Tektronix 422, Instruction Manual

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Operating Instructions—Type 422 AC-DC
hertz are attenuated. In general, AC coupling can be used
fo r most applications. However, if the signal contains un­
wanted low-frequency signals or if the sweep is to be trig­
gered at a low-repetition rate or DC level, one o f the re­
maining TRIGGERING Coupling switch positions w ill pro­
vide a better display.
The triggering point in the AC position depends upon
the average voltage level of the trigger signals. If the trigger
signals occur at random, the average voltage level w ill vary,
causing the triggering point to vary also. This shift o f the
triggering point may be enough so it is impossible to main­
tain a stable display. In such cases, use DC coupling.
AC LF REJ. In the AC LF REJ position, DC is rejected
and low-frequency signals below about 50 kilohertz are at­
tenuated. Therefore, the sweep is triggered only by the
higher-frequency components o f the trigger signal. This
position is particularly useful fo r providing stable triggering
if the trigger signal contains line-frequency components.
Also, in the A L T position of the vertical Mode switch the
AC LF REJ position provides the best display at fast sweep
rates when comparing tw o unrelated signals (TRIGGERING
Coupling switch set to CH 1 & 2).
DC. DC coupling can be used to provide stable trigger­
ing w ith low-frequency signals which would be attenuated
in the AC position, or w ith low repetition rate signals. It
can also be used to trigger the sweep when the trigger signal
reaches a DC level selected by the setting o f the LEVEL
control. When using internal triggering, the setting o f the
Channel 1 and 2 POSITION controls affects the DC trigger­
ing point (CH 1 & 2 position only).
DC trigger coupling should not be used in the A L T dual-
trace mode if the TRIGGERING Source switch is set to CH
1 St 2. If used, the sweep w ill trigger on the DC level of one
trace and then either lock out completely or free run on the
other trace. Correct DC triggering fo r this mode can be
obtained w ith the TRIGGERING Source switch set to CH
1.
Trigger Slope
The TRIGGERING SLOPE switch determines whether
the trigger circuit responds to the positive-going or
negative-going portion o f the trigger signal. When the
SLOPE switch is in the positive-going (J~) position, the
display starts with the positive-going portion o f the
waveform; in the negative-going (~\J position, the display
starts w ith the negative-going portion o f the waveform (see
Fig. 2-11). When several cycles of a signal appear in the
display, the setting o f the SLOPE switch is often
unimportant. However, if only a certain portion of a cycle
is to be displayed, the setting of the SLOPE switch is
important to provide a display which starts on the desired
slope of the input signal.
Trigger Level
The TRIGGERING LEVEL control determines the vo lt­
age level on the triggering waveform at which the sweep is
triggered. When the LEVEL control is set in the + region,
the trigger circuit responds at a more positive point on the
trigger signal. In the — region, the trigger circuit responds at
a more negative point on the trigger signal. Fig. 2-11 illus­
trates this effect w ith different settings o f the SLOPE
switch.
To set the LEVEL control, first select the TRIGGER­
ING source, coupling and slope. Then, set the LEVEL con­
trol fu lly counterclockwise and rotate it clockwise until the
display starts at the desired point.
Trigger Mode
General. The TRIGGERING MODE switch is actuated
by the LEVEL control. When the LEVEL control is pushed
in, the TRIGGERING Mode switch is in the AUTO position
and when the LEVEL control is pulled out, the TRIGGER­
ING Mode switch is in the NORM position.
AUTO. The AUTO position o f the TRIGGERING Mode
switch provides a stable display when the LEVEL control is
correctly set (see Trigger Level in this section) and an
adequate trigger signal is available.
When the trigger repetition rate is less than about 20
hertz, or in the absence o f an adequate trigger signal, the
Sweep Generator free runs at the sweep rate selected by the
T IM E /D IV switch to produce a reference trace. When an
adequate trigger signal is again applied, the free-running
condition ends and the Sweep Generator is triggered to
produce a stable display (with correct LEVEL control set­
ting).
NORM. Operation in the NORM position o f the T R IG ­
GERING Mode switch is the same as in the AUTO position
when a trigger signal is applied. However, when no trigger
signal is present, the Sweep Generator remains o ff and there
is no display.
Use the NORM mode to display signals w ith repetition
rates below about 20 hertz. This mode provides an indica­
tion of an adequate trigger signal as well as the correctness
o f trigger control settings, since there is no display w itho ut
proper triggering.
Horizontal Sweep Rate
The T IM E /D IV switch provides 19 calibrated sweep
rates ranging from 0.5 microsecond to 0.5 second/division.
The V A R IA B LE control provides continuously variable
sweep rates between the settings o f the T IM E /D IV switch.
Whenever the UNCAL light is on, the sweep rate is uncali­
brated. The light is o ff when the VA R IA B LE control is set
to the CAL position.
When making tim e measurements from the graticule, the
area between the second and tenth vertical lines o f the
graticule provides the most linear time measurement (see
Fig. 2-12). Therefore, the firs t and last divisions o f the
display should not be used when making accurate tim e
measurements. Position the start of the tim ing area to the
second vertical line and adjust the T IM E /D IV switch so the
2-19