Oscilloscope Basics

tips for oscilloscope selectionThe oscilloscope is one of the most important tools used in the design, repair and maintenance of electronics. Nevertheless the selection of the right type and configuration for your application is not an easy task. We listed some important points to consider.

1. How do you like to use your oscilloscope? There are different form factors available, such as standalone benchtop instruments, handheld instruments for mobile use, PC-based instruments (needs a PC or controller) and PXI-based instruments (to be integrated into a PXI rack). So first select the form factor.

2. How many analog signals do you need to measure at once? Most oscilloscopes have 2 or 4 analog channels, some have 8 analog channels. Some oscilloscope allow a synchronization of multiple instruments, so more than 8 channels are possible.

3. Do you plan to acquire also digital signals? How many digital signals do you want to watch at once? This determines the number of digital channels you need. Some oscilloscope allow to convert an analog channel in 8 or more digital channels.

4. What is the maximum frequency of the signals you like to measure? Be aware that if your input signal is not a pure sine wave it also contains harmonics. A rule of thumb is that the oscilloscope should have a bandwidth 4 to 5 times higher than the maximum frequency you wish to measure.

5. The same rule applies for the sample rate of the oscilloscope (specified in MS/s = megasamples per second). This should be 4 to 5 times the maximum frequency you want to measure. You should be aware that for scopes two different sample rates are specified: real time (i.e. single shot) and repetitive. This is important if your signal occurs only once and you have to acquire it in one shot. Otherwise if the signal is repeated several times, your scope can take multiple samples of that signal over a certain time with a lower sample rate. Some scopes use interleaving (i.e. two channels or more are combined and used alternately) to achieve higher sampling rates.

6. How long is your signal you have to acquire in one shot? Modern Oscilloscopes store the captured samples in an acquisition memory. This means that for a given sampling rate, the size of this memory determines how long it can capture a signal for before the memory is full. Example: In a single shot acquisition with a sample rate of 5 MS/s a scope with a memory of 1 Mpoints can store a signal with a maximum duration of 200ms (acqisition duration = memory depth / sample rate).

7. Which accuracy and resolution do you need? Normal oscilloscope have a resolution of 8 bits, which means the voltage range is divided into 256 steps. In the 1V range the resolution is therefore 1V/256 = 4mV. Maximum theoretical accuracy is +/-1 bit which equals in this range +/-4 mV (practically the accuracy of the complete analog frontend including probe has to be considered!). There are also oscilloscopes with a resolution of 12 or 16 bit available. In the 1V range the resolution of a 12 bit instrument is 1V/4096 = 244uV.

8. Modern oscilloscopes offer a lot of additional functionality - some of these are included free of charge others are available as options and can probably also be retrofitted. These functions may include: arbitrary waveform generator, spectrum analyzer, protocol analyzer, logic analyzer, datalogging over a long time, advanced trigger functions, synchronization of multiple instruments, math functions (to do complex math operations), battery operation, and more.

9. Do you like to connect your scope to a PC or an other controller or should it be integrated in a test system? Then your scope should have an appropriate interface, such as USB, Ethernet or GPIB.

10. Don't forget the probes! A probe is an essential part of the signal chain and has a great influence on what you are measuring. There are different types of probes available, such as passive probes, active probes, differential probes, and current probes. Criteria for the selection of the best probe are e.g. frequency range, voltage range, isolation voltage, input impedance, attenuation and prbe type (single ended, differential, current probe,...).