Innovations in Agilent X-Series Signal Generators
May 1, 2012
- Precise Imperfections
- Application Power
- Reducing the Cost of Test
- Generate True Performance
- Related Information
Designers of systems that range from communications to wireless networking, from radar to satellite navigation, are constantly seeking greater performance. Clearly, the test equipment they use must keep up with the drive for better performance. In reality, “keeping up” means much more than performance improvements. Even as systems become more complicated and signals use wider bandwidths, there is an imperative to drive down the cost of test to keep products competitive and profitable.This document describes several of the technical innovations in performance and functionality in the Agilent X-Series signal generators and summarizes their significance for various applications and measurements.
Providing industry-leading phase noise and spurious: Triple-loop synthesizer
Single-loop phase-locked loop (PLL) designs are economical and efficient and, when used with fractional-N technology, can provide moderate performance, fine frequency resolution and good frequency agility. The MXG signal generators implement a new triple-loop design and “frequency plan” that results in substantial phase noise improvements close to the carrier and at wide offsets (Figure 1). Three different levels of phase noise performance are available, including two options that improve on the standard phase noise performance. The MXG delivers unmatched phase noise at -146 dBc at 1 GHz and 20 kHz offset and -96 dBc spurs at 1 GHz. Figure 1 shows the “standard” and “best” levels of performance.
Figure 1: Low phase noise (Option UNX) and enhanced low phase noise (Option UNY) provide even greater levels of phase noise performance
In the MXG, the key to improved phase noise is the frequency plan, which is optimized for the triple-loop topology. The frequency plan addresses several attributes: the choice of oscillator and reference frequencies in the synthesizer sum and offset loops, and the associated frequency conversion (mixers and multipliers) and filtering.
The triple-loop approach optimizes frequency choices for effective filtering of undesirable signals pushing them outside the bandwidth of the synthesizer circuits. In the MXG, the plan arranges the frequency references and conversions such that the undesirable mixing products are placed far from the desired frequencies and modest filtering can heavily attenuate them. This approach also allows internal signal levels to be set higher, resulting in relatively lower broadband noise and improved dynamic range.
For less-demanding applications, the X-Series signal generators include a range of alternatives to match cost and performance requirements. For example, the single-loop EXG models provide a moderate performance choice at a lower cost.
Achieving low distortion and high modulation accuracy: Vector channel corrections
In RF communications, seemingly insatiable demand for content continues to drive up data rates, modulation bandwidth and distortion requirements. One recent example is the gigabit speed of the emerging IEEE 802.11ac wireless networking standard in the 5 GHz band. Because the 11ac WLAN combines bandwidths to 160 MHz with constellations as dense as 256QAM, it places severe demands on signal generators because achievable modulation accuracy usually declines with increasing signal bandwidth.
Delivering an unequaled mix of modulation bandwidth and accuracy: Internal channel corrections
The MXG and EXG provide frequency coverage to 6 GHz and modulation bandwidths to 160 MHz or 120 MHz, respectively. This wide modulation bandwidth is available with EVM better than 0.4% and flatness better than ±0.2 dB, ample performance for even the most demanding design tasks.
The X-Series achieves this combination of bandwidth and accuracy through the use of an internal calibration source and factory-calibrated channel corrections that extend from the modulator through the RF output. Together, these technologies minimize I/Q errors to provide high dynamic range and modulation accuracy plus wide modulation bandwidth without user intervention such as manual I/Q adjustment.
Wide modulation bandwidth and high dynamic range provide user benefits when working with signals that are narrower. In today’s crowded spectral environment, receivers must cope with problems such as adjacent-channel interference and multiple types of blocking. For example, a modern smartphone can have three or more transceivers operating at the same time, and must operate in proximity to other devices of similar complexity. The X-Series’ 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 as well.
Optimizing distortion and modulation quality at the DUT: External vector channel corrections
Automatic routines in the MXG and EXG can use information from a remote USB power sensor to build a measurement of the complex (I/Q) frequency response of the path from the signal generator to the DUT. A real-time ASIC can then perform inverse compensation on the modulated signal to correct the complex channel response at the DUT as shown in Figure 2.
Figure 2: The MXG and EXG support customized user I/Q (vector) to compensate for flatness and group delay at the DUT.
This customized vector-correction capability is especially beneficial for wide-bandwidth signals or complex connections between the signal generator and the DUT, which may include filters, amplifiers and switches.
Producing high output power with low distortion
Large signals are needed to directly drive power amplifiers to levels that exercise their nonlinearities, thereby producing harmonics, intermodulation and compression. In a signal generator, high output power is useful only if signals can be produced with very high quality—low harmonic distortion, broadband noise, ACPR, and EVM.
External power amplifiers are sometimes used with RF signal generators but this adds complexity and expense to the test solution. It may also compromise performance by adding additional signal elements such as switching that are outside the signal generator’s calibration loop.
The X-Series signal generators combine output power of up to +27 dBm with low ACPR of up to -73 dBc (W-CDMA TM1, 64 DPCH) and low EVM to provide a one-box solution that is both powerful and pure enough for direct testing of high performance components and systems.
