Specifications

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Video: In a spectrum analyzer, a term describing the output of the envelope
detector. The frequency range extends from 0 Hz to a frequency typically
well beyond the widest resolution bandwidth available in the analyzer.
However, the ultimate bandwidth of the video chain is determined by the
setting of the video filter.
Video amplifier: A post-detection, DC-coupled amplifier that drives the
vertical deflection plates of the CRT. See Video bandwidth and Video filter.
Video average: A digital averaging of a spectrum analyzer’s trace
information. The averaging is done at each point of the display independently
and is completed over the number of sweeps selected by the user. The
averaging algorithm applies a weighting factor (1/n, where n is the number
of the current sweep) to the amplitude value of a given point on the current
sweep, applies another weighting factor [(n – 1)/n] to the previously
stored average, and combines the two for a current average. After the
designated number of sweeps are completed, the weighting factors remain
constant, and the display becomes a running average.
Video bandwidth: The cutoff frequency (3 dB point) of an adjustable low
pass filter in the video circuit. When the video bandwidth is equal to or less
than the resolution bandwidth, the video circuit cannot fully respond to
the more rapid fluctuations of the output of the envelope detector. The
result is a smoothing of the trace, i.e. a reduction in the peak-to-peak
excursion of broadband signals such as noise and pulsed RF when viewed
in the broadband mode. The degree of averaging or smoothing is a function
of the ratio of the video bandwidth to the resolution bandwidth.
Video filter: A post-detection, low-pass filter that determines the
bandwidth of the video amplifier. Used to average or smooth a trace.
See Video bandwidth.
Zero span: That case in which a spectrum analyzer’s LO remains fixed
at a given frequency so the analyzer becomes a fixed-tuned receiver. The
bandwidth of the receiver is that of the resolution (IF) bandwidth. Signal
amplitude variations are displayed as a function of time. To avoid any loss
of signal information, the resolution bandwidth must be as wide as the
signal bandwidth. To avoid any smoothing, the video bandwidth must be
set wider than the resolution bandwidth.