CHECK switch to the vertical deflection system. At zero beat,
the output signal amplitude snaps to a minimum and the cir-
cuit locks the local oscillator to the reference frequency by
feeding a corrective voltage to a Varactor diode in the
oscillator circuit.
FINE RF CENTER FREQ control R1182 tunes the local oscil-
lator over a limited range by changing the DC output level
of Q1180. When the circuit is in a lock condition, any small
shift of the FINE RF CENTER FREQ control is counteracted
by the phase lock circuit.
If the control is further moved
the circuit will lose the lock and the oscillator will jump
to a different frequency lock point. This jump in frequency
is easily seen as a shift in signal position at dispersion set-
tings of 500kHz/div or less,
An isolation switch is used to minimize the loading affect
of the lower band (A & B) phase detectors on the band C
phase detector. The junction at diodes D1170 and D1174
is returned, through a filter network and the Band selector
s w i t c h S W 7 0 , t o + 1 5 0 V . T h e d i o d e s a r e f o r w a r d b i a s e d
when the Band selector switch is in the A or B position. When
the switch is changed to the C position, the diode switch is
open.
Band C Bal and Bands A & B Bal adjustments correct any
imbalance between the phase detectors so the DC output
level of the amplifier remains balanced
as
the Band selector
i s s w i t c h e d b e t w e e n b a n d s . T h e F I N E R F C E N T E R F R E Q
control must be centered for this adjustment.
Sweeper Circuit
This circuit (see Fig. 3-5) provides a swept frequency,
centered at 275 MHz, to the wide band mixer amplifier. The
swept frequency amplitude is constant and the dispersion
can be varied from about O Hz to 100 MHz.
A positive-going sawtooth voltage from the sweep genera-
tor circuit is applied through pin AE to the emitter of Q200.
Q200 is configured as a long-tail amplifier. It converts the
sawtooth voltage input signal to a linear current ramp, which
is applied through the DISPERSION attenuator to one side
of comparator amplifier Q220-Q230.
The output DC level of the current ramp from Q200 is
set by Sweep Center adjustment R203. Dispersion CAL adjust-
ment R208, shunts the dispersion attenuator. It calibrates
the dispersion for the 10 MHz/div position of the DISPERSION
selector by adjusting the output amplitude of the current
ramp from Q200. The remaining positions the selector are
then within instrument specifications.
Two dispersion ranges (MHz/DIV and kHz/DIV) are pro-
v i d e d b y t h e D I S P E R S I O N R A N G E s e l e c t o r R 2 1 0 , w h i c h
selects a different range of resistance values for each position
of the DISPERSOIN selector.
Sweep Comparator.
The sweep comparator containing
Q220 and Q230 compares the current ramp from the dis-
persion attenuator against a current ramp applied to the
base of Q230. The signal applied to the base of Q230 is the
Fig. 3-4. Simplified phase detector circuit.
3-5
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 ...