Melding Measurements With Design Software

Shelley Gretlein, Sherry Hess, and David Hall

Designing RF/microwave circuits and systems has grown in complexity as customers ask for smaller, denser electronic designs with more functions at lower cost. The blend of different technologies has encouraged design engineers to incorporate actual measured data into their computer-aided circuit and system simulations. Powerful assistance is needed, and having computer-aided-engineering (CAE) simulation software that can work seamlessly with high-frequency test equipment can help streamline the design flow on complicated circuits and systems.

A combination of software design and measurement tools can provide the solution needed for incorporating test results into a design flow (Fig. 1). Tools such as NI LabVIEW software from National Instruments and the Microwave Office and Visual System Simulator (VSS) software programs from AWR Corp. can be used together to augment design efforts with measurement results.

The LabVIEW software has introduced new capabilities to traditional simulation software, including new signal processing and instrument control capabilities. This is important for several reasons:

Many microwave designers are producing components designed for a particular wireless standard. Thus, while forward and reverse transmission (S₂₁ and S₁₁) performance is important, engineers must often meet a set of requirements that is tailored to a particular communications standard, such as LTE. In such a case, it is useful to incorporate measurement capability such as error-vector-magnitude (EVM) and adjacent-channel-power (ACP) functionality into a simulator.

Building circuits to operate at microwave frequencies introduces impairments that are not present at frequencies closer to DC. Given the practical limitations of producing smaller-dimensioned physical hardware to operate at microwave frequencies, it is also useful to validate a simulated system by measuring its performance with physical hardware.

One solution that addresses the challenges of increased device and system complexity is to merge the capabilities of traditional configuration microwave design tools with system design software. This approach enables engineers to perform RF measurements much earlier in the RF design flow. LabVIEW is widely recognized as powerful system design software due to its simultaneous capabilities in signal processing, instrumentation control, and measurement and control capabilities. The Microwave Office and VSS software tools focus on RF/microwave system and circuit design, layout, and analysis, respectively.

At NIWeek 2011, National Instruments and AWR demonstrated new tools for integration between LabVIEW and VSS. In this demonstration, an RF power amplifier (PA) was designed and simulated using Microwave Office. The amplifier was a base station PA from Infineon Technologies capable of as much as 250 W CW output power at cellular frequencies. Using AWR's VSS software, the amplifier's performance can be simulated as driven by an LTE waveform. A new VSS feature that enables LabVIEW code integration was used to produce an LTE waveform and measure the amplifier's RF performance.

Using a single VSS system diagram (Fig. 2), a LabVIEW integration block employs a LabVIEW virtual instrument (VI) that has been designed to produce an LTE baseband in-phase/quadrature (I/Q) waveform. In VSS, the LTE waveform is passed into a simulation model of the Infineon base station power amplifier. In VSS, the ideal waveform is amplified and distorted based on a circuit-level simulation performed in Microwave Office. The VSS amplifier model produces an output waveform that simulates the impairments from an actual amplifier. This distorted baseband I/Q waveform is then passed back into LabVIEW, which uses native LTE measurement tools to demodulate the signal and provide standard measurement results.

LabVIEW provides tight integration with existing PXI test instruments from National Instruments, including digitizers, vector signal generators (VSGs), and vector signal analyzers (VSAs). This makes it easy to integrate real-world measurements into a simulation, using a "hardware in the loop" approach to simulation. In software, the original VSS diagram is modified to pass the same LTE waveform to an RF VSG. At the hardware level, the signal generator is physically connected to a driver amplifier, then to the actual amplifier, and finally to a VSA. In the software portion of the design/test system, a new VSS block (the "hardware in the loop" part of Fig. 2) returns a "measured" I/Q waveform (created from the actual test data) that represents the actual performance of the physical amplifier. Figure 3 offers a comparison of the simulated and measured results for the PA in LabVIEW.

The LabVIEW front-panel screen shown in Fig. 3 shows that the "smearing" of the constellation plot and the spectral regrowth are nearly identical between the simulated and measured results. In addition, both parts exhibit nearly identical EVM and ACP performance levels.

As this PA design example illustrates, integration of LabVIEW code into VSS and Microwave Office introduces new signal processing and instrument control capabilities into existing circuit design and system simulation tools. As a result of this demonstration, one can observe the simulated measurement performance in "LTE terms," such as EVM and ACP. This allows for a better understanding of how well the PA adhered to 3GPP LTE test requirements, without sacrificing the RF circuit description responsible for any deviations from these same requirements.

The benefits of this type of "hardware-in-the-loop" design approach include the capability to evaluate a design in the synthesis view as well as in the verification/test view. Using the synthesis view, a circuit designer does not need to wait for the full system hardware to be ready to begin testing. Often, a key piece of the RF circuit hardware is missing, yet the analog baseband input signal is available from existing hardware, and the required characteristics of the output signal are known and specified.

