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Time Domain Setting

 

With Descartes, by reading a Touchstone file as a result of harmonic electromagnetic analysis,
S-parameters can be converted to the results of TDR, which analyzes time domain.
This dialog box allows you to automatically convert the time setting of the TDR analysis to the analysis frequency setting of the harmonic analysis required to obtain the results.

 

Setting Item	Notes
Rise Time of Input Signal	Sets the rise time of the input signal: trise [s]. The input signal is assumed to be stepped wave. Rise time is defined as the time required for a signal to move from 0% to 100%.   Fig. 1: Stepped wave and rise time (trise: rise time, Δt: timestep width)
Number of Timesteps in Rise Time/Timestep Width	Sets the timestep width Δt [s]. Timestep width can be set as the number of timesteps that are included in the rise time.   Number of Timesteps in Rise Time Sets the timestep width by the number of time steps included in the rise time. If the number of time steps in the rise time is nrise, Δt = trise / nrise.   Timestep Width Sets the timestep width Δt [s] directly.
Number of Timesteps/Time	Sets the number of timesteps to draw a result graph. The number of timesteps can also be used as the time axis on the graph.   Number of Timesteps Sets the number of timesteps to draw. In the case of nstep a graph of the TDR impedance over time greater than nstep ×Δt [s] is obtained.   Time A graph of the change of TDR impedance with time greater than the specified time can be obtained.   For the value of the setting time, calculate it roughly based on the length of the signal line for which you want to know the impedance. For example, assume the length of the signal line is 30mm and the relative permittivity of the dielectric surrounding the signal line is 4. The speed of the propagating electromagnetic waves along the signal line is approximately 1.5d8m/s and the traveling distance from the input to the output is 60mm. The required time is at least 400ps. For more details of calculation, see Electromagnetic Analysis Example 27 > [2. The Time When the Impedance Changes].
Calculation of Propagation Speed	Calculates propagation speed and propagation time of the electromagnetic waves.

 

• Fourier transform and inverse Fourier transform are used to convert S-parameters to TDR impedance.
  Since the number of data that can be handled by the Fourier transform and inverse Fourier transform is 2n (n is a positive integer),
  the number of timesteps may not be the number as entered in some cases.
  For example, if the number of timesteps are set to 100, the actual calculation will be for 127 steps.
  Although the number of data (time points) for 100 steps is 101,
  by calculating for 127 steps, the number of data will be 128 (= 27), which makes the Fourier transform and the inverse Fourier transform possible.

 

• If the number of timesteps is set according to the specified time, the specified time may not be exactly as entered due to the time step width.
  For example, if 320 ps is entered for the time when the timestep width is 15 ps, the time will be adjusted to 330 ps (=22 steps x 15 ps).
  Next, the number of timesteps is adjusted to 31 because the number of data must be 32 (= 25) for executing the Fourier transform.
  Finally, the calculation is done for 465 ps (=31 steps x 15 ps).

 

• In the electromagnetic analysis, the accuracy is sometimes low at the frequencies lower than 10MHz.
  When the rise time of signal, timestep width, and number of time steps are entered,
  the minimum frequency, maximum frequency, and number of divisions are automatically calculated. Please make sure that the frequency is not too low.