Postprocessing converts Spice to RF analyzer

Mojy C Chian, Harris Semiconductor, Melbourne, FL -October 15, 2012

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Originally published in the Dec 23, 1993, issue of EDN

Even though engineers designing IC and low-frequency products often use Spice to predict circuit-noise performance, designers of RF and high-frequency products have not been able to use Spice to determine RF-specific noise parameters. These engineers need a simulator that can predict noise figure at a given frequency, minimum noise figure, optimum reflection coefficient for noise, and noise resistance. Typically, these engineers use an RF simulator to determine the noise parameters.

But under the hood, both the Spice and RF simulators use the same noise-analysis algorithms and models. Thus, with a little postprocessing, you can transform the results of Spice noise analysis into terms familiar to an RF engineer. As part of its noise analysis, Spice calculates the individual noise contributions (noise power spectral density, Si, in units of V2/Hz) from each noise generator (resistors and semiconductors) in the circuit. You can use this information to determine noise figure.

Noise figure (F) is a direct measure of signal-to-noise (S/N) ratio degradation caused by the circuit, or

Another definition of noise figure is the ratio of the total available noise power at the output to the available noise power at the output that arises from input noise. Therefore, for a two-port network, if the noise-contribution components of the input and output resistors are SS and SL, respectively, then the noise figure is

where ST is the sum of all noise components. If you must include the noise contribution of the load resistor in the two-port network’s noise specification, then:

Spice supplies SS, SL, and ST. A simple calculation, then, yields the noise figure for a specific frequency. For example, consider the Harris UHFN3 transistor in Figure 1 simulated at 500 MHz. A Spice noise analysis produces the results in Table 1, yielding a noise figure of

Repeating the calculation for different frequencies produces a plot of noise figure versus frequency. You can easily extend this method to include other RF noise parameters.

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