Using Software Defined Instruments to Address the Mixed-Signal Test Challenges of Today’s Software Defined Radios

Wednesday, February 01 2012

Test equipment venders have responded to these changing signal formats by providing digital interfaces to traditionally analog test tools. Today’s vector signal generators, for example, can be equipped with digital signal I/O capability. Based in part on an arbitrary waveform generator, they have the flexibility to recreate, with the right software, user-defined signals within their performance constraints. Further, impairments, such as noise or channel effects, can be modeled into the signal using software processing.

With this versatility, vector signal generators have the flexibility to provide test stimulus for a wide variety of emerging SDR waveforms. Moreover, they can output test signals at RF, IF, analog IQ, digital IF, or digital IQ. For the digital signal output, the generator can utilize a digital signal interface that is reconfigurable to various digital formats and clock rates. The flexibility of an SDI like the vector signal generator enables it to provide a consistent test stimulus to any part of the radio and to independently verify the performance of each component or section.

Flexible SDR Signal Analysis

Flexibility can also be found in SDIs used for SDR signal analysis. As an example, consider the 89600B VSA measurement software, which can operate on many different instrument platforms or analog and digital “front ends” (Figure 1). This flexible measurement tool supports many demodulation formats and measurements. It can run on an RF signal analyzer as well as on a high-performance digital oscilloscope or a logic analyzer. As a result, it provides insight for signals of any format including RF, IF, analog baseband, digital baseband, or digital IF.

A key benefit of being able to consistently measure signals anywhere within the radio with the same test tool is that it allows the engineer to directly compare the signal quality at different test points along a mixed-signal SDR transmitter chain. To better illustrate this point, consider the screenshots in Figure 2, which shows the EVM and constellation measurement of a QPSK radio in various formats using the VSA software running on the signal analyzer, oscilloscope, and logic analyzer. While this is a basic QPSK signal, the concept works for any supported modulation format including more complex OFDMA waveforms such as Mobile WiMAX™ and Long-Term Evolution (LTE), or even custom OFDM waveforms. In addition, MIMO demodulation measurements can be performed with the VSA software by selecting a hardware measurement platform such as a high-performance digital oscilloscope with four phase coherent inputs for two or four-channel MIMO measurements.

As shown in the measurement results in Figure 2, the waveform quality has degraded with an approximate 6% EVM difference between the analog IQ to IF, and approximately a 2% EVM difference between the digital IQ to analog IQ. A closer examination of the results using the 89600B’s detailed analysis functions reveals the cause of the errors. In this case, the majority of error between the IF and analog IQ is due to quadrature error introduced by the IQ modulator. The error introduced between the digital IQ and analog IQ signals is largely the result of dispersion introduced by analog filters located just after the DAC. The digital IQ signal’s 4% EVM is primarily due to the ripple in the passband of the digital filter implemented within the FPGA.

Connecting to the Real World

The versatility of the VSA software is not limited to multiple instrument platforms; it can also be operated within design simulation environments to provide a flexible connection to the real world. In the case of the 89600B VSA, for example, the software can operate within a software design environment such as Agilent’s SystemVue software for electronic system level (ESL) design.

SystemVue creates the radio signals that can be used to model and test Layer 1 PHY architectures, both in simulation as well as when downloaded into test equipment (Figure 3). It enables scenario modeling by adding fading, noise, interferers, and the RF effects necessary for realistic system analysis and early R&D verification. Once in the SystemVue platform, the signals can be used throughout the larger design flow and input into other EDA tools or even used for testing and integration.