New Models Simulate RF Circuits

Its no news to those who simulate that the accuracy of SPICE is directly related to the accuracy of the models. What may be news is that simulation of high frequency circuits well into the gigahertz range is now possible due to the introduction of some new RF SPICE models. 

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To illustrate the improved simulation accuracy, the RF models were used to analyze a 500 MHz oscillator (Figure 1). The  oscillator generates a relatively high power output with a very distorted output waveform (typical at power levels over the 1- 2mW range). The simulation goals were to study the start-up characteristics, oscillating frequency and amplitude, and the resulting harmonic distortion. Two simulations were run. The first using the standard Gummel-Poon BJT model and simple inductor chokes and a second using an mproved 2N5109 model along with a new Intusoft RF bead model from the RF Device Library.

 Above  pproximately 100-200 MHz, the built-in SPICE BJT model, based on the Gummel Poon model, fails to accurately predict the real device performance. The BJT must be remodeled as a subcircuit (Table 1) in order to accurately model the package and bond wire parasitics which are of greater significance at higher frequencies. The improved model includes the package parasitics and matches the s-parameters up to 2 GHz. The RF library was created by Analog & RF Models, specialists in the creation of RF models, for Intusoft [1].

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