Analog Circuit Models Library

Intro:
The Analog Circuit model library of CSIM contains components for modeling analog circuits. This document describes the models and how to use them. The analog component models are located under $CSIM_MODEL_LIBS/analog_circuits. Several example simulations are also included under the examples subdirectory.

Although the components may be useful as-is, they are presently considered more of a curiosity or starting-point which could be further refined, extended, and improved. The component models form examples for how to model analog elements within CSIM. With these models, many other continuous systems can be modeled. For example, thermal heat-flow modeling, mechanical systems (ex. mass/spring/damper), hydraulic, traffic-flows, as well as many other industrial applications involving inertia and proportional/differential/integro (PDI) controllers.

All the existing capabilities of CSIM can be leveraged by these models, including hierarchy, batch and interactive simulation, graphics, and automatic experiment recursion control or design value optimization. Interfacing to higher levels of system models is supported by virtue of the common environment.

Usage:
To use these models, include (by-reference) $CSIM_MODEL_LIBS/analog/Library.sim in your top-level diagram. Draw circuits by snapping-down and connecting the components. The components must be interconnected through junction boxes, which can be shrunk to a virtual dot.

Most components have attributes which are specific to their function. For example, the resistor model has a resistance attribute, capacitors have capacitance, inductors have inductance, and the voltage source has a voltage setting. When you click on the attributes of each component, the appropriate attribute is listed, along with any default value and units. Set the attributes of the components to their appropriate values and click Tools/Build Simulation. If the simulation builds without errors, run the simulation.

Figure 1 - Example circuit model.

A set of example circuit models is included in the analog_circuit model library with the prefix, test.... It is recommended to place meters on test-points of your circuits to study their behavior. Besides giving immediate readout of voltage or current, the meters produce plot files which can be plotted after the simulation with XGRAPH.

Figure 2 - Example voltage plot from meter.

The following analog component models are provided:

Resistor
Capacitor
Inductor

Battery/V+
Switch
Diode

Zenior Diode
SCR
Transformer

Volt-Meter
Amp-Meter

NPN/PNP BJ-Transistors
N-FET
MOS-FET

The switch model provides interactive freedom by allowing you to throw switches while your circuit operates, and to see how your circuit responds. The examples contain demonstrations of this.

Typical applications include:

Amplifier design.
Filter design.
RF circuit design.
A/D-D/A Converter design.
Power-supply design.
Oscillator design.
Cable/Wave-guide design.
Tolerance sensitivity studies.
Failure mode analysis.

A set of icons accompanies the Analog Circuit library. The symbols are automatically attached to the components when you snap them down. Located in $CSIM_MODEL_LIBS/analog_circuits/images are the following schematic symbols:

resistor.ppm	capacitor.ppm	inductor.ppm	switch.ppm
battery.ppm	diode.ppm	zenior.ppm	SCR.ppm
LED.ppm	photodiode.ppm	transformer.ppm	JFET.ppm
MOSFET.ppm	npn_transistor.ppm	pnp_transistor.ppm

Figure 3 - Component icons.

Principles of Operation:
This circuit library is based on the first-principles of circuit laws from Kirchhoff, Thevenin, and Norton, together with a convergent-relaxation method which is distinct from the classic analytical methods applied by other tools such as SPICE. Although virtually invisible in most circuits, the crucial model of this library is the junction model. All component leads must connect to a junction. And Junctions can accommodate any number of connections. The junction model fulfills the basic Kirkoff rule: the net sum of currents following into and out-of a junction must be zero.

If a junction realizes a net in-flow of current (charge), it increases it's voltage until the current-sum returns to zero, and vice-versa for a net out-flow.

All other components, besides the junction, sense the voltage differentials between their leads (ports) which are set by the junctions, and they [the components] respond with current flow amounts into, or out-of, their respective leads or junction connections. For example, the resistor model senses the voltages, v1 and v2, on it's leads, and responds with current i = (v2-v1)/R out its top port, and i = (v1-v2)/R out it's bottom port.

In this way, the circuit models form a self-regulating, auto-convergent system based on a simple dichotomy which can be summarized as:

Junctions sense current-flow and set voltages.
Components sense voltage-differentials and set currents.

(For academic purposes, a dual library could be written based on the opposite combination of principles, like Thevenin current-loops. But the chosen rules are sufficient and convenient, if followed consistently. Other possibilities were considered, such as mixtures of some components which set voltage and others current, or of components which sense and set both quantities. But these approaches seemed unwieldy or quickly became limited.)

Reactive (time-varying) components, such as capacitor or inductor use their corresponding differential/integral function. For example, the inductor's governing equation for current is: i = integral( V/dt ) / L. The capacitor model is i = C * dV/dt.

Units are in basic System-International (SI) units, and their normal derived units: Seconds, Volts, Amps, Ohms, Henries, Farads. A set of convenient macros are included, which cause the RLC circuits to produce the correct time-constants.

	macro mH = * 1.0e-3
	macro uH = * 1.0e-6
	macro pH = * 1.0e-12
	macro mF = * 1.0e-3
	macro uF = * 1.0e-6
	macro pF = * 1.0e-12
	macro Kohm = * 1.0e3
	macro Mohm = * 1.0e6

This allows you to set an inductance to say, 4.3 uH, a capacitor to say, 3.3 uF, or a resistor to 6.8 Kohm.

Examples:

Figure 4 - Example circuit model.

Issues:

The models are not yet as efficient as they could be. They were initially based on a constant time-step, which was set conservatively small. (The first version of this library was for proof-of-concept only.) The time-step is now somewhat variable, but still conservative, based on the rate of state-changes within the circuit. While not-only improving efficiency, this also improves accuracy by decreasing the time-step accordingly during transients. Most notably, the time-step for state updates is determined independently for each component, by virtue of CSIM's unique capabilities. More work needs to be done to optimize the time-step control further.
The models are also less efficient because components transfer current/voltage information on every time-step, instead of only when conditions change. In the future this will be improved.
A variable resistor (potentiometer) component model needs to be added. This would enable additional interaction with circuits beyond the existing switch model.

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