Using “phase noise injection” to optimize for imperfect oscillators
The extremely low phase noise of the MXG is not always a benefit—and a process called phase noise injection enables selective and precise degradation as needed. Signal generators are often used to generate complex modulated RF signals and to substitute for various oscillators or synthesizers in the design process (Figure 3).
Figure 3: Using a signal generator to substitute for an oscillator or synthesizer can help reduce development time
In real-world designs, any improvement to frequency stability is expensive in terms of cost, power and space. As a result, engineers have an interest in creating assemblies with performance that is just good enough. The process of arriving at just good enough is faster and easier if the phase noise of a test source is precisely adjustable.
In the MXG, a real-time signal-processing ASIC customizes phase noise levels for both CW and modulated signals. In an important innovation, phase noise can be adjusted to different levels at different offsets, including the steep slopes of close-in noise, the flat slopes of synthesizer pedestal noise and the shallow slopes of wide offset noise.
This precise substitution capability helps designers avoid the dual hazards of over- or under-performance of their oscillators and synthesizers. Excess performance can lead to expensive designs and longer design cycles, rendering a product uncompetitive. Insufficient performance results in redesigns and product delays and the type of unpleasant surprises that engineers dread most.
OFDM signals provide a good example. Because these signals have extremely close subcarrier spacing, they are sensitive to phase noise, which reduces the orthogonality (independence) of the subcarriers and increases modulation error. Consequently, OFDM transmitters and receivers need good phase noise performance. During product development, the MXG’s injection capability allows selective addition of phase noise in terms of carrier offset, making it an excellent substitute for synthesizers and OFDM transmitters and enabling confident evaluation of receiver tolerance versus real-world transmitter performance. The result is fast, reliable optimization of design cost and performance.
Performing real-time baseband generation, including closed-loop testing
The X-Series signal generators have a powerful internal real-time baseband generator and processor-accelerator ASIC. This supports a large number of real-time applications in cellular communications, wireless networking, audio/video broadcasting, and navigation (GPS & GLONASS). The real-time baseband generator also supports custom or flexible modulation types including dense constellations up to 1024QAM.
Real-time generation supports creation of complex signal scenarios of extremely long durations. Satellite navigation provides an example: the MXG and EXG can generate signals representing up to 32 line-of-sight GPS/GLONASS satellites with real-time control of satellite visibility and power with up to 24 hours of simulation time. Total nonrepeating signal length is a general benefit for virtually any signal type. For example, reducing “time to first fix” requires several minutes of continuous simulation to optimize receiver correlation algorithms.
Another advantage of real-time generation is the ability to do closed-loop testing, which is of increasing importance in the latest digital wireless standards. Closed-loop testing is especially valuable in throughput testing of real-world channels. Two example configurations are 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 more realistic testing of throughput over impaired channels.
Figure 4: Support for closed-loop real-time testing is beneficial when working with the latest digital standards
Leveraging extremely deep arb memory and real-time signal generation in one unit
The replay of an arbitrary waveform file is often a better, simpler way to handle 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 has up to 512 MSa. With 1 GSa, the MXG can provide a minimum of five seconds and as much as hours of continuous signal without repeating depending on sample rate—a capability that generally exceeds conformance requirements and can, in some cases, provide an alternative to real-time signal generation.
The total cost of test is a key metric for most manufacturers, and many take a sophisticated, long-term, “all in” approach to evaluating cost-effectiveness. The X-Series signal generators support this comprehensive approach in many ways.
Providing a range of choices in performance and capability
The multiple MXG and EXG models include both moderate- and high-performance signal sources with analog and vector capabilities. This makes it possible to buy only the performance that is needed for a specific application. The available performance and functionality options expand the envelope of cost, capability and benefit even further.
The least expensive instrument is one you already own
In agile manufacturing, production lines and test resources are repurposed as demand changes. Total cost of test is reduced with long-lasting equipment that can evolve to meet changing needs. That’s why the X-Series signal generators have a flexible hardware and measurement application structure that allows for reconfiguration to adapt as test systems change. With this level of flexibility, the MXG and EXG retain more value over time—whether they are reused, reconfigured or resold.
Achieving durability, long life and infrequent calibration
Once purchased and installed, the ideal piece of test hardware is one that helps a manufacturer meet their goals without further attention. New MXG models are based on a proven design with a target MTBF of 116,000 hours. One source of durability is a low-loss solid-state attenuator that will make countless amplitude changes without wear. It also includes a reverse power-protection circuit that prevents damage or degradation from large signals that reach the instrument output.
Downtime due to removal, calibration and reinstallation means time lost in productive use. That’s why the MXG is designed with a three-year calibration cycle.
Simplifying maintenance: Long warranty, fast repairs, no calibration after repair
The long calibration cycle is matched with a three-year warranty and self-diagnostics that minimize troubleshooting. A refurbished exchange program and warranty parts direct from Agilent minimize repair cost. In addition, it takes less than two hours to replace any part. This self-maintenance strategy, combined with a design that requires no post-repair calibration, ensures that X-Series signal generators return to production even faster.
To know a device’s behavior, a designer will explore many paths. That vision helped inspire the development of the X-Series signal generators. They produce the essential signals—from simple to complex, from clean to dirty—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 is designed to keep pace as system designers continue to drive for better performance.
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