CIRCUIT DESCRIPTION
Introduction
and to the horizontal amplifier circuit for the horizontal
sweep on the CRT. The horizontal CRT beam movement and
The Type 491 Spectrum Analyzer is a swept IF type ana-
the frequency scan in the 2nd mixer are therefore synchro-
lyzer covering the frequency range from 10 MHz to 40 GHz.
nized. This provides the calibrated dispersion and a linear
This section first presents a block diagram analysis, then a
display of the frequency spectrum on either side of the dial
more detailed circuit description of each major section.
center frequency.
The 1st or tunable local oscillator is phase-locked to a
stable crystal-controlled reference frequency by the phase
Basic Description
locking circuit. This stabilizes the local oscillator frequency
and permits narrow 1 kHz/div dispersion settings.
A block diagram of the Type 491 is shown in Fig. 3-1 and
the Diagrams section.
Signals within the frequency range of the Type 491 that
are applied to the RF INPUT are converted by the heterodyne
RF Section
process to the first intermediate frequency. This is a wide
The RF section contains three local oscillator assemblies,
band IF of 150 MHz to 250 MHz. Three selectable local
for each band, and their respective mixers. Two low-pass
oscillators, in combination with selected mixers, provide the
filters (235 and 265 MHz) are switched in series with the
10 MHz to 40 GHz frequency coverage for the instrument. A
signal path between the band A mixer and the IF band-pass
phase lock circuit locks the local oscillator to a stable (internal
filter. Only the 265 MHz low-pass filter is used for bands B
or external) reference frequency. This provides the required
and C, The band selector switch SW70 selects the filters and
stability necessary for narrow dispersion displays.
connects only one oscillator circuit to the +150 volt supply,
One or two [depending on the selected bond) low pass
Only one oscillator is operating for a given band switch
filters (265 MHz and 235 MHz] plus the 150 MHz to 250 MHz
position. The 235 MHz low pass filter attenuates the low
bandpass filter between the first mixer and the wide band
frequency end of the band A oscillator.
IF amplifier, attenuate and isolate local oscillator frequencies
Heater voltage for the oscillators is supplied by the +10
which would generate spurious signals when mixed with the
volt regulated supply. Thus, oscillator frequency drift due
second local oscillator frequency.
to heater voltage variation is minimized. The heater supply
The wide band (150 MHz to 250 MHz) IF response is then
line to V40 and V41 includes a series dropping resistor, R45
swept, in the second mixer, by a swept frequency to generate
and R46, to reduce the voltage far these tubes to 6 volts.
a second IF of 75 MHz. The swept frequency rate of the osci-
Lossy cables (such as W10-W34, etc.) are used to reduce
llator is synchronized to the sweep rate so the CRT display
SWR caused by slight impedance mismatch between circuits.
becomes frequency based with a dispersion window that
Impedance mismatches may be due to coaxial connectors or
depends on how much the oscillator is swept.
other discontinuities.
Center frequency of the swept oscillator is 275 MHz. The
amount the oscillator sweeps depends on the selected dis-
NOTE
persion. At maximum dispersion the oscillator sweeps 225
Lossy cables use steel wire for the center con-
MHz to 325 MHz, which converts all signals within the wide
ductor. These cables are factory-installed and used
band IF to the second 75 MHz IF.
to optimize response flatness and sensitivity. The
Calibrated attenuation is steps of 1 to 51 dB is provided
Iossy cable is identified by the white insulation
by the IF attenuator, The signal is then amplified and applied
coating; the standard 50 W coaxial cable has the
to the 3rd mixer stager where it is mixed with 70 MHz to
clear insulation. Do not interchange these cables.
produce a 3rd IF frequency of 5 MHz, The bandwidth of this
5 MHz IF is varied by means of the variable resolution circuit
Band A: The oscillator frequency for band A is 200 MHz
which provides resolution control from approximately 100
above the RF input dial reading and has a tunable range of
kHz to less than 1 kHz.
210 to 475 MHz. The oscillator uses a ceramic planer triode.
The tuned circuits are ganged together and tuned by the
Video signals from the detector are amplified by the
RF CENTER FREQUENCY control. Frequency tracking of the
vertical amplifier, then applied to the CRT vertical deflection
RF dial is adjusted by variable L and C trimmers, if required.
plates and to the trigger circuit for the sweep generator,
provided the Trigger SOURCE selector is in the INT position.
The band A local oscillator output is applied through a
The sweep generator will free run, or it can be triggered
transmission line transformer T14, to the diodes or balanced
from any one of three selectable sources; line, external and
section of the mixer. Adjustment of R13, C14 and C16 for bal-
internal.
ance greatly reduces local oscillator feed-through. The desired
difference frequency is coupled to the IF amplifier through
The signal from the sweep generator is applied to both
the 235 MHz filter.
the sweeper oscillator through the variable dispersion circuit
3-1
