A guide to oscilloscope selection

Before deciding which oscilloscope they need for the task, there are a number of key selection criteria that users need to take into account. Christian Rau, application engineer at Rigol Technologies EU in Puchheim near Munich, guides us through the facts and features…

Those trying to find an oscilloscope for a specific task will not only have to select from a wide range of providers, but they will also be faced with countless performance features. Examples include bandwidths from 50MHz to 1GHz or even 4GHz; sampling rates between 500MS/sec and 5Gs/sec or more; resolutions of eight bits or 12 bits; minimum input bandwidths between 2mV/div to 500µV/div, and so on.

Today’s oscilloscopes do much more than just providing a single signal. Instead, they offer many functions specifically targeted at complex signals and analyses. Other features such as the measurement rate, memory depth and math functions are designed to make the operator’s task a lot easier.

So, let us take a look into oscilloscope facts, new details and special features.

Bandwidth

The bandwidth specification of an oscilloscope has a significant impact on the quality of the measurement result. The measurement error should also be as low as possible.

It is defined as the frequency at the 3dB point of a sine wave with a given amplitude (e.g. 1Vpp). If the frequency of the sine wave is increased while keeping the amplitude constant, the measured amplitude will decrease. Therefore, the bandwidth of the instrument is defined as the frequency at the point where the amplitude has dropped by 3dB – so, an oscilloscope with a bandwidth of 100MHz will acquire a 100MHz sine wave with an amplitude of 1Vpp at only 0.7Vpp. This equates to an error of 30%. In order to significantly reduce this error, remember that one third of the bandwidth corresponds to an error of 5%, one fifth corresponds to 3% etc. You should therefore use a 300MHz or even 500MHz oscilloscope to measure a 100MHz signal.

Sampling rate

The sampling rate is the second key feature of a digital storage oscilloscope, and it should be 2.5 or three times higher than the analogue signal bandwidth.

According to the Nyquist/Shannon sampling theorem, the signal to be digitized may not include any frequency components higher than half the sampling frequency. In addition, the signal must be sampled at constant time intervals. Alias effects will occur if any of these conditions is not fulfilled.

Therefore, the sampling rate of an oscilloscope used for measuring ‘unknown’ signals should be as high as possible. The sampling rate is particularly important for measuring singular, ‘one-shot’ signals so that no relevant information is missed. Although this is not very critical for repetitive signals, oscilloscopes offer special features for them as well.

Memory depth

The memory depth of an oscilloscope also has a fundamental impact on the quality of the measurement results. The input signal is digitized by the A/D converter, and the resulting data stream is loaded into the oscilloscope’s high-speed memory. Although many users believe that the oscilloscope’s maximum sampling rate applies to all horizontal deflexion settings, the device’s memory space is limited. Therefore, the sampling rate must be reduced with increasing sampling periods, making the sampling process quite sluggish for long timeframes. The larger the memory capacity of an oscilloscope, the longer it can operate at its maximum sampling rate without losing any relevant signal information.

Users must therefore check the impact of the horizontal deflexion setting on the sampling rate. The required memory depth can be calculated by multiplying the sampling period by the sampling rate. Long sampling periods and high timing resolutions require more memory capacity.

Memory depths of more than 100 million points are required to support large amounts of data (see Figure 1, above right). The integrated zoom function enables the user to zoom in on the details of a signal for a closer evaluation. An oscilloscope is supposed to be a valuable tool for this task.

The real-time record, replay and analysis functions offered by all Rigol oscilloscopes can be used to straightforwardly track down any transient spurious pulses outside a specific mask or to detect repetitive glitches. These detection features immediately reveal when and where these transients occurred within a signal. The details can then be inspected using the zoom function, while the statistics feature can be used to evaluate and measure the event. Error evaluation is facilitated by a colour-coded percentage scale (see Figure 2,  right).

Current high-performance oscilloscopes provide convenient evaluation features offering comprehensive mathematical functions and the capability to enter complex mathematical formulae such as  integrals, differentials, logarithms, etc.

Channel count

Selecting the channel count of an oscilloscope is often quite easy because two or four channels are generally available. It must be noted, however, that the sampling rate is divided by two in many two-channel oscilloscopes and divided by four in many four-channel devices. If two separate ADCs

are available, the full sampling rate can be used. In a four-channel device, two channels can be operated at the full sampling rate (e. g. channels one and three), while only half of the full sampling rate will be supported by channels two and four. There is also an impact on the memory depth because only half of the memory will be available if the channel count is doubled.

Capture rate

The capture rate of an oscilloscope defines the frequency used by the device to measure and display the results. The higher the frequency, the higher the probability that transient events including glitches will be captured.

A very deep memory will enable users to capture extensive data streams while the analysis tasks, including the decoding of serial busses or other statistical operations, can be done later.

Serial bus decode

Protocol applications benefit from a large oscilloscope memory.

Serial busses including I²C, SPI, CAN, FlexRay and RS-232 are now increasingly used in applications and technical/electronic devices. Although all major oscilloscope vendors offer protocol decoding functions and the capability to trigger on protocol events, the performance of these programs differs widely.

Those who have had to manually decode a protocol will appreciate the value of a real-time decoding function. Protocol applications are typically offered as software options which can be ordered with the oscilloscope or retrofitted at a later date. Datasheets usually provide detailed information on those supported by a specific oscilloscope family.

In case of the SPI protocol, it is interesting to know the maximum data rate supported by the oscilloscope or whether the support covers two, three- or four-wire SPI or only a subset. In an I²C context, one should also know if the protocol having the read/write bit in the address field is also supported. 

It is often necessary to decode more than one serial bus at the same time. But, how can the oscilloscope in question be configured for this use case? And, how can one switch between busses and select one of them as the trigger source?

Apart from the decoding features, a large memory depth is an important prerequisite for capturing large video signals, among others. It facilitates the recording of a complete signal segment followed by adequate zooming and a detailed post-analysis. It is always important to completely capture all the raw data for any subsequent analyses.

Therefore, the high sampling rate provided by Rigol oscilloscopes significantly improves the display quality. Combined with the brightness modulation feature of the Rigol Ultra Vision oscilloscopes (high-resolution WVGA display, 800 x 400 pixels, 256 brightness levels) even the most delicate details including noise or jitter will become visible.

Let us finally discuss the trigger functions as another significant feature of an oscilloscope which enables precise triggering on protocol events (bus lines, individual bits) or video signals.

Trigger functions

While edge triggering is supported by every general-purpose oscilloscope, additional trigger functions can offer benefits. For developers of circuits with serial data links, oscilloscopes should provide trigger functions for standards including I²C, SPI, RS-232/UART, USB, CAN, or FlexRay. Protocol-oriented trigger functions can then assist measurement engineers in their debugging tasks. Additionally, a glitch trigger feature can trigger on positive or negative glitches or pulses that are either wider or narrower.

Capturing and analysing data

The memory depth and the high capture rate of up to 180,000 waveforms per second offered by Rigol’s Ultra Vision oscilloscopes leave nothing to be desired for glitch detection, failure analysis and serial bus protocol decoding or analysing large amounts of data such as video signals. Furthermore, precise triggering on different signal combinations ensures that the captured data can be analysed and evaluated with maximum precision and reliability.

Rigol Technologies

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