Fig. 6-39. Wide band IF and Mixer tuning adjustments.
bration. These filters require special test equip-
ment and technique to calibrate. Contact your
local Tektronix Field Office or representative.
This procedure does not require a Sweep Gener-
ator, however, a Sweep Generator such as the
Kay Model 121 C Multi-Sweep Generator may be
used to check flatness.
a. Equipment setup is given in Fig. 6-38.
b. Disconnect the cable connector from J120 on the honey-
comb assembly and apply a calibrated 65 MHz signal (—30
dBm) from the signal generator to J120 as described in
step 23.
c. Disconnect the cable connector from J188 (output of
the IF Attenuator). Connect the output of J188 through a
subminiature to BNC adapter and coaxial cable to the Verti-
cal connector of the test oscilloscope.
d. Turn the 20 dB ATTENUATOR switch for the Type 491
to the ON position. Adjust the variable output attenuator
on the signal generator for an approximate 3 mV display
amplitude on the test oscilloscope.
e. Adjust L147 (Fig. 6-39), the 65 MHz trap, for minimum
response to the 65 MHz signal.
f. Remove the signal generator signal and the test os-
cilloscope connection.
Reconnect the Sealectro connectors
and cables to J120 and J188.
9. Apply a calibrated signal within the frequency range of
band B through a 20 dB attenuator (Part No. 011-0086-00)
to band B RF INPUT connector.
h. Set the Dispersion to 10 MHz/div. Tune the RF center
frequency to the applied signal frequency. Adjust the
MIXER PEAKING for maximum signal amplitude. Adjust the
GAIN plus the variable attenuator of the signal generator
for a signal amplitude of 6 divisions.
i. Calibrate the output amplitude of the RF signal from
the signal generator.
j. Tune the signal generator frequency through a 100 MHz
band and check the response flatness of the Type 491. Signal
amplitude should not vary over ±1.5 dB, or a total of 3 dB
from the maximum to minimum amplitude point, with a
constant amplitude input signal to the RF INPUT connectors.
Adjust the MIXER PEAKING for maximum signal amplitude
for each display window.
NOTE
This is not a conclusive check because the local
oscillator power may vary over this frequency
range. Try other input frequencies and oscillator
frequency ranges.
k. If the response flatness is not within tolerance, adjust
C137 and L134 (Fig. 6-39) for optimum sensitivity and flatness.
Adjusting C137 will produce a noticeable affect on the
response slope. Adjust L134 for optimum sensitivity at the
high frequency end of the response.
l. Increase the signal generator frequency to 800 MHz
and tune the Type 491 to this frequency.
m. Adjust the MIXER PEAKING control for maximum
signal amplitude.
n. Adjust C68 (Fig. 6-39) on the band B RF mixer for
optimum sensitivity and bandpass flatness. Tune the signal
across the screen with the RF CENTER FREQUENCY control
to check flatness.
o. Check the display flatness over the frequency range of
the instrument as follows:
NOTE
Each time the signal generator frequency is
changed it will be necessary to recalibrate the
output amplitude.
6-37
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 ...