Linear CAD Software

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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.

Here's an index to our information on linear CAD software:

Synthesis of microwave circuits

Analysis of microwave circuits

Optimization of microwave circuits (now a separate page!)

Physical versus ideal elements

Schematic capture versus netlist interface

Linear CAD software vendors, big and small

Free linear analysis software!

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.

Optimization of microwave circuits

This has been moved to a separate page.

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 skin effect 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". There is merit in having a design suite from one vendor, especially if the interface is seamless between each part of the package.

Here are the "Big Three" for linear simulation:

Applied Wave Research (part of National National Instruments) Microwave Office: AWR has done the unthinkable and provided a "socket" interface that allows the user to plug in the competitors' EM analysis software (i.e. Momentum). Microwaves101 has been known to use MWO for analyzing circuits... since they provided us with a free copy of their product.  Thanks, guys!

Keysight's ADS (Advanced Design System): The URL for this software changes on a daily basis, so go to and start clicking stuff. 

Ansoft Designer rounds out the big three.

Microwave Software is a small business that provides some very low-cost linear analysis software that uses a netlist interface. President Jim Lev tell us the story of his small company:

"What a GREAT site (Microwaves101), but you've TOTALLY overlooked one of the (almost) "founding fathers" of RF/Microwave Circuit Design Software. A fellow (me) who "rubbed shoulders" with "GREATS" like Les Besser, Ulrich Rhode, George Vendelin, Wenzel the filter god, and more. I got Besser his first sale to put the original COMPACT on an HP-3000 at Hughes Aircraft, back when (almost) many of the cars in the parking lot had vacuum tube radios in them. I first used COMPACT on a TTY using an acoustic dial-up modem.

I was there (almost) before "the walls went up," and at a time when Eagleware was known as "Circuit Busters" (ho ho ho - who ya gonna call?) I was ham radio guy K6DGX way back then."

Jim Lev
President - Microwave Software

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Author : Unknown Editor