5
Using
oscilloscopes
It seem superfluous to say that, when using
an
oscilloscope
to
view a waveform, one should choose an instrument appropriate
to the ,job in hand. Yet,
as
explained in the course
of
this chapter,
besides the more obvious requirements ('does it have
a
band-
width wide enough
to
display my signal faithfully?', 'is
it
sensitive
enough to see the very small signal
I
wish to view?') there are
quite a few other considerations that are a little less
obvious.
Some have already been pointed out, notably in Chapter
3,
and
others will become apparent in the course
of
this chapter. We
shall also consider
the case where there
is
no choice and one
is
faced with the task
of
trying
to
obtain some useful information
about
a
waveform with an oscilloscope which
is
hardly adequare
for the purpose.
Use
of
probes
()iiestions
that
people
HC'W
to
using
oscilloscopes oftcn
ask
arc:
'Do
1
always need
a
probe?
If
not, how
do
1
know when
1.0
use
one
and
whrn
not?' The first part
of
Chapter
4
should
havc
provided
a
good
deal
of'
insight into this:
if
you
are
still pu~~1t.d
il
might
be
worth reading again.
But
for
a
short, simple answer, the
author's advice
is
always
to
use a
1O:l
passive divider probe
(correctly
set
up
for
the oscilloscope you are using) as
a
matter
of
habit.
If.
owing
to
the attendant attenuation factor
of
10,
the
signal
you
wish to view gives insufficient vertical deflcction even
with the
Y
input setting
at
its most sensitive position, then it will
be necessary
to
consider whether
it
is possible to depart
from
your standard practice
of
using such a probe.
For example,
if
you are using
a
metre or
so
of
general-purpose,
audio screened
lead
to
connect the signal
to
be viewed
to
the
oscilloscopc,
the
total capacitivc loading on thc circuit
may
wcIl
be
scvcrnl hundrcd picofarads. This will
he
of
no consequence
if
looking
at,
say,
the
secondary voltage
of
a
mains transformer,
and
gerierally
acceptable
for
viewing the output
of
a
hi-fi
amplilier
over the whole
audio
range. However,
200
pF
has a reactance
of
Using oscilloscopes 53
40 k~ at 20 kHz, and you might well get a misleading picture of
a test waveform in one of the earlier high-impedance stages of
the amplifier; worse, the phase shift caused by the additional
capacitance could cause the amplifier to oscillate if the phase
margin of its negative feedback loop is rather sparse. Yet it is
precisely in the earlier small-signal stages that you might want to
avoid the attenuation of a passive divider probe.
Two courses of action are open: 75 f~ coaxial cable as used for
television aerial downleads, for example, has a capacitance of
approximately 60 pF/metre, as against 150-180 pF/metre for an
audio screened lead. So if the connection to the scope can be
made with a mere half metre of coax, the total capacitive input
loading including the oscilloscope's input capacitance can be kept
down to less than 60pF. Alternatively, an active probe as
described in Chapter 4 may provide the answer.
When investigating circuits operating at r.f. a passive divider
probe is essential. Even with it, care must be taken if misleading
results are to be avoided. For example, if a probe is connected
across a tuned circuit, the extra 10-12pF loading of the probe
will change the resonant frequency to some extent. How much
depends upon how much capacitance there is in parallel with the
inductor to start with, but the probe's capacitance will be quite
enough to upset the response of a conventional double-tuned i.f.
stage, resulting in the wrong amplitude being displayed upon the
screen. The effect on an oscillator can be much more dire; not
only will the connection of a probe change the oscillator's
frequency, it will generally cause a reduction in amplitude as
well, and may very likely stop the oscillator altogether. The
reason for this is that the impedance seen 'looking into' the probe
is 10Mf~ at d.c. only, and falls with frequency (see Figure 4.3). At
several MHz it may be down to a few hundred kilohms or even
lower, imposing damping on the tuned circuit to which it is
connected. In the case of an oscillator, of course, there will
generally be several volts peak to peak of signal available, so if the
oscilloscope is reasonably sensitive (i.e. has a 5 or 10mV/div
range) it will be possible to use a x l00 probe with an input
capacitance of about 1 pF. If a x l 0 probe is all that is available, it
is possible to achieve much the same result by connecting a 1.2 pF
54 Oscilloscopes
capacitor in series with the probe tip. The result will be a sort of
x100 probe that is not exactly calibrated and will not work at low
frequencies. However, it will permit you to monitor oscillators,
and tuned circuits generally, without affecting them unduly. In
fact if it is only required to monitor the frequency and waveshape
of the oscillator, the 1.2pF capacitor can be dispensed with
entirely and the tip of the xl 0 probe simply held very close to, but
not actually touching, the tuned circuit.
