-only characterization assumes the output phase can be determined from the harmonic indices of the input and output frequencies. In certain rare cases, this can be ambiguous. For example, if the input frequency is fund1 and the output frequency is 2*fund1, then the simulator assumes the output signal is generated by doubling the input frequency and so the input phase is doubled. However, if this 2*fund1 output frequency is actually generated by mixing with another LO source at the fund1 frequency and the phase relationship is supposed to be linear, then the Fast Cosim results will be incorrect. If the mixer LO is operating at an independent fund2 frequency, with a mixer output at fund1 + fund2, then the HarmGain of 1.0 is correctly determined. So, as with the IQ demodulator, if there are circuit sources operating at the same frequency as the input signal, caution must be used when enabling this Fast Cosim mode, and the 2D amplitude and phase characterization may be required.

In addition to the swept amplitude characterization, the Fast Cosim characterization also includes a small signal Harmonic Balance frequency sweep. In this case, the input amplitude is set to 0, and the small signal frequency is swept between +/- 0.5/ TimeStep. Note that even though the input amplitude is set to 0, a nonlinear analysis is still being done so any frequency translation effects due to internal mixers will be fully captured. An impulse response representing this frequency response can then be generated and placed (at the user's choice) on the input or the output of the nonlinear block. An additional delay can be added to the frequency response, so that the impulse can be made a more accurate representation of the frequency response. The user should set the number of frequency points high enough that any frequency response deviations are sufficiently sampled (a minimum of 4 samples every 360 degrees). The maximum duration of the impulse response will be approximately 0.75/ FreqSpacing, where

N is determined so that 2 ^N is just larger than the user-specified number of frequency points.

In addition to the amplitude and frequency response characterization, nonlinear noise characterization is also done. A single value for the equivalent input noise for each output is determined, then added to the input signal prior to the above nonlinear and frequency response effects. In the case of multiple outputs, these equivalent noise sources are uncorrelated. So any correlation of the Analog/RF subcircuit added noise between multiple outputs is lost during this Fast Cosim mode. This noise characterization is implemented by selecting Turn On All Noise in the Envelope parameter group in the Options window. Normally, the frequency points used for noise characterization are the same as for small-signal response characterization in which a linear frequency sweep is used. Such a linear frequency sweep may require too many frequency points to properly capture narrow band noise such as 1/f noise. To handle 1/f noise characterization efficiently you can choose to set an independent log sweep on the Envelope controller's Fast Cosim tab in the Characterization Options.

In certain cases, the time spent doing this characterization can be eliminated if the user requests that the simulator use previous characterization. Once this mode is selected, then the user must make sure that the previous characterization is still valid, and that circuit parameters have not been changed (perhaps by optimization), biases have not changed, carrier frequencies have not changed, etc. The circuit should not have changed its connectivity within the WTB analysis environment. Also, the format and data in the dataset must be in the same expected configuration as when it was written by simulator.

In addition to the characterization and implementation portion of the Fast Cosim mode, there is also a user-selectable verification step. If the user specifies a non-zero verification Stop Time, then the normal Circuit Envelope simulation is performed in parallel with the fast cosimulation predictions. The error over this verification is then calculated and output to determine how well these predictions are performing. If the behavior is unacceptable, as determined by the Accept Tolerance, then the Fast Cosim will be disabled and only the normal Circuit Envelope results will be used. Clearly, if used, this verification time should be set long enough to include a representative portion of the input signal. This may need to take into account the fact that many sources, due to channel filtering, take a while to generate their full amplitude outputs.

Limitations

While you may have selected the Fast Cosim mode, it may become disabled if:

• The RF DUT has more than one input from the WTB model or the input is not for an RF signal (non-zero carrier frequency).
• The Circuit Envelope Step is not an integer submultiple of the WTB_TimeStep. See the specific WTB documentation to understand how the WTB_TimeStep is set. WTB_TimeStep cannot be set directly by the user and is set by derivation from the other WTB model parameters.
• Verification was enabled but performance did not meet an acceptance level.
• The Envelope analysis includes an oscillator analysis.
• The input frequency was a mixing term and not harmonically related to a single fundamental.
• The input frequency does not exactly match an Envelope analysis frequency.
• There was a problem reading the previously generated dataset.

Additional limitations of the Fast Cosim mode, that may not be automatically detected by the simulator unless verification is enabled, include:

• Envelope analyses parameters such as the Freq parameters should not be swept or optimized.

Setting Optional Model Parameters

The optional WTB model parameters are listed after Required Parameters in the parameter dialog box. Scroll down to see all of the parameters. Some parameters control the simulation run time, which you will use primarily to modify the simulation run time; otherwise, you rarely need to modify them.

Some parameters are for measurements listed in Required Parameters. Some of those parameters will also control the simulation run time. You may need to change parameters for the measurements you have selected.

For detailed information about setting these parameters, click Help in the parameter dialog box.

Performing Sweeps, Monte Carlo/Yield Analysis, and Optimizations

WTB model parameters can be included in Parameter Sweep, Monte Carlo/Yield Analysis, and Optimization Analysis.

Note The SimInstanceName must always use "WTB1" for sweep or optimization controller regardless of the Envelope controller's instance name.

For information about Parameter Sweeps, see Parameter Sweeps and Sweep Plans in the Using Circuit Simulators documentation.

For information about Monte Carlo/Yield Analysis and Optimizations, see Using Monte Carlo Yield Analysis in the Tuning, Optimization, and Statistical Design documentation.