Choosing an oscilloscope


With an overwhelming number of products on the market, each boasting
widely varying specifications, choosing the right oscilloscope can be a
daunting task. And with price tags of anything up to €60000, investing in
the correct product for your application is essential. This guide suggest
some criteria to consider before approaching a sales engineer. In addition,
it reveals some common pitfalls to avoid.

The basic function of an oscilloscope is to plot a graph of the voltage of
an electrical signal against time. Today's oscilloscopes are mostly digital
instruments and use analogue-to-digital converters to change the signal
voltage into digital sample points. Although some vendors offer analogue
products, digital storage oscilloscopes (DSOs) now have the lion's share
of the market. For customers reluctant to abandon tried-and-tested analogue
technology, many digital scopes can simulate an analogue display.

State-of-the-art DSOs are based on silicon germanium (SiGe) technology.
Although it was originally used only in high-end oscilloscopes, this
technology has gradually filtered down to low- and medium-end devices.
As such, the customer now benefits from a compact internal SiGe chip
that can accurately acquire complex signals. A SiGe chip also boasts
faster signal processing and the ability to acquire higher-frequency signals.
After speaking to several vendors, OLE has drawn up some questions you
should ask yourself before you make a purchase.

What range of frequencies do I need to measure?
This is the first and most important question to consider before buying
an oscilloscope. The key pieces of information to be aware of are the
range of frequencies that you are trying to measure, and whether the
signal is repetitive or single-shot.

The frequency of the signal that you want to measure determines the
"bandwidth" of the oscilloscope - in other words, the frequency range
that your oscilloscope is able to measure accurately. Without adequate
bandwidth, the oscilloscope will not be able to resolve high-frequency
changes. To calculate the bandwidth you require, vendors suggest that
you multiply the highest-frequency component of your signal by five.
For example, to measure an 800 MHz signal accurately requires an
instrument with a bandwidth of around 4 GHz.

The oscilloscope market can be divided into three different classes:
Low-end instruments that cover the 100-500 MHz range
Middle-of-the-range devices for 500 MHz to 2 GHz signals and
High-end DSOs that can measure from 2-7 GHz.

What sample rate do I require?
All DSOs contain a sampling clock that plots the voltage of the incoming
signal at a specific time. The sample rate is simply the number of data
points that the instrument acquires per second. To accurately reconstruct
a waveform, your DSO's sam-pie rate should ideally be around 3-5 times
the frequency of your signal. It is important to check whether the sampling
rate changes when signals are acquired simultaneously on two or more
input channels.

Low-end oscilloscopes are available with sample rates of around 100-500
megasampies per second (MS/s); mid-range products can sample 500 MS/s
- 2.5 GS/s; and the higher-end products are capable of sampling up to 20 GS/s.

What internal memory size do I need?
The internal memory size goes hand-in-hand with the sample rate and the
bandwidth; choosing the correct oscilloscope involves carefully balancing
all three quantities. DSOs store samples in an internal buffer memory. For
a given sampling rate, the size of the internal memory determines the
period over which the oscilloscope can acquire a signal before its memory
is full. A high sample rate is worthless if there is insufficient memory in
place to support it. Vendors typically offer between 100 kB and 1 MB per
channel, although products are available that offer 100 MB per channel
should your application demand it. Additional memory is an option that
can be discussed with a sales engineer.

Can I incorporate my own signal analysis software?
Vendors now offer so-called custom DSOs, which enable users to integrate
their own measurements, parameters or analysis into their oscilloscope.
Functions can be written in programs such as Visual Basic, Matlab, Mathcad
or Excel and seamlessly inserted into the instrument as a regular function
which can then be used in conjunction with other standard signal-analysis
features.

What resolution do I require?
Standard DSOs offer 8-bit resolution. This means that the vertical voltage
axis on the display is split into 256 intervals. For applications requiring
higher resolution, precision oscilloscopes offering 12-bit resolution or
greater are available.

How much should I pay?
An oscilloscope is an investment. It is important to consider your future
needs as well as the immediate requirements of your application.
Generally speaking, the higher the bandwidth of the oscilloscope, the
more expensive it is. The size of the screen also influences the overall
cost. While the majority of products on today's market will have a colour
LCD screen, sizes vary. Low-end scopes tend to have 5-6 inch diagonal
displays, increasing to 8.4 inch diagonal for medium-range oscilloscopes
and 10.4 inch diagonal for the high-end models.

Specifically, a basic, lightweight, portable commodity instrument will cost
around € 1000-5000. Low and medium-range oscilloscopes cost € 5000-25 000,
while top-of-the-range instruments can cost anything between € 25 000 and
€ 80,000.

Do I want an Integrated printer?
Some DSOs come complete with an integrated colour printer that plots
the signal stored in the buffer memory. Alternatively, a printer can be
attached to a DS0 and data can be transferred and printed across a
computer network. DSOs can also be controlled remotely across a network.

What warranties and support services do vendors offer?
The length of warranty offered is likely to vary from one firm to another.
While some vendors only offer a year's cover, others will offer up to three
years as standard.
It is also crucial to look at the length of time for which the vendor guarantees
support. The best deals can offer you support for up to seven years.
A long-lasting support agreement ensures the company has the parts
available to fix the product, even if it is no longer available to buy.

For more information,please contact...
LeCroy at
www.lecroy.com
Tektronix at
www.tektronix.com
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