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Design Notes

NONLINEAR/LINEARIZED NOISE FIGURE USING SWITCH VIEWS

Switch Views and Switch Lists provide an easy way for a designer to toggle between different versions of a model while maintaining a single layout cell for that instance.  This could include linear models, nonlinear models, EM data, measured results, etc.  In the attached project, the Switch Lists are used to show how a single subcircuit can have both a linear and nonlinear representation.  For a more detailed description of Switch Views and Switch Lists, see Help > Open Example and filter on the keyword "switch_views".

The discussion in the project focuses mainly on the Switch Views and Switch List concept as it applies to this circuit. Note how Switch Views and Switch Lists allow the designer to make significantly more simple projects.  This project has one schematic to perform all of the analysis.

Overview

There are several methods that can be employed to calculate the linear and non-linear performance of active devices with noise depending on the class of model that is available to the designer.  In this project, designs using both linear S-Parameters and a nonlinear Gummel-Poon models.  The nonlinear model includes the noise parameters. 

For all the nonlinear schematics, the amplifier is biased at VCE = 3V and IC = 20mA and component values optimized for Input & Output Match.  This is important as the Touchstone data file used for the linear simulation has the S-parameters and noise parameters specified at this specific operating point.

Switch Views In This Project

• “Device” Schematic - contains the Touchstone file for the amplifier.  In the project tree it can be seen that it also has a Switch View called “Nonlinear.”  This view contains the Gummel-Poon model.  Note that it is important that both the default view model and the switch view models have the same number of ports and a consistent port numbering scheme.

• My_DC_Voltage_Source Schematic - contains an open circuit.  The reason is that when using the Touchstone file (recall that the Touchstone file is the default model for the “Device” schematic) there is no reason to have any DC bias supplies in the circuit as these will only cause an unnecessary DC operating point to be solved.  In the project tree it can be seen that this schematic also has a Switch View called “Nonlinear.”  This view contains a DC voltage source.  Note that it is important that both the default view and the switch view have the same passed parameters.

Switch Lists In This Project

The Switch Lists that have been configured for this schematic can be seen by selecting Simulate > Manage Switch Lists.  There is one Switch List configured called “Nonlinear” and in the View List field it can be seen that this list only looks for the “Nonlinear” Switch View.  Note that if the “Nonlinear” Switch View is not found that the default model will be used.

When making measurements there is a new drop down menu called “Configuration.”  This is where the different Switch List can be chosen.  As will be seen below, this concept will be used to look at both the linear and nonlinear noise performance of this device.  The Measurements will point to the same Schematic, but will use the “Default” configuration for the linear device and the “Nonlinear” configuration for the nonlinear device.

LNA_Model Schematic

Contains the LNA circuit.  Note that the active device is represented by the “Device” Schematic and the voltage source is represented by the “My_DC_Voltage_Source” schematic.  Thus, this one schematic can be used to analyze both the linear and nonlinear performance of the circuit by using a Switch List to determine which Switch View is used for a given Measurement.

An NLNOISE block is added to the schematic to computer nonlinear noise.  The NFstart and NFend both set to 0.01 GHz, which indicates a 0.01 GHz offset from the fundamental frequency.  The noise must be computed at an offset so that the noise signals can be differentiated from the RF signals.

Also on the schematic is a V_NSMTR(noise voltage meter) which is used in conjunction with the NV Measurement to compute noise voltage at the desired node.

DC Annotation

DC current and voltage is annotated on all schematics.  Note that this annotation using the “Nonlinear” Switch List and, thus, are using the nonlinear models.  DC annotation for the linear models wouldn’t make sense because the touchstone file is already biased and the DC source was turned into an open circuit for linear analysis.

LNA Gain Graph

Shows the linear gain of the “LNA_Model” schematic using both the linear and nonlinear models.  It also shows the nonlinear gain of the “LNA_Model” schematic.  The difference between linear and nonlinear gain is that drive level does not impact the linear gain, but it does impact the nonlinear gain.  Thus, the compression that can be seen on the graph.  Note the nonlinear model is selected by using the “Nonlinear” Switch List in the Measurement. 

Linear Noise Figure Graph

Shows the linear noise figure of the “LNA_Model” schematic using both linear and nonlinear models.  Not the nonlinear model is selected by using the “Nonlinear” Switch List in the Measurement.

Match Graph

Shows the linear input and output reflection characteristics of the “LNA_Model” schematic using both linear and nonlinear models.  Not the nonlinear model is selected by using the “Nonlinear” Switch List in the Measurement.

Nonlinear and Linearized Noise Graph

There are 3 measurements NFSSBN for the upper sideband, NFSSBN for the lower sideband and NF (linear).  Note that for the case of an amplifier where the NFStart and NFEnd frequencies are the same that the upper and lower sideband nonlinear noise figure will also be roughly the same.  The upper side band is at the RFFreq + NFStart and the lower sideband is at RFFreq - NFStart.

This graph shows the key difference between nonlinear and linearized noise.  With linearized noise, the drive level of the amplifier does not impact the noise.  With nonlinear noise, the drive level does impact the noise.  The graph shows that the nonlinear noise is higher than the linear noise (which is expected for a compressed amplifier - the compression can be seen on the “LNA Gain” graph).

Noise Voltage Graph

This graph shows noise voltage as a function of frequency.  Note that noise voltage is measured using a V_NSMTR(noise voltage meter).  This  Measurement uses the “Nonlinear” Switch List and, thus, the nonlinear model.

Notes

The performance of the linear and nonlinear circuits show good correlation for the noise, gain & match measurements; Graphs “LNA Gain,” “Linear Noise Figure” and “Match.”  This means that either model is valid, but they both have different uses.  The nonlinear model is required for nonlinear performance metrics like compression, OIP3, and nonlinear noise.  The linear model is good for fast Monte Carlo analysis or design methods like the Cripps test bench.

Schematic - LNA_Model

Graph - LNA Gain

Graph - Linear Noise Figure

Graph - Noise Voltage

Graph - Nonlinear and Linearized Noise

Graph - Match