Agilent Spectrum Analysis Basics - Agilent Technologies

Noise sidebands: Modulation sidebands that indicate the short-term
instability of the LO (primarily the first LO) system of a spectrum analyzer.
The modulating signal is noise, in the LO circuit itself and/or in the LO
stabilizing circuit, and the sidebands comprise a noise spectrum. The mixing
process transfers any LO instability to the mixing products, so the noise
sidebands appear on any spectral component displayed on the analyzer
far enough above the broadband noise floor. Because the sidebands are
noise, their level relative to a spectral component is a function of resolution
bandwidth. Noise sidebands are typically specified in terms of dBc/Hz
(amplitude in a 1 Hz bandwidth relative to the carrier) at a given offset from
the carrier, the carrier being a spectral component viewed on the display.
Phase noise: See Noise sidebands.
Positive peak: The display detection mode in which each displayed point
indicates the maximum value of the video signal for that part of the
frequency span and/or time interval represented by the point.
Preamplifier: An external, low noise-figure amplifier that improves system
(preamplifier/spectrum analyzer) sensitivity over that of the analyzer itself.
Preselector: A tunable bandpass filter that precedes the input mixer of
a spectrum analyzer and tracks the appropriate mixing mode. Preselectors
are typically used only above 2 GHz. They essentially eliminate multiple and
image responses and, for certain signal conditions, improve dynamic range.
Quasi-peak detector (QPD): A type of detector whose output is a function
of both signal amplitude as well as pulse repetition rate. The QPD gives
higher weighting to signals with higher pulse repetition rates. In the limit,
a QPD will exhibit the same amplitude as a peak detector when measuring
a signal with a constant amplitude (CW) signal.
Raster display: A TV-like display in which the image is formed by scanning
the electron beam rapidly across and slowly down the display face and
gating the beam on as appropriate. The scanning rates are fast enough to
produce a flicker-free display. Also see Vector display and Sweep time.
Reference level: The calibrated vertical position on the display used as
a reference for amplitude measurements. The reference level position is
normally the top line of the graticule.
Relative amplitude accuracy: The uncertainty of an amplitude
measurement in which the amplitude of one signal is compared to the
amplitude of another regardless of the absolute amplitude of either.
Distortion measurements are relative measurements. Contributors to
uncertainty include frequency response and display fidelity and changes
of input attenuation, IF gain, scale factor, and resolution bandwidth.
Residual FM: The inherent short-term frequency instability of an oscillator
in the absence of any other modulation. In the case of a spectrum analyzer,
we usually expand the definition to include the case in which the LO is
swept. Residual FM is usually specified in peak-to-peak values because
they are most easily measured on the display, if visible at all.
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