Modern Tools for Today’s Radio
By John Barfuss, Agilent Technologies

Radios, and the instruments used to test them, have both benefited from advances in digital signal processing (DSP) and converter technology. The result is more versatile designs that can be adapted to multiple uses because most of the functionality is implemented in software and firmware rather than hardware.

This article will discuss the versatility in today’s test tools and how that versatility contributes to the design and test needs of the modern radio.

Software Defined Instruments (SDI)
A software defined instrument is analogous to the software defined radio (SDR). The core functionality of an SDR is implemented in digital signal processors (DSPs), field programmable gate arrays (FPGAs), and/or general purpose processors (GPPs) and can be changed by modifying the software and firmware. As a result, the radio isn’t limited to just one function but is flexible and can be modified to support a new standard or to operate as a different kind of radio altogether. Further, for a true SDR, the software stands on its own and can be transported to different hardware platforms.

Modern test instruments are versatile and can perform many functions by simply adding or changing software. Measurements traditionally done with hardware circuitry are now implemented in DSP. The result is instruments that are largely software based. And, as we will see in this paper, in some cases the measurement software can stand on its own and be transported to radically different platforms.

An example of a software defined instrument is a spectrum analyzer that incorporates an all digital intermediate frequency (IF) section.

One such instrument is the MXA signal analyzer from Agilent. In the analog front-end of this instrument, the input signal is down converted to an IF, and then digitized. From there, the core functions of the analyzer are implemented using DSP. With the right software, the analyzer operates as a complete swept tuned spectrum analyzer. But, it can also be an FFT analyzer, a modulation analyzer, or a full-fledged Vector Signal Analyzer (VSA). You can expect more functions to be added to the MXA in the future.

This does not mean that the MXA hardware is completely generic. As found with the software defined radio, cost, function, and performance restraints shape the hardware. SDIs are not without their practical boundaries.

As an aside, a type of generic SDIs exist in the form of synthetic instruments. Synthetic instruments build on the idea of a general purpose SDI by using modular high performance building blocks, including digitizers, frequency converters, processing units, and software to synthesize all measurements. The modularity of these components allows them to evolve independently. Due to the large upfront costs of developing such a test system, today’s synthetic instruments are used primarily for large military projects that may mandate the use of them to meet long-term cost, reuse, and maintenance criteria. For the purpose of this article, I won’t go any further into synthetic instruments other than to make mention of them as part of the big picture in instrumentation trends.

Changing Signal Forms
A challenge with testing modern radios is the changing signal formats. A conventional radio transmitter typically uses a baseband integrated circuit (IC) that outputs an analog baseband signal which modulates an IF, which is then upconverted to RF and amplified.

Today, more of the radio is being implemented in the digital realm. It is therefore more common to be working with digital representations of the signal, such as digital IQ or digital IF. These digital signals can be formatted in various ways, such as parallel or serial, two’s complement or binary, or packetized in the form defined by one of the digital interface standards such as DigRF. Note that these “digital” signals are not the same as digital data. In other words, the ones and zeros do not represent the data directly. Rather, the digital values represent a digitized form of the modulated analog waveform.

Test equipment vendors are responding to these changing signal formats by providing digital interfaces to traditionally analog test tools.

Consistent Measurements Everywhere
An example of a traditional analog instrument with digital signal I/O capability is the Agilent ESG signal generator. The ESG has the versatility to provide test stimulus in any format needed. Not only can the instrument output signals at RF, but it can also provide the same test signals at IF, analog IQ, digital IF or digital IQ. For the digital signal output, the generator utilizes a digital signal interface module (DSIM) that is reconfigurable to various digital formats and clock rates. (see Figure 1)

The power in such a solution is the ability to provide consistent test stimulus to any part of the radio and to independently verify the performance of each component or section. Since the ESG is arbitrary waveform generator (AWG) based, it has the flexibility to recreate, with the right software, any signal within its performance restraints. Further, impairments, such as noise or channel effects, can also be modeled into the signal using software processing.

Similar flexibility also exists on the analysis side. (see Figure 2) The following example is one of transportable measurement software that can operate on different platforms.
The Agilent 89601A VSA software is a flexible measurement tool that supports many demodulation formats and measurements.

This transportable VSA software is not only able to run on its native signal analyzer but can also operate on the oscilloscope and the logic analyzer. By doing so, its measurement capabilities are unlocked and can provide insight for signals of any format including RF, IF, analog baseband, digital baseband, or digital IF.

A great benefit of being able to consistently measure the signals anywhere within the radio with the same test tool is that it allows you to directly compare the signal quality in different parts of the radio.

To illustrate, measurement screen shots A, B, and C show the error vector magnitude (EVM) and constellation measurement of a QPSK radio at IF, analog IQ, and digital IQ respectively, using the VSA software running on the signal analyzer, oscilloscope, and logic analyzer. This is a basic QPSK signal, but the concept works for any supported modulation format including WLAN, WiMax, CDMA, GSM or generic QAM signals.

The measurement results show that we gain about 6% EVM error going from analog IQ to IF and about 2% EVM error going from digital IQ to analog IQ. (see Figures A, B, C) Closer examination using the detailed analysis functions of the VSA software reveal the cause of the errors. In this case, the majority of the error between the IF and analog IQ is quadrature error introduced by our IQ modulator. The error gained between our digital IQ and analog IQ signals is largely the result of dispersion introduced by analog filters located just after the DAC. The 4% EVM of our digital IQ signal is primarily due to the ripple in the passband of the digital filter implemented within our FPGA. The key point is that being able to compare measurement results at different locations in the radio helps isolate the source of the errors.

Connected Solutions – Tying Simulation with the Real World
The versatility does not stop with multiple instrument platforms. The VSA measurement software, can also operate within a software design environment.

In the case of the Agilent VSA software it has direct support for the Agilent ADS and The Mathworks Simulink simulation and design environments.

Figure 4 is an example of an ADS simulation of the RF section of a radio. Note the VSA icons placed into the model. By adding the VSA tool into the simulation, an engineer is able to measure signals with the same algorithms and functions that will eventually be used to test the hardware implementation of the simulated circuit.

Notice, in the example above the VSA software is acting as both a measurement tool and a source within the simulation. This is possible because of the record and playback capability of the VSA that allows it to record, store, and play back signals in either the physical world or in simulation. The capability allows designers to test their simulated system using real-world signals. The connection from simulation to the real world is a powerful tool in transitioning designs from the software development environment to the real world.

In summary, the flexibility of today’s test tools greatly improves the efficiency of radio designers by providing flexibility to use common measurement tools throughout the radio and through all stages of development. This flexibility in test complements the trends in modern radio designs that utilize more DSP, require greater functionality and more rapid development.

About the Author
John Barfuss, an applications engineer for Agilent’s Aerospace & Defense business, joined Agilent in 1999. Prior to working for Agilent, John was a test engineer for 3Com Corporation. He graduated from the University of Utah with his BSEE in 1997 and his MBA in 2000.

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