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Over modulation will produce extra sideband frequencies.
The spectrum is very similar to multi-frequency modulation.
Over modulation, however, is usually distinguished from the
multi-frequency modulation by: 1) The spacing between over-
modulated sidebands is equal, while, multi-frequency side-
bands may be arbitrarily spaced, unless the modulating fre-
quencies are harmonically related; 2) The amplitude of the
sidebands decreases progressively out from the carrier, but.
the amplitude multifrequency modulated signals is determined
by the modulation percentage of each frequency and can be
arbitrary.
Frequency Modulated Spectrum
FM measurements are generally measurements that deter-
mine; the modulating frequency, amplitude of the modulating
signal or frequency deviation, and index of modulation.
A typical FM spectrum is shown in Fig. 2-26. The exterior
modulation envelope resembles a c
OS
2
curve, and identifies
the signal as frequency modulation.
Frequency Deviation Measurement
There is no clear relationship between spectral width and
deviation, because in theory the FM spectrum approaches
infinity. In practice however the spectral level falls quite
rapidly as shown in Fig. 2-26B.
Accurate deviation measurements can be made if the
modulating frequency and the modulation index (where the
carrier goes to zero) are known.
Modulation Index =
Carrier deviation
Modulating frequency
Values of modulation index corresponding to zero carrier
amplitude are listed in Table 2-1.
TABLE 2-1
Accurate carrier null is essential for accurate measurement.
Pulse Modulated RF Spectral Measurements
A visual examination of the pulse modulated spectrum can
check a
number of the characteristics about a transmitting
and modulating device. Some of these characteristics are:
1, The transmitting oscillator stability can be checked by
noting the degree of frequency shift as described previously.
F i g . 2 - 2 7 . P u l s e m o d u l a t i o n d i s p l a y ,
i l l u s t r a t i n g p u l s e w i d t h a n d
PRF measurements.
2-23
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 ...
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Page 192: ......
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 203: ...9 1 ...
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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 ...