Operation
Auto Setup
Q)
will always attempt to arrange the dis-
play so that two to five complete cycles appear, with
the amplitude set so that the height
of
the trace
is
roughly two to five screen divisions.
It
also selects
auto trigger to ensure that the screen
is
frequently
updated and a trace
will be visible.
Adjusting the Trace
The trace can be altered in two main ways: horizontally and
ver6;ally. Basic horizontal adjustments involve altering the
sweep rate
of
the trace, so
that
the image
on
the screen
stretches
or
contracts horizontally. The main vertical
adjustment
is
the height
of
the displayed signal - i.e. the
volts
per
screen division.
Horizontal Adjustment (A TIME/DIV)
To
alter the sweep
rate
of
the trace, for example
to
look
more closely at part of the image, the A TIME/DIV paddle
is
used (the B TIME/DIV paddle is discussed in Section
1.3.). With the 4kHz signal applied as discussed earlier,
Auto
Setup may have set the sweep rate to
50µ.s
per
screen
division horizontally - i.e. each horizontal screen division
represents
50µ.s
worth
of
the input signal. This
is
shown by
the
'TBA=50µ.s' near the top
of
the display. The
'TBA'
means that the A timebase is in use; two timebases are
available for each channel, namely A and B.
Use of the B
timebase
is
discussed in Section 1.3.
A
TIME/DIV
@ This
is
a five position paddle which con-
trols the sweep rate
of
the trace. A gentle push causes
a
!ilow
change in the sweep rate, whilst a firmer push
will cause a faster change
in
the sweep rate.
Given that the current sweep rate is
50µ.s
per
screen divi-
sion, a single push and release
of
the paddle to the right will
change the 'timebase' from
50µ.s
per
division to
20µ.s
per
division.
The
displayed signal will stretch accordingly. A
second single push
to
the right will change the timebase
again, this time to
10µ.s.
If
you keep pushing the paddle to the right, you will find
that the fastest timebase
is
20ns
per
screen division.
On
the
4071
operating
in
dual channel mode, the 250ns per division
range
is
not available. You may notice that at 100,
50
and
20ns
per
division, the oscilloscope builds up the image
gradually. This
is
because it uses a method called 'Equiva-
lent Time Sampling'
or
ETS for the faster timebases.
It
takes a number
of
random samples
of
the signal and adds
them to the display to
build up the final result. This
is
only
useful for repetitive input signals.
If
you now push the paddle
to
the left you
will
see the
timebase change
in
the opposite direction. The maximum
time
per
division
is
20s; the 4070 will take over 3 minutes to
acquire a full trace at this speed. However, assuming you
are applying the 4kHz signal mentioned earlier, when the
timebase reaches
50ms, an 'alias' becomes visible.
Section 1
Aliases
An alias
is
a false image.
The
4070
is
a digital oscilloscope
and so takes frequent samples of the input signal
in
order
to
update the trace. Thus it may take one sample from one
point on the input waveform and the next sample from a
point
slightly further along
on
the next wave.
It
will then
display the wave as being much longer than it really
is.
The
effect
is
similar in principle
to
the false motion
of
wagon-
wheels as seen on a television.
The
4070 does however have a feature which will often help
you to spot
aliases.
If
the input signal peaks
or
trc1.ghs at
any point between samples, a peak/trough can be dis-
played. This Max-Min function
is
discussed further
in
Sec-
tion 2.4.
Vertical Adjustment (VOL TS/DIV)
Each
of
the channels has its own set
of
vertical controls.
The
main control in each case
is
the VOLTS/DIV (volts
per
divi-
sion) paddle. Again this is a five-position switch. A
gentle
push either up
or
down will result in a slow single step
change in the sensitivity
of
the instrument. A firmer push
will cause a more rapid change.
VOL
TS/DIV
@ This
is
a five-position paddle.
It
adjusts
the volts
per
division. Pushing the paddle upwards
will adjust the sensitivity as far as 5V/div and the
height of the image
will be seen
to
decrease in accor-
dance with the changes. In the
other
direction, sen-
sitivity can be increased as far as 2mV
per
division,
increasing the height
of
the screen image.
Note: The 4070 can accept
input
signals
with
a peak
voltage as
high
as
plus
or
minus
400V. Larger signals
can cause damage
to
the internal circuitry.
AC/Gnd/DcG)
This button controls the type
of
coupling between the input
signal and the
4070. Though
DC
is
the most generally
applicable, auto setup will normally set this control to
AC
to facilitate sensible triggering.
Howev<;1,
if when using the
instrument you find that there
is
a permanent vertical offset
in
the display - i.e. the signal
is
consistently displayed too
high
or
too low on the screen - you could try pressing this
button once to set it to
AC. This will cause the 4070 to
remove any
DC
component which may be causing the
offset. Repeated pressing
of
this button will cause it to cycle
through the three settings AC, Ground and DC.
With this control set to DC, the 4070 can display signals
in
a frequency range
of
OHz(DC) to
lOOMHz.
On
AC, the
range
is
lOHz to
lOOMHz.
With the setting on Gnd
(ground) a
OV
reference only
is
displayed (the input signal
is
not connected to ground
or
coupled to the trace
in
this
case).
This control is discussed
in
more detail
in
Section 1.2.
7
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