Tektronix 434, Инструкция по эксплуатации

Страница: 24 / 222

Operating Instructions—434
rem aining T R IG G E R C O U P L IN G s w itch p o s itio n s w ill
p ro vid e a b e tte r disp la y.
The trig g e rin g p o in t in the A C p o s itio n depends u pon
th e average voltage level o f the trig g e r signals. I f the trig g e r
signals o c c u r at ra n d o m , th e average voltage level w ill vary,
causing th e trig g e rin g p o in t to vary also. T h is s h ift o f the
trig g e rin g p o in t m ay be enough so it is im possible to m a in ­
ta in a stable d isp la y. In such cases, use DC c o u p lin g .
L F R EJ. In th e LF REJ p o s itio n , DC is rejected and
lo w -fre q u e n c y signals b elow a b o u t 3 0 k ilo h e rtz are a tte n u ­
ated. T h e re fo re , th e sweep is triggered o n ly b y the higher-
fre q u e n c y co m p o n e n ts o f th e trig g e r signal. T h is p o s itio n is
p a rtic u la rly useful fo r p ro v id in g stable triggering if the
trig g e r signal c o n ta in s lin e -fre q u e n c y co m p o n e n ts. A ls o , in
th e A L T p o s itio n o f th e V e rtic a l M ode s w itch th e L F REJ
p o s itio n provides th e best d is p la y at fa st sweep rates w hen
com p a rin g tw o u n re la te d signals (T R IG G E R C O U P L IN G
s w itch set to CO M P).
H F R EJ. T h e H F REJ p o s itio n passes all lo w -fre q u e n c y
signals betw een DC and a b o u t 50 k ilo h e rtz . Signals outside
the above range are a tte n u a te d . When trig g e rin g fro m
c o m p le x w aveform s, th is p o s itio n is useful fo r p ro v id in g
stable d isp la y o f th e lo w -fre q u e n c y co m p o n e n ts.
DC. DC c o u p lin g can be used to p ro vid e stable trig g e r­
ing w ith lo w -fre q u e n c y signals w h ic h w o u ld be a tte n u a te d
in th e A C p o s itio n , o r w ith lo w re p e titio n rate signals. It
can also be used to trig g e r th e sweep w hen th e trig g e r signal
reaches a DC level selected b y the setting o f th e L E V E L
c o n tro l. W hen using in te rn a l trig g e rin g , the se ttin g o f th e
Channel 1 and 2 P O S IT IO N c o n tro ls a ffe c t th e DC
trig g e rin g p o in t o f a d isp la y w h e n th e T R IG G E R SO U RC E
sw itch is in th e COMP p o s itio n .
DC trig g e r c o u p lin g should n o t be used in the A L T d u a l­
trace m ode if the T R IG G E R S O U R C E sw itch is set to
COMP. I f used, th e sweep w ill trig g e r on the DC level of
one trace and th e n e ith e r lo c k o u t c o m p le te ly or free run
on th e o th e r trace. C o rre c t DC trig g e rin g fo r th is m ode can
be obta in e d w ith the T R IG G E R S O U RC E s w itch set to CH
1.
W ith b o th th e L F REJ and H F REJ b u tto n s pushed,
o n ly those signals between a p p ro x im a te ly 30 k ilo h e rtz and
50 k ilo h e rtz w ill be passed to th e T rig g e r G e n e ra to r w it h ­
o u t being a tte n u a te d .
Trigger Slope
T h e T R IG G E R SLOPE sw itch determ ines w h e th e r the
trig g e r c irc u it responds to th e positive-going o r negative­
going p o rtio n o f th e trigger signal. W hen th e SLOPE sw itch
is in th e positive -g o in g (+) p o s itio n , the d is p la y starts w ith
th e positive-going p o rtio n o f the w a v e fo rm ; in th e negative­
going (—) p o s itio n , th e d isp la y starts w ith th e negative-
going p o rtio n o f the w a ve fo rm (see Fig. 2 -6 ). When several
cycles o f a signal appear in th e d is p la y , th e setting o f the
SLOPE sw itch is o fte n u n im p o rta n t. H ow ever, if o n ly a
c e rta in p o rtio n o f a cycle is to be d isp la ye d , th e se ttin g o f
th e SLOPE sw itch is im p o rta n t to p ro vid e a d isp la y w h ic h
starts on the desired slope o f th e in p u t signal.
Trigger Level
T he T R IG G E R L E V E L c o n tro l dete rm in es th e voltage
level on th e trig g e rin g w a v e fo rm at w h ic h the sweep is
triggered. When th e L E V E L c o n tro l is set in th e + region,
th e trig g e r c irc u it responds at a m o re p o sitive p o in t on th e
trig g e r signal. In the — region, the trigger c ir c u it responds a t
a m ore negative p o in t on the trig g e r signal. Fig. 2-6 illu s ­
trates th is e ffe c t w ith d iffe r e n t settings o f the SLOPE
s w itc h .
T o set th e L E V E L c o n tro l, fir s t select th e T R IG G E R
source, c o u p lin g , and slope. T h e n , set the L E V E L c o n tro l
fu lly c o u n te rc lo c k w is e and ro ta te it c lo ckw ise u n til the
display starts at the desired p o in t.
Sweep Mode
A U T O . The A U T O p o s itio n o f th e Sweep M O D E sw itch
provides a triggered d isp la y w hen the L E V E L c o n tro l is
c o rre c tly , set (see T rig g e r Level in th is section) and an
adequate trig g e r signal is available.
When th e trig g e r re p e titio n rate is less th a n a b o u t 10
h ertz, or in the absence o f an adequate trig g e r signal, the
Sweep G e n e ra to r free runs at the sweep rate selected b y the
T IM E /D IV sw itch to produce a reference trace. When an
adequate trig g e r signal is again a pplied, th e fre e -ru n n in g
c o n d itio n ends and th e Sweep G enerator is triggered to
produce a stable d is p la y (w ith c o rre c t L E V E L c o n tro l
s e ttin g ).
N O R M . O p e ra tio n in th e N O R M p o s itio n o f th e Sweep
M O D E s w itch is th e same as in the A U T O p o s itio n w hen a
trig g e r signal is a p p lie d . H ow ever, w hen no trigger signal is
present, th e Sweep G e n e ra to r rem ains o f f and there is no
d isp la y.
Use the N O R M m ode to disp la y signals w ith re p e titio n
rates b e lo w a b o u t 10 h e rtz . T h is m ode provides an in d ic a ­
tio n o f an adequate trig g e r signal as w e ll as the correctness
o f trig g e r c o n tro l settings, since there is no d isp la y w ith o u t
p ro p e r trig g e rin g .
2-10