Vector signal analyzer

Bctwriter (talk) 03:23, 9 October 2009 (UTC)

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

Manufacturers of modern VSAs tend to focus on features that allow users to reduce their measurement and data acquisition times while building in the flexibility necessary to adapt to changing wireless standards. A leading trend in the VSA market is the use of “software-defined radio” (SDR) architecture, defined as a radio communication system that uses software to modulate and demodulate radio signals. Such systems 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, with the A/D and D/A converters as key elements of the SDR system. The speed and resolution of the converters determine how much analog frequency conversion is required. Converters need sufficient resolution (bits) to produce or capture the modulation data adequately; more complex modulation formats require converters with even greater resolution. The speed of the converters limits the maximum signal frequency that can be created or sampled.

Digital signal processing, another key element of SDR, performs several functions traditionally performed with analog circuitry, including frequency conversion, modulation, demodulation and filtering. Digital signal processing 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 allow for easier 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. Additionally, VSAs employing SDR techniques allow for improved throughput due to faster frequency switching and signal analysis. (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.) Test equipment manufacturers leverage the capability of leading-edge, commercially available signal processing devices and achieve instrument-level performance from them, thus reducing 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

• Measure Phase-Locked Loops With A Spectrum Analyzer
• Beware Of Spectrum Analyzer Power Averaging Techniques
• Selecting the right signal analyzer
• Test Software: A Hands-On Encounter With SignalMeister
• RF-communications software spans signal generation and analysis
• LXI speeds gigahertz measurements
• Software Shapes Next-generation RF Instrumentation
• Instruments test MIMO data transmissions