Oscilloscopes: Your questions, our answers.

The required bandwidth is determined by the highest signal frequency to be measured. If the bandwidth is set too low, the oscilloscope cannot detect any high-frequency changes. The amplitude may be distorted, and the edges are difficult to see, so that signal details are lost. When analysing analogue signals, the specified oscilloscope bandwidth should be at least three times as large as the highest sine wave frequency. When analysing digital signals, we recommend a bandwidth of five times the highest clock frequency to be measured. 

Attention: The measuring system consists not only of the oscilloscope, but usually also of a probe. Both together form the relevant system bandwidth.

Most oscilloscopes can measure several signals simultaneously and display them on the screen. Each signal is fed into a separate channel. Models with 2, 4, 6 or 8 channels are common. Mixed signal oscilloscopes (MSO) also have 8 or 16 digital inputs for the display and analysis of time-correlated analogue and digital signals.

In an oscilloscope, the sample rate refers to the number of measuring points per unit of time with which the oscilloscope's A/D converter converts the analogue signal input values into digital data (samples per second, Sa/s). The higher the sample rate, the better the temporal resolution and thus the details recognisable in the signal curve. For oscilloscopes with more than one channel, the sample rate may be reduced if several channels are used. With a longer time base setting, the sample rate is reduced according to the available memory depth.

The signal acquisition rate indicates the speed at which the measuring device acquires a signal (specified in waveforms per second, wfms/s). The number of detections depends, among other things, on the set time resolution (seconds per scale division) and the memory depth. An oscilloscope requires a certain amount of time to process the data after each sampling cycle. The shorter this processing time is, the faster the oscilloscope resamples a signal and can therefore detect sporadic signal anomalies more quickly, for example. With multi-channel oscilloscopes, the signal acquisition rate may be reduced if several channels are in use.

The number of scans is limited by the memory depth: To be able to utilise the maximum sampling rate for a longer time range, you need a large memory. The memory requirement is calculated using a simple formula: storage depth = recording duration x sampling rate. However, this only applies if each input channel of the oscilloscope has the same memory depth. For sporadic signal events, it is advantageous to have a memory as deep as possible and a high signal acquisition rate to be able to store as many signal sequences as possible.

The relationship between bandwidth and rise time can be established using a constant "k": rise time = k / bandwidth, where "k" depends on the oscilloscope. For oscilloscopes with a bandwidth > 1 GHz, a value between 0.4 and 0.45 typically applies for k. For a bandwidth of < 1 GHz, k = 0.35.

The vertical resolution of an oscilloscope indicates the accuracy with which the A/D converter converts an input voltage into digital values. Usually, 8 to 16 bits are used. The effective amplitude resolution of some oscilloscopes can be improved using digital calculation methods, for example in high-resolution acquisition mode (Hi-Res mode).

The vertical sensitivity describes how much the oscilloscope can amplify a weak input signal before it is displayed on the screen. It is usually specified in millivolts per scale division (mV/div). For example, if the vertical sensitivity is set to 5 mV/div, this means that each vertical division on the screen represents a voltage of 5 millivolts. The correct setting of the vertical sensitivity is important to display the signal precisely on the screen. If the sensitivity is too high, the signal may be overdriven and if it is too low, the display may be too weak or inaccurate.

Your oscilloscope should also fulfil future measurement requirements to be economical. Modern devices allow, for example, the number of channels, bandwidth, or memory depth to be expanded. You can also add application-specific measurement functions or increase the performance and functionality of the device with software options, probes, application modules and corresponding accessories.

Special current probes are available for measuring current with an oscilloscope. A so-called Rogowski coil or a Rogowski current transformer is used for alternating current measurement. Rogowski current transformers are robust and enable AC measurements up to 100 kA. Oscilloscope current clamps are ideal for direct current measurements. These are generally suitable for currents up to 2 kA. Alternating current measurement is also possible, but only up to 100 MHz. Oscilloscope current clamps are available as accessories.

Well-known oscilloscope manufacturers include:

• GW Instek
• Keysight
• NI (National Instruments)
• Pico
• Rohde&Schwarz
• Tektronix

The functions of a digital oscilloscope are based on dedicated software. Some manufacturers provide software free of charge, while others offer highly complex and individually programmable solutions. Stand-alone oscilloscopes are often supplied with sample programmes, USB oscilloscopes require special software. The software solutions differ depending on the design, manufacturer, measuring task and product.

A digital oscilloscope is usually made up of the following basic elements:

• Probe
• Analogue input channel or channels
• AC/DC coupling
• Attenuator
• Preamplifier
• Scanner
• Trigger unit
• A/D converter
• Processor
• Monitor for signal display

The criteria for purchasing an oscilloscope include technical requirements and specifications as well as budgetary conditions and company-specific purchasing conditions. Your measurement application defines the technical properties of the oscilloscope. Depending on the budget, compromises may have to be made when purchasing. However, these compromises should not affect the decisive parameters. The 12 most important purchase criteria for an oscilloscope are:

• Bandwidth
• Number of channels
• Vertical resolution
• Sample rate
• Rise times
• Compatibility (probes etc.)
• Trigger functions
• Memory depth
• Automated measurement functions
• Simple operation
• Expansion options
• Interfaces
  

The measurement application determines how many channels an oscilloscope requires. As a rule, 2 to 4 analogue channels are sufficient for simple signal analysis. For very complex applications, 8 input channels can be useful, for example to monitor several sensors and actuators simultaneously. For example, 8-channel oscilloscopes are used to analyse the alternating current of electric motors by measuring the current and voltage on the three phases (= 6 channels) and the remaining 2 channels are available for trigger conditions. An oscilloscope requires additional digital input channels for the validation of digital and mixed signals.

