Tektronix 576 - Page 31

secon d ,
or
one
ste
p per
2
cycles of
t h
e
Collector supp ly .
Again, each ste p
occurs at t he beginning of α Collector
Su pp ly cycle . (Th
is rate s hould be used w h en t he PO-
LARITY
switch is
set to AC .)
P
ressing t he 2 Χ RAT E but-
ton
produces α Ste
p
Generator rate of 240 ste p s/secon d , 2
ste p s p er cycle
of t
he
Collector Su pp ly . In t h is case ste p s
occur at
bot
h
t
he
beginning an d t h
e
p ea k of α Collector
Su p
ply
cycle . If t he 2 ΧRAT E an d .5 ΧRAT E buttons are
p ressed
togeth er, t he Step Generator rate is t h e normal rate
of
120
ste
p
s/secon d except t h at t he ste p s occur at t he pea k
of eac h Collector Su pply cycle rat h er t h an at t he beginning
as in normal rate operation .
T h e STEP F AM I L Y buttons d etermine w h et h er ste p
families are generate d re p etitively or one family
at α
time .
P
ressing t h e REP ST EP F A M I L Y button turns t he
Ste
p
Generator on an d p rovi d es repetitive families
of
ste ps .
Wh en t he SI N G LE ST EPF AM I L Y button is p
us
he
d , one
ste p family is generated an d t h e Ste p Generator turns off .
To get anot h er ste p family, t h e SI NGLE button
must be
p resse d again .
T he OFFS E T
b
u
ttons an d
t h e
O
FFS E T M UL T control
allow
cu rrent
or voltage to be
eit her
add
e
d or subtracted
from t he Step Generator out pu t . T h is causes t he level
at
w h
ic
h
t h
e
ste
ps begin,
to
be s h ifte
d
eit her in
t h
e
d
irection
of
t he ascen
d
ing ste ps (ai
d
ing) offset,
or
in t he
opp
osite
d irection of t h e ste p s (opp osing) offset . Wh en t h e Z ER O
OFFS E T button is p us hed , t he step family is generate d at
its nomal level w here t he
zero
ste p level is eit h
er
0
mA or
0
V an d t he O FFSET MUL T control is in h ibited
.
Wh en t h e
AID O
FF
S
E
T button is p resse d
,
current or voltage may be
ad ded to t
h e
Ste
p
Generato r out p ut using t he ST
EP
MUL T
co n trol
.
T
he
amo u nt of cu rrent or voltage a dde d to t he
Ste p Generator out put w hen t he AID button is p resse d is
eq ual to t he setting of t h e ST EP M UL T control times t he
setting of t h e AMPL IT U D E switch . T he ST EP MUL T con-
trol
h
as α continuous r ange of 0 to 10 times t h
e
setting of
t h e AMPL IT UD E switch . P ressing t he O PP OS E O FF S E T
button
allows
eit h
er
cu r
re n
t or voltage to be subrtracte d
from t he Step Gene r
ator output,
t h e
amount subtracte
d
d
etermine
d by t
h e
ST
EP MUL
T control
.
Table
2-5
s h ows
t h
e p
olarity of t he offset cu rrent or voltage for t he two
p
olarities
of t
h e
Ste
p
Generator output .
O pp osing offset is most useful w hen gen erating voltage
ste p s to test fiel d effect transistors . Wh en cur r ent steps are
h aing generated, t he
maximum
opposing ν nltage is limite d
to ap proximately 2 volts . T
h
is voltage limiting p
r otects t h e
base-emitter j unctio n of α bi- p olar transistor from reverse
brea k down .
T h e ST EP /O FF S E T P O L A R ITY I NVER T button allows
t he Step Generator output (bot h steps an d offset) to be
inverted from t he p ola r ity at w h ic h it was set by t he
P
O L A-
R ITY switc h . It h as no effect w h en t h e Terminal Selector
switch is set to B AS E GR OUND E D . Caution s h oul d be
exercise d w h en using t h is button to ca u se reverse cu rrent to
flow between t he base and emitter terminals .
V
oltage limit-
Stand ar dTest
F
ixture
O p erating
Instructions-Ty pe 576
TA BLE 2-5
P olarity of Offset for P olarity of
Step Generator Out put
ing
occurs, w h
en current
steps are being generate d , only
w
h
en t he O
PP
OS
E
O FFS
E
T button is presse d .
Wh en one of t h e PUL
S
E
D ST
EP
S buttons is presse d ,
steps are generate d in pulses having
d
urations
of eit h er 300
μ s or 80 μs (offset is unaffecte d ) . Pulsed operation is use-
ful w hen testing α d evice at power levels w h ic h mig h t
d
am-
age t h e d evice if applied for α sustained lengt h of time .
P
ulse d ste
p
s of α 300μ s duration occur w h en t h e 300 μ s
PUL S
E
D ST
EP
S button is p resse d . Wh en t h e 80 μ s
PUL S E D ST EP S button is presse
d , t h
e
duration of t h e
p ulse d ste p s is 80
μ
s
.
Wh en
eit h
er
t h
e
300 μs button or t h e
80
μ s
button is presse
d ,
t h e
Collector Su p ply mo de is auto-
matically set to DC . If
t
he
300 μ s an d 80 μs buttons are
p resse d togeth er, t he Collector
Su p ply remains in t h e nor-
mal mo d e an d 300
μ s pulsed steps are pro d uce d . In all t he
p
reviously me n tioned cases, t h e pulses occur at t h e pea k of
t he Collector Supply swee p an d t h erefore
only t he normal
an d .5 times normal Step Generator rates are available for
use .
T h
e
Stan d ar d Test F ixture, w h ic h sli d es into t h e front
of
t h e
Ty p e 576, provi des α means of connecting t he Collector
Su pp ly out p ut, t he Ste p Generator output an d t h e
d isplay
am p lifiers
to t
he
d evice to be teste d .
T he Terminal Selector switch , locate d
on t
he
Stan d ar d
Test F ixture, d etermines
t
he
state of t h
e
base and t h e emit-
ter terminals of t he d evice u n
d
er
test . T
h e
switch has two
ranges : EM ITT ER
GR OUND
E
D an d BAS
E
G R OUN D E D .
In t h
e
EM ITT ER G R OUN D E D ra n
ge,
t he emitter terminal
is connecte d to ground and t he Terminal Selector switch
d etermines t he state of t h e base terminal . Wit h t he switch
set to ST EP G EN, t h
e
Ste
p
Generator out p ut is ap p lie d to
t h e
base
terminal .
In t
he
OPEN (O R ΕΧΤ ) p osition, t he
base terminal is left open . In t h is case
measurements may
be ma de with t h e base terminal left o p en or wit h an exter-
nally generate d signal a pp lie d to it t h roug h t h e ΕΧΤ BAS E
2- 23
Step O FFSET
Offset
Ge n erator Buttons
P olarity
: Cu rrent Voltage
P ositive AID P ositive P
ositive
going
P ositive OPPOSE
N egative N egative
going
Negative AID
N egative N egative
going
N egative
O
PP
OS E P ositive P ositive
going
Ι