Full access to the state-of-the-art of circuit simulation for RF/microwave applications is made available to final year project student willing to submit a thesis project on circuit design for wireless applications.

Thanks to an educational donation from Agilent EESOF, a certain number of full license bundles of Advanced Design Systems (ADS^®) are available within the ARCES laboratories and on several machines of the computing laboratory of the Department of Electronics, Informatics and Systems (DEIS). This represents a valuable tool that allows students to start getting acquainted with the complex ADS design environment during their courses. A wide range of research subjects is also available for final year project students, typically spending between three and five months working on large-signal circuit simulation problems for circuit design for wireless telecommunications.

An updated selection of such educational and research activity is summarized in the following page.

S-parameters for power amplifier design

 The design of medium power amplifiers in the GHz frequency range is tackled by optimizing the input and output matching networks based on available S-parameter data of the active device.
A class-A amplifier example was designed using the Philips BFG198 Bipolar Transistor device, with tabulated s-parameters available from the manufacturer. The schematic of the amplifier is shown in Fig. 1, when short-circuit stubs are adopted for implementing a narrow-band impedance conjugate matching and the biasing network.
The amplifier was realized on an FR4 substrate, using a T-tech Quick Circuit 5000 milling machine for rapid circuit prototyping, available at the ARCES laboratories. The layout was compiled in Agilent Momentum and is shown in Fig. 2. A picture of the realized circuit in shown in Fig. 3.
The experimental characterization of the amplifier was performed using an Agilent 8753 Vector Network Analyzer for a frequency dependent observation of the small-signal gain, and an Agilent E4402B Spectrum Analyzer to monitor gain compression and output power performances. Figure 4 is a comparison between circuit simulation and measurements for the amplifier gain vs. frequency.

Figure 2 – Layout of the class-A conjugate matched narrow-band amplifier compiled in Agilent Momentum layout tool.

Figure 3 – Picture of the realized narrow-band class-A amplifier.

(a)

 

(b)

Figure 4 – Measured (a) and simulated (b) small-signal gain of the class-A amplifier around the design frequency of 1 GHz (bias current 20 mA).

RF-MEMS model extraction.

 RF-MEMS devices such as switches and variable capacitors are more and more emerging as a viable technological solution for achieving compact, low-cost and highly reconfigurable circuit blocks for wireless telecommunications applications. This relatively new Microsystems technology discipline still gathers a lot of interest from the research community, in particular for issues related to device compact modeling at RF-microwave frequencies.
The simulation and optimization capabilities of Agilent ADS have been here used as a model extraction tool in order to obtain lumped element circuits able to predict the terminal characteristics of RF-MEMS devices which have been realized and experimentally characterized on-wafer. S-parameters are the optimization goals, and the circuit topology is chosen in order to maintain a reasonable physical interpretation of the equivalent circuit based on the actual device geometry.
An example of a modeled device is shown in Fig. 5. This is an ohmic contact based switch with separate actuation electrode, realized in RF-MEMS technology from ITC-irst, Trento, Italy. The equivalent circuit topology for the device is schematized in Fig. 6, together with the simulation and optimization control instances of ADS.
After optimization of all circuit parameters, both the UP-state and the DOPWN-state configuration of the RF-MEMS switch can be predicted with good accuracy by the lumped element circuit, as shown in Figures 7 and 8.

A Reconfigurable RF-MEMS LC-Tank Circuit

Reconfigurable passive circuits can be obtained by exploiting a standard RF-MEMS surface micromachining technology integrating inductors, MIM capacitors and ohmic contact RF-MEMS switches. The accurate prediction of the resonant frequency of an LC-tank circuit for RF oscillator applications is a critical modeling task due both to the complex topology and to the presence of deformed membrane layers. In this project, an accurate modeling of such a circuit is demonstrated with the use of ADS Momentum. The simulation is performed for both up-state and down-state RF-MEMS switch state, giving results in a very good agreement to the measured data. Of particular interest is the choice of layer structure adopted for the description of the ohmic metal-metal contact between the actuated membrane and the underlying electrode. A successful circuit behavior prediction is also obtained from circuit simulation with measured s-parameter data of the RF-MEMS switch included as a data based 2-port component.

Verification Result