Vector signal analyzer: Difference between revisions

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[[File:VSA screenshot.PNG|right|400px|thumb|A vector signal analyzer display featuring a constellation diagram, demodulation error data, signal spectrum, and the real-time measured signal]]

{{unreferenced|date=September 2009}}

TheA '''vector signal analyzer''' (VSA) is aan toolinstrument that canmeasures performthe manymagnitude and phase of the sameinput measurementsignal andat characterizationa taskssingle thatfrequency within the [[spectrumIntermediate analyzerfrequency|IF]] [[Bandwidth (SAsignal processing)|bandwidth]] can,of butthe itinstrument. canThe alsoprimary performuse manyis to make in-channel measurements, moresuch as [[digitalerror vector magnitude]], code ___domain power, and [[demodulationspectral flatness]], on known functionssignals.

Vector signal analyzers are useful in measuring and [[Demodulation|demodulating]] [[Modulation#Digital modulation methods|digitally modulated signals]] like [[W-CDMA (UMTS)|W-CDMA]], [[3GPP Long Term Evolution|LTE]], and [[Wireless LAN|WLAN]].<ref>National Institute of Standards and Technology [http://www.eeel.nist.gov/kate_papers/R13_NIST_TN1546_Modulated_Signal_(Web)1.pdf], ''Measurement to Support Modulated-Signal Radio Transmissions for the Public-Safety Sector'', p. 15, April, 2008, accessed July 19, 2011.</ref> These measurements are used to determine the quality of [[modulation]] and can be used for design validation and compliance testing of electronic devices.

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.

==Operation==

==Trends in Vector Signal Analyzer Design==

[[File:Vsa block.PNG|right|400px|thumb|A vector signal analyzer block diagram featuring a down-convert stage, a digitizing stage, and a [[digital signal processing|DSP]] and display stage]]

The vector signal analyzer spectrum analysis process typically has a down-convert & digitizing stage and a [[digital signal processing|DSP]] & display stage.

===Down-convert and digitize stage===

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.

A vector signal analyzer operates by first down-converting the signal spectra by using [[superheterodyne receiver|superheterodyne techniques]].

A portion of the input signal spectrum is [[Superheterodyne receiver#High-side and low-side injection|down-converted]] (using a [[voltage-controlled oscillator]] and a [[Frequency mixer|mixer]]) to the center frequency of a [[band-pass filter]]. The use of a [[voltage-controlled oscillator]] allows consideration of different carrier frequencies.

After the conversion to an [[intermediate frequency]], the signal is [[electronic filter|filtered]] in order to band-limit the signal and prevent [[aliasing]]. The signal is then [[digitizing|digitized]] using an [[analog-to-digital converter]]. [[Sampling rate]] is often varied in relation to the frequency span under consideration.

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.

===DSP and display stage===

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.

Once the signal is digitized, it is separated into [[quadrature phase|quadrature]] and in-phase components ([[I/Q data]]) using a quadrature detector, which is typically {{cn|date=February 2024}} implemented with a [[Hilbert transform#Discrete Hilbert transform|discrete Hilbert transform]]. Several measurements are made and displayed using these signal components and various [[digital signal processing|DSP]] processes, such as the ones below.

==== Signal spectrum from FFT ====

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.

{{main|Fast Fourier transform}}

A [[fast Fourier transform|FFT]] is used to compute the frequency spectrum of the signal. Usually there is a [[window function|windowing function]] option to limit [[spectral leakage]] and enhance frequency resolution.<ref>Keysight [https://stgwww.keysight.com/cn/zh/assets/7018-02679/technical-overviews/5990-6405.pdf], ''Keysight Vector Signal Analysis Basics'', Published September 30, 2019.</ref> This window is implemented by multiplying it with the digitized values of the sample period before computing the FFT.

==== Constellation diagram ====

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.

{{main|Constellation diagram}}

A [[constellation diagram]] represents a signal modulated by a digital [[modulation]] scheme such as [[quadrature amplitude modulation]] or [[phase-shift keying]]. This diagram maps the magnitude of the [[Quadrature phase|quadrature]] and in-phase components to the vertical and horizontal directions respectively. Qualitative assessments of signal integrity can be made based on [[Constellation diagram#Interpretation|interpretation of this diagram]].

==== Error vector magnitude ====

==External Links==

{{main|Error vector magnitude}}

By representing the quadrature and in-phase components as the vertical and horizontal axes, the [[error vector magnitude]] can be computed as the distance between the ideal and measured constellation points on the diagram. This requires knowledge of the modulated signal in order to compare the received signal with the ideal signal.

===Typical functionality===

* [http://electronicdesign.com/Articles/Index.cfm?ArticleID=18152 Measure Phase-Locked Loops With A Spectrum Analyzer]

Typical vector signal analyzer displays feature the [[spectrum]] of the signal measured within the [[Intermediate frequency|IF]] [[Bandwidth (signal processing)|bandwidth]], a [[Quadrature amplitude modulation|constellation diagram]] of the demodulated signal, [[error vector magnitude]] measurements, and a [[time-___domain]] plot of the signal. Many more measurement results can be displayed depending on the type of modulation being used (symbol decoding, [[MIMO]] measurements, radio frame summary, etc.).

* [http://www.mwrf.com/Articles/Index.cfm?ArticleID=14305 Beware Of Spectrum Analyzer Power Averaging Techniques]

* [http://rfdesign.com/microwave_millimeter_tech/test_and_measurement/selecting-signal-analyzer/index.html Selecting the right signal analyzer]

==References==

* [http://www.evaluationengineering.com/features/2009_april/0409_test.aspx Test Software: A Hands-On Encounter With SignalMeister]

* [http://www.ednindia.com/article-23645-rfcommunicationssoftwarespanssignalgenerationandanalysis-Asia.html RF-communications software spans signal generation and analysis]

* [https://www.ewh.ieee.org/r2/baltimore/Chapter/Comm/VSA_COMSOC.pdf Communications Systems Analysis Using Hardware and Software-Based Vector Signal Analyzers]

* [http://www.tmworld.com/article/CA6495736.html LXI speeds gigahertz measurements]

* [http://www.wirelessdesignmag.com/ShowPR~PUBCODE~055~ACCT~0014400~ISSUE~0611~RELTYPE~PR~PRODCODE~V0140~PRODLETT~A.html Software Shapes Next-generation RF Instrumentation]

===Footnotes===

* [http://www.tmworld.com/article/CA6488248.html Instruments test MIMO data transmissions]

{{Reflist}}

[[Category:Measuring instruments]]

[[Category:Electronic test equipment]]

[[Category:Signal processing]]

{{electronics-stub}}

[[es:Analizador vectorial de señales]]