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HP 811 OA Service Gui
d
e
TIG
ER
Cur
r
ent Source
Thi s secti on i s on schematic 5E.
T
r
igge
r
a
nd
St
r
o
b
e O
u
tputs
7-22
F
u
n
ctio
n
al Descriptio
n
Th e TI GER I C is curre nt-con trolled . The greater t he curre n t, t he
shorter the
p
er iod . Th e curren t source is divid ed into 2 sect i on s, α
l
o w
c
u rrent
p
at h and α high cu rre nt
p
ath . Th e low c urrent
p
at h
provides 10%ο o f that
p
rovid ed by the hig h cu rrent
p
ath.
Control
Con trol i s
p
rovi ded by α
D
- Α conv erter,
U
503 . To o btain the correct
relat i onship b et wee n the digital control data an d t he curre n t n eeded
f
or controllin g t he TIG ER IC, th e DA C must operate in 1/χ
m
ode .
In stead of
a
pplyin g
f
eed b ack to pin 16 on
U
503, it is applied to pin
15, cau sing 1/ χ op erat i on. Pin 16
i
s g iven α v o ltage reference via
U
501 and
U
502 A.
The output of t h e DA C is applied to
U
501 B whi ch is connected to
the drain
o
f Q501 . C urren t flowing through Q501
i
s mirrored th rough
Q502,
p
ro vi ding th e hi gh-c u rrent
p
ath. Q503 acts in
t
he sa me
w
ay,
except t hat choice of res i stor
R
514 limi ts current flo w to 10% of
t
he
current throug h Q502, thus
p
roviding t he
l
o w-curre n t
p
at h.
The
l
o w-c urrent
p
at h i s perm ane ntly co nnected to th e TIG ER I C
and th e hi gh -cu rrent p ath is switched by U505 bet w ee n 0 V
a
nd the
output, dep ending on t h e requi red p er i od range . Th e hi g h-curre nt
p
ath i s u sed for ran ges <10 n s .
T
his sect ion is on schem ati c 5F.
I
t
p
rovi des TTL or
E
CL comp atibility for the
T
rigger an d Strobe
o utputs . Thi s is do ne by swi tch ing the range of o utput v oltage
in an
ou tput stage co nsist ing of two di fferential
a
mpli fiers.
As the
T
rigger an d Strobe o utput stages are identical , o nly t he
c irc uitr y for the
T
r igger o u tput will b e described .
Multiple xer
U
601
p
ro vides switching signals to tran sistors Q605 an d
Q607 . Whe n Q605 i s on it switches res istors in p arallel to increase .
th e curre nt flo win g t hrough t h e first differential
a
mplifier and wh en
Q607
i
s on i t switch es Q604 off in t he secon d differential
a
mpli fier,
t hus setting the stage to o perate at
E
CL level s. Q608
i
s also swi tch ed
off at thi s
p
oin t, and Q606 is co nducting .
When the outpu t stage is requi red to op erate at TTL level s Q608 is
swi tched on a n d this biases Q606 off . Q605 and Q607 are swi tch ed
off by U 601 .
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