Center frequency (radio frequency or intermediate fre-
quency)-That frequency which corresponds to the center
of the reference coordinate.
Center frequency range (radio frequency)-That range of
frequencies which can be displayed at the center of the
reference coordinate. When referred to a control (e.g., Inter-
mediate Frequency Center Frequency Range) the term indi-
cates the amount of frequency change available with the
control.
Dispersion (sweep width)-The frequency sweep excursion
over the frequency axis of the display can be expressed as
frequency/full frequency axis, or frequency (Hz)/division in
a linear display,
Display flatness-Uniformity of amplitude over a rated
dispersion, The maximum variation in amplitude response
over the maximum dispersion is a measure of display flatness
(usually in units of dB).
Drift (frequency drift)-Long term frequency changes or
or instabilities caused by a frequency change in the spectrum
analyzer local oscillators.
Drift limits the time interval that a
spectrum analyzer can be used without retuning or reset-
ting the front panel controls (units maybe Hz/s, Hz/°C, etc).
Dynamic range (on screen)-The maximum ratio of signal
amplitudes that can be simultaneously observed within the
graticule (usually in units of dB).
Dynamic range (maximum useful)-The ratio between the
maximum input power and the spectrum analyzer sensitivity
(usually in units of dB).
Frequency band-A range of frequencies that can be cov-
ered without switching.
Frequency scale-The range of frequencies that can be
read on one line of the frequency indicating dial.
Incidental frequency modulation (residual frequency modu-
lation)—Short term frequency jitter ar undesired frequency
deviation caused by instabilities in the spectrum analyzer
local oscillators. Incidental frequency modulation limits the
usable resolution ond dispersion (in units of Hz).
I n c r e m e n t a l l i n e a r i t y - A t e r m u s e d t o d e s c r i b e l o c a l
aberrations seen as non-linearities for narrow dispersions.
Linear display-A display in which the vertical deflection
is a linear function of the input signal voltage.
Linearity (dispersion linearity)-Measure of the comparison
of frequency across the dispersion to a straight line frequency
change. Measured by displaying a quantity of equally spaced
(in frequency) frequency markers across the dispersion and
observing the positional deviation of the markers from an
idealized sweep as measured against a linear graticule.
Linearity accuracy, expressed as a percentage, is within
X 100% where
i s m a x i m u m p o s i t i o n a l d e v i a t i o n
and W is the full graticule width.
Maximum input power-The upper level of input power
that the spectrum analyzer can accommodate without de-
gradation in performance (spurious responses and signal
compression). [Usually in units of dBm).
Minimum usable dispersion-The narrowest dispersion ob-
tainable for meaningful analysis. Defined as ten times the
incidental frequency modulation when limited by “incidental
frequency modulation” (in units of Hz).
Phase Lock-The frequency synchronization of the local
oscillator with a stable reference frequency.
Resolution-The ability of the spectrum analyzer to re-
solve and display adjacent signal frequencies. The measure
of resolution is the frequency separation (in Hz) of two equal
amplitude signals, the displays of which merge at the 3 dB
down point. The resolution of a given display depends on
three factors; sweep speed, dispersion and the bandwidth of
the most selective (usually last IF) amplifier.
Resolution bandwidth-The -6 dB bandwidth (with Gaus-
sian response) of the analyzer, with the dispersion and sweep
time adjusted for the minimum displayed bandwidth of a CW
signal. Resolution and resolution bandwidth become syno-
nymous at very long sweep times,
Optimum resolution-The best resolution obtainable for a
given dispersion and a given sweep time, Theoretically or
mathematically:
Optimum resolution bandwidth—The bandwidth at which
best resolution is obtained for a given dispersion and sweep
time. Theoretically and mathematically:
Optimum
dispersion (in Hz)
resolution
bandwidth
sweep time [in seconds)
Safe power level-The upper level of input power that
the spectrum analyzer can accommodate without physical
damage (usually in units of dBm).
Scanning velocity-Product of dispersion and sweep re-
petition rate (units of Hz/unit time).
Sensitivity-Rating factor of spectrum analyzer’s ability to
display signals.
1. Signal equals noise: That input signal level (usually
in dBm) required to produce a display in which the signal
level above the residual noise is equal to the residual noise
level above the baseline. Expressed as: noise =
twice noise.
2. Minimum discernible signal: That input signal level (usu-
ally in dBm) required to produce a display in which the sig-
nal is just visible within the noise.
Skirt selectivity-A measure of the resolution capability of
the spectrum analyzer when displaying signals of unequal
amplitude. A unit of measure (usually in Hz) is the band-
width at some level below the 6 dB down points. For ex-
ample: 10 dB, 20 dB or 40 dB down.
Spurious response (spurii, spur)-An erroneous display or
signal which does not conform to the indicated frequency or
dial reading. Spurii and spur are the colloquialisms used to
mean spurious response (plural) or spurious response (singu-
lar) respectively. Spurious responses are of the following
types
:
1. IF feedthrough-Signal frequencies within the IF pass-
band of the spectrum analyzer that are not converted in the
first mixer but pass through the IF amplifier and produce dis-
2-3
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 ...
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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 ...