Fig. 2-18. Spectrum of an amplitude modulated signal. Sideband
amplitude is ½ the percentage of modulation. This spectrum shows
1 0 0 % m o d u l a t i o n .
3. Decrease the dispersion to open the screen. Keep the
signal centered on screen with the IF CENTER FREQ controls.
If the local oscillator should lose its lock condition when the
dispersion settings are 100 kHz or less, the signal will dis-
appear from the screen. A slight adjustment of the FINE RF
CENTER FREQ control will usually return the signal to the
display.
4. If two or more high frequency (upper scale) signals are
to be resolved, they can be moved on the display without
losing phase lock by adjusting the INT REF FREQ control.
Recorder Out Connector
Signals on the display may be recorded by connecting to
the RECORDER output connector on the rear panel. A linear
output is provided when the VERTICAL DISPLAY switch is in
t h e L O G a n d L I N p o s i t i o n s . W i t h t h e D I S P L A Y s w i t c h i n
the SQ LAW position, the output to the RECORDER con-
nector is square law.
Control Setup Chart
Fig. 2-17 is a control setup chart for the front panel of the
Type 491. This figure may be reproduced and used as a test
setup record for special applications or procedures. It may
also serve as a training aid to facilitate control operation.
SPECTRUM ANALYZER DISPLAYS
The Spectrum Analyzer displays a plot of signal amplitude
as a function of frequency. With this type of display, in
the frequency domain, individual frequency. components in
the signal can be displayed and readily analyzed. This
section describes some basic spectrum analyzer displays.
Fig. 2-19. Formation of a spectrum.
F is the fundamental or carrier
f r e q u e n c y , F
1
and F
2
are the modulating frequencies.
Spectrum of Amplitude Modulation
When a single frequency (CW) signal is amplitude-modu-
lated by a single frequency, two additional frequencies will
be generated; the carrier plus the two side bands. See Fig.
2-18. The amplitude of either sideband with respect to the
carrier voltage is ½ the percentage of modulation. The
frequency difference between the carrier and either sideband
equals the modulating frequency.
Figure 2-19 illustrates how a spectrum is generated when
a fundamental carrier frequency F is modulated by two
frequencies F
1
and F
2
.
The sideband spectrum af multiple frequency amplitude-
modulated signal spectrum is determined by the modulating
frequencies. To resolve this complex spectrum, the analyzer
resolution bandwidth must be less than the lowest modulating
frequency, or the bandwidth must be less than the difference
between any two modulating frequencies, whichever is the
smaller.
In wideband amplitude-modulation such as television pic-
ture information, the spectrum analyzer may be used to
measure the sideband energy distribution and modulation
bandwidth.
The amplitude modulated signal spectrum will therefore
furnish the following information: 1) Fundamental or carrier
frequency, 2) modulation frequency or frequencies, 3) modu-
lation percentage, 4) sideband energy distribution and 5)
modulation bandwidth. Other characteristics which may
be evaluated are; degree of incidental FM (evidenced by
signal jitter),
nonlinear modulation, and over-modulation.
These characteristics will be described in more detail with
other types of spectrum display patterns.
2-17
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 ...
Page 88: ......
Page 90: ...Fig 6 1 Test equipment recommended for calibration of the Type 491 6 2 ...
Page 138: ......
Page 192: ......
Page 195: ...SECTION II B 3 ...
Page 196: ......
Page 200: ......
Page 201: ...SECTION 9 DIAGRAMS MECHANICAL PARTS LIST ILLUSTRATIONS ACCESSORIES ...
Page 202: ......
Page 203: ...9 1 ...
Page 204: ...9 3 ...
Page 205: ...9 5 ...
Page 206: ...9 7 ...
Page 207: ...9 9 ...
Page 208: ...9 11 ...
Page 209: ...9 13 ...
Page 210: ...9 15 ...
Page 211: ...9 17 ...
Page 212: ...9 19 ...
Page 213: ...9 21 ...
Page 214: ...9 23 ...
Page 215: ...9 25 ...
Page 216: ...9 27 ...
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 ...
Page 225: ......
Page 226: ...PIN 028017 000 ...