Tektronix 2715, Руководство пользователя

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2715 User Manual
1–1
Introduction
This section introduces you to the basic concepts of an RF spectrum analyzer and
its uses. This section also briefly describes the features of the 2715 Cable TV
Spectrum Analyzer.
What is a Spectrum Analyzer?
There are several types of spectrum analyzers, but we will describe only the
heterodyne, or scanning, spectrum analyzer. A scanning RF spectrum analyzer is
essentially a radio receiver.
Tune a conventional FM broadcast receiver from one end of the band to the
other. As you tune, plot the reading of the signal level meter versus frequency.
The graph you produce is a frequency domain representation, or spectrum, of the
FM broadcast band. The graph tells you at which frequencies the signals occur
and how strong they are. If stations are too close together, you will not be able to
get an independent meter reading for each individual station, and you will hear
them simultaneously. This is because the intermediate frequency (IF) filter of the
receiver has a bandwidth that is too wide to separate (resolve) the stations.
What you have just done is tune or scan the FM broadcast band with a resolution
bandwidth equal to the bandwidth of the IF filter in your receiver. If you plot
your measurements on graph paper with one centimeter divisions, making each
division equal 1 MHz, the span/division of the resulting plot is 1 MHz/division.
If you stop tuning, the receiver no longer spans a range of frequencies, but is
fixed at the currently selected frequency; it is in zero span mode. The output of
the receiver depends on the signal coming through the IF filter at the selected
frequency. If you plot the amplitude of the signal as a function of time (or view it
on an oscilloscope-type display) you create a time-domain representation (signal
amplitude versus time).
A spectrum analyzer performs similarly to the receiver except that the scanning
is usually performed automatically (and faster than you do it manually) and there
is a selection of IF bandwidths or resolution bandwidths to choose from.
Multiple resolution bandwidths are needed because in some cases you may want
to separate closely-spaced, narrow-band signals, while in others you may want to
examine signals with larger bandwidths. There is a maximum speed at which a
band can be accurately scanned with a resolution bandwidth of a given width
(generally, the smaller the resolution bandwidth, the slower the speed). The
spectrum analyzer can automatically select the fastest speed for you.
You can find additional information about basic spectrum analyzer concepts and
definitions in Tektronix application note 26W-7037-1, Spectrum Analyzer
Fundamentals.