Circuit simulators for circuit analysis in graduate engineering courses

Circuit simulators are extensively used as an aid in many courses at the graduate level in many different engineering and applied sciences programs. SPICE (Simulation Program with Integrated Circuits Emphasis) based software programs have been used for long due to their traditional market position. If we focus on circuits analysis and linear systems subjects, the features that are required from a given simulator can be found in student/limited versions of commercial EDA (Electronic Design Automation) suites or in freeware/open source codes. In this contribution, we analyse and compare the most revelant characteristics of a representative set of the software packages that are commonly adopted in these courses, focusing on the Spanish University system. For this purpose, the analysis (transient, DC and AC) of a typical second order passive low-pass filter is approached making use of each one. Then, we give some comments and recommendations, based on our own expertise, always taking into account the particular circumstances within a given academic scenario.


Introduction
Electrical circuits and linear systems are in the curriculum of all the programs in engineering and applied sciences.A proper teaching and understanding of these topics are key in the correct development of the career ot the students (Johnson, Butcher, Ozogul, & Reisslein, 2014).Simple circuits containing few components can be analytically approached with regular graduate mathematics (complex numbers, arrays and calculus) or experimentally studied within basic workbenches.When circuits and systems become more complexes, or as complementary supporting tool, software simulators enter into play (Gimeno et al., 2016).
Circuit simulators have been used as pedagogical aids for long (Pota, 1997).The software SPICE (Simulation Program with Integrated Circuits Emphasis) was originally developed in 1973 by Donald O. Pederson and Laurence W. Nagel at the University of California, Berkeley.Since then, it has become a globally accepted reference in this field (Prigozy, 1989), being recognized in 2011, in its 40 th anyversary, as an IEEE Milestone (Nagel, 2011).
Nowadays, different vendors provide circuit simulators embedded in electronic design software suites with a large number of improved features including differend kind of analysis (transient, frequencial, noise …), behavioral models for nonlinear (electronic) elements, printed circuit board design and development tools, high frequency capabilities and many others.Of course, not for free.In this scenario, a good election of a circuit simulation software for electrical circuits and and linear system at the graduate courses in engineering and applied sciences programs has turned out a major issue in the definition of the curriculum.
Up to our knowledge, the use of a particular circuit simulator has been analysed in a particular topic (Hart, 1993) or in a particular university (N. A. 0. Demerdash, 1993), but no as a comparative approach among different ones used in different universities.In this paper, we make a comparison among different options that can be found, giving recommendations depending on the particular case.After defining the scope of the study in section 2, the considered method is described in section 3.Then, conclusions are provided.

Scope of the study
As before stated, we will focus on circuit analysis and linear systems subjects included in the first courses of engineering and applied sciences programs.Slightly changing from one case to another, in the most of these courses the following topics are treated: -Linear components: R, C and L.
-Transient, DC and AC regimes.
-One or Two ports networks.Quadripoles.Transfer functions.
-Magnetic coupling.Lineal transformers, -Laplace transform and Fourier analysis (optional) -Transmission lines (optional) -Passive filters (optional) -Linear applications of operational amplifiers (rare) The topics marked as optional are common in electrical engineering programs but are often skipped in science oriented courses.Then, the considered circuit simulator should include all the above mentioned capabilities.We have limited our study to circuit simulators that are currently being used or have been used in the recent past in graduate courses of engineering programs in Spanish universities, as detailed in Tab. 1.All of them fall withing the student version or the open software schemes.

PSpice
The most of current and past circuit simulators are based on the original kernel of SPICE, developed at Berkeley in the seventies, being SPICE3F5 the last freely released version.In fact, since then, commercial versions with enhanced tools have evolved based on the original SPICE code.PSpice was the first PC oriented Spice simulator, firstly released in 1984 by MicroSim.In 1998, MicroSim was acquired by OrCAD, which itself was subsequently purchased by Cadence Design in July 1999.In this way, the last version of MicroSim PSpice (8.0) was discontinued in 1998, but a lot of institutions are still using it, assuming that no updates or new models will be available.

