Tektronix 454A - Page 40

O p er at i ng I nst r uct i ons- 454zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ‘ /
R 454A
T h e t r i gger i ng
p
oi nt i n
t h e AC posi t i on de p en d s on t he
aver age vol t age
l evel of t he t r i gger si gnal . I f t h e t r i gger
si gnal s
occur i n ± r a nd om f as h i on, t h e aver age vol t age
l evel
wi l l var y, causi ng t he t r i gger i ng p oi nt t o var y
al so . T h i s s h i f t
of t he t r i gger i ng p oi n t may be enoug h so
i t i s i m p ossi bl e t o
mai n t ai n ± st a b l e d i s p l ay . I n suc h cases,
use
DC
cou p l i ng .
LF REJ . I n
t h e LF REJ posi t i on, DC i s r e j ect e d an d
si gnal s
bel ow about 50 k i l o h er t z ar e at t enuat e d . T h
er ef or e,
t he swee p wi l l be t r i gger e d onl y by t h e h i g h er - f r e
q uency
com p onent s of t he si gnal . T h i s p osi t i on
i s par t i cul ar l y
usef ul f or p r ovi d i ng st abl e
t r i gger i ng i f t he t r i gger cont ai ns
l i ne- f r e q uency
com p onent s . Al so, i n t he A L T p osi t i on of
t
he M
OD
E swi t ch , t h e LF REJ p osi t i on p r ovi d es t
he b est
di s p l ay at f ast sweep r at es w h e n com p ar i ng
t wo unr el at e d
si gnal s ( T RI GG ER swi t c h set t o N O RM) .
HF
REJ . T he HF REJ p osi t i on p asses al l l ow- f r equency
si gnal s bet ween about 30 h er t z and 50 k i l o h
er t z .
DC
i s
r e j ect ed an d si gnal s out si d e t he above r ange ar e
at t e nu at ed .
Wh en t r i gger i ng f r om com p l ex wavef or ms,
t h i s p osi t i on i s
usef ul f or p r ovi d i n g st abl e
d i s p l ay of t h e l ow- f r equency
component s .
DC . DC cou p l i ng can be used t o p
r ovi de st abl e
t r i gger i n g wi t h l ow- f r equency si gnal s
w h i c h woul d b e
at t enuat ed i n t he AC
posi t i on, or wi t h l ow- r epet i t i on r at e
si gnal s . T he LEVEL
cont r ol can be ad j ust ed t o p r ovi d e
t r i gger i ng
at t he desi r e d DC l evel on t he wavef or m . Wh e n
usi ng i nt er nal t r i gger i ng, t he set t i ng of t he CH 1 and CH 2
P OSI TI ON cont r ol s af f ect t he DC
t r i gger i ng l evel ,
DC
t r i gger cou p l i ng s houl d not be used i n t he A L T
d ual - t r ace mo d e i f t he T R I GG ER swi t ch i s set t o N O RM . I f
used, t he swee p wi l l t r i gger on t h e DC l evel of
one t r ace and
t h e n ei t h er l oc k out com p l et el y
or f r ee r un on t he ot h er
t r ace . Cor r ect DC t r i gger i ng
f or t h i s mode can be obt ai ne d
wi t h t h e TRI GG
ER swi t c h set t o CH 1 O NL Y .
2- 22
Tr i gger L
evel
T h e Tr i gger i ng LEVEL co n t r ol d et er mi nes t he vol t age
l evel on t he t r i gger i ng wavef or m at w h i c h t he swee p i s
t r i gger e d . Wh en t he LEVEL cont r ol i s set i n t he + r egi on,
t h e t r i gger ci r cui t r es p onds at ± mor e p osi t i ve poi nt on t he
t r i gger si gnal . Wh e n t h e LEVEL cont r ol i s set i n t
h
e -
r egi on, t he t r i gger ci r cui t r es p on d s at ± mor e negat i ve poi nt
on t he t r i gger si g n al . F i g . 2- 11 i l l ust r at es t h i s ef f ect wi t h
di f f er ent set t i ngs of t h e S L O PE swi t c
h .
To set t he L EVEL co n t r ol , f i r st sel ect t h e Tr i gger i ng
SO UR C E, CO UPL I N G and S L O PE a nd M OD E . T hen set
t h e LEVEL cont r ol f u l l y cl oc k wi se a n d r ot at e i t cl ock wi se
unt i l
t
h
e d i s p l ay st ar t s at t
he d esi r e d p oi nt .
H i g h - F r e q uency
St abi l i t y
T
h e HF STA B co n t r ol ( ‘ on l y) i s most usef ul at
f r e q uenci es above about 40 mega h er t z or wi t h si gnal s
w h
i c
h
cont ai n h i g h - f r e q uency component s . I f ± st abl e
di s p l ay
cannot be o bt ai ned usi ng t he ‘
LEVEL cont r ol ( t r i gger
si gnal must h ave ade q uat e am
p l i t ude) , ad j ust t h e HF STA B
con t r ol
f or mi ni mum h or i zont al j i t t er i n t he di s p l ay .
‘ Swee p Tr i gger e d L i g h t
T he ‘ S WEEP TR I G' D l i g h t p
r ovi des ± conveni ent
i n d i cat i on of t he condi t i on
of t he ‘ Tr i gger i ng ci r c u i t . I f
t he
‘
Tr i gger i ng cont r ol s ar e cor r ect l y ad j ust ed wi t h an
ade q uat e t r i gger si gnal ap p l i e d , t h i s l i g h t i s on . H owever , i f
t he ‘ LEVEL cont r ol i s mi sad j ust ed, t h e ‘ CO UPL I N G or
‘ SO UR CE swi t c h es i ncor r ect l y set , or t h e t r i gger si gnal
t oo l ow i n ampl i t u d e, t he ‘ S WEEP TR I G' D
l i g h t wi l l be
of f .
T
h
i s
f eat ur e
can
be use d as ± gener al i ndi cat i on of
cor r ect t r i gger i ng . I t i s p ar t i cul ar l y usef ul w he n set t i ng up
t he t r i gger ci r cui t s w h e n ± t r i gger si gnal i s avai l abl e wi t h out
± t r ace d i spl ay on t he CR T ; i t al so i nd i cat es t h at
t h e ‘
sweep i s cor r ect l y t r i gger ed
w hen o p er at i ng i n t he ’
( D EL
AY E
D S
WEEP) mode .
‘ Swee p M ode
Tr i gger Sl o p e