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Using the verification/test view, a circuit design can perform system simulations early and continuously throughout the design, verification, and test cycle starting with the simplest models, barely representing a real system. As the design evolves during the product development process, simple models can be replaced by more accurate ones, allowing the RF circuit and subsystem design team to constantly verify system compliance with digital and baseband components. Finally, as development and prototyping progress, real and simulated hardware can be "swapped" to perform troubleshooting, verification, and system verification.

Because of the unique impairments that exist at RF/microwave frequencies, it is important to model an RF system as part of the design process. The Microwave Office and VSS programs help capture these impediments in two ways. Thermal and other nonlinear effects, including those due to baseband memory, can be simulated at the system and circuit levels in VSS and Microwave Office, respectively. Physical effects can be captured and analyzed by electromagnetic (EM) solvers and incorporated into both the Microwave Office and VSS simulations.

Design engineers at Infineon and other circuit and system developers are starting to explore the possibilities of applying hardware-in-the-loop techniques to their design strategies. Having the link between simulation software and test hardware can shave time off design cycles, by eliminating separate steps between simulation and measurement. Also, vendors can ship part models and evaluation boards to customers based upon where they are in their development cycle.

It is crucial for designers to see how successfully their simulations can predict fabricated hardware performance. With PAs playing such a critical role in wireless communications, both in performance and cost, Infineon and other equipment providers will increasingly look to solutions like tightly integrated measurements and test in the RF design flow.

Linking LabVIEW And VSS

Prior to the acquisition of AWR by National Instruments, RF/microwave designers had been using tools like Microwave Office and VSS together with LabVIEW to perform co-simulations on their designs, send measurement files, and create and compare the accuracy of different models. At times, the software programs would be used as checks against each other, in an effort to achieve the most accurate RF/microwave design in the shortest time possible.

Honeywell's Louis Brown, who has been using AWR and LabVIEW software for many years, recently posted an online comment (available at http://signal-integrity.tm.agilent.com/2011/national-instruments-joins-the-party/#comment-46008) stating, "I think the acquisition will work well since both companies listen to their customer and put customer needs first and foremost. I have been using LabView and NI hardware since. Great quality software and hardware; bugs were not an option in the frigid, offshore environment I was running my radar. I have been using LabView ever since, mostly for controlling test equipment for open test setups of my RF products, and hobby use, such as a home-brew SDR and ham radio use (audio filtering and spectro-grams for crowded CW bands)."

Integration today can be realized either as pulling model files from VSS into LabVIEW or bringing LabVIEW measurements and RF IP into VSS. On the IP front, LabVIEW software provides a flexible platform to keep pace with emerging communications standards. While engineers have long used LabVIEW to automate instrumentation, communications IP in the form of add-on toolkits in LabVIEW enable engineers to test many RF devices using a software-defined, virtual instrumentation approach. Unlike traditional RF signal generators and analyzers that contain fixed personalities for standards such as GSM/EDGE, WCDMA, LTE, wireless local area network (WLAN), and more, VI provides equivalent measurement functionality while maintaining an open software architecture. The open toolkit approach to software-defined RF test produces many benefits that include the following:

Measurements for a variety of cellular and wireless connectivity standards including GSM/EDGE, WCDMA/HSPA/HSPA+, LTE, WiMAX, Bluetooth, and ZigBee; integrated DC, digital, and baseband measurements for the same system; and significantly faster measurement times delivered by multicore central processing units (CPUs).

LabVIEW toolkits for wireless standards use virtual instrument handles to create baseband waveforms, apply I/Q impairments, perform measurement, and set RF characteristics. While RF toolkits and the corresponding example programs are explicitly designed to operate with PXI RF instrumentation, they can also be used with other instrumentation and to simulate physical-layer RF characteristics.

Historically, LabVIEW and VSS software integration for such measurement correlation was performed through file sharing or ActiveX calls. However, as RF design and test engineers demand more integration between design and test functions, VSS and LabVIEW now offer more tightly coupled integration. An online video (available at http://www.youtube.com/watch?v=RCXSsS9Otc0) shows how simulated results for a PA can be compared with measurements based on PXI test hardware. The demonstration uses an in-work, integration node from the VSS diagram. This is a native VSS block that calls LabVIEW directly via the VI server interface. The tight coupling (see figure) makes the model and analysis easier within the AWR software frameworkensuring a more productive design time. Development teams for programs such as Microwave Office and LabVIEW continue to refine the integration among the various software tools based on ideas from designers; they encourage new ideas to be shared at www.ni.com/ideas.

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