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Nexxim Simulator > Discrete Time Simulation of Signals and NoiseIn discrete time simulation, all waveforms (signal and
noise) are represented in a discrete (sampled) form. This implies that
baseband signal and noise waveforms may be simulated using the discrete
signals The Nyquist criterion states that if a continuous time signal S(t) is sampled at a rate that is at least twice its highest frequency content (i.e., bandwidth), this signal may be completely reconstructed from its sampled version S(nts). The minimum sampling rate needed to completely recover a continuous signal from its sampled version is known as the Nyquist rate. It is the responsibility of the user to be aware of the signal S(t) bandwidth. It is highly recommended that the actual simulation sampling rate be chosen well above the Nyquist rate for more accurate simulations. This added accuracy typically results in a longer simulation time. Most bandpass signals encountered in practical communications systems are narrowband signals. A narrowband bandpass signal has a bandwidth that is much less than its carrier frequency. The Nexsys discrete time simulator can utilize envelope modulation techniques where only the complex envelope of the bandpass signal or band-limited noise is sampled, as opposed to sampling the carrier as in Nexxim transient analysis. As a result, the Nexsys simulation sampling rate may be orders of magnitude less than that of a Nexxim simulation. This translates into a much faster simulation time. The information of a continuous bandpass signal and noise may simply be recovered from knowledge of their corresponding sampled complex envelope signals:
and
respectively, as well as the carrier frequency fc. Thus, the information of a continuous bandpass signal and noise may be recovered from the triplets (IS(nts), QS(nts), fc) for the signal information and (IN(nts), QN(nts), fc) for the noise information. To ensure that the Nyquist criterion still holds for narrowband bandpass signals, the user needs to choose a simulation sampling rate that is at least twice the highest frequency content (bandwidth) of the complex envelope .
The
choice for the simulation sampling rate fs affects the outcome of
the simulation. Choosing a simulation sampling rate fs
that is higher than 2fc results in the sampling
of the carrier. In this case, the bandpass signal with a complex envelope
representation will be converted to a baseband real signal of the form: Conversely, time domain simulation results for bandpass signals with a sampling rate
may be viewed only in terms of their complex envelopes.
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Ansoft Designer / Ansys Designer 在线帮助文档：
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= Waveform simulation time step. 
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