Tektronix 2235 - OSCILLOSCOPE DISPLAYS; INTRODUCTION; BASELINE TRACE; Horizontal

G
ENER A L O PER ATI N G
I NF O RM ATIO N
T h
e g
r
atic
u le is internally mar ked on t h e faceplate of
t he
crt to ena
b le accu rate meas u reme n ts wit h o u t pa
r allax error
(see
Figure 3-1) . It is mar ked wit h eig h t vertical a n d te n ho
r i-
zontal ma j or
divisions . Eac h ma j o r division is divi ded into
five su b
divisions . T he vertical deflectio n factors and
h orizon-
tal timi ng a
r
e cali
br ated to t he gratic u le so t h at accu
r
ate
meas u reme
n ts ca n be ma de di r ectly from t he crt . Also, p e
r-
cen tage mar
k ers for t he measureme n t of rise an d
fall times
are located on
t he left si de of t he graticule .
G R OUN
DI N
G
T h e most r eliable
signal measureme n ts a r e made w hen
t h is instr u me n t a nd t h e unit under test are
connected by α
common reference (grou n d lead),
i n
addition
to t he signal
lea d o r p robe . T he pro b e's gro un d lea d p
rovides t he b est
grou n di ng met h od for
sig n
al
interco n nectio n and e n sures
t he maximum amount of
signal-lea d s h iel d ing in t he p robe
cable . Α separate grou
n
d lea d
can also b e connected from
t he unit under test to t he oscillosco pe
G N D connector lo-
cated on t he front p an el .
SIG N A L CO NNE CTIO N S
Generally, pro b es offer t h e most
convenient means of
connecting an input signal to t h e inst
r ument . T h ey are
sh ielded to pr event p ic kup of electromag
n
etic
i n terference,
and t he supp lie d 10 Χ p robe offers α
hig h in p ut imp eda nce
t h at minimizes circuit loading . T h is allows t
h
e
circ u it un der
test
to op erate wit h α minim u m of c h ange
fr
om
its normal
condition as measurements are b eing made .
Coaxial ca
b les may also be used to connect signals to
t h e
in p
ut
connectors, bu t t hey may h ave considera b le effect
on t he accuracy of α
d is p layed waveform . To main tai n t he
origi n al
fre
q uency
ch aracteristics of an ap p lied signal, o
n ly
h
ig h -quality, low-loss coaxial cables s h o u l d b e u sed . Coax-
ial cables s
hou l d b e terminated at both ends in t h eir c
h
arac-
te
r istic im p edance . If t h is is not p ossible, use suita
b le
impe d a nce-match i ng devices .
Section 3-2235 ΑΝ / U S M -488 Ope
r
ato
r
s
O
PER ATI N G CO
N SID ER ATIO
N
S
G R ATIC ULE

I NPU T CO
UPL I N
G
CA P ACITO R PRE C
H A R
GI
N
G
Wh e n t he In p ut Cou p ling switc
h
is set
to G N D, t he i np ut
signal is co nn ecte d to ground
t h ro ugh t he inp u t cou p li ng
capacitor in series wit
h
α
1 ΜΩ resistor to form α
p r ec h arging network . T h is network allows t
h
e
i np
ut cou
p ling
cap acitor to c ha r ge to t he average d
c-voltage level of t h e
signal applie d to t he pr obe .
T h
us
any large voltage
transients t h at may acci d entally b e ge
n
e
r
ated
will n ot be
app lied to t he am p lifier in pu t
when t h
e
Inp u t Co up li ng switc h
is move d from G ND to AC .
T h
e p
r
ec
h
arging
networ k also
p rovides α measure of protectio n to t h e exte rn al ci
r cuitry
by
r educing t he curre n t levels t h at can b ed
r
aw
n from t he ex-
ter n al ci r cuit r y duri ng capacitor c
h arging .
1ST O R LEF T

11TH O R R IG HT
V ERTICA L

VER TICA L
GR ATIC
ULE

G R ATIC ULE
L I NE

L I NE /
R
IS
E A N D

C EN T ER
F A LL TIME

T ER

H O R IZO N TA L
C EN
ME AS UREMEN T

GR ATIC ULE
PER C EN TAG E
VER TICA L
L I NE
MA RKER S
G R ATIC
ULE
L I NE
F igure 3-1 . G r atic u le meas ureme n t ma rk i n gs .
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