ESI VideoBridge 2150, Service Manual

Page: 157 / 332

2 . 1 0 CAPAC I TANCE MEASUREMENTS WITH EXTERNAL DC BIAS
( C ode s 1 a n d - 1 )
A D C b i a s o f u p t o + 5 0 V c a n b e a p p l i e d t o t h e r e a r p a n e l b i a s
t e rm i na l s ( ob s e r v e po l a r i ty ) . T h e B i a s V o l t a g e i s n o t a pp l i e d t o t h e
u n k n ow n u n t i l t e s t c o d e 1 i s e n t e r e d . M e a s u r e m e n t s w i t h b i a s a r e
a va i l a b l e f o r c a pa c i t a n c e on l y . B i a s s upp l y m u s t h a v e l ow r i p p l e w i t h
i n t e rna l c u r re n t l i m i t o f 1 00 mA a n d i t s o u tp u t i mp e d a n c e mu s t be l e s s
t ha n 5 0mQ. L e a k a g e c u r re n t t h roug h t h e u n kn ow n c a n be me a s u r e d by
s a mp l i n g t he c u r r e n t f ro m the b i a s s o u r c e to t he b i a s t e rm i na l s w i th a
l ow i mp e d a n c e a m me t e r . I f t h e b i a s s o u r c e i mp e d a n c e i s n o t l ow
comp a r e d t o t h e u nk n own , a bypa s s capa c i t o r w h o s e i mpeda n c e i s 1 /5 o f
t h e r a n g e re s i s t o r a t t he ope ra t i n g f r eq u e n cy c a n b e c o nn e c t e d a c ros s
t he b i a s t e r m i na l pos t s .
E L E C TR I CAL SHOCK HAZARD E X I STS WHEN A B I A S SUPPLY I S
C ON N E C TED TO THI S I NSTRUMENT . WHEN A N EXTERNAL BIAS SUPPLY
I S ATTACHED , THE B I AS VOLTAGE IS PRESENT ON THE REAR PANEL
B NC CONNECTORS .
USE ONLY BIAS VOLTAGE S UP TO 5 0VDC AND B I AS
SUPPL I ES CURRENT L I M I TED AT 1 00mA . DO NOT TOUC H , CONNEC T , OR
D I S C O NNECT THE UNKNOWN COMPONENT OR BNC CABLES WHI L E A BIAS
VOLTAGE IS APPL I ED .
A D C b i a s capa b i l i ty o f + 2 00V i s a va i l a b l e by o r de r i n g a n S P 5 2 4 0
op t i on . T h e S P 5 2 4 0 i s i de n t i ca l t o t h e 2 1 5 0 a n d 2 1 6 0 e x ce p t f o r t he
e x t e n d e d b i a s capa b i l i ty . S e e S e c t i on A . 4 f o r m o r e i n f orma t i on .
U se t he f o l l ow i ng p r o c e d u re w h e n mea s u r i n g D C - b i a s e d c a pa c i t o r s .
STEP 1 . L oo s e n t h e b l a c k t e r m i na l caps a n d r e move t h e s h o r t i n g s t rap
b y pu l l i ng i t up and p i vo t i ng i t ou tw a r d . T h e s t rap may be
h e l d i n p l a c e by t i g h t e n i ng t he t e rm i na l c ap s .
STEP 2 . I n s ta l l bypa s s capa c i t o r i f n e e d e d ( o b s e r v e p o l a r i ty ) .
STEP
3 .
C o n n e c t t h e e x t e rna l b i a s i ng supp l y t o t h e i n s t rume n t ' s re a r
pane l b i a s t e rm i na l s ( ob s e rve po l a r i ty ) .
2 - 1 1 9
e l s l i 2 1 5 0 / 2 1 6 0 5 /8 5