Agilent’s pure, precise signal generators help designers create next-generation wireless devices
From 9 kHz to 6 GHz, the analogue and vector versions of Agilent’s MXG and EXG X-Series signal generators (Figure 1) provide industry-leading performance in five areas: phase noise and spectral purity, bandwidth, error vector magnitude (EVM), adjacent-channel power ratio (ACPR) and output power. The MXG and EXG take that performance into new areas with advanced capabilities such as real-time baseband signal generation that supports applications in cellular communications, wireless networking, audio/video broadcasting, and navigation (e.g., GPS and GLONASS). Through these contributions, the MXG and EXG enable testing that reveals the true performance of components, receivers, power amplifiers, and other electronic devices.
Delivering wide modulation bandwidth — with accuracy
One recent example is the gigabit speed of the emerging IEEE 802.11ac wireless networking standard. It operates in the 5-GHz band and uses 256QAM modulation at bandwidths up to 160 MHz. This places a heavy burden on the signal generators used in device testing because the achievable level of modulation accuracy typically declines as signal bandwidth increases.
To meet this need, the MXG uses advanced modulator circuits and innovative calibration techniques to overcome the I/Q and frequency-response problems that generally worsen with increasing bandwidth. This enables the MXG to provide modulation bandwidths up to 160 MHz with flatness up to ±0.2 dB and EVM up to 0.4% (Figure 2).
One key to this combination of bandwidth and accuracy is the use of an internal calibration source and factory-calibrated channel corrections that extend from the modulator to the RF output. Together, these technologies minimize errors in amplitude, group delay and I/Q, thereby ensuring excellent flatness and modulation accuracy. The combination of calibration source and channel corrections also provides wide modulation bandwidth accuracy without requiring user intervention such as manual I/Q adjustment.
This can be useful in the testing of smartphones, which can have three or more transceivers running at the same time while other similar devices are operating nearby. The MXG’s wide-bandwidth modulator can produce a test signal composed of a complete spectral segment that includes the desired signal plus adjacent and alternate channels as potential interferers—and perhaps spurious or transient signals, too.
Delivering pure signals at high output levels
In applications such as the development of multi-carrier power amplifiers (MCPAs), large signals are needed to drive an amplifier into its nonlinear region to produce harmonics, intermodulation and compression. In a signal generator, high output power is useful only if test signals can be produced with high quality in areas such as harmonic distortion, broadband noise, ACPR and EVM.
The MXG and EXG combine output power of up to +27 dBm with low EVM and low ACPR of up to -73 dBc (W-CDMA TM1, 64 DPCH). This provides a one-box solution that is pure and powerful enough to support direct testing of high performance MCPAs as well as components and systems.
External power amplifiers are sometimes used with RF signal generators; however, this adds cost and complexity to the test solution. It will also degrade performance by adding amplitude error that is outside the signal generator’s calibration loop.
Applying excellent performance to complex signals
To help developers quickly create high-quality signals that meet the needs of specific standards and measurements, the MXG and EXG are compatible with Agilent Signal Studio software. This suite of signal-creation tools addresses cellular communications, wireless connectivity, audio, video, positioning, tracking, and general-purpose applications.
Signal Studio has two operating modes: waveform playback mode and real-time mode. Waveform playback mode supports two levels of functionality, basic and advanced. These are used to create and customize waveform files of finite length by configuring signal parameters, calculating the resulting waveforms and downloading the files for playback by an MXG, EXG or other Agilent instrument. Basic capabilities support creation of partially coded, statistically correct signals for stimulus/response measurements. Advanced options support creation of fully channel-coded signals for analysis of, for example, receiver BER, FER, BLER and PER.
In real-time mode, a graphical interface provides a direct instrument connection for parameter transfer and closed-loop or interactive control during signal generation. Continuous data or closed-loop testing is becoming increasingly important in the testing of several wireless standards: 3GPP LTE TDD and FDD; digital video; Global Satellite Navigation Systems; GSM/EDGE; 3GPP2 CDMA; and 3GPP W-CDMA. All these are supported by the corresponding versions of Signal Studio.
Enhancing real-time baseband generation
To support real-time mode, the MXG and EXG include a powerful internal real-time baseband generator and processor-accelerator ASIC. The baseband generator also supports custom or flexible modulation types including constellations as dense as 1024QAM.
These capabilities support creation of complex signal scenarios of extremely long durations. In satellite navigation, an MXG or EXG can generate signals representing up to 32 GPS+GLONASS satellite channels with real-time control of satellite visibility and power with more than 24 hours of simulation time (Figure 3). In DVB applications, this solution supports up to two hours of playback or continuous PN23 data sequences.
Real-time generation is especially useful during throughput testing of real-world channels. An example configuration is shown in Figure 4. In testing such as LTE HARQ, the X-Series signal generators can receive TTL-level feedback signals and reconfigure the transmission signal while maintaining the link. This enables highly realistic testing of throughput over impaired channels.
Simplifying signal simulation
The replay of an arbitrary waveform file is often an easier way to handle highly customized signal-simulation applications. In such cases, an important but simple technical advance is deep waveform memory: the MXG has up to 1 GSa and the EXG up to 512 MSa.
With 1 GSa, the MXG’s playback capacity ranges from seconds to hours of a continuous signal without repeating, depending on the selected sample rate. This capability generally exceeds conformance requirements and can, in some cases, provide an alternative to real-time signal generation.
Generating true performance
To know a device’s behavior, a designer will explore many paths. That vision helped inspire the development of the X-Series signal generators. These instruments produce the essential signals—from simple to complex, from clean to impaired—that enable detailed testing of components and receivers at and beyond their limits. From the pure and precise MXG to the cost-effective EXG, the innovative X-Series signal generators are designed to keep pace as system designers continue to drive for better performance in RF communications and connectivity.
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