Trace finding
When using an oscilloscope to view waveforms, you will generally
have some idea of what to expect. Thus, if examining TTL or CMOS
logic gates operating at a clock frequency of 1 MHz, you would use
a x l0 divider probe and set the scope's Y sensitivity to 0.1 or
0.2 V/div, d.c. coupled, giving a 1 or 2 V/div sensitivity on the
screen. The timebase speed would be set to, say, 1 b~s/div.
However, it can happen that you do not know the appropriate
settings, either because of lack of information on the circuit
under test, or because owing to a fault the waveform is not what
you would expect. Some scopes (e.g. that featured in Chapter 3)
have an 'auto everything' feature, which will test the peak-to-
peak amplitude of the input signal and select a suitable Y
sensitivity range, also checking the frequency or repetition rate of
the input waveform and selecting a suitable timebase speed to
show several complete cycles. Such an oscilloscope is very handy
for a technician of limited experience, or a repair man fault-
finding on a complicated piece of equipment, though certain
types of waveforms- those with an extreme mark/space ratio or
many high-frequency components- can result in a non-
optimum display. But the majority of oscilloscopes do not have
this feature. Let us suppose, then, that when the input signal is
connected, the trace disappears from the screen of the oscillo-
scope. The more expensive type of scope (and increasingly
nowadays the cheaper models also) will have a trace finder
button: pressing this has the effect of restoring the trace to the
screen regardless of the control settings, albeit in a defocused
form. But its use should become unnecessary when you know
how to drive an oscilloscope properly.
Using oscilloscopes 55
The commonest cause of a 'lost trace' is connecting a signal
with a large d.c. component to the scope with the Y input d.c.
coupled and the input attenuator at too sensitive a setting. So if
you don't know what to expect, set the trace to the centre of the
screen, set the Y input to a.c. coupled and the input attenuator to
the least sensitive setting- usually 20 or 50V/div. It will then
need a very large signal voltage to lose the trace, especially if
using a 10:1 probe! In fact, with a.c. coupling, connecting a large
d.c. voltage will move the trace up (or of course down, if the
voltage is negative), but the trace will then slowly return to the
centre of the screen. This is so even if the attenuator is at one of
its more sensitive positions, although in this case it could take
many seconds before the trace returns to the screen.
You can still lose the trace even with the Y input a.c. coupled,
if the input attenuator is at too sensitive a setting. Take, for
example, a 1 kHz TTL squarewave a.c. coupled to an oscilloscope
set to a sensitivity of 5 mV/div: even using a 10:1 probe, the tops
of the waveform will be off the top of the screen and the bottoms
below the bottom edge of the screen. Although parts of the rising
and falling edges will be on-screen, they will be so rapid as to
leave too faint a trace to be seen. If the scope has a trace-finder or
locate button, pressing this will show lines of dashes near the top
and bottom of the screen, but if you always follow the sound
practice of setting the Y input to an appropriate setting if known,
or to the least sensitive setting if not known, you need never lose
the trace in the first place.
The trace can also be lost through inappropriate settings of the X
timebase controls. Suppose, for example, that you apply a 100 Hz
sine wave to an oscilloscope, with suitable settings of the Y input
controls but with the timebase speed set to 1 ~s/div. When the
timebase triggers the trace will be complete in 10 ~s (assuming the
screen has 10 horizontal divisions). At the end of the sweep the
trace will remain blanked for the next 9.99 ms until triggered by
the next cycle; see Figure 5.1. With the trace blanked for 99.9 per
cent of the time, it will be invisible, and on many cheaper scopes
will remain so even if the intensity control is turned up. Only
oscilloscopes with a high writing speed (see Chapter 9) will cope
with this situation. The rule therefore is that if you do not know
56 Oscilloscopes
Figure 5.1 100Hz sine wave displayed with l l~s/div sweep speed. The 10t~s
segment is not to scale, having been exaggerated for clarity. The display shows
one-thousandth of a cycle, which would in practice be too dim to see
the frequency of the waveform you wish to examine, set the
timebase speed to one of its slower positions, say 2 ms/div.
This leaves just one tricky case to watch out for: a narrow pulse
occurring at a low repetition rate, say lOOns wide at 100pps
(pulses per second). At 2 ms/div sweep speed the pulses will be too
narrow to see and the trace will appear indistinguishable from the
straight line produced by the auto brightline circuit. The test here is
simply to switch to normal trigger, which disables the brightline
circuit. Now the trace will only appear when the trigger control is
set to that part of its travel covered by the input pulse.
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