F i g . 3 - 6 . S i m p l i f i e d d i a g r a m o f t h e t r a n s f o r m e r ( T 3 3 0 - T 3 3 1 ) c i r -
cuit from the swept oscillator to the push-pull amplifier Q340-Q350.
so the dirve signal to the amplifiers is a balanced push-
pull signal.
Transformers T343 and T354 in the collector circuit of
Q340 ond Q350 provide a 4:1 impedance transformation
from the collectors of the transistors to the output transformer
T347.
Transformer T347 converts the push-pull signal to a single-
ended output signal. Push-pull amplification, plus filtering
through the low pass filter circuit of L358-C358 and L348-
C348 reduces the harmonic content in the swept frequency
output signal.
Diode D334 in the base voltage divider circuit provides
the temperature compensation for transistors Q340-Q350.
The single-ended output signal is coupled through a 2:1
impedance transformer T363, to the mixer in the Wide Band
IF. The output signal is also applied, through two feedback
loops, to frequency ond amplitude control circuits.
Frequency Discriminator.
Two frequency discriminators
for each position of the DISPERSION RANGE selector SW365
provide an output voltage signal to the frequency discrimi-
nator comparator Q260. The output voltage from the com-
parator is a ramp voltage that is proportional to the sweep
oscillator frequency. It is applied to one side of the com-
parator Q230-Q220.
The MHz/Div discriminator consists of two matched diodes,
D373 and D376, at the input ends of two transmission lines.
The transmission lines are
1
/
8
wavelength long at the center
frequency (275 MHz). One line is open ended and appears
capacitive, the other line is shorted and appears inductive,
at the center frequency. As the input frequency to the dis-
criminator increases, the transmission line input impedance
nears the characteristics of a ¼ wavelength line. The shorted
transmission line input impedance increases, the open ended
line input impedance decreases. This produces proportionate
changes to the output signal from the diodes. Signal output
from diode D376 becomes more negative, and signal output
from D373 becomes less negative. This push-pull drive is
applied to the comparator Q260 and converted to a single
ended output signal for the sweep comparator.
Thermal balance is achieved by balancing the current
differential through both sections of the transistor. The com-
mon emitters are connected to a constant current source
Q270. Current (approximately 3 mA) is established by the
voltage drop across the emitter resistance R274.
The IF CENTER FREQ (R256) and the FINE (R259) controls
sum in a DC voltage with the differential signal from the
comparator to allow positioning of the IF center frequency
(200 MHz or a frequency close to 200 MHz) to the center of
the horizontal sweep.
The amplitude of the ramp signal to the sweep comparator
is a function of the DISPERSION RANGE switch SW210 and
t h e D I S P E R S I O N s e l e c t o r S W 3 6 5 s e t t i n g . T h i s a m p l i t u d e
determines the frequency deviation swing of the sweep
oscillator band.
Fig. 3-7. Frequency vs Voltage curves for kHz/DIV discriminator cir-
cuit.
3-7
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 ...