For many engineers, choosing a new oscilloscope can be daunting — there are hundreds of different models to choose from with widely varying costs and specifications. This article guides you through the maze of considerations and will hopefully help you avoid making what could prove to be an expensive mistake.
First Things First
The first step in choosing an oscilloscope is not to look at adverts or scope specifications, rather it is to invest some time thinking about what you will use it for and where.
- Where will the scope be used (on the bench, at a customer’s site, under the hood of a car)?
- How many signals are needed to measure at once?
- What are the maximum and minimum amplitudes of signals that are needed to measure?
- What is the highest frequency of signal needed to measure?
- Are the signals repetitive or single shot?
- How are the signals to be viewed in the frequency domain (spectrum analysis) as well as the time domain?
Armed with the above knowledge, a user can begin to consider what oscilloscope will be best for the applications.
Analog vs. Digital
The focus of this article is on Digital Storage Oscilloscopes (DSOs) as they represent the majority of new oscilloscopes purchased today. Before describing what to look for in a digital oscilloscope, it is necessary to begin by at least touching on analog.
Most electronics engineers will have used an analog scope at some time and will be familiar with its layout and operation. In fact, many people purchasing oscilloscopes today are replacing analog with digital.
Although there are still some engineers who love the look-and-feel (let alone the warmth) of analog scopes, they have few, if any, features that cannot be surpassed by a DSO.
If you are still tempted by an analog scope you will find your choice limited. Only a few manufacturers still make analog scopes; some of the models still on sale are based on rather old technology and often have very limited performance.
Buying a second hand analog scope may initially seem like good economic sense but, before doing so, check for the availability of spares, as high repair costs can make the purchase a false economy.
There are other criteria that have added weight to the analog vs. digital debate. DSOs:
- Are small and portable
- Have the highest bandwidths
- Have single shot ability
- Have color displays
- Provide on-screen measurements
- Have simple user interface
- Provide storage and printing
Modern DSOs, with their PC connectivity, can also be fully integrated into Automatic Test Equipment (ATE) systems. In addition, the DSO is often used as the front-end of a high speed data acquisition system, making the cost per channel much more economically viable.
The first feature to consider is bandwidth. This can be defined as the maximum frequency of signal that can pass through the front–end amplifiers. It therefore follows that the analogue bandwidth of your scope must be higher than the maximum frequency that you wish to measure (real time).
Bandwidth alone is not enough to ensure that a DSO can accurately capture a high frequency signal. The goal of scope manufacturers is to achieve a specific type of frequency response with their designs. This response is known as the Maximally Flat Envelope Delay (MFED). A frequency response of this type delivers excellent pulse fidelity with minimum overshoot, undershoot and ringing. However, since a DSO is composed of amplifiers, attenuators, ADCs, interconnects, and relays, MFED response is a goal that can only be approached and never met completely.
It is worth noting that most scope manufacturers define the bandwidth as the frequency at which a sine wave input signal will be attenuated to 71% of its true amplitude (-3 dB point). Or, to put it another way, they allow the displayed trace to be 29% in error of the input before calling it a day.
Remember also that, if your input signal is not a pure sine wave, it will contain higher frequency harmonics. For example, a 20 MHz pure square wave viewed on a 20 MHz bandwidth scope will be displayed as an attenuated and distorted waveform. As a rule of thumb, try to purchase a scope with a bandwidth five times higher than the maximum frequency signal you wish to measure. Unfortunately, high bandwidth scopes are expensive, so you may have to compromise here.
On some scopes, the quoted bandwidth is not available on all voltage ranges, so check the data sheet carefully.
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