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CST – Computer Simulation Technology

Direct Transient Co-Simulation of a Step Recovery Diode (SRD) Pulse Generator

The unique network co-simulation feature in CST STUDIO SUITE® enables a direct transient simulation of a 3D structure in CST MICROWAVE STUDIO® (CST MWS) with non-linear lumped components or circuit networks in CST DESIGN STUDIO™ (CST DS). This article shows the application of this feature to the simulation of a Step Recovery Diode (SRD) pulse generator.

The interface between the 3D EM field simulator and the circuit simulator is facilitated by the use of discrete ports. This approach is suitable for the analysis of sub-nanosecond pulse generators, frequency multipliers with high multiplication factor, etc. An arbitrary waveform of the excitation signal can be used.

Figure 1: CST DS Schematic with co-simulated CST MWS model

The pulse generator, shown in figure 1, consists of coplanar waveguide (CPWG) layout including a helical coil, two lumped capacitors and a step-recovery diode (SRD). A spice model of the step-recovery diode SMMD840 is used which also includes the parasitic elements of the SMD package. The pulse generator is excited with a harmonic signal at 60 MHz passing through low-pass filter (capacitor 68 pF and helical coil) reaching a SRD connected in parallel. A strong non-linearity of the SRD produces a very sharp peak, which is additionally filtered by DC blocking capacitor at the output (1.5 pF)....

Figure 2: Voltage and Current versus Time waveforms

The circuit co-simulation is activated by the “MWS Co-simulation” button in the transient simulation task menu in CST DS, while the total simulation time is given by “Tmax” parameter. The input and output signals are recorded using probe P1 and probe P2 respectively. It is necessary to define differential ports of the CST MICROWAVE STUDIO block in the CST DS schematic for correct handling of differentially connected discrete ports 3,4 and 5 in 3D model.

Figure 2 shows the waveforms of the input voltage and current as well as the output voltage. The ratio of the duration of the input signal (60 MHz) to the monopulse output can be clearly seen. The width of the monopulse, 0.45 ns, can be read from the probe results. The sub-nanosecond pulse is produced by the SRD when the current direction changes from forward to reverse. The step-recovery effect occurs close to the maximum derivative of the current (at about 16 ns).

Figure 3: Power spectrum density

The Power Spectrum Density (PSD) of the output voltage signal can be obtained using “1D Template Based Postprocessing“ (1D Result from 1D result/PSD). It can be seen that the maximum of the PSD figure is approximately at 1500 MHz resulting in the multiplication factor of 25 (1500 MHz / 60 MHz).

The application of the network co-simulation feature has been demonstrated on an SRD pulse generator. This feature is unique in that it allows a full 3D structure to be correctly co-simulated with a non-linear circuit network.

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