Computer-aided Design of MMIC Variable Attenuators

Computer-aided Design of MMIC Variable Attenuators

Variable attenuators are used widely in telecommunications and electronic warfare applications to adjust the signal level or compensate for intrinsic gain variations in operating temperature. This article discusses the design, fabrication and testing of a 3 to 7 GHz, voltage-controlled MMIC attenuator and demonstrates the circuit simulation, test equipment control and data-acquisition capabilities of the MMICAD™ Suite of computer-aided engineering (CAE) and computer-aided test (CAT) software. In this application, MMICAD's user-defined modeling feature is key to achieving a satisfactory design.

Salam Dindo, Roman Meierer and Randall North
Optotek Ltd.
Kanata, Ontario, Canada

GaAs MESFETs at zero drain bias have been used as variable resistors to construct a T-type attenuator.^1,2 The impedance matching condition and attenuation as a function of the resistance values for the series and shunt MESFETs are shown in Figure 1 and Table 1 , respectively.

This idealized model does not take into account the series and parallel parasitic capacitances that are present in real MESFETs. To account for the influence of these parasitics properly, a bias-dependent common gate zero drain bias model was developed for a cell library of MESFETs using MMICAD's data-acquisition capabilities.³ An automatic network analyzer and a probing station were used to extract the S parameters of the MESFETs under bias. MMICAD allows for the optimization in real time to an equivalent circuit model. By using the software to control a DC-programmable power supply source, the bias on the MESFET was ramped automatically while the optimization results were extracted and stored in files. Finally, the tabular data of each element in the model as a function of bias were fitted numerically with pinchoff voltage as the variable.

The schematic of the model used is shown in Figure 2 and the element values for a 600 mm MESFET are listed in Table 2 . The schematic was generated using MMICAD LAYOUT, a component of the MMICAD Suite software. A distributed RC line was used to simulate the characteristic of the channel region of the device, including voltage-dependent values for the distributed resistance and capacitance. Lumped elements are included to account for the parasitics of the extrinsic MESFET. (These elements are not bias-point dependent.)

To illustrate this technique, S-parameter measurements for a drain voltage of Vd = 0 V were performed on a 600 mm-wide device for gate voltages ranging from 0 to 1.1 times the pinchoff voltage Vp. The measured S parameters were used to determine the extrinsic element values and the variation of the voltage-dependent values for the resistance and capacitance of the distributed RC line. The dependence of these parameters was fitted to the data in order to obtain a polynomial expression, as shown in Figure 3 . The MMICAD simulation file listed in Appendix A shows this equivalent circuit incorporated as a user-defined model. Measured and simulated S parameters at 20 percent of pinchoff for a 600 mm transistor are shown in Figure 4 . Models were developed for the entire range of gate widths in the MESFET cell library for use in MMICAD as user-defined models.

All MESFETs in the attenuator operate in the passive mode by controlling the MESFET's linear operating region with gate bias. For each circuit topology, two independent gate biases are used: one to control the series MESFETs and one to control the shunt MESFETs. The circuits use MESFETs as variable resistors; in the simplest approximation, the models are resistors and capacitors in parallel. R varies as a function of bias and MESFET width. C varies slowly with bias, but is a function of MESFET width. Using the MESFET nonlinear model, the T-attenuator performance was optimized as a function of gate length from 3 to 7 GHz using MMICAD. The optimal configuration in terms of profile, insertion loss and matching was determined to have two series 600 mm transistors and one shunt 600 mm transistor. On the basis of switching speed, resistor values in series with the gate were optimized to be 2000 W; 20 pF capacitors were chosen for coupling and bypass.

The final MMICAD optimized electrical schematic created using MMICAD LAYOUT is shown in Figure 5 . Figure 6 shows the simulated insertion loss and matching, both as a function of bias. MMICAD LAYOUT allows for bidirectional layout generation from the MMICAD simulator netlist. The design was laid out using 0.5 mm design rules. The final chip layout is shown in Figure 7 . The chip size measures 1040 x 925 mm. The attenuator MMICs were fabricated in house and the attenuator performance was evaluated by mounting the MMICs on a gold-plated brass test jig with APC 3.5 connectors. S-parameter measurements as a function of bias were made on a network analyzer using MMICAD for power supply control and data acquisition. The bias points were chosen to match the simulated flat attenuator profile. The results, shown in Figure 8 , closely match the simulated performance. Because deep-channel, high current MESFETs were chosen for this design, power handling does not appear to be a problem.

Conclusion
A T-type 3 to 7 GHz attenuator has been designed and processed successfully using the MMICAD CAE/CAT software and standard components from a MESFET cell library. Attenuator performance was measured using MMICAD to acquire data from a network analyzer. Measured results agreed closely with the simulated performance.

Acknowledgment
The automatic network analyzer used was the model 360 from Anritsu Wiltron, Morgan Hill, CA. The cell library is a product of Optotek Ltd.

References
1. Y. Tajima, T. Tsukii, R. Mozzi, E. Tong, L. Hares and B. Wrona, "GaAs Monolithic Wideband (2 to 18 GHz) Variable Attenuators," 1982 IEEE MTT Symposium Digest, IEEE, New York, 1982, pp. 479–481.

2. G.S. Barta, K.E. Jones, G.C. Herrick and E.W. Strid, "A 2 to 8 GHz Leveling Loop Using a GaAs Active Splitter and Attenuator," 1986 IEEE Microwave and Multimeter-wave Monolithic Circuits Symposium, IEEE, New York, 1986, pp. 75–79.

3. R. Pucel, "Signal and Noise Properties of GaAs Microwave MESFETs," Adv. Electron. Electron Phys., Vol. 38, 1975, pp. 193–265.