If you are involved in electronics, you probably used an
oscilloscope before. Today, owning an oscilloscope or replacing an existing oscilloscope
has become more affordable and desirable as the price-performance of
oscilloscope has improved significantly. There are many oscilloscopes out there
in the market. No matter who you talk to, everyone will have a favorite brand
or preference that they swear by. Selecting your handheld oscilloscope has to
do with what you are looking for in a scope. Do you want something that is
reliable and accurate, something that is rugged and durable, or something that
is cheap and simple to use? In this post, I’ll go through the key factors that
I think you should consider before making a buying decision.
Bandwidth
Every oscilloscope has a bandwidth-limited filter. This
filter will attenuate or remove frequencies higher than the bandwidth designed
in the trigger path, the acquisition path and the display path. When selecting
an oscilloscope, the rule of thumb is to select one with a bandwidth of at
least three times more than the highest frequency that has to be captured.
To illustrate this point, below is a 20MHz square wave
signal captured on a 20MHz and 200MHz oscilloscope respectively. Notice that
when the measurement was done using an oscilloscope that has a bandwidth of equal
or less than 3x the frequency of the signal, the shape of the signal is
distorted.
Figure 1.
A 20MHz square wave signal on a 20MHz bandwidth Scope (left) and 200MHz
bandwidth Scope (right).
Sample Rate and memory depth
Sampling rate and memory depth are interrelated. An oscilloscope
with deeper memory depth lets you see more details over a longer period of time
span compared to an oscilloscope with lesser memory. This is so because the
duration that an oscilloscope acquires a signal is determined by:
Acquisition time = Memory depth/ Sample rate
Let’s look at an example of two oscilloscopes that are sampling
at 2GSa/s, but one with a memory depth of 10k points and another with 2M
points. You would notice that the oscilloscope with a 10k points memory cannot
sustain its maximum sampling rate as the acquisition time increased above certain
value. On the other hand, the oscilloscope with a deeper memory can maintain
its sampling rate over a longer period of time. Figures below show a same
signal captured by oscilloscope with 10k points and 2M points respectively.
Figure 2.
Captured waveform using scope with 10k points of memory (left) and 2M points of
memory (right).
At a glance, the waveforms
appear to be identical on both oscilloscopes. However, when you zoom in on both
oscilloscopes, you would notice some details are missing on the oscilloscope
with 10k points of memory. This will result in you not catching a glitch or
overlooking an event that could be detrimental to the system you’re
troubleshooting or evaluating.
Figure 3.
A zoom in on waveform captured on 10k points of memory (left) and 2M points of
memory (right).
Number of channels
Handheld oscilloscopes often come in single, dual or 4
channels. Single channel oscilloscopes offer a simple and lower ownership cost.
However, dual and 4 channel oscilloscope are usually preferred as they let you
see two or more signals simultaneously. Apart from input channels, oscilloscopes
with a dedicated triggering channel offer an additional input terminal for
triggering the oscilloscope to start making measurements. Imagine if you are
measuring the phase delay of a differential output on an amplifier. You would
need a dual channel oscilloscope at the very least and probably start the
measurement through the oscilloscope’s triggering input.
Channel isolation
If your work involves making measurements on power control
circuits such as uninterrupted power supplies, motor controllers and industrial
equipment, floating measurements are not a strange to you. For those who are
not involved in these areas, a floating measurement is defined as a measurement
that is not ground referenced. When
using a handheld oscilloscope to make floating measurements, you must ensure
the oscilloscope has isolated channels. A handheld oscilloscope with isolated
channels does not connect the outer layer of the input channels to ground
directly and hence supports floating measurements. This reduces the risk of
accidental short circuits and offers a wide range of applications. In case
isolated channel’s oscilloscope is not available, be sure to use a differential
probe to make differential measurement. Using a standard probe by connecting
the ground connector of the standard probe to any of the floating points
essentially pulls down the probed point to the earth ground, which often causes
spikes or malfunctions on the circuit. Hence, a differential probe is used to
look at signals that are referenced to each other instead of earth ground and
to look at small signals in the presence of large DC offsets or other common
signals such as power line noise.
Safety Ratings
Safety rating is another important aspect to consider when
choosing a handheld oscilloscope. Electrical test and measurement equipment are
assigned to 4 different categories from I – IV. When it comes to selecting the
right handheld oscilloscope, it’s really depends on the environment where you
will be using your oscilloscope. If you
find yourself making measurements on distribution boards, circuit breakers, or
stationary motors with permanent connection to the fixed installation, then you
are working in CAT III environments and require tools that are certified under
this category or higher. For a list of Measurement
Category, you may refer to American National Standard Institute website (www.ansi.org) for IEC 61010-1 Standards or
simply jump to the Wikipedia at http://en.wikipedia.org/wiki/Measurement_category
for a quick reference.
Misc Factors
Perhaps after considering the hard specs, the next factor to
consider is the size and type of display on the handheld oscilloscope. Display
may not be high on your list of priorities, but it is the thing you are going
to stare at and displays all the details that the handheld oscilloscope captures,
so it deserves a little thought. A handheld
oscilloscope that offers a colorful display makes reading the waveform more
comfortable. A high-resolution display and wider viewing angle is important to
display more relevant information. For
example, when you zoom into a particular area of a waveform, the ability to
also view the overall waveform is a useful feature.
Free bundled software comes as a plus for handheld tools
such as oscilloscope. The software certainly will make your life a lot easier
when you need to transfer the waveform and data captured to your computer for
post analysis. Another plus point for handheld oscilloscopes is the additional
built-in capability such as DMM and logger functions. These functions allow you
to do more with one instrument and save you the hassle of carrying additional
instruments while on the job site.
These are the specs and features that you may want to
consider up front. But, I would suggest you to also look at the intangibles
such as warranty period, the level of support and the reputation of the brand
that would also differ from one brand to another. Some brands do offer loaner
units for you to evaluate. Try asking for one to test it out and do not forget
to evaluate the bundle software for the handheld oscilloscope if it does come
with one.
I’m sure that
whatever you decide to buy you will be happy with. If you need any help
specifying a handheld scope or have questions just list them in the comments
section below. Do share with us your experience with your handheld
oscilloscope. We would love to hear your opinions too!
No comments:
Post a Comment