Sample rate refers to how frequently a digital oscilloscope takes a sample of the signal. The faster the oscilloscope samples, the lesser the details lost while reconstructing the signal. In order to accurately reconstruct a signal and avoid aliasing, the Nyquist theorem states that the signal must be sampled at least twice as fast as its highest-frequency component. This theorem assumes an infinite record length and a continuous signal, but no oscilloscope can offer infinite record length. Therefore sampling at only twice the rate of highest-frequency component will not suffice.
The sampling rate that you require to accurately read your signal will majorly depend on the method used for reconstructing the signal—also called interpolation. For accurate reconstruction using sin(x)/x interpolation, sample rate should be at least 2.5 times the highest-frequency component of your signal. Using linear interpolation, the sample rate should be at least ten times the highest-frequency signal component.
The oscilloscope cannot store infinite number of samples as assumed by Nyquist equation. Record length determines the time that can be captured by each channel of the oscilloscope:
Time captured = Record length/Sample rate
Normally, entry-level oscilloscopes come with 2 to 2.5k points, which is more than enough. In general, the greater the record length, the better. To capture and debug serial bus, you need record length as high as 1M points.
External triggers allow you to stabilize repetitive waveforms and make them appear static on the display by continuously displaying the same section of the input signal. Buy an oscilloscope that offers a wider range of triggering.
Generally, oscilloscopes are available with edge- and pulse-width trigger. Advance triggering options like A and B sequence, video, communication (CAN, SPI, etc) and logic triggering can help you debug faster.
Waveform capture rate
Sample rate indicates how fast an oscilloscope samples the input signal within one waveform, but the waveform capture rate refers to how quickly an oscilloscope captures a waveform. An oscilloscope with higher waveform capture rate captures fast transients better.
Number of channels
While deciding the number of channels, look at your future needs too. Increase in the number of channels adds a lot to the cost. Beware of those manufacturers who offer more channels by compromising the sample rate.
Probe specifications are as important as the oscilloscope specifications because all measurements will start from the probes. The probe’s bandwidth should match the scope’s bandwidth. Here also the five times thumb rule will work perfectly.
The probe will be in direct contact with the circuit, so the circuit should not overload it. Otherwise, the measurement will not be correct. Resistive loading greater than 10 mega-ohms and capacitive loading less than 10 pico-farads is acceptable.
Specifications related to measurement convenience
Easy-to-use user interface, advanced automated measurements and convenient operation can really speed up your work. Below are the specifications that you need to check for measurement convenience.
Automated waveform measurements
Automated waveform measurement function helps you get numerical readings without any hassles. Most basic digital oscilloscopes provide this function where you can choose parameters like amplitude, frequency and time period, or have all of them together on the screen. Now if you measure any waveform, the display will show the graph along with numerical values of the parameters that you have selected.
This function provides automatic calculations for mathematical functions like integration, differentiation, FFT and logarithm.
The oscilloscope should be easy to use with an intuitive menu. This helps a beginner to immediately get started with his work. With dedicated knobs for frequently used functions, reading the waveforms becomes handy.
Convenience of waveform navigation and analysis is highly important as, at the end of the day, you would want to navigate the waveform that you have captured, and would also want to analyse it to extract useful information. Look for options like zoom and pan, interesting mark points and search options. Such features reduce the time to debug applications to a great extent.
In-built automated test
In-built automated tests like pass/fail test are sometimes very useful to quickly identify a problem. Such tests monitor your system continuously and inform about an error through a message on the display or some sound. Some oscilloscopes also log the history of these test results, which can be very handy to analyse the errors that occurred in the system.
Connectivity of an oscilloscope is highly important as you will always need to analyse and share the measurements. It can also help a lot while documenting your work. A lot of interfaces like GPIB, Ethernet, RS-232 and USB are available in oscilloscopes. Some advanced oscilloscopes also let you edit your documents and print them, or send via the Internet.
Specifications related to physical construction
Buy an oscilloscope that suits your work environment. For harsh environments, you need a rugged scope. Below-mentioned specifications can help you judge an oscilloscope for your work environment.
Smaller displays are very uncomfortable. It becomes very difficult to analyse some high-frequency signals or bus on a small display. Look for an oscilloscope that has comparatively bigger display size. Colour displays are pleasing to the eyes and make it easy to distinguish signals by representing them in different colours. Entry-level oscilloscopes are generally available with a 14.4cm (5.7-inch) display.
Dimensions and weight
Dimensions and weight should be carefully selected depending on your work environment. If you work in the field, you will not be able to carry around a big oscilloscope. Taking measurements with it will also be very uncomfortable. So look for a small-size and light-weight oscilloscope.
Feel interested? Check out other posts on oscilloscopes.