"This time we'll have to run the entire design in parallel," the ambitious engineering manager announced to the skeptical crowd of engineers and designers that had gathered for the project kickoff. "Our beloved and highly realistic management has given us the task of cutting our design times in half - so the RF guys will design their circuits at the same time that you lay out the board for the analog and digital modules," he added with a glued-on confidence.
"Sure, go ahead, it's your dream," the team muttered, united in an obvious lack of conviction.
As the plot tightens to insufferable tension, we'll take a look at how the ambitious team ended up in this precarious situation and how they can succeed in meeting their aggressive performance target.
Incorporating RF Circuits
As in most cases, when designing mixed technology, the RF modules had been designed on specialized RF design tools and after being tweaked into perfection, the circuit was presented to the board design team who now had to fit the RF module onto an already overcrowded PCB. The digital and analog engineers would express concerns that RF energy will leak into their circuits and jeopardize safe operation. As if magic, the RF engineers have the exact same concerns- - from the opposite point-of-view. To make matters worse, the only means of integration between RF design and board design was by ASCII IFF translators.
This had led to that form of integration referred to as "voice operated cursor" (RF designer commands the mouse arm of the PCB designer) was commonly used. Not only did this add significant time to the project but in addition, both the RF designer and the PCB designer spent significant time doing the same job.
This story was especially common when the RF designers had to design custom metal shapes for their RF circuits.
Pretty much every design ended up in multiple iterations as late in the game problems are discovered through full board RF verification that the RF module, when implemented on the real (non ideal) PCB, no longer met the performance requirements. You may ask "Why would this be found late in the design process?" The tools for verification are at hand in the RF design tool in the form of electromagnetic extraction of the real PCB features. The culprit is the tool integration. Generating the data for electromagnetic modeling was extremely time consuming and always demanded a lot of manual set up. Hence, it was not practical to run this validation too frequently. The cost is added design cycles.
So much for the history ; let's look at how the team can cut significant time off the schedule and at the same time reach closure at a predictable schedule and predictable quality.
Designing in parallel " or "concurrent design" as it's also called, is the right move. In a sequential design flow we will undoubtedly add cycle time for once but in the case of an analog/digital/RF mixed technology design, we also risk adding cycles, which is even more costly. The concurrent design flow is a good match when it comes to RF design because of its iterative nature. You design an RF module using parametric circuit level models, which regard ground planes as ideal and don't take coupling to surrounding circuits into consideration. Then as you implement the circuit on the PCB, you are typically forced to change it somewhat to make it fit into the available board space. This requires re-simulation and further tweaks of the circuit. The benefits of a concurrent design flow are several.
First, the obvious benefits of applying multiple designers concurrently on the same design database are that multiple designers, each expert in different disciplines such as RF, digital, analog, can work in parallel to dramatically cut design time. Less obvious are the benefits that follow from the possibility to keep the RF modules constantly validated as the circuit and PCB evolves. By constantly keeping the performance within specifications through repeated RF circuit-level simulations as the circuit is being adapted to the sparse board space, followed by detailed electromagnetic level simulations closer to design closure, let you reach closure faster and achieve a circuit solution that works in the context of the real board " with non-ideal ground planes, multiple circuit modules in close proximity and lots and lots of ground vias. You will also be able to know the tolerances for success and tune the RF circuit shapes so that the manufacturing tolerances has as little impact as possible on the overall RF performance thus increasing yield and squeezing costs.
To achieve a concurrent design flow, you need a tool integration that is so robust and so fast that it takes practically no time to copy a design from the RF design environment to the board environment, and back again, to validate changes. The previous IFF based solutions could never reach that level of integration as the tool sets are too different.
