| Five key tips for using an oscilloscope |
Five key tips for using an oscilloscope
- essential tips for using an oscilloscope or scope and ensuring the
best measurements are made using this type of test equipment.
The oscilloscope is one of the most widely used pieces of
test equipment. It is used my many engineers almost every day and as a
result it sometimes helps to recap some simple but important tips that
enable the best to be gained from this type of test equipment.
When making measurements using an oscilloscope there are
a number of pitfalls that result in measurements not being as accurate or
useful as they might otherwise be. Taking a few simple precautions when
using an oscilloscope, it is possible to ensure that the measurements taken
are as accurate as possible and that the measurements are as useful as
possible.
Some of the precautions result from knowing the
limitations of using an oscilloscope. However other precautions can be
reflected in the way the oscilloscope is used, and the equipment, including
probes that are used with the test equipment.
Beware of oscilloscope bandwidth limitations
As with any item of test equipment used for making
measurements of signals and waveforms the bandwidth of the oscilloscope is
particularly important. If the oscilloscope is to be able to reproduce the
image of the waveform accurately then it must have sufficient bandwidth to
accommodate the frequencies within the signal.
For sinusoidal type waveforms, measurements should not be
made near the 3dB bandwidth as operating in this area will give a 30%
reduction in the signal level. For digital measurements, the expected rise
time of the signal can be used to determine the bandwidth requirements of
the oscilloscope.
Ensure the correct triggering
In order that any waveform can be seen clearly when using
an oscilloscope, it is necessary to ensure that the oscilloscope triggers
correctly. If it does not trigger properly then the waveform will not be
seen clearly. In view of the fact that it is necessary to view many complex
waveforms on an oscilloscope, it is not always possible to enable it to
trigger correctly when using the automatic trigger facility taken from the
input channels. When looking at the best methods it is necessary to consider
using the external trigger, and using a pulse of other suitable waveform
from another point on the circuit under test. In this way it is often
possible to gain a much better signal so that the oscilloscope can display
the optimum image of the waveform.
Use the right oscilloscope probe
In the same way that the performance of any piece of test
equipment can have its performance limited by some of the peripheral
equipment, the same is particularly true when using an oscilloscope. While
it is common to focus on the specification of the oscilloscope as this is
the item into which the major investment is made. However the performance of
the oscilloscope probe is just as important. If a poor oscilloscope probe is
used then the performance of the whole test equipment will be impaired.
The oscilloscope probe should provide a simple way of
presenting the signal on the board or item under test to the input of the
oscilloscope, and ideally being totally transparent. A typical probe will
consist of three major elements: probe tip; length of shielded wire; and a
compensation network.
The most common types of oscilloscope probe are the
passive probes. Of these there are two major types that are used namely a X1
and a X10. The X1 probe presents the signal as it is to the oscilloscope.
Normally the oscilloscope input impedance is 1 Mohm. However this can load
the device under test and distort the waveform. In addition to this tip
capacitance of a x! oscilloscope probe can be as high as 100 pF. To overcome
some of these problems and load the circuit under test less, a X10 probe can
be used. Providing an input impedance of 10 Mohms and a capacitance which
may be typically around 10pF, it will distort the waveform much less. Where
even greater levels of performance are required, active oscilloscope probes
may be sued. Having the active element very close to the tip of the
oscilloscope probe, these have much higher levels of input impedance and
lower levels of capacitance.
Remember to calibrate the oscilloscope probe
When using an oscilloscope, it is very easy to plug the
oscilloscope probe in and start to make measurements. Unfortunately
oscilloscope probes need to be calibrated before they are sued to ensure
that their response is flat. There is a built in calibrator on virtually
every oscilloscope for this purpose. It provides a square wave output, and
there is a small preset adjustor on the probe. With the oscilloscope probe
connected to the output of the calibrator the shape of the waveform
displayed on the screen should be adjusted until it is perfectly square. If
the high frequency response of the probe is down then the edges of the
square wave will be rounded. If it is up then the square wave edges will
show overshoot.
Although a simple adjustment, it is essential that it is
undertaken to ensure that the performance of the probe is correct.
Beware using ground clips for high speed measurements
Oscilloscope probes normally come with a clip that is
removable, and an earth or grounding clip that provides the earth return to
the circuit under test. The clip can normally be taken to a convenient earth
test point on the board. While this is perfectly adequate for most low
frequency measurements and tests, when high speed tests are to be made, it
is not satisfactory. The wire to the clip introduces inductance and this can
introduce ringing into the circuit and this affects the measurements.
The only way to overcome this problem is to significantly
reduce the length of any return path, and also reduce the length of wire to
the actual probe tip. This can be done by removing the ground clip and its
associated wire. In addition to this the clip on the centre of the probe can
also be removed. This leaves a short exposed coaxial style connection. The
centre connection or pin is for the signal and the surrounding connection is
for a direct connection to an earth plane. While this may present some
physical problems in making connections, it provides a far better level of
electrical performance.
Using the oscilloscope
Oscilloscopes are easy to use, and with the developments
in recent years oscilloscopes, along with many items of electronic test
equipment have become even more versatile. It is possible to use them to see
waveforms that might not have been at all easy even a few years ago. However
the basic knowledge of using oscilloscopes is only gained from years of
using them, but it is hoped that these few tips will enable them to be used
in a better way and to be able to gain more from them.
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