HFSS TDR options事宜
Transforming Frequency To Time Domain
HFSS uses a discrete Fourier transform (DFT) to approximate a continuous time transform, with the frequency step size and upper limit determining the corresponding quantities in the time domain.
(HFSS完成时域和频域转换的两个重要参数：频率步进和频率上限（信号带宽））
The default time step and maximum time are given by:
t_step=1/(2*fmax ) t_max=1/(2*fstep)
Time resolution is controlled by the upper frequency in the sweep. The maximum time is controlled by the frequency resolution of the sweep.
（时间分辨率由频率上限f_max决定，TDR曲线plot时间t_max由频率步进f_step决定）
While tmax is fixed by the choice of frequency step and cannot be increased after the simulation, tstep, or the time delta, can be reduced from the default value within the TDR Options Dialog. Decreasing the time delta does not increase the bandwidth of the frequency data, but it does more closely approximate the band-limited continuous time spectra we have so far discussed.
（rise time可改，越小的rise time，那么离散傅里叶变换越接近连续傅里叶变换，更接近真实的情况）
Although decreasing the time delta will increase the time required to perform the DFT, the time required is rarely significant. Additionally, a smaller time delta has a significant benefit.
Applications
When simulating a TDR plot, we want the fastest edge possible for the bandwidth of our simulation, subject to our preference for oscillation control. Therefore an edge with a risetime of zero is a good choice. Fig. 13 shows TDR plots of a transmission line with several impedance discontinuities. The results for a rectangular and Hanning window with a 20GHz bandwidth are compared with those for a Hanning window with a 50GHz bandwidth, which will necessarily be more accurate due to the higher bandwidth, and can be used as a reference. In all cases, the time step was set substantially lower than the default.
Fig. 13 shows that the rectangular window effectively captures the sharp impedance transitions, but also displays spurious oscillation. The 20GHz Hanning window does not suffer any oscillation, but gives less resolution on the sharp edges. These results are consistent with the step response characteristics of the different windows we have previously shown.
同样的带宽f_max=20GHz，rectangular矩形窗的时间分辨率高，但是抖动大，Hanning窗几乎无抖动但是时间分辨率低，都意味着不能准确定位通道的TDR，产生错误的结论，合理的应该是使用hanning窗+f_max=50GHz。然而f_max=50GHz，将耗费太多的仿真资源。
TDR加载各种窗口效果图：
阶跃信号加载hamming窗、hanning窗和Bartlett窗后，时域曲线无抖动但是边缘很缓慢，参考上面的例子，需要很大f_max才能获得足够的精度；
阶跃信号加载矩形窗、Welch窗和Blackman窗后，时域曲线存在抖动且以矩形窗的抖动最厉害，参考上面的例子，综合使用Blackman窗+一个合适的f_max就可以获得较准确的结果。
