Tektronix 434, Инструкция по эксплуатации

Страница: 22 / 222

Operating Instructions—434
by a b o u t 24 pF. When the signal is coupled to the in p u t
th ro u g h a coaxial cable, the effective in p u t capacitance is
increased. The actual in p u t capacitance depends upon the
typ e and length o f cable used and the frequency o f the
signal.
Ground Considerations
Reliable signal measurements cannot be made unless
both the oscilloscope and the u n it under test are connected
together b y a com m on reference (ground) lead in a d d itio n
to the signal lead or probe. A lth o u g h the three-w ire AC
power cord provides a com m on connection when used w ith
e q uipm ent w ith sim ilar power cords, the ground loop
produced may make accurate measurements impossible.
The ground straps supplied w ith the probes provide an
adequate ground. The shield o f a coaxial cable provides a
com m on ground when connected between tw o coaxial
connectors (or w ith suitable adapters to provide a com m on
ground). When using unshielded signal leads, a com m on
ground lead should be connected fro m the 434 chassis to
the chassis o f the e q uipm ent under test.
In p u t Coupling
The Channel 1 and 2 In p u t C oupling switches a llo w a
choice o f in p u t coupling methods. The type o f display
desired and the applied signal w ill determ ine the coupling
to use.
DC C oupling can be used fo r m ost applications. This
p o sitio n allows measurement o f the DC co m ponent o f a
signal. It m ust also be used to display signals below about
100 hertz (10 hertz w ith a 10X probe) as th e y w ill be
attenuated in the AC position.
W ith AC C oupling, the DC c o m p o n e n t o f the signal is
blocked by a capacitor in the in p u t c irc u it. The low-
frequency response in the AC p o sitio n is a bout 10 hertz
(—3 dB p o in t). T herefore, some low -frequency a tte n u a tio n
can be expected near this frequency lim it. A tte n u a tio n in
the fo rm o f w aveform t i l t w ill also appear in square waves
w h ich have
low -frequency com ponents. AC coupling
provides the best display o f signals w ith a DC co m ponent
w hich is much larger than the AC com ponents.
The GND p o sitio n provides a ground reference at the
in p u t o f the 4 34 w ith o u t the need to e xte rn a lly ground the
in p u t connectors. The signal applied to the in p u t connector
is in te rn a lly disconnected, b u t n o t grounded, and the in p u t
c irc u it is held at ground p o te n tia l. T his also allows pre­
charging the in p u t coupling capacitor to prevent application
o f high-am plitude voltage spikes to the a m p lifie r in p u t.
D eflection Factor
The a m o u n t o f vertical d e fle ctio n produced by a signal is
determ ined b y the signal am p litu d e , the a tte n u a tio n fa c to r
o f the probe (if used), the setting o f the V O L T S /D IV
sw itch and the setting o f the Variable V O L T S /D IV c o n tro l.
The calibrated d e fle c tio n factors indicated by the V O L T S /
D IV switches apply o n ly when the Variable V O L T S /D IV
c o n tro l is set to the calibrated d e te n t p o sitio n.
The Variable V O L T S /D IV c o n tro ls provide c o n tin u o u sly
variable (uncalibrated) vertical d e fle c tio n factors between
the calibrated settings o f the V O L T S /D IV switches. The
Variable co n tro l extends the m axim um vertical d e fle ctio n
fa c to r o f the 434 to at least 25 v o lts/d ivisio n (10 V posi­
tio n ).
The 434 Oscilloscope features vertical d e fle c tio n scale-
fa c to r sw itching. When probes having a scale-factor sw itch ­
ing connector are used, the co rre ct d e fle c tio n fa c to r can be
read d ire c tly fro m the in s tru m e n t fr o n t panel.
Dual-Trace O peration
A lte rn a te Mode. The A L T po sitio n o f the V e rtica l Mode
switch produces a display w h ich alternates between
Channel 1 and 2 w ith each sweep o f the C RT. A lth o u g h the
A L T mode can be used at all sweep rates, the CHOP mode
provides a m ore satisfactory display at sweep rates below
a bout 0.5 m illis e c o n d /d iv is io n . A t these slow sweep rates,
alternate mode sw itching becomes visually perceptible.
Proper internal triggering in the A L T mode can be
obtained in either the COMP or the CH 1 po sitio n o f the
T R IG G E R SOURCE sw itch. When in the COMP p o sitio n,
the sweep is triggered fro m the signal oh each channel. This
provides a stable display o f tw o unrelated signals, b u t does
n o t indicate the tim e relationship between the signals. In
the CH 1 p o s itio n , the tw o signals are displayed showing
tru e tim e rela tio n sh ip . I f the signals are n o t tim e related,
the Channel 2 w aveform w ill be unstable in the CH 1
SOURCE switch p o sitio n.
Chopped Mode. The CHOP p o sitio n o f the V e rtica l
Mode sw itch produces a display w hich is e le ctro n ica lly
switched between channels. In general, the CHOP mode
provides the best display at sweep rates slower than about
0.5
m illise co n d /d ivisio n or whenever dual-trace, non-
repetitive phenomena is to be displayed. A t faster sweep
rates, the chopped sw itching becomes apparent and may
in te rfe re w ith the display.
Proper internal triggering fo r the CHOP mode is p ro ­
vided w ith the T R IG G E R SOURCE sw itch set to CH 1. If
the COMP p o sitio n is used, the sweep circu its are triggered
2-8