CIRCUIT AND ELECTROMAGNETIC (EM) simulators now offer many capabilities to make software prototyping successful, including accurate models. To provide an example of how such models can assist the design process, Modelithics, Inc. offers a nine-page application note, "Accurate Non-linear Models Enable Successful PIN Limiter Design," which focuses on the design and validation of a 1.8-GHz PIN-diode limiter circuit.
The design uses Aeroflex/Micrometrics MLP7100 limiter diodes (CS19-1 package), mounted on a 16-mil-thick Rogers 4003 microwave laminate. The design process leverages a nonlinear model from the Modelithics NLD Library version 3.1, extracted from a series of measurements that included capacitancevoltage (C-V), current-voltage (I-V), RF impedance, and small- and large-signal S-parameters. The package is included via a user-level input parameter in the model. At 4 GHz, package parasitics resulted in a ~2-dB difference in the return and insertion loss.
For the limiter design, a dual-diode configuration was chosen. The diodes were arranged in an anti-parallel pair and attached to either side of a microstrip line. The small-signal, S-parameter sweep results indicate an input impedance of about 38.8 ohms at 1.8 GHz. A matching circuit should therefore be added for best performance. To confirm the broadband accuracy of the modeling approach, measured and simulated small-signal, S-parameters were compared and agreement between them reached. A plethora of graphics, such as screen captures from Agilent's Advanced Design System (ADS), are included in the application note.
Modelithics, Inc., 3650 Spectrum Dr., Suite 170, Tampa, Fl 33612; (813) 866-6335; Fax: (813) 558-1102, internet: www.modelithics.com.