Tektronix 576 - Page 43

Circuit Descri p tio n -Ty p e 576
VA R IA BLE
CO LLE CTO R
S UPPL
Y
VO L TS
Τ 300 Τ 301
* Br i d ges d r aw n f r o m P O L A R ITY switc h set το
+
( ΝΡΝ
)
2 κν
Bri
d
ge*
500
V
Br i d
ge*
M
OD
E
assembly is use d for rectification . Operation in t h e 1500
volt range is similar to op eration in t h e 350 volt range
exce p t t h at t he 2 kilovolt b ri dge is used for rectification .
T he P O L A R ITY switc h (see t he Collector Su
pp ly
sc hematic) allows t h e choice of t h ree differe
n
t swee p
out-
puts from t he collecto r s upply by c
h
a
nging t h
e
out p ut con-
nections on t h e rectifier bri d
ges . T h e p ossible out puts are
positive-goi ng
+(
ΝΡΝ ) or negative-going
-( ΡΝΡ ) f u ll-wave
rectifie d sine waves or unrectifie d si n e waves
(AC) . In all
cases t h e p eak amplitu de of t h e collector swee p
is co n trol-
le d by t h e VA R IA BLE CO LLE CTO R S UPPL Y
control and
t h e MAX PEAK VO L TS switch .
T h
e
M OD E switch allows t he choice of two different
Collector S
u pp ly out puts : t he no r mal collector sweep as
h
as been
p reviously me ntioned and α DC collecto r voltage
out p
ut
. Wh en t h e M OD E switch is set to DC (ANTI L OO P )
or LE
A K
AG E
(EM ITT
ER C
URREN T) t he MAX PEAK
V
O LTS
switch p ic k s
one of
four resistor-ca p acitor com b i-
nations
w
h ic h is co nn
ected between t h
e
collector swee p
outp u t and t he
current
return in
p
ut
. T he pur p ose of t h ese
capacitors
is
to
h ol d t h
e
collector swee p voltage at α con-
stant
DC level
set by
t
h
e
V A R IA BLE CO LLE CTO R SUP-
PL Y cont r ol . T h is
h
olding is done
b y ch argi n g t h e cap acitor
up to
maxim u m p
ea
k
voltage as
set by t he VA R IA BLE
CO LLE CTO RS UPPL Y cont rol
an
d
keep i n g
t
h
em
charged
with t h e re p etitive collector sweep . T
he result
of
c h arging
these h oldi n g cap acitors is α dot on
t h
e
C R T rat h er t h an t he
normal
swee p .
3-2
F ig . 3-1 . Sim p lified sc h e m atic o f collector sup p ly circuit .
In series with t he collector swee p are series resistors
R345 t h roug h R 355 . T he interco
n nected
MAX PEAK
VO L TS and PEAK PO WER W ATTS switc h es ad d t hese
resistors in series acco r d ing to t he amou n t of p ea k collector
c u rrent d esired . T
h
e amount of t h is current is determine d
by t he maxim u m power dissi p ation rating of t he d evice
under test .
L ooping
T h ere is α certain amou n t of non-discrete cap acita nce
associated with t he collector s u
pp
ly w h ic h cau ses
an
effect
known as loop ing .
P
art of t h is un d esire d ca p
acitance is
stray cap acitance, w h ic h provi d es an AC curre
n t p
at
h b
e-
tween t he collector supp ly and c h assis g r oun d . T
h
e trans-
former and t h e guard box also ex h ibit some un d esired
capacitance between t h e guard box p oten tial (common
return p oint connecte d to guard box) and ch assis groun d .
Fig . 3-2 Α s h ows t h at t h ese two cap acita n ces form α d ivi der
for AC current, t he center of t he d ivider being connecte d to
t h e vertical amplifier .
During transitions of t h e collector swee p , some
current
will
be
tra n smitte dby t h is un d esire d cap acitance, by p
assing
t
h e d evice un d er test . T h is current, h owever, is sensed by
t h e vertical amplifier alo ng with t h e collector curren t and
causes
t
he rea d ing of collector curren t on t h e C R T to be
i
n
correct . Wh en
t h e collector swee p rises, t h e u n desire d cur-
re
n
t will
start p ositive a nd d ecrease to zero as t he collector
swee p r eac
h
es
its pea k .
As t he swee p falls, t he stray
curren t