Tektronix 576 - Page 56

t hrough R165, R166 an d R167 . As t he voltage ste p s in-
creaset h roug h 0176 and Q184 or t h roug h Q172 and Q180,
t he current increases t h roug h t h e current limiting resistors .
T h is curren t increase cau ses t h e voltage dro p across t h e
resistors to increase . If p ositive-going steps are being p ro-
d uce d , t h is increase in voltage d ro p is tra n smitted t h roug h
Q176 and Q169 to t he junction of R 166 an d R 167 . As t he
voltage d rop increases, t he voltage at t h is j unctio n p oi
n
t
goes down .
Wh
en t
h
e voltage reac
h
es
about -2 .3 volts,
D165 forwar d biases, clam p ing t he voltage at t h e base of
Q169 . T h is p revents
ge
neration of furt h er ste p s . Wh en
negative-going ste p s are being p ro duced , t he dro p across t he
curren t limiting resistors is transmitte d t h roug h t hree base-
emitter j unctio n s, Q180, Q172 and Q169, to t he j unction
of R 166 an dR 167 . As voltage d rop i n creases, t he voltage at
t h e collector of 0164 goes up . Wh en t h is voltage reach es
+12 .5 volts, Q169 is saturate d , an d agai n no furt her ste p s
can be generate d . T he C URREN T L I M IT switch d etermines
t h e number of resistors to be inclu ded i n t he current lim-
iting resistance, t herefore d etermining t he amount of cur-
rent necessary to eit h er turn on D165 or saturate Q169 .
Ci r
cuit Desc r i p tio n -Ty pe 576
VER TICA L A N D H
O
R
IZO
N TA
L DIS PL AY
Signal Sensing an d Dis p
lay Sensitivity
O n ce t he Collector Su pp ly and t he Step Generator Out-
put h ave
been a pp
lie
d to t he device un d er test, measure-
ments
of t
h
e voltages
and
currents seen at t h e terminals of
t he d evice un der test may be d is p laye d on t he vertical and
horizon tal
axes of
t h
e CRT . T hese measureme n ts are ma de
by first sensing
t
he current or voltage t h roug h current
sensing resistors or voltage d ivi d ers, t hen am p lifying t he
meas u rement wit h t he d is p lay amplifiers a nd app lyi ng t h e
measurement to t he d eflection plates of t he C R T . T he p osi-
tio n s of t he H O RIZON TA L , t he M OD E and t he Termi n al
Selector switch es d etermine w h ic h meas u rements are ma de .
Collector Current Sensing . If t h e M OD E switch is set to
eit h er N O RM or DC, collector current
(
Ις
)
is measure d on
t h
e vertical
axis of t he CR T .
Collector current is meas u re
d
by p lacing α resistor between groun d and t h
e
current return
to t h
e
collector su pp ly a nd measuring
t
h
e voltage develope
d
across t h is resistor (see F ig . 3-8 an d F
ig . 3-9) . By varying
F ig .
3-9
. Sim p li f ied sc h emat i c o f Di s p lay Sensit i v i ty Switc hi ng and Standard T est Fixture sc h ematics f or meas u rement o f collector current
(IC) and base-emitter
voltage
( νΒΕ )
on
emitter-base vo l tage
( νΕΒ ) .
3- 1 5