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HP 811 OA Se
r
vice Guide
7- 3 2
Fun
ctional Desc
r
i
p
tion
Table 7-2 . W avefo
rm
Amplit
u
de a
nd
C
u
rre
n
t So
u
rces
Vernier
Ι
L
arge ι
M
edium Ι
S
mall
r
ange
source
sou rce
source
0
. 1V-0
.39V(
off Ι
off
Ι
on
0 .4V - 1.99 V
off
on
on
2 .O
V
-
l
0 .O V
on
on
on
DA C
U
300, buffered by U 301C an d U302 D sets α reference vol tage
at Q300 coll ector. Thi s is applied to Mul tiple xer
U
304 and switched
to t he current so u rces t hat are on. C urrent sources that are off are
connected t hroug h U 304 to α fixed reference vol tage at
R
306 . Thi s
appli es α high
c
urrent to th e vern ier cell causing it to switch off.
Good commo n m ode re
j
ecti on is
p
rovi ded by running t he c irc ui tr y
from α single +5
V
supply.
Ot her circui try in t hi s fun cti onal uni t
p
rov i des Alph a control for the
S LAVE I C to stabili se gain again st temp erature.
Output Co
n
trol
Thi s sect i on is on schemati c 61.
Th e HP 8110 Α
o
utput is de v eloped across α cascode stage. This is
m
o ni tored by α l e v el sense line using
R
403 and diodes C R400 and
C R401, and wi th α
p
ower se nse lin e
u
sing
R
404 and
R
405 .
Wh en th ere i s no externa l load attached, but the instr um e n t is
op erat ing, rel ay Κ401 connects α
l
oad comp rising L405,
R
406 -
R
411
and C407 .
If th e
m
aximum
o
ut put v oltage of 21V i s exceeded, output e nable
rel ay Κ 403
o
p erates, switch ing t h e o u t put to an inter nal 50 Ω
l
oad,
R
412, represe ntin g t h e exter nal
l
oad .
Κ402 conn ects t he second channel (wh e n fitted)
f
or Α+Β operation.
Diodes C R400 an d CR401
p
rovi de
p
eak detect ion.
Offset Generator
Thi s secti o n i s on schemati c 6 J.
DA C
U
500, buffered by U501B , switches on upp er or
l
ower curre nt
sources, p ro vidin g
p
os i tiv e or negativ e offset for the output w aveform .
The t wo curre n t sources are similar in des ig n , eac h ha vin g two
sources in p arallel, delivering equal current . Two sources are re qui red
b
ecause of p o wer d i ss ip ation .
T
ran s istors Q552 - Q555
f
orm α cascode stage.
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