F i g . 2 - 1 4 . T o t r i g g e r t h e a n a l y z e r f r o m t h e d i s p l a y r e q u i r e s 0 . 2
d i v i s i o n s o f s i g n a l . T u n e t h e s p e c t r u m n u l l p o i n t a w a y f r o m t h e
sweep starting point, with the RF CENTER FREQUENCY control.
the FINE RF or IF CENTER FREQ control to move the sweep
start off a spectrum null point. See Fig. 2-14.
When the SOURCE switch is in the LINE position, the dis-
play is triggered from a sample of the power input line
frequency. This feature provides a stable display when the
signal is time-related to the line frequency.
External triggering requires a signal equal to or greater
then 0,2 V, within the frequency range of 20 Hz to 100 kHz,
to trigger the sweep. The signal is applied to the TRIG IN
connector on the rear panel. External triggering will pro-
vide a stable display when the internal signal triggering is
unstable. It may also be used to slave the analyzer to a
recording device.
The SLOPE switch selects the positive or negative-going
portion of the triggering signal. The LEVEL control selects
the required signal amplitude to trigger the sweep for single
sweep operation.
RF Center Frequency Tuning
The dial and the analyzer are tuned through the fre-
quency range of each band by the RF CENTER FREQUENCY
control. The dial frequency calibration is accurate to within
± (2
MHz + 1%
of the dial reading) when the FINE RF CEN-
TER FREQ and the IF CENTER FREQ controls are centered.
The RF CENTER FREQUENCY control is supplemented by
a FINE RF CENTER FREQ, ten turn control, that provides a
fine tuning adjustment through a limited frequency range
on either side of the dial frequency, or the RF center fre-
quency. This allows fine tuning when operating on the high
frequency scales with narrow dispersion, or fine adjustment
to establish a phase lock condition.
Fig. 2-15. Adjusting FINE RF CENTER FREQ control for phase lock
operation.
When searching a frequency band, set the MIXER PEAK-
ING control to SEARCH position and tune slowly through
t h e b a n d w i t h t h e R F C E N T E R F R E Q U E N C Y c o n t r o l . T h i s
ensures that signals of sufficient power within a tunable range
will be observed. See Mixer Peaking. After the signal has
been located adjust the MIXER PEAKING to optimize signal
amplitude.
N O T E
MIXER PEAKING control must not be in the SEARCH
position when making relative amplitude measure-
ments and should be adjusted for maximum signal
amplitude.
Phase Lock Operation
The 1st local oscillator can be phase locked to either an
internal 1 MHz reference oscillator or an external freguency
source when it is applied to the EXT REF FREQ IN connector.
Locking the local oscillator to a stable frequency, such as the
internal 1 MHz crystal controlled oscillator, reduces the local
oscillator incidental frequency modulation and frequency
drift. This allows narrow dispersion and high resolution set-
tings for signal analysis.
The frequency range of an external reference frequency
is 1 MHz to 5 MHz, and amplitude limitations are 1 to 5
volts peak to peak. The external signal for phase lock
operation is applied to the phase lock circuit when the INT
REF FREQ control is turned ccw to the OFF OR EXT REF FREQ
IN position.
The LOCK CHECK pushbutton applies the output of the
phase lock amplifier to the vertical display system. The out-
put of the phase lock amplifier contains the following: (1)
Beat frequency signals between the local oscillator and the
reference frequency when the oscillator frequency is very
close to a lock with the reference frequency. (2) A DC ref-
2 - 1 4
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 ...