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5-33 The t e s t s e t u p s illustrated in Figures 5-4A
and 5-4B a r e g e n e r a l l y not a c c e p t a b l e for measur-
ing spikes; a differential o s c i l l o s c o p e is n e c e s s a r y .
Furthermore, t h e measurement c o n c e p t of Figure
5-4C must be modified if a c c u r a t e s p i k e measure-
ment
is
t o b e achieved:
5 0
l l
TERMINATION
OSCILLOSCOPE
AC
ACC
AC
GN D
ACC
GNO
50
n
TERMINATION
Figure 5-6.
Noise Spikes T e s t Setup
1. As shown in Figure 5-6, two c o a x c a b l e s ,
must b e s u b s t i t u t e d for the s h i e l d e d two-wire c a -
ble. In addition a high voltage protection network
(Figure 5-5) a s d e s c r i b e d i n paragraph 5-31 should
b e c o n n e c t e d i n s e r i e s w i t h t h e power supply out-
put.
2.
Impedance matching r e s i s t o r s must b e
included t o eliminate standing w a v e s and c a b l e
ringing, and t h e c a p a c i t o r s must b e connected t o
block t h e DC current path.
3.
The length of the t e s t l e a d s outside the
c o a x is c r i t i c a l a n d must b e kept a s short a s pos-
sible; the blocking c a p a c i t o r and t h e impedance
matching r e s i s t o r should b e connected d i r e c t l y
from the inner conductor of the c a b l e t o the power
s u p p l y terminals.
4.
Notice t h a t the s h i e l d s of the power
supply e n d of the two c o a x c a b l e s a r e not con-
n e c t e d t o t h e power s u p p l y ground, s i n c e s u c h a
connection would give r i s e t o a ground current
path through t h e c o a x s h i e l d , resulting in a n er-
roneous measurement.
5.
The measured n o i s e s p i k e v a l u e s must
b e doubled, s i n c e t h e impedance matching r e s i s -
t o r s constitute' a 2-to-1 attenuator.
6.
The n o i s e s p i k e s observed on t h e o s c i l -
l o s c o p e should b e l e s s than 2.5mV p-p.
5-34 The c i r c u i t of Figure 5-6 c a n a l s o b e u s e d
for the normal measurement of low frequency
ripple and noise; simply remove t h e four terminat-
ing r e s i s t o r s a n d the blocking c a p a c i t o r s and sub-
s t i t u t e a higher gain v e r t i c a l plug-in in place of
the wide-band plug-in required for s p i k e measure-
ments.
Notice t h a t w i t h t h e s e c h a n g e s , Figure
5-6 becomes a two-cable version of Figure 5-4C.
5-35
TRANSIENT RECOVERY TIME
Definition: The time "Xu for output
voltage recovery t o within "Y" mil-
l i v o l t s of the nominal output volt-
a g e following a "Z" amp s t e p change
in load current -where:
"Y" is specified s e p a r a t e l y for e a c h
model but is generally of the same
order a s t h e load regulation speci-
fication.
The nominal output volt-
a g e is defined a s the DC l e v e l half
w a y between the s t a t i c output volt-
a g e before and after t h e imposed
load c h a n g e , and
"Z" is the specified load current
change, normally e q u a l t o the full
load current rating of the supply.
5-36 Transient recovery time may be measured a t
a n y input line voltage combined with a n y output
voltage and load current within rating.
5-37
Reasonable c a r e must be taken in switching
t h e load r e s i s t a n c e o n and off. A hand-operated
switch in s e r i e s w i t h the load is not adequate,
s i n c e the resulting one-shot d i s p l a y s are difficult
t o observe on most o s c i l l o s c o p e s , and the a r c
energy occuring during switching action complete-
l y masks t h e d i s p l a y with a n o i s e burst. Transis-
tor load switching d e v i c e s a r e expensive if rea-
sonably rapid load current c h a n g e s a r e t o be a-
c hieved.
A mercury-wetted relay, a s connected in the
load switching circuit of Figure 5-7 should b e
u s e d for loading and unloading the supply. When
t h i s load switch is connected to a 60 Hz AC input,
t h e mercury-wetted r e l a y w i l l open and c l o s e 60
times per second.
Adjustment of the 25K control
permits adjustment of the duty c y c l e of the load
current switching and reduction in jitter of the
o s c i l l o s c o p e display.
5-38
To c h e c k the transient recovery time, pro-
c e e d a s follows:
a .
Connect t e s t setup a s shown in Fig-
ure 5-7.
b.
Set the VOLTAGE thumbwheel switches
for 9OVdc.
c .
C l o s e the line switch on the repetitive
load switch setup.
d.
Set the o s c i l l o s c o p e for internal sync
and lock on e i t h e r the positive or negative load
t r a n s i e n t spike.
e.
S e t the vertical input of the o s c i l l o s c o p e
for AC coupling s o t h a t small DC l e v e l c h a n g e s in
t h e output voltage of t h e power supply w i l l not
c a u s e the d i s p l a y t o shift.
f.
Adjust the vertical centering on the
scope s o t h a t the t a i l e n d s of the no load and full
load waveforms a r e symmetrically displaced about
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