Do you need a real time oscilloscope or a sampling oscilloscope?

Real-time oscilloscopes capture a signal in a single pass, while sampling oscilloscopes, sometimes called digital communication analyzers or equivalent-time sampling oscilloscopes, take multiple samples over several periods of a signal, allowing for a slower sampling rate and potentially higher resolution. Both are subcategories of digital storage oscilloscopes (DSOs).

Real-time scopes are ideal for capturing one-shot events or intermittent signals, while sampling scopes excel at analyzing repetitive signals, especially those with high bandwidths. Sampling scopes often include dedicated analysis features and can support superior accuracy for time-domain waveform analysis.

Bandwidths over 100 GHz are available for both types of scopes. To accurately capture the signal, real-time scopes need an analog-to-digital converter (ADC) with a sample rate much higher than the bandwidth, which limits the typical vertical resolution to 8 or 12 bits in most cases. Sampling scopes capture the signal over several periods and can use an ADC with a lower sample rate and can have vertical resolutions up to 16 bits.

How does that work?

A sampling scope can capture signals up to the analog bandwidth of the instrument independently of the sample rate by collecting the needed number of samples across several triggers. The collected samples are used to recreate the captured waveform. Real-time scopes don’t need to recreate the signal since they capture the entire signal in real time and in a single pass (Figure 1).

Figure 1. Comparison of real-time sampling (top) and equivalent-time sampling (bottom) for waveform reconstruction. (Image: BK Precision)

A slower and lower-cost digitizer can be used with sampling scopes due to the repetitive nature of the signal being captured. For example, a sampling scope is available with a bandwidth of 350 MHz using a 100 MS/s sampling rate on each of its four channels.

On the other hand, a real-time scope with an analog bandwidth of 400 MHz and a 1 GS/s sample rate can deliver a usable bandwidth of 400 MHz, while a real-time scope with an analog bandwidth of 400 MHz, but a 100 MS/s sample rate, has a usable bandwidth of 40 MHz.

Application considerations

Sampling oscilloscopes and real-time oscilloscopes have a lot of applications in common. Both types can be used for producing eye diagrams, histograms, and jitter measurements. A significant area of difference is the tradeoff between precision and flexibility.

For example, sampling oscilloscopes are higher precision instruments, and their lower cost makes them more suitable for use in production, standards testing, and qualification environments. They excel at analyzing high-speed, repetitive signals, particularly in the context of optical and electrical transceivers.

The greater flexibility of real-time oscilloscopes can make them ideal for use in laboratories and repair operations where the demands on the instrument can vary widely. They are great for capturing and analyzing a wide variety of signals, including single-shot events, transients, and non-repetitive signals that sampling oscilloscopes are not suited for.

Clock recovery is an important function in many oscilloscope applications, and it’s an area of significant differences. Sampling oscilloscopes need to get a clock signal from the device under test (DUT) or require hardware clock recovery. Real-time instruments don’t require an external trigger and can use software clock recovery.

Timing is another area where the tradeoff between flexibility and precision shows up. A hardware clock (found on sampling scopes) can run much faster than a corresponding software solution (found on real-time scopes). A hardware clock is also more accurate than a software solution.

As a result of its ability to reconstruct a signal with high fidelity, a sampling oscilloscope is better suited for analyzing harmonics and signal distortion. Real-time oscilloscopes are not the optimal choice for analyzing the detailed frequency content of repetitive signals.

Real-time oscilloscopes are useful for analyzing the time-domain characteristics of a signal, like pulse shapes and rise times, but they are not as effective for detailed harmonic analysis. Sampling oscilloscopes, on the other hand, generally have a higher dynamic range that supports accurately measuring the harmonics as well as the fundamental frequency, even if the harmonics are significantly lower in amplitude.

Table 1. Performance comparison of sampling scopes versus real-time scopes. (Table: Keysight)

Summary

The choice between sampling and real-time scopes comes down to application and performance priorities. There’s a lot of overlap; both types can be used for producing eye diagrams, histograms, and jitter measurements. The deciding factor often comes down to the tradeoff between the precision delivered by sampling scopes versus the flexibility of real-time units.