March 2008
System Level Analysis:
Time to Sideline the Spreadsheet
By Joel Kirshman
Marketing Segment Manager, Wireless System Design, AWR

As a teenager, when my father watched me hacking away at repairing something on my car with a screwdriver or some other common household implement, he would always point out that I could do myself a favor by using the tool designed specifically for the job. Of course, that would first require buying or otherwise locating the tool, which I decided would take too much time. And besides, I had the screwdriver. I usually got the job done, albeit with a few cuts and scrapes and a developing bad humor, in a rather long time. It wasn’t an elegant solution, but it worked. However, once my stubborn nature gave way to reason (years later) I learned that I could indeed save time and anguish and make repairs in a more effective fashion if I simply did as he had recommended in the first place.

Tens of thousands of design engineers are essentially working the way I once did by using a common generic tool (the Excel spreadsheet) to determine how their designs will perform at the system level. Some of these spreadsheets are truly marvels of ingenuity, packed with macros that take as many variables as possible into consideration in order to get a reasonable first-pass approximation of the circuit’s likely performance “in situ.” But spreadsheets, as generic tools, cannot be expected to deliver results as accurate as a tool designed for the purpose. For example, they cannot incorporate the effects of image noise, phase noise, or mismatch between components into the performance equation, and these omissions can make all the difference in optimizing the design to maintain RF link quality, for example.

Spreadsheets are also generally the creations of a single individual who has spent dozens if not hundreds of hours customizing them, so other designers who attempt to use them will have no idea what mysterious factors are embedded inside. Errors in the spreadsheet also tend to become further and further embedded over time, ultimately making them virtually impossible to identify. But perhaps the most severe shortcoming of the spreadsheet approach is that it takes an enormous amount of valuable time to deliver acceptable results, which can only be obtained after multiple, manual iterations. In short, the spreadsheet is simply the wrong tool for the job. But it sure is handy.

Unfortunately, through no fault of designers, it’s typically been the only tool available. The fact is that while developers of digital and circuit-level EDA tools have highly refined their products to deliver exceptionally accurate results in a very short time, system level tools have been given short shrift. In other words, designers have spent hundreds of hours elevating the spreadsheet approach to an art form because EDA vendors had nothing better to offer, or at least not at a reasonable price. It’s an embarrassing shortcoming for those of us in the EDA business and one that AWR is remedying.

The RF Budget Analysis Tool, which has been part of our company’s Visual System Simulator (VSS) product for more than a year, replaces the archaic spreadsheet approach with “the right tool for the job.” It lets designers make RF cascaded measurements throughout a communications link, including gain, noise figure, third-order intercept, and image noise. The interaction of components within a transceiver design can be evaluated before components are connected at the circuit level. It can typically do this in a few minutes, where a similar analysis on a spreadsheet (if indeed it could be done at all) would take hours and still produce inferior results.

Let’s use two MMIC receiver front ends as an example. Each one has a different image noise rejection filter along with a bandpass filter, low-noise amplifier, quadrature mixer, low-pass filter, and 50-ohm termination. Using a seventh-order image noise rejection filter produces an RF link with a 3.4 dB noise figure, which is 0.2 dB better than achieved by the front end using a third-order filter. However, it also costs more and similar performance could possibly be achieved for less money by using the third-order filter and an LNA with a lower noise figure. The RF Budget Analysis Tool within VSS can evaluate both scenarios in a few minutes, while also providing a yield analysis and keeping track of changes to other figures of merit along the way.

VSS also has a frequency domain simulation tool called RF Inspector that helps designers root out the causes of intermodulation products in the RF link, including the effect of conversions, harmonics, intermodulation, and both thermal and phase noise. It identifies the contributions of every component in the circuit to the production of unwanted signals, so designers can work on the biggest contributors first.

To illustrate the benefits of this tool, we can use as an example a circuit that downconverts the desired 2.1 GHz signal to an intermediate frequency. The circuit is exposed to unwanted interfering signals at 1.4 GHz and 2.4 GHz that can replicate undesired signals generated by other nearby applications. The RF Inspector tool evaluates the effects of the unwanted signals and illustrates the ramifications of improper filtering ahead of the downconversion process. Placing a marker on a particular tone immediately shows the associated power level and signal generation “heritage” at that frequency. The entire frequency spectrum can be monitored, or just the signal, the phase noise of the RF link, or the effect of thermal noise. It’s also able to generate a table with color-coded flags that identify the desired signal along with intermodulation and distortion products.

All of this analysis can be performed in a few minutes, and since it is performed by “independent” software tools (rather than a highly-customized Excel spreadsheet), it is effectively “transportable”. Not only do these tools far more comprehensively evaluate the true performance of a circuit at the system level, they do so in a tiny fraction of the time required by the spreadsheet approach (not even including the time required to create the spreadsheet itself) at a much earlier stage of the design process, where changes can more cost-effectively be made. The inevitable result is a reduction of the design cycle and a commensurate reduction in cost. The RF analysis tools in VSS fill a long-standing and much-needed void in the RF and microwave designer’s toolkit. They’re the right tools for the job.

AWR
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