Drive your Evolution with the Agilent PXA Signal Analyzer

The Agilent PXA signal analyzer is an evolutionary replacement for existing high-performance spectrum analyzers (Figure 1). It combines extensive compatibility features with outstanding RF and microwave performance for new and "legacy analyzer replacement" applications in aerospace, defense and commercial communications. Agilent innovations such as noise floor extension (NFE) and a variable frequency IF output are among the technologies described in this backgrounder, along with their benefits in new and legacy analyzer migration applications.

Taking Advantage of Moore's Law in Unexpected Areas

Unlike the exponential growth pace set by digital circuits, analog circuits have tended to see improvements come slowly--and at considerable cost. RF engineers correctly understand Moore's Law to apply to digital circuits, such as processors and memory, and understandably wish it applied more directly to analog circuits as well. RF circuits are more linear and therefore not as amenable to the advances characterized by Moore's law. Even Dr. Moore himself saw this issue from the beginning, noting that "Integration will not change linear systems as radically as digital systems" in his famous 1965 article.[1] So while digital circuits experience a veritable growth explosion, advancements on the RF side are hard won.

Nonetheless, RF engineers have enjoyed some very substantial gains in recent years, particularly in the speed, resolution and fidelity of ADCs and DACs. Combining these advances with the DSP capabilities made possible by improved processors and memory has yielded some advanced, modern spectrum/signal designs that include all-digital IF sections with improved RBW filters, relatively wide analysis bandwidths, and advanced processing capabilities such as digital modulation analysis.

Despite these advances, some critical instrument performance parameters have yielded much more slowly to semiconductor-based performance improvements:

• Analyzer noise floor, which is variously described in terms of displayed average noise level (DANL) or noise figure
• Third order distortion (often described as third order intercept or TOI)
• Phase noise (analyzer phase noise floor)
• Amplitude accuracy (both single-frequency and integrated band power)

Collectively, these figures define fundamental spectrum analyzer performance. The Agilent PXA combines both analog and digital technologies to provide tangible improvements in the most important aspects of spectrum analyzer performance.

Noise floor extension

An analyzer's own noise floor is a limiting factor in several types of measurements. As one key example, noise limits the low end of dynamic range as well as the accuracy of measurements on small signals. When large signals are present, it is common to increase analyzer attenuation as way to limit distortion due to overload; however, this brings small signals closer to the analyzer's own noise, and thus the analyzer noise limits the amount of attenuation. Noise can also have a large impact on measurement speed, potentially requiring narrower RBWs for measurements such as spur searches--and narrow RBWs require slower sweeps due to increased settling time.

As a result, reducing the analyzer's own noise level can help both low-level and high-level measurements. Alternatively, it can allow low-level measurements to be made without using a preamplifier, which would reduce the TOI performance or other elements of dynamic range.

The PXA combines real-time measurement processing with an unprecedented characterization of the analyzer's own noise to allow that noise to be accurately removed from measurements. The improvement from noise floor extension (NFE) varies from about 3.5 dB for CW and pulsed signals to approximately 8 dB for noise-like signals, and up to 12 dB or more in some applications. Typical DANL at 2 GHz is --161 dBm without a preamp and a remarkable --172 dBm with the preamp. An example of the improvement from NFE is shown in Figure 2. For more detail, please refer to the upcoming article by Gorin.[2]

Of course, lower noise is not the only way to improve dynamic range in an analyzer, and the PXA has been designed to measure higher-level signals with lower distortion as well. The input includes a new RF front-end microcircuit design with a higher TOI point (better than 20 dBm at 2 GHz) to provide higher dynamic range for measurements such as transmitter ACPR. The expansion of both ends of the dynamic range envelope build on each other.

Alternate low-noise input path

At microwave frequencies, any sort of signal routing or switching results in signal path loss. This loss goes up significantly with frequency and, although it can be characterized and corrected to preserve amplitude accuracy, it still affects the noise floor or noise figure of the analyzer, along with the accuracy of low-level signal measurements. Preamplifiers can compensate for this loss and improve signal/noise for small signals, but they can cause distortion from larger signals present in the measurement. In the PXA, an optional low noise path (LNP) allows lossy elements normally found in the RF input chain to be completely bypassed for highest sensitivity without a preamplifier in the microwave bands above 3.6 GHz. See Figure 3.

In working with many different engineers (mainly in automated test) who use high performance signal analyzers, we've discovered a couple of essential truths. First, that they're torn between the guaranteed compatibility assured by continuing to use (and find support for) their existing solutions and the desire to move to new, more supportable solutions that are, in so many ways, more capable: performance, speed, functionality and interfaces, for example. Second, that the level of compatibility they need is multifaceted and difficult to achieve when moving to a new solution.

It's tempting to see the issue of compatibility as primarily a matter of code and performance compatibility. Those are certainly significant challenges, but the myriad other difficulties can best be expressed by six key capabilities supported by the PXA.

