How to choose an oscilloscope
Choosing a new oscilloscope can be a difficult task with so many different models available. Here are some pointers to help you make an informed decision and know what to look for.
Before you rush out and buy your new oscilloscope, think about your needs:
- Where are you going to use the scope.
- How many signals are you going to measure at once.
- What are the amplitudes of the signals you will typically measure.
- What is your highest frequency needed.
- Do you need to measure repetitive or single shot signals.
- Do you need a FFT (spectrum analyser) function (signals in the frequency domain).
Analog vs. Digital Oscilloscopes
You might still love the analog scopes, but in today’s digital world their features cannot match the modern digital storage oscilloscope (DSO). Moreover, the analog models still available make use of old technology with often very limited performance. The availability of spares for second hand analog scopes might also be a problem.
The advantages of a Digital Oscilloscope are obvious:
- Portability and small size
- Higher bandwidth
- Single shot measurement possible
- User-friendly interface
- On-screen measurements
- Colour display
- Storage and printing capability
Digital scopes also provide the possibility for high-speed data acquisition and to be integrated into ATE (automatic testing equipment).
What to consider in your choice of oscilloscope:
This is the maximum frequency of signals that can pass through the front-end amplifiers of the scope. The analogue bandwidth of your oscilloscope must be higher than the max frequency that you need to measure in real time. Keep in mind that the input signal will never be a pure sine wave, but will contain higher frequency harmonics.
Also useful to know is that most oscilloscope manufacturers allow a 29% error of the input in the displayed trace.
Additionally, sometimes the quoted bandwidth is not available for all voltage ranges, so read the technical specifications carefully.
Considering the above, ideally you should try and purchase an oscilloscope with a 5 times higher bandwidth than the highest frequency signal you need to measure. Economics will determine what you can afford, so you might have to compromise on bandwidth.
The Nyquist criterion states that the sampling rate must be at least twice the maximum frequency that you want to measure: for a spectrum analyser this may be true, but for a scope you require at least 5 samples to accurately reconstruct a waveform.
There is a difference between real time and equivalent time sampling (ETS), also called repetitive sampling. Because ETS mode works by building up the waveform from successive acquisitions, ETS only works if the signal is stable and repetitive – at least 5 samples.
Note that some scopes have different sampling rates depending on the number of channels in use. In single-channel mode the sampling rate could typically be twice than in dual channel mode.
Memory depth is one of the most important, but least understood aspects of a DSO. The size of the buffer memory determines how long it can capture a signal before the memory is full. It also follows how important the relationship between memory depth and sampling rate is. An oscilloscope with a high sampling rate but small memory will only be able to use its full sampling rate on the top few time bases.
So, for example, in a scope that is capable of sampling at 100 MSa/s, a 1k buffer memory would limit the sampling rate to 5 MSa/s (1k / 200µs) even though the scope is capable of sampling at 100 MSa/s.
Resolution and Accuracy
Most digital oscilloscopes offer 8 bit resolution, and can detect at best 0.4% signal change, which is adequate for viewing digital signals, but not for viewing analogue signals, especially when using the FFT function (spectrum analysis).
For applications such as audio, noise, vibration and monitoring sensors (temperature, current, pressure) an 8 bit oscilloscope is often not suitable and you should consider 12 or 16 bit alternatives.
Accuracy is generally not thought of as very important, as you can make measurements within 3 – 5% accuracy with a DSO. If you need better accuracy a multimeter will do the job. Of course a higher resolution, or precision scope will offer 1% or better accuracy.
A scope’s trigger function synchronises the horizontal sweep at the correct point of its signal, which is essential for clear signal characterisation. You can stabilise repetitive waveforms and capture single-shot waveforms with your trigger controls. The basic trigger options of all DSO’s are source, level, slope, pre- or post trigger. Scopes differ in the more advanced trigger functions. Whether you will need these depends on the signals you need to measure. For digital signals pulse trigger is useful, and for tracking intermittent faults an auto recording function is a great help.
Application specific triggers like, disk drive testing, most often come at an additional cost in the form of a firmware or software upgrade.
Input Ranges and Oscilloscope Probes
A typical oscilloscope offers selectable full scale input ranges from ±50 mV to ±50 V. For higher voltages 10:1 and 100:1 attenuating oscilloscope probes are used. More important is how small the voltage range is for the signals that you want to measure. If you regularly measure small signals (less than 50 mV), consider a scope with a 12 or 16 bit resolution. A 16 bit scope has 256 times the vertical resolution of an 8 bit scope, so it is possible to zoom in on millivolt and microvolt level signals.
You should preferably use the probes that match or exceed the bandwidth of your scope. Wherever possible, use the 10:1 attenuation setting on your probes as this minimises loading on the circuit under test and increases the overload protection should you accidentally connect to a high voltage. For high voltages the safest is to use a differential isolating or high-voltage probe, which does not come standard with oscilloscopes.
PC-based or USB Oscilloscopes
USB oscilloscopes are becoming increasingly popular as they are less expensive than the traditional bench-top scope. By utilising a computer, they offer the advantages of a large colour display, fast processor, disk drives and keyboard use. Another big advantage is the ability to seamlessly export data to spreadsheets.
External PC-scopes that connects to a PC is the most common, as internal PC-oscilloscopes, in the form of a plug-in PCI-card, has the problem of noise from the PC, and also does not offer the possibility of portability between different computers.
External PC-scopes overcome the noise-issue by keeping all the analog electronics outside the PC. They can be used with either desktop or laptop PC’s.
When choosing between different oscilloscopes, check the following:
Get a demo or watch some user videos on the different options you are considering.
If getting a demo, make sure the demo is done with actual signals that you want to measure, and not just signals that show the scope in a good light.
Check what is included in the cost – software (also PC-software for bench-top scopes), scope probes, cables and also, importantly, upgrades. These can all add up.
Look for a money-back guarantee from the vendor.
Check the length of the warranty, and also the vendor policy regarding a lending unit while repairs are done.
Find independent user reviews on the internet.
In order of priority, consider Bandwidth, Sampling rate (real-time and/or equivalent time) and then Memory depth.
Note: bandwidth and sampling rate are not upgradeable options on most DSOs, so once you’ve bought the oscilloscope, you are stuck with your choice.