How to select the right handheld oscilloscope

Aside from characterizing microelectronics to troubleshooting process controls, multi-industrial automation, automotive-service industries and facility maintenance, handheld oscilloscopes can also offer the performance of bench oscilloscopes in a mobile and rugged form factor.

Generally, handheld oscilloscopes integrate test tools specifically designed for field use where the conditions are dangerous, rigorous and precarious. It has to be noted, however, that handheld oscilloscopes are not designed equal. Hence, it is advantageous for users to compare some of its important specifications, such as bandwidth, sample rate and memory depth, before deciding on which one to purchase.

Users should also take into consideration the number of channels, channel isolation capability and safety ratings of the handheld oscilloscope.

Most oscilloscope users erroneously believe that bandwidth determines the highest frequency that can be captured. However, oscilloscope’s bandwidth refers to the frequency that the input signal is attenuated by three dB, which translates to around -30 percent amplitude error. This implies that signals cannot be accurately captured near the oscilloscope’s bandwidth.

When choosing a handheld oscilloscope, users should pick one with a bandwidth that is five times more than the highest frequency that has to be captured.

Another mistaken assumption is that an oscilloscope with a higher sample rate is better than one with a lower sample rate. In truth, an oscilloscope offering higher sample rate does not necessarily offer any extra benefit when compared to an oscilloscope with a lower sample rate.

For a faithful reproduction of a signal on an oscilloscope, Nyquist’s sampling theorem provides that the oscilloscope’s sample rate should be more than double the signal’s highest frequency. This means that if a 40 MHz signal is to be captured, the sample rate of the oscilloscope should be at least 80 MSa/s.

Captured data are stored in the oscilloscope’s waveform buffer and the size of its buffer memory is known as the memory depth. An oscilloscope with deep memory allows users to keep a higher sample rate for a longer period of time.

The memory buffer is quickly filled at high sample rates. Thus, an oscilloscope with a limited memory depth and a high sampling rate of up to two GSa/s can collect data only if it reduces its sample rate. This will prevent the oscilloscope from capturing waveform accurately since the sample points are placed too far apart from each other. However, an oscilloscope that offers deeper memory can sustain its maximum sampling rate, allowing it to accurately capture the waveform.

Oscilloscopes are available in two-channel and four-channel varieties. Each channel allows users to record and measure one signal at a time. In deciding which oscilloscope to purchase, a user should determine the number of signals the user wanted to display on the oscilloscope.

Oscilloscopes with four independently isolated channels are generally used when simultaneously viewing three or more signals, such as when troubleshooting three-phase applications like industrial motors, high-power inverters and variable-frequency motor drives. These kinds of oscilloscopes are rarely used and are usually more expensive. Most of the time, users will only need two channels.