Consumers may buy specific wireless phones and smartphones because of their eye-catching applications, but it’s the RF and microwave technology hidden inside that makes the whole wireless experience possible. Of course, that’s hardly breaking news to engineering teams feverishly trying to cram transceivers that operate in multiple bands and serve Bluetooth, WiFi, and cellular standards into a tiny form factor, faster, and for less money than the “last time”. Just the task of making all this wizardry work harmoniously together is a major undertaking.
The need to satisfy these conflicting requirements in a timely fashion has brought system-level analysis from its former position at the end of the design process to the forefront where it can do the most good, at the least cost, with the least design disruption. System-level analysis performed “up front” almost invariably results in fewer problems at the “back-end”, where modifications become far more difficult, expensive, and time-consuming to remedy.
Industry use of system-level analysis has lagged behind the level of “mainstream” high-frequency circuit design tool usage. It’s not surprising that many designers have created their own spreadsheets in Excel to perform these analyses. The case can obviously be made that a general-purpose office tool isn’t suited for such tasks, but as thousands of microwave engineers who painstakingly created fantastically-complex spreadsheets can attest – they can indeed account for numerous system-level effects.
However, even the best of spreadsheet cannot analyze the performance of an RF link in the detail required by today’s complex wireless communication systems. They cannot fully exploit nor correctly calculate the full impact of factors such as cascaded noise figure and image noise that determine a receiver’s RF link quality. They also cannot account for the effects of component mismatches, import circuit designs or S-parameter data files, or include a useful library of components.
They have no spectrum display or the ability to identify the cause of a spurious or intermodulation product. And of course, spreadsheet programs were created for wholly different types of analysis, so there is no “microwave-specific” user interface. What’s probably most vexing is that the spreadsheet “design flow” is an evolving creation, the latest version of which downstream users may be unaware of. This allows erroneous data or specifications to be unknowingly propagated forward.
That said, the benefits of using a software tool created specifically for system-level analysis are significant and merit consideration. For example, Visual System Simulator (VSS) software from AWR addresses the aforementioned shortcomings of spreadsheets. With this system-level tool, a designer can delve deep into the architecture of a communications system, analyze virtually every factor than can destroy its performance, and optimize performance by making prudent design and component choices. It can be used from the behavioral level, progressing to the component level with a circuit-level tool such as Microwave Office or Analog Office software. The final step would be to verify system performance using actual measurements.
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Of course, for designers, moving from a customized spreadsheet (and macros) that took countless hours to develop and is rightly a source of accomplishment to a dedicated system-level tool is akin to a making a major lifestyle change. It is also tough to move away from the back-end process of breadboarding and testing in the laboratory. However, growing system complexity, time-to-market pressure and an economy that does not tolerate inefficient and slow product development requires system integrated to consider using faster, modern design software. The advantages of using a dedicated system-level tool are spelled out in the AWR white paper: Upfront RF Planning Speeds System-level Analysis available at this web site
(http://www.mwjournal.com/News/article.asp?HH_ID=AR_6610 ).
Advanced system-level planning lets designers account for variables that are not feasible with a spreadsheet. For instance, today’s powerful system simulators should not be stand-alone solutions but should incorporate circuit-level details where necessary. Such details include device nonlinearities, coupling and other parasitic behaviors. VSS does so through full integration with AWR’s Microwave Office. Together they provide a complete solution, from concept through verification with baseband and microwave portions of the design evaluated together so that all possible component interactions can be considered.
Modern system-level tools can perform lab-like verification as well. VSS incorporates test benches dedicated to specific wireless standards, such as GSM/EDGE, DVB-H, DVB-T, W-CDMA, IEEE-802.11a/b/g, and WiMAX. Each one includes standard-specific measurement criteria that are applied to ensure compliance with an applicable standard. To accompany this feature, the company’s TestWave software allows circuits, subsystems, and even complete systems to be evaluated using actual standards-based waveforms generated by signal generators, and evaluated by vector signal analyzers, spectrum analyzers, vector network analyzers, and other external instruments. Complex test routines can be created, configured, and controlled by the TestWave software and the results compared with a simulation performed in VSS
As discussed in the white paper, there are two basic components within VSS to address design at the system level. RF Budget Analysis allows standard cascaded RF measurements such as gain, noise figure, and third-order intercept (including image noise) to be performed throughout the project to help ensure proper component selection. The other - RF Inspector (RFI) makes it possible to identify the cause of impairments such as intermodulation products or spurious signals anywhere in the signal chain, and then isolate and eliminate them.
RF Inspector can best be described as a frequency-domain circuit simulator. It includes the effects of conversions, harmonics, intermodulation, and mixer leakage (LO-to-IF, IF-to-RF and RF-to-IF). The RF link can be simulated with CW or modulated signals. RF Inspector can determine contributors to frequency content, identify their path, examine the voltage, current, and power of each one, and classify them as signal of interest, distortion, or interference (Figure 5).
The full spectrum, the individual input signal, or the spectrum of the distortion products can be monitored in real time. In addition, the software can monitor the spectral content of any node after simulation, so another simulation isn’t required just to monitor the output of another node. Double click on a tone to open up the RFI dialog box and detailed information appears about the content of the selected spectrum component. Flags of different colors identify the desired signal, intermodulation products, and distortion products. These are just a few of the features a modern system-level tool can provide that go far beyond what a spreadsheet can accomplish.
Summary
The current wireless system design environment makes it essential that the entire communications link be viewed as a whole, from baseband to microwave, so that all variables potentially effecting performance can be considered. These variables must be identified and dealt with as early as possible, since problems discovered later can potentially drive a design out of its time-to-market window, notwithstanding the added cost and design iterations. While many designers will no doubt continue to use spreadsheets for system-level analysis, their inherent limitations can prove costly. The transition to a dedicated high frequency system-level tool will pay dividends immediately by reducing cost and design time, and ensuring that the finished product is technically superior and manufacturable.