Tektronix 491, Maintenance Manual

Page: 77 / 226

6. Check Saw Out Signal Amplitude
a . R e q u i r e m e n t — S A W O U T s i g n a l a m p l i t u d e i s 7 0 t o
90 mV.
b. Connect the test oscilloscope probe to the SAW OUT
connector on the rear panel of the Type 491. Adjust the
test oscilloscope triggering controls for a triggered sweep
ramp display.
c. Measure the amplitude of the sawtooth waveform.
Must measure between 70 and 90 mV.
d. Disconnect the test oscilloscope from the SAW OUT
connector.
7. Check External Triggering
a. Requirement—Sweep circuit must trigger on an exter-
nally applied signal of 0.2 V within the frequency range of
20 Hz to 100 kHz.
b . A p p l y t h e o u t p u t o f t h e A u d i o S i g n a l G e n e r a t o r
through a coaxial cable and T connector to the rear panel
TRIG IN connector. Monitor the applied signal with the test
oscilloscope by connecting a coaxial cable between the T
connector and the oscilloscope vertical Input connector.
c. Set the Signal Generator frequency to 20 Hz and adjust
the output control for a signal amplitude of 0.2 V peak to
peak.
d. Change the Type 491 SOURCE switch to EXT position.
Adjust the LEVEL control and check for a triggered sweep
with the SLOPE switch in either the + or - positions. Must
trigger with a 20 Hz, 0.2 V externally applied signal.
e. Change the Signal Generator frequency to 100 kHz
and adjust the output control for a 0.2 volt peak to peak
signal.
f. Adjust the LEVEL control and check for a triggered
s w e e p w i t h t h e S L O P E s w i t c h i n e i t h e r t h e + o r - p o s i -
tions. Must trigger with a 100 kHz, 0.2 volt signal.
g. Remove the externally applied signal and T connector
from the Type 491 TRIG IN connector.
8. Check Line Triggering
a . R e q u i r e m e n t - S w e e p w i l l t r i g g e r o n L I N E w i t h t h e
LEVEL control centered and the SLOPE switch in either posi-
tion.
b. Set the SOURCE switch to LINE position.
c. Check for a triggered sweep with the LEVEL control
centered and the SLOPE switch in either the + or - posi-
tions.
9. Check Sweep Timing
a. Requirement-Sweep timing accuracy must be within
±3% of indicated TIME/DIV selector Position.
b. Apply 10 ns and 1 ms time markers from the Time-Mark
Generator through 40 dB attenuator to band A, RF INPUT
connector. Set the front panel controls as follows:
D I S P E R S I O N R A N G E k H z / D I V
D I S P E R S I O N 100 kHz/div
R E S O L U T I O N 100 kHz (fully cw)
I F A T T E N U A T O R
O F F
V E R T I C A L D I S P L A Y L I N
Band Selector
A
T I M E / D I V 1 ms
VARIABLE CAL
S O U R C E L I N E
LEVEL Triggered sweep
c . A d j u s t t h e G A I N c o n t r o l f a r a s i g n a l a m p l i t u d e o f
approximately 6 divisions.
d. Decrease the DISPERSION to 0, keeping the signal
centered on screen with the IF CENTER FREQ controls.
e. Switch the SOURCE switch to INT position and adjust
the LEVEL control for a triggered display.
f. Adjust the position control to position the 1st marker to
the 1st graticule line (see Fig. 5-1), then check the sweep
timing for each position of the TIME/DIV selector. Marker
input, TIME/DIV selector and type of display is listed in Table
5-1.
Fig. 5-1. Time markers aligned to check sweep timing.
g. Return the TIME/DIV selector to 1 ms position, then
apply 10 ns and 5 ms markers to the Type 491.
h. Turn the VARIABLE control fully counterclockwise.
i. Check-A minimum of five (5 ms) markers should be
displayed within the 10 division graticule width. Variable
control range 2.5:1.
j. Return the VARIABLE control to the CAL position.
5-3