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
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
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