Aut o Tr i g . T
h
e AU TO
T R
I G
posi t i on of t he ‘ S WEEP
The Tr i gger i ng S L O PE swi t c h det er mi nes w h et h er t he

M OD
E swi t c h p r ovi des ± st a b l e d i s p l ay w h en t he ‘ LEVEL
t r i gger ci r cui t r es p onds on t he p osi t i ve- goi ng or
negat i ve-

cont r ol i s cor r ect l y set ( see Tr i gger L evel i n t h i s sect i on)
goi ng por t i on of t h e t r i gger si gnal . Wh
e n
t h
e
S L O PE
swi t ch  an d ± t r i gger si gnal i s avai l abl e . T h e ‘ S WEEP TR I G' D l i g h t
i s i n t he
+ ( p
osi t i ve- goi ng)
p osi t i on, t h e d i s p l ay st ar t s wi t h  i ndi cat es w h en t h e ‘ Swee p Gener at or i s t r i gger ed .
t h e p osi t i ve- goi ng por t i on of t he wavef or m ; i n t h e -
( negat i ve- goi ng) p osi t i on, t he d i s p l ay st ar t s wi t h t he
negat i ve- goi ng p or t i on of t h e wavef or m ( see
F i g . 2- 11) .

Wh en t h e
t r i gger
r e p
et i t i on r at e i s l ess t h an a b o u t 20
Wh e n sever al cycl es
of ± si gnal a pp ear i n t h e di s p l ay, t he

h er t z,
or
i n t h
e a
b
sence of
an
a de quat e t r i gger si gnal , t h e ‘
set t i ng of t h e S LO PE swi t c h i s of t en uni m p or t ant .

Swee p Gener at or f r ee r uns t o pr od uce ± r ef er ence t r ace .
H owever , i f onl y ± cer t ai n por t i o n of ± cycl e i s t o be

Wh en an ade q uat e t r i gger si gnal i s agai n a p p l i e d , t h e f r ee-
d i s p l aye
d , cor r ect set t i ng of t he S L O PE swi t c h i s i mpor t ant

r u nni ng co n di t i on en
d
s an
d
t
he ‘ Swee p Gener at or i s
t o p r ovi de ± di spl ay w h i c h st ar t s on t h e d esi r ed sl o pe of t h e

t r i gger e d t o p r oduce ± st abl e d i s p l ay ( wi t h cor r ect LEVEL
i np ut si gnal .

co n t r ol set t i ng) .