erence level of the output amplifier. This DC level changes
as the FINE RF CENTER FREQ control is rotated and shifts
I
Phase lock operation is established as follows:
Fig. 2-16. Typical LOCK CHECK displays
the local oscillator frequency a slight amount. It also affects
the vertical position of the display baseline. Thus, by de-
pressing the LOCK CHECK button and slowly turning the
FINE RF CENTER FREQ control (Fig. 2-15), the operator will
observe the baseline of the display shift until a lock mode is
reached. The baseline will then remain stationary over a
portion of the control range as the circuit holds the local
oscillator locked to the reference frequency. Turning the con-
trol further causes the circuit to lose its lock and the baseline
jumps from the locked position.
Beat frequency signals are usually displayed just before a
lock point is reached. See Fig. 2-16. However, through part
of the frequency range, the phase lock operation may be
very positive and the local oscillator will jump from one lock
mode to another without displaying the beat signals or the
smooth shift of the display baseline between lock points.
When the DC operating level of the phase lock amplifier
reaches either extreme (top or bottom of the graticule area)
the operation of the amplifier becomes non-linear and com-
pression of the beat signals will be noted. Phase lock opera-
tion becomes difficult to achieve. The displayed DC level
thus aids in setting a phase lock condition within the linear
operating range of the phase lock amplifier.
Part of the input signal is coupled through and displayed
when the LOCK CHECK button is pushed. This permits the
operator to re-establish a particular lock point that may
be lost because of oscillator drift or other reasons. The opera-
tor adjusts the FINE RF CENTER FREQ control while observ-
ing the display until the signal is again at a particular lock
point (the point where the baseline or the signal position
locks).
The local oscillator fundamental frequency locks in 1 MHz
steps, (from one lock mode to the next) then the internal
1 MHz reference frequency is used for phase lock operation.
This produces gaps of as much as 5 MHz in the upper fre-
quency scale, where the upper harmonic of the local oscilla-
tor is used. Continuous tuning through these gaps is provided
by the INT REF FREQ control. Rotating the control through
its range pulls the crystal controlled reference frequency ap-
proximately 1 kHz. This is sufficient to shift the local oscil-
lator frequency through these gaps and maintain phase lock
operation.
1. Tune the desired signal to the center of the display with
the RF CENTER FREQUENCY control.
2. Depress the LOCK CHECK button and adjust the FINE RF
CENTER FREQ control for a lock indication within the center
(4 div) of the graticule. if the lock indication or beat signal
is outside the linear operating range of the amplifier (base-
line of display at the top or bottom of the graticule), center
the display with the FINE RF CENTER FREQ control, then
adjust the RF CENTER FREQUENCY control to shift the signal
towards a beat mode. Adjust the FINE RF CENTER FREQ con-
trol while observing the desired signal for phase lock opera-
tion, then release the LOCK CHECK button.
2-15
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 ...