CST – Computer Simulation Technology

Transient Co-Simulation of a Hybrid Ring Mixer with Matching Network

The integration of active components and system level simulation is an outstanding feature for users of CST STUDIO SUITE®. In this article, a 180° Hybrid junction is employed in the popular mixer topology. The Hybrid itself is simulated in CST MICROWAVE STUDIO® (CST MWS) and the diodes and matching circuits are incorporated in CST DESIGN STUDIO™ (CST DS). Simulation results compare well with measurements and optimization of the matching networks within CST DS is used to improve the overall performance.

Figure 1: Full 3D CST MWS Model of the hybrid Mixer with CST DS Diodes

The 3D model simulated in CST MWS is shown in Fig. 1. As a postprocessing step, the diodes were added to the model using CST DS and simulations were performed to obtain S-Parameter, time domain waveforms, and frequency domain spectral content....

Figure 2: 3D Simulation S-Parameter results

Figure 2 shows the simulated S-parameters of the 180° Hybrid junction.

Figure 3: Animated 3D Surface Current Plot

An animation of the surface current at 2.8 GHz is shown in figure 3. This animation can be created effortlessly with the aid of the video animation macro in CST MWS.

Figure 4: Schematic View of the CST MWS / CST DS Co-Simulation

The diodes for mixing and the package parasitic components were included in CST DS as shown in Fig. 4.

Figure 5: Simulated (Green) and Measured (Red) S-parameter results from CST DS Cosimulation

Figure 6: Simulated (Green) and Measured (Red) S-parameter results from CST DS Cosimulation

The aforementioned results are shown in Figs. 5 - 8. Figures 5 and 6 show good correlation between measured and simulated results.

Figure 7: Time Domain Voltage Waveform at IF Port

As a confirmation of proper operation, the time domain voltage waveform is shown in Fig. 7. The IF frequency was chosen to be very low relative to the RF and LO to facilitate its identification in the time domain waveforms. It should be noted that the design frequency of the 180° Hybrid is 2.8 GHz and the LO frequency should match. The proper operating conditions are shown in Fig. 8 along with the simulated spectral content at the IF port.

Figure 8: Frequency Domain Spectral Content

As shown in Fig. 8, the output spectrum is cluttered with harmonics of, and dominated by, the RF and LO inputs.

Figure 9: CST DS Circuit with Matching Network

The performance can be improved by including impedance matching networks at all three ports as shown in Fig. 9. By inspecting the S-parameters of Figs. 5 and 6, one can obtain a decent starting point for the values of the matching elements, however, all three ports must be matched simultaneously at three different frequencies. Such a task would prove difficult and time consuming. Instead, the built in optimizer of CST DS was used to generate the optimal solution. The optimized values are also shown in Fig. 9 and the optimized S-parameters are in Fig. 10.

Figure 10: S-Parameters after DS Matching

It is apparent from Fig. 10 that the optimization worked quite well, providing over 60 dB return loss at all ports. Re-simulating the spectral content at the IF port generated the plot shown in Fig. 11.

Figure 11: Simulated spectral content at IF port after matching in DS

Inspecting Fig. 11 shows that the matching network dramatically improved the performance of the mixer. The output spectrum is now dominated by the IF and the RF/LO and their harmonics are supressed.

In this article, it was shown how CST MWS and CST DS can be combined to analyze and optimize the design of a 180° Hybrid mixer. Comparisons with measured results were made and found to correlate well. Optimized impedance matching networks at the input and output ports provided a simultaneous solution and improved mixer performance.

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