Tektronix 491, Руководство по обслуживанию

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29.
a.
b.
with
Check Dynamic Range of Vertical Display
Modes
Equipment setup is shown in Fig. 6-35.
Apply a 200 MHz signal from an RF Signal Generator
a calibrated variable output attenuator to band A RF
I N P U T c o n n e c t o r . S i g n a l a m p l i t u d e s h o u l d b e l e s s t h a n
- 4 0 d B m
c . A d j u s t t h e G A I N c o n t r o l a n d t h e V a r i a b l e O u t p u t
Attenuator for a display amplitude on the Type 491 of 8
divisions.
d. Increase the output attenuation of the Signal Generator
until the signal is just visible (about 0.5 minor divisions).
Note the difference in attenuator reading between the full
screen display and the 0.5 minor division display.
e. Check the dynamic range af each VERTICAL DISPLAY
switch mode to the following specifications:
L I N ;
≥ 2 6
d B
L O G ;
≥ 4 0
d B
S Q L A W ;
≥ 1 3
d B
30. Check Accuracy of IF ATTENUATOR dB
Selectors
Accuracy of the IF ATTENUATOR dB selectors is checked
at the factory to insure they are within the 0.1 dB/dB spec-
ification. Any change in this tolerance should be a large
one and due to component failure. Step attenuators with
rigid specifications are, therefore, not recommended. How-
ever, if the user desires to precisely measure the error of
the dB selectors, he must accurately calibrate the recom-
mended equipment or use an attenuator with more rigid
specification.
a. Equipment setup is shown in Fig. 6-35.
b. Apply a 200 MHz signal, that is 10 dB below 1 mW,
from the signal generator through a
2×
Attenuator (6 dB), a
Tens and Units Step Attenuator and a
10×
Attenuator (20 dB)
to the Type 491 RF INPUT connector.
c. Set the Tens Step Attenuator for 20 dB and the Units
Step Attenuator for 12 dB attenuation.
d. Adjust the GAIN control for a signal amplitude of 6
divisions on the analyzer.
e. Check the accuracy of the IF ATTENUATOR dB se-
lectors as follows:
1. Switch the 1 dB ATTENUATOR switch to on and switch
out 1 dB attenuation through the Units Step Attenuator.
2. Check the display amplitude. Must equal 6 divisions
± 0 . 7 m i n o r d i v i s i o n s .
3. Switch the 1 dB ATTENUATOR switch to the OFF posi-
tion, then check the remaining IF ATTENUATOR dB steps as
directed in Table 6-6a.
T A B L E 6 - 6 a
S p e c t r u m A n a l y z e r
L i m i t ( . 1 d B / d B )
S i g n a l A m p l i t u d e
I F A T T E N U A T O R
Switch on Units T e n s
1 dB 11 20 5.93 to 6.07 div
2 dB 10 20
5.86 to 6.14 div
4 dB
8 20 5.7 to 6.3 div
8 dB
4 20 5.5 to 6.6 div
16 dB
6 10 5.0 to 7.2 div
20 dB
2 10 4.8 to 7.5 div
The 1 and 2 dB measurements are very difficult because of
signal stability and the noise level. Over these small levels
the square law mode may be used. This expands the screen
changes for the same level change by the square power as
listed in Table 6-6b. Use reference signal amplitude of 5 divi-
sions to avoid going outside the graticule area.
T A B L E 6 - 6 b
dB 1 2 4 8 1 6 2 0
Signal 4.96
4.75 4,5 4.0 2.8 2.0
Amplitude
to to
to to to
to
limits
5.06 5.25 5.5 6.0 7.3 8.0
An alternate method which is not as accurate but is suf-
ficient for most applications is as follows:
1 . A p p l y a - 6 0 d B m , 2 0 0 M H z s i g n a l f r o m t h e s i g n a l
generator to the RF INPUT connector. Adjust the Spectrum
Analyzer GAIN control for a signal amplitude of 5 divisions.
2. Switch the 1 dB ATTENUATOR switch on and adjust the
signal generator Attenuator control to return the signal ampli-
tude to 5 divisions.
3. Check the new reading of the Attenuator dial. Should
read -50 dBm ±0.1 dBm.
4. Turn the 1 dB ATTENUATOR switch to OFF. Check the
remainder of the IF ATTENUATOR selector steps as directed
in Table 6-6c.
T A B L E 6 - 6 c
S p e c t r u m A n a l y z e r
R F G e n e r a t o r A t t e n u a t o r
I F A T T E N s w i t c h O N
C o n t r o l S e t t i n g
2 dB - 5 8 d B m ± . 2 d B m
4 dB
- 5 6 d B m ± . 4 d B m
8 d B
- 5 2 d B m ± . 8 d B m
16 dB
-44 dBm ± 1.6 dBm
20 dB
-40 dBm ± 2.0 dBm
31. Check Attenuation Range of IF GAIN
Control
a. Equipment setup is given in step 30.
b. Turn the GAIN control fully counterclockwise. Adjust
the Signal Generator variable output attenuator for an 8
6-33