Vector signal analyzer

The vector signal analyzer (VSA) is a tool that can perform many of the same measurement and characterization tasks that the spectrum analyzer (SA) can, but it can also perform many more digital demodulation functions.

The SA and VSA operate in different manners. These operational differences can result in measurement errors if these differences aren't properly considered while making a measurement.

Trends in Vector Signal Analyzer Design

Because the manufacturers of mobile handsets and wireless components are under constant pressure to reduce costs, address new technologies, and increase their manufacturing flexibility, the manufacturers of modern vector signal analyzers (VSAs) have tended to focus on features that will allow their customers to reduce their measurement and data acquisition times while building in the flexibility necessary to adapt to changing wireless standards.

One of the leading trends in the VSA market is the use of “software-defined radio” (SDR) architecture, which can be defined as a radio communication system that uses software to modulate and demodulate radio signals. These systems can achieve high flexibility at a lower cost than traditional analog designs.

In the purest sense, digital-to-analog (D/A) and analog-to-digital (A/D) conversion would occur at the carrier frequency and no analog up- and down-conversion would be required. Today’s SDR applications typically have at least one analog up- and down-conversion stage. Clearly, the A/D and D/A converters are key elements of an SDR system. The speed and resolution of the converters will determine how much analog frequency conversion is required. Converters need sufficient resolution (bits) to produce or capture the modulation data adequately, and more complex modulation formats will require converters with even greater resolution. The speed of the converters will limit the maximum signal frequency that can be created or sampled. Converter technology continues to advance, providing higher combinations of resolution and frequency.

Digital signal processing is another key element of SDR because it performs several functions traditionally performed with analog circuitry, including frequency conversion, modulation, demodulation, and filtering. Digital signal processing also allows better performance than analog designs by supporting functions such as waveform pre-distortion and decimation. Pre-distortion of transmitted waveforms takes into account the known non-linearity of the analog circuitry and modifies the baseband waveform to compensate for it, producing a better quality modulated signal.

VSAs employing SDR techniques offer advantages to both equipment manufacturers and their customers:

• Easy upgradeability to new communication standards. Signal generation and analysis are largely performed by routines programmed into the digital signal processor. When new standards emerge, it’s easy to create new DSP programs for the new functions and distribute them to the owners of existing instruments via firmware upgrades.
• Improved throughput due to faster frequency switching and signal analysis. Wide bandwidth A/D converters and fast DSP devices can process large FFTs very efficiently. For example, a DSP-based analyzer can provide measurement times several orders of magnitude faster than traditional spectrum analyzers, under conditions of wide spans and narrow resolution bandwidths. Direct digital synthesis provides significantly faster frequency switching than traditional approaches allow. Fast frequency switching will improve the throughput of both signal generators and signal analyzers.
• Faster time to market for test instruments. Test equipment manufacturers can leverage the capability of leading-edge, commercially available signal processing devices and achieve instrument-level performance from them. This reduces the amount of development required for test instruments dramatically. Also, the basic digital design can be shared across a range of instruments, further reducing development costs.

Today’s VSAs are often designed to work in tandem with vector signal generators (VSGs) designed using the same SDR architecture as part of an integrated test system.

External Links

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