To go to our main CAD page to go to a page on free EDA software What do we mean by 'linear' CAD software? This is simulation stuff that obeys 'normal' transmission line theory with closed-form equations rather than finite-element analysis of Maxwell's equations, does not produce results that are have multiple frequency solutions (like mixers) or does not produce results that are functions of power (non-linear simulations). We'll also stay away from the topic of filter synthesis, and anything to do with SPICE. Granted, almost all CAD programs cross these boundaries today, but we will deal with these topics separately to make the discussion more focused. Surabhi 2000 software. Here's an index to our information on linear CAD software: (now a separate page!) Synthesis of microwave circuits Synthesis is the 'art' of selecting a network that will provide a useful circuit response to a given design problem. For example, selecting an L-C network in a lumped-element filter. Most synthesis is done 'by hand', meaning the designer looks for a similar circuit built by a previous engineer, and reverse-engineers his matching networks. There is one exception: there are a lot of software packages that do filter synthesis quite well (which we will one day cover on a separate page). Analysis of microwave circuits Analysis is where the software takes your network and all of the component values and crunches through a daunting amount of math to provide you with a description of the response of your network, for example, the calculating S-parameters of an amplifier over frequency. Other typical outputs that you can get from linear simulations include gain and return loss (in decibels or in magnitude), group delay. VSWR, and various stability factors for amplifier circuits. ![]() Computer Aided Design program for Microwave Circuits, developed at the California Institute of Technology. PUFF 2.1 + DOSBOX - LINK How to install PUFF. Optimization of microwave circuits This has been moved to a Netlist versus schematic capture interface In software driven from a netlist, the user defines the nodes of the network and assigns them unique numbers. All 'modern' linear analysis products use schematic capture as the interface. Using a schematic capture interface, the user grabs and moves itty-bitty pictures of the overall circuit, such as capacitors and transistors.
Most people prefer this interface. The errors that are most prevalent are due to mistakes due to misreading tiny fonts. Note to software vendors: no one's eyesight ever got better doing computer-aided design! Ideal versus physical elements Ideal transmission lines are analyzed as lossless structures of fixed characteristic impedance (no dispersion) with lengths measured in electrical degrees at one frequency. Ideal lumped elements have no real resistive loss, and no parasitic components that cause unwanted resonances. Physical elements are 'more real', such as transmission lines realized in microstrip or stripline with finite conductor heights and losses associated with the and surface roughness. Quite often a design will start out using ideal transmission lines and lumped elements, because the designer can better apply microwave theory this way. Then later when he/she improves the design fidelity using inductors with finite Q, capacitors with series resonant frequencies, and microstrip transmission lines with loss and dispersion, the design takes one step backward as the ideal response is degraded. Getting the design to provide nearly the ideal response by optimizing all of the physical elements is why you get the big bucks! Linear analysis versus nonlinear analysis Linear simulation implies that power and voltage levels have no effect on the outcome of an analysis. ![]() For filters and small-signal amplifiers like LNAs, this is good enough. For power amplifiers and frequency conversion devices such as mixers, you can't finish a design without nonlinear analysis. One common method of non-linear analysis is harmonic balance. We've started a page on the topic, here. Time domain versus frequency domain Everyone knows that using a little advanced math you can describe a linear network response in either the frequency domain or the time domain. Some linear analysis software can convert back and forth, which is great if you are trying to analyze filter responses to pulsed RF. Linear CAD software vendors big and small Today, it is almost impossible to split off the linear simulation tools from all of the other stuff, since all of the vendors want to sell you an entire 'design suite'.
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