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===Operation===
[[Image: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 & Digitize Stage===
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|voltage-controlled oscillator]] and a [[Frequency mixer|mixer]]) to the center frequency of a [[band-pass filter]]. The use of a [[Voltage-controlled oscillator|voltage-controlled oscillator]] allows for 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.
===DSP & Display stage===
Once the signal is digitized, it is separated into [[quadrature phase|quadrature]] and in-phase components using a quadrature detector, which is typically implemented with a [[Hilbert transform#Discrete Hilbert transforms|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'''
{{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. This window is implemented by multiplying it with the digitized values of the sample period before computing the FFT.
'''Constellation Diagram'''
{{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]] 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'''
{{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===
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