OrCAD PSpice
OrCAD PSpice took over MicroSim PSpice with its 9.1 release, including a student vesion, which become as popular as its predecessor, remaining as the reference in the academic environment.Now we are with the 16 th version.

MicroCAP
First version of MicroCAP (from Spectrum Software) was released in 1982 both for Apple II and IBM platforms.Since then, it has followed a compiting evolution with PSpice, up to the current 11 th version, also releasing the corresponding student versions.

LTSpice
LTSpice is a SPICE based simulator, schematic capture and waveform viewer, courtesy of Linear Technology (LT), also providing electronic models of their own components.With a lot of support from LT and general users, it display comparable features with PSpice or Microcap.With the purchasing of LT by Analog Devices in 2017, the free distribution of LTSpice could be questioned.

QUCS
The Quite Universal Circuit Simulator (QUCS) is a free-software electronics circuit simulator, released under the GPL scheme.It is intented as an easier-to-use tool, when compared with OrCAD or MicroCAP suites.It is powerful, easy-to-use and with a high number of additional featres such as transmission lines analysis, Verilog/VHDL support or allowing the use of different simulation kernels.

gEDA
It was launched as a project for providing Linux users of a good EDA suite, including Gnucap as the circuit simulator.Being an excellent tool, it will be not considered in the current study.

Web based simulators
Nowadays, open web-based circuit simulators can be also considered for circuit analysys (Weyten, Rombouts, & De Maeyer, 2009).Among the most popular, we can name PartSim (-PartSim,‖ 2018), CircuitLab (-CircuitLab,‖ 2018) and EasyEDA (-EasyEDA,‖ 2018) as excellent options.The main concern on web based simulators is that there is not warranty of having it available for long, what is a handicap for the programation of future courses.Circuit simulators for circuit analysis in graduate engineering courses

Full versions
Some vendors offer university programs at a reasonable price, giving access to full versions of the software.ADS from Keysight, AWR from National Instruments or Virtuoso/Systems from Cadence are valid examples of this policy.These software packages are very powerful and all of them include tools for circuit analysis.Nevertheless, they focus on specific fields of electronics (high frequeny or microelectronics) and they require advanced training efforts, not justified at first courses.In addition, the number of released licensed sites in these university programs are usually limited, which is often sufficient in postgraduate courses, but hardly compatible for scheduling different laboratory groups.

Method
In order to highlight the above exposed considerations, a representative example is approached with the different considered circuit simulators.We will consider a second order low-pass passive filter, and we will perform the typical DC, transient and AC analysis.Such kind of circuits are frequently used as testbenches for the evaluation of simulation tools and academic approaches (Coppens, Van den Bossche, & De Cock, 2017).
The results are collected in Table 2.
With proper teching guidance, freshman year students are capable to perform any of the proposed analysis in less than an hour, regardless the considered simulator.That is to say, a laboratory session dealing with the full analysis of this circuit (schematic, transient and frequency analysis) can be easily completed in three hours, which is a common duration of the laboratory sessions.If mixed experimental/simulation laboratory sessions are considered, it is a good practice to dedicate the first laboratory session as introductory both for the basic instruments and the used simulator.
In any case, all the considered circuit simulators have demonstrated its good performance in the different analysis listed at the beginning of Section 2. Parametric analysis, use of combined equations, exportation of results, compability with netlists of different origins and many other are common built-in features.In summary, all the considered simulators perfectly match the requirements for circuit analysis courses.In this regards, we should note that, in addition to the technical capabilities of the software, other characteristics play an important role in the selection process.Academically speaking, we consider as valuable the following characteristics, with valorations collected in Tab.3:  Freely available for the students  Good amount of supporting documents, tutorials …  Good learning curve, allowing students to focus on the problem, not on the tool. Useful for other subjets: analog/digital electronics, VLSI design, HF circuits …