Today's RF circuit design process is frequently stalled by the inability of the engineer to characterize the electrical performance of the components or interconnects associated with a given design. Successful development efforts depend on accurate characterization of the components and input signals used for design simulation and validation testing. In the past, modeling has been addressed through advancements in measurement technology such as the vector network analyzer, and more recently through electromagnetic simulation.
Both approaches in isolation have certain short comings. For instance, measurement data requires the fabrication of test structures, requiring additional development time and manufacturing costs. While electromagnetic simulation offers a significant speed-up in component modeling, it is not capable of providing response data for structures with unknown physical properties such as a purchased component. Additionally, the system and component developers are often interested in defining the modulation scheme or specifying the budget for individual components to achieve an overall desired system performance. While what-if scenarios are best investigated in the design environment, the same waveforms used in simulation must drive the device under test if correlation between the two is to be achieved.
A practical design flow supports flexible component representation ranging from measured data to electromagnetic simulation as well as a reliable and efficient exchange of model and source data between the simulation and test environments. The need to integrate data flows between advanced design environments and automatic test systems has prompted a new level of integration.
High-frequency design tools such as Ansoft Designer, coupled with high-level development tools such as National Instruments (NI) LabVIEW, target the complexity of accurate source and component modeling and design verification by linking behavioral models for simulation directly to automated test system results. Together, products like these lay the groundwork for a product-development infrastructure that closes the loop between virtual prototyping and hardware verification.
Configuring Ansoft Designer and NI LabVIEW to share information and complete the design/verification product-development cycle is demonstrated in the following example of a measurement-based model simulation of an amplifier/filter chain driven by a digitally modulated 320MHz signal. Error Vector Magnitude (EVM) and Magnitude Error Ratio (MER) measurements are used to characterize the signal chain.
A Measurement-Based Behavioral Model
In this example, a commercially available broadband amplifier module is integrated with a pair of surface acoustic wave (SAW) filters as part of a transmitter chain. The design focus is on determining if the selected components provide sufficient linearity to meet the system requirements for EVM and MER. The amplifier being used is a Mini-Circuits ZKL-2R7 and has a rated P1dB of +13dbm (output) with 25 dB of gain. To drive the amplifier into compression, we will need approximately -10 dBm of input power. The two SAW filters are placed back-to-back and have a total loss of approximately 40 dB. These SAWs have significant phase and amplitude ripple.
Figure 1. Modeling RF filter effects
The intent of the design/test setup will be to validate the use of a measurement-based behavioral model for use in digital modulation simulations and accurate component representation. The amplifier/filter chain is characterized by its nonlinear response using am-to-am and am-to-pm measurements as a function of CW signal drive level and frequency. These frequency-dependent measurements are made in LabVIEW and are used directly by a nonlinear behavioral model embedded in an Ansoft Designer system schematic. A mixer is added to the Ansoft block diagram to represent upconversion to the final carrier frequency.
Figure 2. Characterizing the device under test
After characterizing the non-linear frequency response of the device-under-test (DUT), digital modulation functions built in LabVIEW are used to measure the EVM and MER of the device when the signal source feeds it a 16 QAM signal, centered at 320 MHz, with a 3 Mbit/sec data rate. These results are displayed in the LabVIEW environment and imported into the Ansoft Designer environment for display and comparison to the simulated response of the behavioral model.
This allows the designer to validate the DUT representation in the simulation. In the simulation, the receiver strips off the carrier and de-modulates the signal for comparison with the original signal to derive the EVM, MER and Bit Error Rate (BER) responses. In addition, the results of compression can be observed by feeding the amplifier with an input signal of -8 dBm.
Generating the Modulated Source
Both Ansoft Designer and LabVIEW provide the signal processing to define the specific data bit-stream passing through the system. To ensure correlation between simulation and measurement, both the Ansoft Designer project and LabVIEW VI have been configured to import and export the generated waveform for use in either environment. In Ansoft Designer, two analysis setups are used to toggle the source between the Ansoft generated waveform and the LabVIEW VI generated one. While it is important to synchronize the waveforms used in both measurement and simulation, creating duplicate sources in both environments simply demonstrates that the capability is available in each. Ansoft Designer provides the modulation schemes and impairments associated with standards such as WCDMA, IEEE 802.11, GSM and EDGE.
The source is created with a 3Mbps pseudo-random bit generator passed through a 16-level Quadrature Amplitude Modulator (16QAM), normalized, smoothed with a root-raised cosine filter and centered at 25MHz. This signal is then be upconverted to be centered at 320 MHz. Ansoft Designer and LabVIEW both generate an ASCII text file of complex I&Q data, which is either played out through an RF signal source to the actual DUT or passes through an upconverter model in simulation. The design and simulation setup has been configured to calculate these values for both the Ansoft Designer and NI LabVIEW generated waveforms.
Figure 3 Analyzing the power sweep
Figure 4. The Spectral Response
Figure 5. Eye Pattern predicts Bit Error Rate
The commercial: Flexible Modeling of Sources and Components
Ansoft Designer and NI LabVIEW are two well-established software packages that offer virtual prototyping and measured data acquisition, respectively. LabVIEW is a graphical programming environment that allows its users to create customized automatic test systems, known as virtual instruments. It is easily configured to perform signal acquisition, measurement analysis and data presentation based on a single test instrument or on an entire test system. LabVIEW also allows the user to provide this measurement data for direct use in Ansoft Designer.
Ansoft Designer is a high-frequency electronics-design suite that integrates a 3D planar electromagnetic solver with circuit and system simulation. Ansoft Designer allows engineers to incorporate highly accurate physical designs based on electromagnetic simulation alongside flexible (electrical or functional) behavioral models. These component models allow externally generated data to define component behavior. Both products are capable of formatting data for mutual access so that source, model/DUT and simulation/measured results are easily shared between environments (Figure 6).
Figure 6. How Designer and LabVIEW Interface
The integration of Ansoft Designer and LabVIEW delivers benefits to engineers and designers throughout the product-development cycle. The integration between these two environments allows the user to select the best tool at each step of the development cycle and seamlessly exchange results between the two environments. LabVIEW provides the most complete, vendor-neutral connectivity to instrumentation, and both environments offer a wealth of signal processing and analysis tools. The integrated use of both environments allows the engineer to develop products quicker, with higher quality, and at reduced cost.