Deep compatibility mode with customized settings: In many ATE applications, form/fit/function replacement of a legacy analyzer is a key goal. In such cases, a good starting point is the PXA's code compatibility with the HP 8566/68, 856x and PSA. Existing test programs often depend on the coupling of measurement settings and timing parameters that are specific to a particular analyzer. The PXA compatibility mode allows many parameters to be individually set to compatible values or allowed to be optimized for new measurement capabilities. Four examples are worth noting:

• Traditional 1/3/10 RBW/VBW steps vs. fine 10% steps available with a modern digital IF
• Span/RBW/VBW/sweep time coupling rules for analog filters vs. coupling optimized for digital filters
• Exclusively swept mode vs. optimized swept/FFT operation
• Instrument preset/reset settings matching legacy instruments vs. PXA normal preset

Sweep time minimum limit: In ATE systems, faster is not always better and, in some cases, speed improvements may impair compatibility. The HP 8566, for example, had a minimum sweep time of 20 ms while the PXA sweep time can be 1 ms or faster. When faster sweep speeds may cause problems, the PXA can be set to automatically limit them to 20 ms.

Quick access to newer features: Users who need the compatibility features still have ready access to new capabilities with fast switching between compatibility and native modes. This eliminates the need to choose between high compatibility and access to important new features.

Multiple, configurable IF outputs and video outputs: In some applications, compatibility depends on analyzer outputs such as video and IF. The PXA has a user-programmable IF output to closely approximate multiple special IF options available for the PSA-Series spectrum analyzers, including 21.4 MHz and 70 MHz IF outputs. The PXA's IF output is also programmable in frequency from 10 to 75 MHz, in 500 kHz increments. For wideband applications such as radar pulse characterization the PXA has a log video output with fast rise time and wide bandwidth (>140 MHz nominal). This makes the PXA ideal for custom applications in which wideband external envelope and video detection is desired. In addition, the PXA offers a narrowband video output that includes the effects of RBW and VBW settings. This output is specifically designed to mimic the Y-axis recorder output of the HP 8566B.

Error log: When the PXA is using the N9061A Remote Language Compatibility application to emulate the 856xE/EC spectrum analyzers it generates a log file of any unrecognized commands. This allows the test engineer to review errors and determine if they are important to their application. If necessary, the test engineer can modify the test program set (TPS) or engage Agilent to provide support for the missing command. The PXA can then be set to ignore any remaining unrecognized commands, typically those that are meaningless in the context of the new analyzer.

Attenuator offset mode: This mode adds 12 dB of transparent attenuation to any manually or automatically selected input attenuation. It allows the PXA to respond to overloads in a manner similar to that of an analyzer with an analog IF. The attenuation avoids IF overloads, which cause more dramatic problems in digital IF analyzers. Because the PXA has better DANL, it can afford this extra attenuation. The result is better performance for high-level signals and, to provide measurement equivalence, similar appearance of signal level relative to baseline noise.

Information security

Modern analyzers have a large amount of nonvolatile memory to support measurement accuracy and analysis features, and for user convenience. This can be a security concern, especially if the analyzer must be moved between secure environments or from secure to unsecure environments. In the PXA the task of instrument "sanitizing" is easy and dependable through the use of a removable hard disk drive. The drive is user-removable without tools, and extra disk drives are available for fast swapping with non-classified hard disk units when needed. An equivalent solid-state hard disk is also available for applications with more stringent environmental requirements. No complex "memory erase" procedures are required because there is no other user-accessible nonvolatile memory in the instrument.

Multiple measurement bandwidths and wide bandwidth

For the replacement of many legacy analyzers, the PXA's base 10 MHz measurement bandwidth is sufficient--and the best economic value. For other applications, the PXA can be ordered with--or upgraded to--bandwidths of 25 MHz, 40 MHz and 140 MHz. Remarkably, the enhanced bandwidth does not extract a cost in performance. The PXA's 400 Ms/sec 14-bit ADC is coupled with proprietary ASIC technology for real-time I/Q corrections and data processing, resulting in up to 75 dB of spur-free dynamic range and typical flatness of ±0.4 dB as shown in Figure 4. In addition, the PXA is architected to support even wider bandwidths in the future.

A Platform for the Future

To provide a modular, future-ready architecture, the PXA includes seven internal expansion slots, a generous 450-W power supply, and easily replaceable/upgradable CPU and disk drive assemblies. With the PXA, there is no need to choose between compatibility, supportability, and the highest levels of signal analysis performance and speed. The PXA extends the Agilent X-Series foundation of an open PC architecture, transportable measurement applications, proven measurement algorithms, and a straightforward, consistent user interface. The result is an evolutionary replacement for existing high-performance spectrum and signal analyzers.

Note: Some of the capabilities or features mentioned above are optional or are provided as part of measurement applications products from Agilent.

[1] "Cramming more components onto integrated circuits" Electronics, Volume 38, Number 8, April 19, 1965, available at: ftp://download.intel.com/museum/Moores_Law/Articles-Press_Releases/Gordon_Moore_1965_Article.pdf

[2] "Noise Floor Extension: Improving the Dynamic Range of Spectrum Analysis" Joe Gorin, available in this press kit, not yet published]

 

Agilent Technologies Inc. (NYSE: A) is the world's premier measurement company and a technology leader in communications, electronics, life sciences and chemical analysis. The company's 18,000 employees serve customers in more than 110 countries. Agilent had net revenues of $5.8 billion in fiscal 2008. Information about Agilent is available on the Web at www.agilent.com.

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