A high level of user-friendliness is important for several reasons:

• Efficiency: A high level of user-friendliness enables the user to access the desired functions and settings quickly and efficiently. This saves time and makes it easier to carry out measurements. 
• Reliability of results: An intuitive user interface reduces the risk of operating and measurement errors. Clear instructions, a well-organised menu structure and well-placed controls help you to make the right settings and measurements. 
• Productivity: An easy-to-understand oscilloscope makes it easier for new users to quickly familiarise themselves with the device and work productively. 
• Economic efficiency: Modern oscilloscopes offer a wide range of functions for sometimes complex measurement applications. A user-friendly design enables the user to understand these functions, use them effectively and fully utilise the performance of the device.
• Ease of use: The arrangement of the operating elements and the design of the screen influence the comfort of operation, even over longer periods of time.

A distinction is made between a real-time oscilloscope and a sampling oscilloscope (equivalent time oscilloscope). With some oscilloscopes, the sampling method can also be selected. During sampling, part of the input signal is converted into electrical values. The value of the displayed sampling points corresponds to the amplitude of the input signal at the time of sampling. A digital oscilloscope displays a series of sampling points with the measured amplitude on the y-axis and the time on the x-axis. This reconstructs the input signal.

A real-time oscilloscope can display the current values present at the A/D converter. Real-time sampling is ideal for signals whose frequency range is less than half the maximum sampling rate of the oscilloscope. The real-time oscilloscope can also be used to record one-off or sporadically occurring, transient signals. This requires a very high sampling rate. Real-time sampling generally requires a high memory depth to store the signals after digitisation.

A sampling oscilloscope requires a recurring signal to display it after several sampling runs. Equivalent time sampling thus enables the acquisition of signals whose frequency components are significantly higher than the sampling rate of the oscilloscope. Repetitive signals are reconstructed by capturing a small amount of information with each repetition.

Digital oscilloscopes are mainly divided into Digital Storage Oscilloscopes (DSO), Digital Phosphor Oscilloscopes (DPO), Mixed Signal Oscilloscopes (MSO), Mixed Domain Oscilloscopes (MDO) and Digital Sampling Oscilloscopes. USB oscilloscopes, sampler-extended real-time oscilloscopes and modular PXI oscilloscopes are also widely used, especially for laboratory applications.

The digital storage oscilloscope enables the recording and visualisation of one-off events, so-called transients. The signals can be displayed and analysed on the oscilloscope itself or on an external PC. A DSO offers permanent signal storage and comprehensive signal processing.

Digital phosphor oscilloscopes use high signal acquisition rates to display and analyse a signal. With their capabilities, signals can be reconstructed precisely. Compared to a DSO, DPOs are better able to recognise transient events in digital systems, e. g. runt pulses, glitches or edge errors, and also offer extended analysis options.

The mixed-signal oscilloscope has digital input channels in addition to the analogue channels. It combines the measurement and analysis functions of a DPO with the basic functions of a 16-channel logic analyser, including the decoding and triggering of parallel and serial bus protocols. The MSO is suitable for fast troubleshooting in digital circuits as it enables both analogue and digital signal display.

A mixed-domain oscilloscope combines several device functions, including those of a spectrum analyser and function generator. It therefore enables the time-correlated display and synchronisation of signals from the digital, analogue and RF ranges.

In a digital sampling oscilloscope, the positions of the attenuator or amplifier and sampling bridge are reversed compared to the DSO/DPO architecture. The input signal is sampled before attenuation/amplification and converted to a lower frequency. This results in a very high bandwidth. The disadvantage is a limited dynamic range. In addition, no protective diodes can be used before the sampling bridge, as this would limit the bandwidth. This reduces the input voltage of a sampling oscilloscope to around 3 V compared to 500 V for other types of oscilloscopes.

USB oscilloscopes require an USB connection to an external PC and a corresponding software application to control the oscilloscope and display measurement curves. The devices are particularly light and compact and are therefore also suitable for mobile use. USB sampling oscilloscopes and USB real-time oscilloscopes are available on the market.

SXRTO oscilloscopes combine the advantages of real-time and equivalent time sampling. Its architecture utilises two independent cyclic processes: If a sampling sequence is triggered by a trigger event, the oscilloscope takes as many samples as possible. The oscilloscope measures the time interval between the trigger time and the first sampling time. All subsequent sampling times are determined by the internal sampling generator. If a trigger event is repeated, the sequence starts again. As the occurrence of the trigger event and the sampling generator are not time-correlated, a sampling sequence takes place at different times in relation to the trigger. By measuring the time intervals, the individual sampling values can be precisely assigned to each sampling sequence on the time axis. This type of signal sampling (= random sampling) leads to a very high sampling rate.

PXI (PCI eXtensions for Instrumentation) is an industry standard in measurement and automation technology. A PXI system consists of three hardware components: Chassis, controller and peripheral input/output modules for data acquisition, control, and regulation, including oscilloscopes. The oscilloscope modules can be integrated into the PXI platform and replace conventional desktop devices. PXI oscilloscopes are flexible, software-based measuring devices. They offer users a particularly powerful and scalable test system.