Keithley 153, Инструкция по эксплуатации, Страница: 21 / 51

MODEL 153
CIRCU I T
DESCRI PT IO�
S E CT I O N 4 .
C I R CU IT D E S C R I PT I O N
4 - 1 . G E:'ERAL .
a . T h e
Ke i t h l e y
Mod e l
1 5 3
t-t i c rovo l t - Amme t e r co n s i s t s o f a p ho t o - mo d u l a t o r , a c amp l i f i e-r ,
a n d
d emod u l a t o r s y s t em f o l l owed
b y a d c c a t ho d e
f o l l owe r . F e e d b a c k
i s
a pp l i ed t o t h e
wh o l e l o op .
b . S i g n a l s b c l ow
I v
o
l t
a r e a
p p
l
i
e
d
d i r e c t l y
t o
t h e i n p u t
f i l t e r ;
t h o s e
a b o v e
1
v o l t
a r e
a t t e nua t e d
1000 : 1
\" i th
a r e s i s t i v c
d iv i d e r .
A l l s i g n a l s pa s s t h rou !; h
t h e
I m,- p a s s
i n pu t f i l t e r wh i c h a t t enua t e s und e s i reab l e a c s igna l s . T h e f i l t e r h a s a
3 d b
c u t o f f a t
4 c p s . T h e f
i
l
t
e
r
e
d d c s i g n a l en t e r s t h e ph o t o - modu l a t o r , wh e r e
i t
i s c ompa r e d
t o t il e
f e ed b a ck
vo l t a g e . T h e d i f f e r e n c e
or � r r o r s igna l i s mod u l a t ed
a t
4 0
c p s , a m
p l
i
f
i
e
d b y
t h e
tuned a e
amp l i f i e r ,
and s y n c llronou s l y d emod u l a t ed t o r ega i n d c . T h e
d emodu la t e d
s i£ ­
na l
i s
f
i
l t e red and u s ed t o d r ive the ou t pu t , me t e r and f e edb a c k .
c .
The
Mod e l
1 5 3
opera t e s a s
a n
amme t e r b y mea s u r ing t h e vo l t a g e drop a c ro s s a res i s ­
t o r through whi c h the inp u t current f l ows . The r e s i s t o r u s ed and t h e i n s t rume n t
vo l t a g e
rang e d e t ermines the curren t s en s i t iv i t y .
NOTE
C i r c u i t d e s igna t i ons r e f er t o s chema t i c d iagram
1 7 7 7 1H.
4 - 2 . I NPUT CIRCUIT .
a .
The sett ing
o f the
FUNCTION
SWi t ch , 5 2 , d e termin e s the inpu t res i s tan c e o f t h e Mod e l
153 .
W
i t h t h e Swi tch s e t t o VOLTS INPUT
R- 2M , resis tor R1 3 3 i s
c onnec t ed t o the h
i
g h
in­
put t ermina l , and the inpu t r e s i s tance is approxima t e ly two megohms for a l l ranges . Se t ­
ting the
S
w
i
t ch t o
OPEN increases
th e in pu
t
res i s tanc e of each range t o t h a t shown i n
Tab le
1 .
For the 3-vo i c anu
lI .Lgll�c ral1g�l:I ,
i..il�
LIIVUL
1 E:l:l i ti tance
i s
200
megohms , as
d e termined
by res i s tors R1 34 and R1 35 .
For
the l ower rang e s the inpu t r e s i s tance i s n o t a
f ixed
re s i s tan c e , but a
synthe tic
re s i s t a nc e o
bt
a i ne d
by us ing
high
f
eed b
a
c k f a c t o r s .
b . For the 3 -vo lt and h ighe r range s , the input s ig
n
a l i s a t t enua t e d 1000 : 1 b y a re s i s ­
t ive d ivider , consi s t ing o f res i s tors Rl 34
to
R137 .
c . The inpu t s igna l i s
t hen f i l
t e r e d b y the low pas s input f i l t er t o remov e unwa n t e d
a c s igna l s
from
the
vo l tage
b eing mea s ured . The f i l t e r cons i s t s of r e
s i
s
t o
r
s
RI0 2 , RI0 3
and RI04 and capac itors C I0 1 , C 10 2 and C I0 3.
'''PU T
A�v��"T
�[SISTOIIS
I
I
I
I
LOW PASS
I"PUT
'ILl(R
PHOTO·
MOOUUlTOII
�M4"l MoD 'UNC T lOOO SWITCHES
I
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __
F IGURE 9 .
Mod e l
1 53 Block
Diagram.
1 3