CAUTION
The Type 491 should not be operated with these
voltage selector switches in improper positions.
Operation of the instrument with incorrect voltages
will either blow the protective fuses or the instru-
ment will not operate properly.
The Voltage Range Selector located on the rear panel
permits the instrument to operate on line voltages above and
below the nominal 115 or 230 volts. Each selection provides
correct regulation through an overlap voltage range into the
next higher or lower range. It is best to select a range with
Spectrum Analyzer Terms
its center voltage near your nominal line voltage, thus pro-
viding adequate regulation over a plus and minus devia-
The following glossary of spectrum analyzer terms is pre-
tion of the input line voltage.
sented as an aid to understanding the terms as they are used
in this manual.
The following procedure will prepare the instrument for
operation at your average input line voltage:
Spectrum Analyzer-A device that displays a graph of the
relative power distribution as a function of frequency, typi-
1. Remove the cover assembly over the selectors by
cally on a cathode-ray tube or chart recorder.
unscrewing the two cap screws, then pull the cover with the
attached fuses away from the panel.
Types: Real-time and non real-time.
2. Pull the Line Voltage Selector out and turn the connector
A real-time spectrum analyzer performs a continuous
around to plug it back into the correct position.
analysis of the incoming signal, with the time sequence of
events preserved between input and output.
3. To change the regulating range, pull out the Range
Selector bar, slide it to the desired range and plug it into
A non-real-time spectrum analyzer performs an analysis
the correct position. Select a range which is centered about
of repetitive events by a sampling process.
the average line voltage to be applied to the instrument.
See Fig. 2-3.
Methods: Swept front end and swept intermediate fre-
quency.
4. Re-install the cover. Make certain the cover fits firmly
against the rear panel, so the line fuses are seated in their
A swept front end spectrum analyzer is a superheterodyne
sockets, and tighten the two cap screws.
spectrum analyzer in which the first local oscillator is swept.
A swept IF spectrum analyzer is a superheterodyne spec-
5. Before applying power to the instrument, check that the
trum analyzer in which a local oscillator other than the first
indicating tabs on the switch bars are protruding through
is swept.
the correct holes for the desired voltage setup.
2-2
