This FAQ provides some answers to the most common questions about circuit simulators and EM simulators. If you can't find the answer for your question, please send it to firstname.lastname@example.org I will try to add the question and answer to this page.
[Q] What is Full-Wave Electromagnetic simulator ?
[A] Full-wave Electromagnetic(EM) simulator is
a computer program for simulating the EM wave propagation subject to different
boundary conditions (different geometries). The full-wave simulator solves
the Maxwell's equations either in
frequency domain or in time domain.
[Q] Why we need Full-Wave Electromagnetic simulator ?
[A] The law of natural tells us that the behavior of an object depends on the electrical size instead of physical size. Suppose we have a piece of metal of arbitrary shape. For very low frequencies (with respect to the size of the metal), the voltage and current are approximately the same. But when the frequency becomes higher, the behavior of the metal becomes very complicated. A good example is a piece of metal with a cross on top of a ground plane. Depending on the frequency, this configuration can be a short circuit, a low-pass filter, or even an antenna. Since it's not easy to predict the high-frequency behavior by looking at it directly, a full-wave simulator which directly solves the Maxwell equations is necessary.
[Q] It seems Full-Wave Electromagnetic simulators
are getting popular these days,
are they new ?
[A] No, they are probably older than your father. The Full-wave methods have been there for many years.
[Q] If they have been around for a long time, why weren't they widely used before?
[A] The reason is very simple -- computer technology. Full-wave EM simulators require lots of computational powers and resources which were not available cheaply 10 years ago.
[Q] Is there Half-wave Electromagnetic simulator ?
[A] Probably not. But there are some other types of methods such like quasi-static analysis which simplifies the circuit by its static behavior and extends the static model to high frequency. These methods were popular twenty years ago but the accuracy depends on many factors (which means it's not easy to do).
[Q] How about popular software packages such like EEsof Libra and MDS? they can do high frequency simulation too!
[A] The popular simulators such like Libra and MDS are what EM society refered as circuit simulators. This type of simulators treat a component as a network of lumped element (capacitor, inductor, resisotr...) in time domain or frequency domain. By connecting all the components with their network expressions, the response of the entire circuit can be obtained by solving the resultant network. The advantage of this method is that it is much faster than solving the circuit using full-wave simulators.
[Q] I am confused! You said the circuit simulators replace the circuit components by their network expression, then why I need full-wave simulator?
[A] The question is how do we obtain the network
expression for each component? There are several ways, one way is to measure
circuits with various parameters and then find the network expression.
You can easily see how much effort and money have to be put there to get
an accurate network expression. Another way is use the
full-wave simulator and let the computer do it!!
[Q] So does that mean the circuit simulator must have the network expression before we can use it to simulate the circuit?
[A] You've got it!!
[Q] Now I understand, if the circuit components that I want to use can not be found in the circuit simulator's library, then I must use EM simulator to get the network expression!
[A] Not necessasarily, you can try to find the
formula or equivalent circuit expression from somewhere and put those into
the circuit simulator. If you can't find reliable one, then the good way
is to use EM simulator to obtain the network
expression (which is easier than building thousands of circuits and measuring those).
[Q] What's the advantage and disadvantage for both circuit simulator and EM simulator?
[A] This will be general only, don't be too picky!
Circuit Simulator -- Much faster, can do linear, nonlinear simulations, optimizations. But its accuracy depends on the circuit component models, and it might have problem for the circuits with strong coupling and radiation.
EM simulator -- Slow, computationally hungry, may have problem for large circuits. But it's more accurate!
[Q] Do I need to have both simulators?
[A] This is the most common question people ask!
The answer is that both simulators are necessary for a sophisticated high-frequency
circuit design!! But if you are only designing circuits with very traditional
circuit components for which
you can easily find in circuit simulator, then you might not need EM simulator. But if you are only designing non-traditional components for which you can't find in the circuit simulator, then what you need is an EM simulator
[Q] If I only use the circuit provided by the circuit simulator, no non-traditional circuit components, then I don't have to use EM simulator, is this correct?
[A] Yes, if you can make sure that you will never
use non-traditional circuit components and the circuit components are placed
far enough and no housing, crosstalk effects..... Also you have to be careful
about the accuracy of the circuit
component models since most of the models have limitations such as they are only valid within a certain frequency range, for some certain substrate thicknesses and dielectric constants. Be sure to check the manual came with the circuit
simulator. If you can not be so sure that the model is valid for your purpose, EM simulator is necessary for getting the information!
[Q] Can you show some real examples on the necessity of full-wave simulator?
[A] Sure, there are tons of them. One example is that one day I was sitting in a talk, the speaker was working in a very famous semiconductor company doing power amplifier design. PA design is typically very difficult, but he mentioned that he simulated the entire circuit thouroughly using full-wave simulations to take all the parasitic effects into account and put all the info into circuit simulator. The result was quite good compared to measured result. Another example that I heard is an EM guy joined an RFIC research group and discovered that all the planar spiral inductors (on CMOS) that the group used before were all wrong since they didn't use full-wave simulator, and the measurements showed the same thing.
[Q] So what's the conclusion?
[A] Full-wave simulator simulates the EM wave propagating in the circuit (usually a small portion of the entire big circuit) and gets the solution. Circuit simulator uses the information obtained from full-wave simulator and connects hundreds of them to get the solution for the entire circuit!! Which one to choose depends on the circuits you are designing!!