The Evolving Role of RF Test and Modeling Tools for RFICs
Executive Interview with Stephane Dellier, co-founder of AMCAD-Engineering
1. How and when did AMCAD Engineering get started? What’s the background of the company?
The company get started in December 2004 and will turn 10 this year. We (the 3 co-founders) were PhD students at the XLIM laboratory / University of Limoges in France. Freshly graduated we created the company to answer an unsatisfied need from industrial partners on characterization and modeling services. Our first customer was the French Space Agency (CNES) which is still among our customers today. Nowadays, the R&D team is composed of 10 PhDs, highly skilled in different fields, RF measurements, component modeling, circuit modeling, RF design, and software development.
2. I see you are partners with Maury Microwave, how did this partnership arise and what does it entail?
Because AMCAD was concentrated on compact modeling, our target was to develop a load pull solution dedicated to transistor's model validation. To run accurate simulations, the compact model need to reproduce the transistor's input impedance variation versus power level and load matching conditions. As a consequence, our aim was to develop a real time VNA based load pull solution, which start to become a mainstream in the load pull industry now.
To spread this technology, Maury Microwave was a natural partner for us, because of their state-of-the art tuner technology, load pull expertise, and market penetration as the leaders in load pull. At the same time, Maury was looking for partners to increase their portfolio of measurement solutions and complete the design flow. The alliance between Maury, AMCAD and Anteverta, 3 high tech companies, is a real asset. Together, we can offer unrivaled measurement speed, and enhanced measurement valorization.
3. The company offers software products to characterize RF and microwave circuits as well as characterization services themselves. Tell us about the software products, how they work, what type of data do they input and what data do they provide and how do designers use this data?
The software we provide was originally developed for our own needs for the service activity, and grew up with the needs and ideas of our customers. This IVCAD software is now a comprehensive solution, offered by our partner Maury Microwave, made of several modules at different levels. At component level pulsed IV/RF measurements are used to extract compact transistor models, and vector-receiver Load-Pull is used to validate and refine these models dedicated to advanced power amplifier design. At circuit level STAN tool helps to analyse your circuit design and check the stability for linear and non-linear operating conditions. And at system level it offers "state of the art" behavioral modeling capabilities for power amplifiers, mixers, limiters, opening new roads for system level simulations when using complex modulated RF signals.
4. Do you offer training programs to use your products?
Training is part of measurement system installation provided by Maury Microwave. In addition we provide on-demand training when the customers use our modeling tools. As each customer’s situation is unique, this part is closely defined between the customer and our modeling experts. Basically, the story starts when we sell the modeling services. Because of the accuracy of our models, then, the customers want to have and use this solution internally
5. What are some of the trends in device (FET) modeling these days? Are companies more or less able to perform their own internal modeling these days?
Designer requirements on the accuracy and flexibility of the device models are always increasing. This is directly linked to the increasing complexity of the targeted applications and signals. In the same time the effects highlighted on semiconductor technologies in maturation (eg. GaN) brings new challenges to the modeling engineers. There are some foundries and OEMs with strong modeling group but this is not yet the majority of them because it implies dedicated equipment and experts. This is part of our commitment to democratize suitable solutions for this complex but necessary step of an advanced design flow.
6. So you support linear S-parameter models, nonlinear device models, X-parameters and system behavioral models. Which ones seem to be in highest demand and why?
At component level, even if people have been curious about behavioral models based on Poly Harmonic Distortion (PHD) models, compact model is still the preferred solution as it is much more flexible and comprehensive, including various effects and allowing different kind of simulations. You can afford what-if analyses and get the best performances from a given technology, while securing your design with complete stability analysis. At system level, gone are the days when system architects can be satisfied with Excel spreadsheets, and the demand for system behavioral models is growing up very quickly. Behavioral modeling is now a hot topic for AMCAD, because our customers want to check to specifications at the system level before launching an expensive production. While a lot of efforts have been done to improve the system simulators, existing behavioral models of RF circuits used by these simulators are still suffering from a lack of accuracy when dealing with complex modulated signals. Indeed, these circuits can reveal some nonlinear memory effects, triggered by different time constants. Basically, we are dealing with simultaneous High Frequency Memory Effects, caused by the RF carrier, and Low Frequency Memory effects caused by the RF envelop of the modulated signal. Again, these developments are made in close cooperation with our customers and partners. As example, AMCAD has an ongoing collaboration with Thales Systèmes Aeroportés, a global technology leader for the defence, security, and aerospace markets, who is using our Multi-Harmonic-Volterra (MHV) models for PAs used in Electronically Scanned Array (AESA) applications.
7. Your models are available for all RF circuit simulators such as ADS and Microwave Office? Are there differences between the models for either simulator? Is compatibility between simulators something customers are looking for?
Our models are supported in the two main RF design tools that are ADS and Microwave Office. The models are nearly the same in both tools. When we are working with foundries, they need to have the models in both tools because they need to support their own customers on each tool. This is different with OEMs which generally have their design team working on one preferred RF design tool.
8. For the companies that don’t perform their own modeling, what kind of services do you provide?
We provide comprehensive modeling services, including all the measurements (pulsed IV/RF, Load-Pull) needed for model extraction and validation. We have a 700sq ft dedicated clean room for on-wafer measurements. The extracted models can be either really dedicated to a specific application (it is often the case when an OEM is not satisfied enough with the foundry model), or more generic, that is to say validated on wider operating conditions (bias, frequency, temperature...) and scalable (this is what foundries need to integrate into their PDKs).
9. Do you have device libraries already characterized that are available for licensing?
No we have not chosen this business model, because this one is more dedicated to passive device modeling, while AMCAD's strength is more focused on active device modeling field. The libraries of transistors are mainly provided by the foundries, which have a better process tractability to maintain their design kits. Our target is compact modeling improvement rather than modeling production.
10. What do designers get from Pulsed IV measurements that they can’t get from non-pulsed measurements?
The question is how does one accurately characterize a technology that suffers from self-heating and trapping (eg. GaN), and how does one develop accurate linear and nonlinear transistor models? The pulsed IV measurements allow to characterize specifically the thermal effects and trapping effects and get precious information for accurate modeling. This is capital for the designers because it will affect significantly the RF performances, even on more mature technologies, or ones without trapping effects (LDMOS).
11. Has GaN modeling become a big market opportunity for AMCAD and if so, are there unique issues to modeling GaN devices?
Definitively yes, the company grew up in the same time as this technology maturation, and even if part of our business concerns other technologies such as GaAs or Si LDMOS, GaN is and will mostly be our mainstream market. Modeling issues on GaN have been cited above, these are trapping and thermal effects, but significant progress have been done on this recently along with the technology improvements. Another challenge is at the system level, to see how these effects will affect the overall circuits behaviour, and how to take it into account in system level models, when you apply modulated signals in complex sub-systems.