Section 5—492/492P Service Vol. 1 (SN B030000 & up)
THEORY OF OPERATION
This section of the manual describes the circuitry in the
492/492 P Spectrum Analyzer. The description begins with a
general and functional description related to a block diagram
of the major systems within the 492/492P. This is followed
with a detailed description of the circuitry within each sec
tion; for example, the Display section.
The number in the diamond refers to the corresponding
schematic diagram number. Note that these same numbers
are included on diagrams. Schematics of all major circuits
are in Volume 2, section 8.
Φ
FUNCTIONAL AND GENERAL DESCRIPTION
What It Does
The 492/492P Spectrum Analyzer accepts an electrical
signal as its input and displays the signal's frequency com
ponents on a crt. Signals can be applied directly to the RF
INPUT or, if the analyzer is equipped for external mixer op
eration, to an external mixer, which extends the measure
ment range of the 492/492P.
frequency of each span is set by the FREQUENCY control.
The frequency range of each span is set by the SPAN/DIV
control. The power level represented by the vertical deflec
tion is set by the REFERENCE LEVEL control; this control
causes the microcomputer to change the input RF
attenuator or IF gain, or both, to bring signals within the
display range.
The display of the frequency components of the input
signal appears on the crt as a graph where the horizontal
axis is frequency and the vertical axis is amplitude. The dis
play can also be plotted on a chart recorder using rear-panel
connectors. The 492/492P, when equipped with Option 02,
can transmit the display digitally via the IEEE 488 bus.
Manual operation of the 492/492P Spectrum Analyzer is
accomplished through the front-panel knobs and switches.
The 492P may also be operated via the IEEE 488 bus using
a straightforward language format.
How It Works
The Functional Block Diagram is located at the front of
the Diagrams section. It relates the major sections in the
instrument and shows the main signal paths. Refer to the
diagram while reading this general description.
The 492/492P operates as a swept, narrow-band receiv
er. As it sweeps a range of frequencies, it moves the crt
J
beam horizontally. When it detects a frequency component
of the input signal, it deflects the beam vertically. The center
First, Second, and Third Converters
In the 492/492P Spectrum Analyzer, this swept-frequen-
cy analysis is achieved with a triple-conversion super
heterodyne technique.
Each of the three frequency converters consists of a mix
er, a local oscillator, and appropriate filters. Only one fre
quency can be properly converted in each mixer and pass
through all bandpass filters and reach the detector. This
analysis frequency can be changed simply by changing the
frequency of any of the local oscillators in the converters.
The first converter, usually referred to as the front end,
converts the input signal frequency to an intermediate fre
quency (IF) which may be either 829 MHz or 2072 MHz de
pending on which band is in use. Although the internal mixer
covers signals from 50 kHz to 21 GHz, an external mixer
may be used for analysis through the millimeter wavelengths
(unless this capability is deleted by Option 08). If the
492/492P is equipped with Option 01, a preselector and a
lowpass filter attenuate unwanted signals when the internal
mixer is used. This prevents most images and spurious
responses.
REV AUG 1981
5-1
