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Ansoft Designer / Ansys Designer 在线帮助文档：

System Simulator >
Discrete Time Analysis >
   Setting Discrete Time Simulation Control Parameters with Convolution >
       Step 1: Choosing The Simulation Time Step           

Step 1: Choosing The Simulation Time Step

Choosing the simulation time step t_s is not a direct process in discrete time signal analysis.

For a typical wireless communications system, the user always begins by setting the bit rate for binary data sources (or sampling rate if a source happens to be a waveform source).

Binary source components in Designer (e.g., BSRC) have a parameter that determines their output bit rate. In a typical baseband modulation process, binary bits (at a user-defined bit rate) are mapped unto information symbols (to yield a given symbol rate). Each symbol is then represented by a user-selected number of samples (typically by upsampling or repeating each symbol) to finally yield a desired sampling rate f_s and the corresponding simulation time step . These samples are then filtered to yield the discrete baseband modulation waveform S(nt_s) described above.

When choosing the simulation time step t_s, caution should be exercised to preserve the Nyquist criterion for the signal S(t). In other words, the user must ensure that the discrete signal S(nt_s) (at any point in the system) has at least a Nyquist sampling rate or higher, where the Nyquist sampling rate is equal to twice the bandwidth of the continuous signal S(t).

At the same time, the simulation time step must be chosen in accordance with the Nyquist criterion for system bandwidth. In other words, if the bandwidth of a filter or electrical sub-design is BW, then

Good results may be obtained for values of .

In conclusion, the simulation time step t_s must be chosen in accordance with the expected (baseband or bandpass) signal S(t) and (baseband or bandpass) system bandwidths. For most practical applications, the signal and system bandwidths are of the same order, but in general, t_s at any point in the system must be chosen in accordance with the larger of the signal bandwidth and system bandwidth at that point.

In some applications, the user may be interested in generating direct waveforms without having to convert binary information to symbols and symbols to samples. The Designer system has a good number of waveform sources that can generate a variety of periodic and transient waveforms. In addition, arbitrary waveforms may be imported for the discrete time system analysis from MATLAB, WinIQSim and other system simulators by means of an external waveform file (refer to the Sources section).

All waveform source components in the Designer system have a parameter that determines the desired sampling rate f_s, which in turn will set the desired simulation time step (refer to the AM and PLL2 examples)

As an example, reconsider the second order (type 1) PLL2 project under the directory Examples\System. Note that the sample rate parameter for the complex constant source (CCONST) is set to 50kHz (well beyond the Nyquist rate of the electrical sub-design or loop filter of the Phase Locked Loop). This sampling rate implies a time step of 20. After analyzing the project, the PLL time domain response shown below in Figure 7 will be displayed.

If the sample rate of the CCONST source is readjusted to 20KHz and the project is reanalyzed (ignore warning message which is explained later), the resulting change in time domain response for the PLL can be seen below in Figure 8 in terms of the “smoothness” of the output curves.

As can be seen from the previous two figures, choosing a higher sampling rate results in more accurate simulation results.

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