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O P E R A T I N G
P R O C E D U R E
3.4 PRF VS PULSE DURATION.
3.4.1 DUTY RATIO LIMITS DURATION ACCURACY
There i s no restriction on the duty ratio of the
Type 1398-A. (Duty ratio i s the ratio of the pulse
"
on"
time to the total time of the period established by the
prf setting; duty ratio in percent
=
prf xduration
x
100.)
Therefore, the PULSE DURATION controls may be
mistakenly s e t for a duration longer than the total
period (period is the reciprocal of prf ). The instrument
cannot be damaged by such settings, but the user may
be confused by the resulting oscilloscope display.
Refer t o Table 3-1 for duration accuracy versus duty-
ratio specifications.
3.4.2 DURATION LESS THAN 50% OF PERIOD.
The accuracy of the DURATION control settings
i s preserved if the duty ratio i s 50% or l e s s ( p u l s e
duration i s 50% or l e s s of total time of period). Table
3-2 l i s t s the 50%-of-period figures for e a c h P R F control
setting.
3.4.3 DURATION GREATER THAN 50% OF PERIOD.
When the pulse occupies more than 50 percent of
the total period, the dial reading of duration i s erron-
eous. T h i s effect i s due to insufficient recovery time
for the pulse-forming circuits but can
be
circumvented
by the reversal of the OUTPUT PULSE polarity.
For example: A 1-kc, 600 p s positive pulse is
desired. From Table 3-2 i t can be s e e n that 600
ps
exceed the 500 p s maximum duration given for a PRF
control setting of 1 kc/s ( a c t u a l duty ratio
103
x
6
x
10
x l o 2
=
60%). But, from Figure 3-2, it a l s o c a n be
s e e n that a positive duration of 600
,US
can be obtained
TABLE 3 - 2
50%-OF-PERIOD FIGURES
I
50%
of Period (Durations inaccurate
P R
Con'r01
/ m DORATON controi s e t t i ~ g s
I
(AF
control fully clockwise)
longer than those listed)
3.4.4 DURATION EQUAL TO PERIOD.
When the DURATION controls c a l l for a pulse
exactly equal to the pulse period, the instrument f a i l s
completely, and both duration and prf are indeterminate.
3.4.5 DURATION GREATER THAN PERIOD.
When the duration i s s e t longer than the pulse
period, the pulse timing circuits will "count down
",
producing one pulse for each 2 , 3 , 4 ,
...
n input periods.
In general, the pulse duration will not be precisely
controllable due to lack of recovery time. However,
such frequency division may be useful in some applica-
tions and i t should be remembered that the Type 1398-A
can be used a s a frequency divider of arbitrary s c a l e
by such operation.
if connection i s made to the -OUTPUT PULSE binding
3.5 ~ R E C A U T ~ O N S FOR VERY LONG
post rather than to the + O U T P U T PULSE binding post
OR VERY SHORT PULSES.
and if the DURATION controls are s e t t o 400 p s pulse
( t o t a l period minus 600 p s ) . The duty ratio for a 1-kc,
3-50' GENERAL-
400-ps pulse i s 40%; therefore the DURATION control
When pulses of very long or very short duration
accuracy i s preserved.
are t o be produced and observed, s p e c i a l attention must
~ P E R I O D = I O O O ~ ~ ~
+OUTPUT PULSE
OV
-
(INACCURATE)
-
6 0 V
j
4
1
I
DURATION CONTROLS SET TO 600ps
DURATION INACCURATE BECAUSE DUTY RATIO EXCEEDS 50%
~ P E R I O D = ~ O ~ ~ ~ ,
,
-OUTPUT PULSE
OV-
(
ACCURATE)
-
6 0 V
f'
11598-21
DURATION CONTROLS SET TO 400ps
DURATION ACCURATE BECAUSE DUTY RATIO IS LESS THAN 50%
F i g u r e
3-2.
Circumvention of
50%
duty-ratio l i m i t a t i o n a t
1
kc/s
by r e v e r s a l
o f O U T P U T P U L S E p o l a r i t y .
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