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Real-Time or Sampling Oscilloscope – Key Comparisons

What is the best oscilloscope for your application? The following areas will help you make an accurate and informed decision. Today’s complex electronics industries require a broad spectrum of test equipment, with oscilloscopes being one of the most fundamental tools used by engineers and technicians. Oscilloscopes provide design and manufacturing engineers with critical insights to signal properties suggesting additional design work needed, targeting manufacturing issues, or performing compliance and protocol testing per international standards. Oscilloscopes fall into two groups, real-time oscilloscopes and sampling oscilloscopes (also called equivalent-time oscilloscopes) and it is important to understand the difference between the two types. Real-time oscilloscopes digitize a signal in real-time. Imagine a repetitive AC signal - the real-time oscilloscope acts like a camera, taking a series of frames of the signal during each cycle. The amount of frames the real-time oscilloscope captures depends upon the bandwidth, memory depth, and other attributes that we will soon discuss. A sampling oscilloscope, on the other hand, takes only one shot of the signal per cycle. By repeating this one shot, but at slightly different time frames, the sampling oscilloscope can reconstruct the signal with a high degree of accuracy.

The following topics can help you better evaluate which kind of oscilloscope will best suit your needs.

Trigger
Sampling oscilloscopes are designed to capture, display, and analyze repetitive signals. If your oscilloscope solution needs to capture a single random event within your waveform, a real-time oscilloscope should be selected. Whether you are looking at intermittent signals during product design or manufacturing, real-time oscilloscopes allow you to trigger on a specific event such as a rising voltage threshold, a set up and hold violation, or a pattern trigger. The real-time oscilloscope will capture and store continuous sample points around these triggers and update the display with the captured data.

Bandwidth
The frequency of your signal under test and the harmonics within it will determine the bandwidth of the oscilloscope that will fit your needs. Sampling and real-time oscilloscopes cover a wide bandwidth range and there is a lot of overlap. A sampling oscilloscope can acquire any signal up to the analog bandwidth of the oscilloscope regardless of the sample rate. But a real-time oscilloscope must gather a significant number of samples after the initial trigger to accurately display a waveform. A typical rule of thumb for a real-time oscilloscope bandwidth is 2.5 times your signal frequency to reproduce your signal with the best fidelity. So you can get by with an effectively lower bandwidth scope using a sampling scope as long as you have the trigger mentioned in the previous section.

Memory Depth
Oscilloscope memory depth is an important specification for only real-time oscilloscopes. A real-time oscilloscope captures an entire waveform on each trigger event. To do this the real-time oscilloscope captures a large number of data points in one continuous record. For a real-time oscilloscope, the memory is directly tied to the sample rate. The more memory you have, the more samples (sample rate) you can capture for each waveform. The higher the sample rate, the higher the effective bandwidth of the oscilloscope. There is a simple calculation to determine the sample rate given a specified time base setting and a specific amount of memory (assuming 10 divisions across screen): Memory depth / ((time per division setting) * 10 divisions) = sample rate (up to the max sample rate of the ADCs). This memory depth concept does not apply to sampling oscilloscopes because only one instantaneous measurement of waveform amplitude is taken at the sampling instant.

Analog to Digital Converter Bits
Sampling oscilloscopes can have as high as a 14-bit analog-to-digital converter (ADC). Consequently, they have a very large dynamic range, which enables viewing signals ranging from a few millivolts to a full volt without the need for attenuation. This allows sampling oscilloscopes to maintain very low noise levels at all volts per division settings. A real-time oscilloscope is limited in its dynamic range to 8 - 10 bits depending upon the model, but typically will show an effective bit number of around 6 – 8 bits respectively. Because of a real-time oscilloscope’s lower signal-to-noise ratio, it is designed with attenuators to correctly display signals at specific volts per division settings.

Frequency Response
Frequency response is another key consideration in your selection criteria. Sampling oscilloscopes do not use digital signal processing (DSP) correction, so the frequency response rolls off slowly and looks more Gaussian in shape. Real-time oscilloscopes can implement DSP to correct their frequency response. For instance, Keysight’s S-Series oscilloscope has a very flat frequency response across its bandwidth, which means its gain will not vary by more than 1 dB across the entire band.

Clock Recovery
The clock recovery component of an oscilloscope measurement is used for building real-time eyes, mask testing, and jitter separation. A recovered clock is a reference clock within the oscilloscope and used for measurement comparisons. Keysight’s Sampling oscilloscopes provide an accurate software-based clock recovery system. In many applications, real-time oscilloscopes have a software clock recovery and selectable hardware clock recovery frequencies. Please note that the advantage of a software clock recovery is that it is not prone to the hardware errors, and will land its edges where they need to be regardless of the data rate.

Applications
Sampling oscilloscopes, like real-time oscilloscopes, offer eye diagrams, histograms, and jitter measurements. With high bandwidths, modularity, and lower pricing, they typically fit manufacturing environments better than real-time oscilloscopes.

Many of Keysight’s sampling oscilloscopes have modular systems consisting of a mainframe and various electrical, optical and TDR modules. This allows the end user to customize measurement hardware to fit their solution. Sampling oscilloscope electrical and TDR channels can be integrated into a module to reduce cost or remote heads can be used to improve accuracy. Optical channels are always integrated creating a well-controlled 4th-order Bessel-Thomson frequency roll-off.

When making jitter measurements clock recovery systems play a large role. Understanding the clock recovery algorithm and the jitter transfer function used will help you determine your final oscilloscope selection. The sampling oscilloscope has a slightly lower jitter and a higher dynamic range making it ideal for characterization in a controlled environment assuming that your signal is repeatable. However, real-time oscilloscopes are great if you need to trigger on difficult to find events. Real-time oscilloscope users can choose from a long list of compliance, protocol triggering and decode, and analysis applications including jitter.

Form Factor
Your solution may require an oscilloscope solution with a specific size or configuration (form factor) to fit your needs. Keysight has both sampling and real-time scope solutions in a variety of form factors, from standard desk top and rack mountable frames to faceless (no screen) module solutions in a variety of AXI or PXI configurations. See the links below for sampling and real-time options.

Summary
On the surface there is a lot of overlap between sampling and real-time oscilloscopes but the differences in capabilities and performance that we have discussed can help you make an informed decision to tailor a selection to your specific application.

If you require measurements of a repetitive waveform with lower jitter and a higher dynamic range, a sampling oscilloscope is a good choice. In addition, sampling oscilloscopes have an advantage of a lower initial cost and modular upgrades, making them well suited for electrical and optical manufacturing test applications. Real-time oscilloscopes come in a variety of bandwidths, include the ability to capture single-shot events as well as repetitive signals.

Both Keysight sampling and real-time oscilloscopes are available in frequencies from 1 GHz to 50 GHz and beyond with a variety of modular and frame options to fit your specific requirements.

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