fuses which are positioned correctly when the correct selec-
tion is made for either 115 or 230-volt nominal operation.
A Spectrum Analyzer is an instrument that graphically
Fig. 2-3 shows
the power input panel on the rear of the
presents a plot of signal amplitude as a function of frequency
instrument and the voltage range and nominal line voltage
for a selected portion of the spectrum. The Type 491 is
designed to provide a spectral display of the frequency
distribution of electromagnetic energy within the frequency
range of 10 MHz to 40 GHz. Signals are displayed as a
spectrum on a CRT screen with signal energy plotted on the
vertical axis against frequency on the horizontal axis.
This type of display provides the following information;
The presence or absence of signals, their frequencies, fre-
quency drift, relative amplitude of the signals and the nature
of modulation if any, plus many other characteristics.
This section of the manual describes the function of the
front and rear panel controls and connectors, power supply
connection, and a procedure for first time operation, to intro-
duce the operator to the operational functions of the controls.
The remainder af the section then describes operation tech-
nique with some measurement applications and signal
Front Cover and Handle
The front cover furnished with the Type 491 provides a
dust tight seal around the front panel. Use the cover to
protect the front panel when storing or transporting the
instrument, The cover also provides storage for the external
waveguide mixers and other accessories. See Fig. 2-1.
Removing or replacing the dust cover on the instru-
ment may be hazardous, if the" instrument is lifted
out of, or slid into, the dust cover. To remove or
replace the dust cover, set the instrument on a
bench or table, then slide the cover off or on the
instrument. The instrument may also be set on the
front panel cover and the dust cover slipped on or
off the instrument. Do not set the instrument on
the front panel controls.
The handle of the Type 491 can be positioned to carry
the instrument or it can be positioned at several angles to
serve as a tilt-stand. To position the handle, press in at both
pivot points (see Fig. 2-2) and adjust the handle to the
desired position. The instrument may also be set upright
on the rear panel feet for operation or storage.
The Type 491 can be operated from either 115 or 230-volt
nominal line voltage with a range from 90 to 136 VAC or
180 to 272 VAC. Two selector-type connectors on the power
input panel may be positioned to accommodate these volt-
Fig. 2-1. Accessory storage provided
in the front
cover of the Type
age ranges. The selection assembly also includes two line