SECTION 3
CIRCUIT DESCRIPTION
Introduction
The Type 491 Spectrum Analyzer is a swept IF type ana-
lyzer covering the frequency range from 10 MHz to 40 GHz.
This section first presents a block diagram analysis, then a
more detailed circuit description of each major section.
Basic Description
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
process to the first intermediate frequency. This is a wide
band IF of 150 MHz to 250 MHz. Three selectable local
oscillators, in combination with selected mixers, provide the
10 MHz to 40 GHz frequency coverage for the instrument. A
phase lock circuit locks the local oscillator to a stable (internal
or external) reference frequency. This provides the required
stability necessary for narrow dispersion displays.
One or two [depending on the selected bond) low pass
filters (265 MHz and 235 MHz] plus the 150 MHz to 250 MHz
bandpass filter between the first mixer and the wide band
IF amplifier, attenuate and isolate local oscillator frequencies
which would generate spurious signals when mixed with the
second local oscillator frequency.
The wide band (150 MHz to 250 MHz) IF response is then
swept, in the second mixer, by a swept frequency to generate
a second IF of 75 MHz. The swept frequency rate of the osci-
llator is synchronized to the sweep rate so the CRT display
becomes frequency based with a dispersion window that
depends on how much the oscillator is swept.
Center frequency of the swept oscillator is 275 MHz. The
amount the oscillator sweeps depends on the selected dis-
persion.
At maximum dispersion the oscillator sweeps 225
MHz to 325 MHz, which converts all signals within the wide
band IF to the second 75 MHz IF.
Calibrated attenuation is steps of 1 to 51 dB is provided
by the IF attenuator, The signal is then amplified and applied
to the 3rd mixer stage
r
where it is mixed with 70 MHz to
produce a 3rd IF frequency of 5 MHz, The bandwidth of this
5 MHz IF is varied by means of the variable resolution circuit
which provides resolution control from approximately 100
kHz to less than 1 kHz.
Video signals from the detector are amplified by the
vertical amplifier, then applied to the CRT vertical deflection
plates and to the trigger circuit for the sweep generator,
provided the Trigger SOURCE selector is in the INT position.
The sweep generator will free run, or it can be triggered
from any one of three selectable sources; line, external and
internal.
The signal from the sweep generator is applied to both
the sweeper oscillator through the variable dispersion circuit
and to the horizontal amplifier circuit for the horizontal
sweep on the CRT. The horizontal CRT beam movement and
the frequency scan in the 2nd mixer are therefore synchro-
nized. This provides the calibrated dispersion and a linear
display of the frequency spectrum on either side of the dial
center frequency.
The 1st or tunable local oscillator is phase-locked to a
stable crystal-controlled reference frequency by the phase
locking circuit. This stabilizes the local oscillator frequency
and permits narrow 1 kHz/div dispersion settings.
RF Section
The RF section contains three local oscillator assemblies,
for each band, and their respective mixers. Two low-pass
filters (235 and 265 MHz) are switched in series with the
signal path between the band A mixer and the IF band-pass
filter. Only the 265 MHz low-pass filter is used for bands B
and C, The band selector switch SW70 selects the filters and
connects only one oscillator circuit to the +150 volt supply,
Only one oscillator is operating for a given band switch
position. The 235 MHz low pass filter attenuates the low
frequency end of the band A oscillator.
Heater voltage for the oscillators is supplied by the +10
volt regulated supply. Thus, oscillator frequency drift due
to heater voltage variation is minimized. The heater supply
line to V40 and V41 includes a series dropping resistor, R45
and R46, to reduce the voltage far these tubes to 6 volts.
Lossy cables (such as W10-W34, etc.) are used to reduce
SWR caused by slight impedance mismatch between circuits.
Impedance mismatches may be due to coaxial connectors or
other discontinuities.
N O T E
L o s s y c a b l e s u s e s t e e l w i r e f o r t h e c e n t e r c o n -
ductor. These cables are factory-installed and used
t o o p t i m i z e r e s p o n s e f l a t n e s s a n d s e n s i t i v i t y . T h e
I o s s y c a b l e i s i d e n t i f i e d b y t h e w h i t e i n s u l a t i o n
coating; the standard 50 W coaxial cable has the
clear insulation. Do not interchange these cables.
Band A:
The oscillator frequency for band A is 200 MHz
above the RF input dial reading and has a tunable range of
210 to 475 MHz. The oscillator uses a ceramic planer triode.
The tuned circuits are ganged together and tuned by the
RF CENTER FREQUENCY control. Frequency tracking of the
RF dial is adjusted by variable L and C trimmers, if required.
The band A local oscillator output is applied through a
transmission line transformer T14, to the diodes or balanced
section of the mixer. Adjustment of R13, C14 and C16 for bal-
ance greatly reduces local oscillator feed-through. The desired
difference frequency is coupled to the IF amplifier through
the 235 MHz filter.
3-1
Summary of Contents for 491
Page 4: ...i i Fig 1 1 The Type 491 Spectrum Analyzer ...
Page 24: ...2 16 Fig 2 17 Control set up chart ...
Page 34: ...Fig 3 1 Function block diagram of the Type 491 3 2 ...
Page 42: ...Fig 3 10 Block diagram of the video detector and vertical amplifier 3 1 0 ...
Page 48: ......
Page 53: ...Fig 4 4 Power Supply Circuit board assembly with wiring color code 4 5 ...
Page 54: ...Fig 4 5 Horizontal Display circuit board assembly showing color code to pin connectors 4 6 ...
Page 55: ...Fig 4 6 IF Control board assembly Wiring color code to pin connector 4 7 ...
Page 64: ...Fig 4 20A Tube subassembly removal procedure 4 16 ...
Page 65: ...Fig 4 20B Tube subassembly installation procedure 4 17 ...
Page 68: ...Fig 4 22 Power supply board assembly with component call out 4 2 0 ...
Page 69: ...Fig 4 23 Vertical Amplifier and Blanking board assembly with component call out 4 21 ...
Page 70: ...Fig 4 24 IF control board assembly with component call out 4 22 ...
Page 71: ...Fig 4 24 IF control board assembly with component call out 4 23 ...
Page 72: ...Fig 4 25 Horizontal display board with component call out 4 24 ...
Page 73: ...Fig 4 25 Horizontal display board with component call out 4 25 ...
Page 74: ...Fig 4 26 Phase lock board with component call out 4 26 ...
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Page 90: ...Fig 6 1 Test equipment recommended for calibration of the Type 491 6 2 ...
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Page 195: ...SECTION II B 3 ...
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Page 201: ...SECTION 9 DIAGRAMS MECHANICAL PARTS LIST ILLUSTRATIONS ACCESSORIES ...
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Page 217: ...FIG 1 9 29 ...
Page 218: ...FIG 2 REAR 9 31 ...
Page 219: ...9 33 FIG 3 IF CHASSIS PHASE LOCK ASSEMBLIES ...
Page 220: ...FIG 4 POWER CHASSIS 9 35 ...
Page 221: ...FIG 5 TIME DIV SWITCH OSCILLATOR ASSEMBLIES 9 37 ...
Page 222: ...FIG 6 CRT SHIELD ASSEMBLY 9 39 ...
Page 223: ...FIG 7 CABINET ASSEMBLY HANDLE 9 4 1 ...
Page 224: ...FIG 8 491 STANDARD ACCESSORIES FIG 8 491 STANDARD ACCESSORIES 9 43 ...
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Page 226: ...PIN 028017 000 ...