![Tektronix 491 Maintenance Manual Download Page 20](http://html1.mh-extra.com/html/tektronix/491/491_maintenance-manual_1077911020.webp)
Fig. 2-10. 1 MHz MARKERS OUT (Phase lock reference) connector.
D I S P E R S I O N 1 M H z / d i v .
d . A d j u s t t h e D I S P E R S I O N - C A L f o r 1 m a r k e r / d i v i s i o n .
Use the Horizontal POSITION control or the IF CENTER FREQ
control to align the markers to the graticule lines. Dispersion
is calibrated over the center 8 divisions of the display. See
Fig. 2-10.
Video Filter Operation
The video filter restricts the video bandwidth so that noise
or beat signals are reduced. This application is very useful
when analyzing signals close to minimum resolution band-
width. Fig. 2-11 shows the apparent increased resolution
when the VIDEO FILTER is turned on. It does restrict the use-
able sweep rate, because of the filter time constant, to about
50 ms/div or slower.
Vertical Display Modes
The appearance of the displayed signal depends to a
great extent on the setting of the VERTICAL DISPLAY switch.
For example; to accentuate the side lobes of a signal, the
LOG (40 dB full screen) position should be used, as com-
pared to the SQ LAW (13 dB full screen) position. Fig, 2-12
illustrates the effect of each display mode or each position
of the VERTICAL DISPLAY switch.
The LOG position increases the dynamic range of the dis-
play by attenuating large amplitude signals more than small
amplitude signals. This produces a display which approxi-
mates a logarithmic response curve. The circuit is basically
a compression circuit, and is most effective when there are
large signal amplitude differences.
Fig. 2-11. Integrating the display with the video filter.
The SQ LAW (power) position provides a display that is
approximately proportional to the square of the input sig-
nal amplitude. The display, therefore, approximates the input
signal power. This is basically an expansion circuit to ac-
centuate small amplitude differences.
Selecting the Sweep Rate
The sweep rate for wide dispersion coupled resolution set-
tings is usually set just above the visual flicker setting; how-
ever, as the DISPERSION is decreased the sweep rate will
begin to affect the resolution and sensitivity of the analyzer,
as described under Obtaining Optimum Resolution. There-
fore, as the DISPERSION settings are reduced the sweep
rate should also be reduced to maintain sensitivity and resolu-
tion.
2-12
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 ...