Oscilloscopes in the application of secondary technical indicators play what role
Bandwidth Definition
Bandwidth metrics are certainly important. For designers who are constantly pushing the limits of high-speed serial bus architecture, bandwidth has always been at the top of their list of considerations when purchasing an oscilloscope.
However, bandwidth itself is only one metric that describes the frequency response of an instrument (the frequency at which a sine wave rolls off -3 dB). Two oscilloscopes with the same bandwidth rating may have very different rise times and completely different responses to complex waveforms. Isn't there a need to carefully nudge some of these metrics or features to better facilitate the buyer's decision?
There are two ways to answer this question, one is the true rise time performance of the oscilloscope and the other is the behaviour of the instrument in digital signal processing (DSP) mode.
Analogue rise time is a function of the oscilloscope's bandwidth. It attempts to simply calculate the rise time from the bandwidth using textbook formulas, which are the basis of some published rise time metrics. Guest-observed rise times provide a better basis for measurements, both with and without DSP enhancements. Every engineer understands the importance of rise time response. Measuring the difference between measured rise time and calculated rise time is to understand what is being said.
Oscilloscope Triggering and Signal Complexity
The term "high-speed measurement" has various meanings in terms of sub-nanosecond edges and fast clock rates. It is sometimes overlooked that these high-speed measurements are often very complex measurements. Capturing a code in the data stream involves judgement, luck, estimation, guesswork... or the right choice of trigger function.
Oscilloscope triggering determines what can be captured, viewed and measured using the instrument, a function that is as important as bandwidth and sampling rate. Trigger systems have their own different set of specifications. Trigger paths are generally tributaries of the main input signal path and should reflect many of the same environmental characteristics, such as sensitivity, jitter, and so on. Another indicator of trigger performance is the range of trigger types, that is, the conditions that can be defined when a trigger occurs.
Related "Secondary" Metrics
Until now, the technical metrics we have discussed have usually been secondary to the primary metrics of bandwidth, sampling rate, and so on. But the fact is that there are many other parameters that are often considered secondary issues in the oscilloscope evaluation process that can either facilitate or hinder a tight engineering schedule.
For many serial standards, embedded clock recovery is the basis for oscilloscope eye diagram analysis, which also provides support for measurements such as clock-to-data recovery (CDR as shown in Figure 3). Designers working with embedded clock signals should look beyond the primary metrics and consider the ways in which oscilloscopes can make clock recovery faster, easier, more flexible, and more repeatable.
Application requirements have always guided the direction of choice. Can the oscilloscope be used for overhaul or conformance measurements? What clock recovery mechanisms are available? Can oscilloscopes recover clocks in real time and display dynamic eye diagram features?
Most high-end oscilloscopes offer one of two methods of clock recovery, software-based clock recovery or hardware-based clock recovery. Software clock recovery is generated from stored acquisition data. For conformance testing using procedures such as the TDSRT-Eye automated conformance test and analysis software, the software approach is recognised as the tool of choice.
It is possible to use phase-locked loop (PLL)-based clock recovery for real-time eye chart acquisition, but here, too, the metrics need to be carefully pushed: can the PLL (which can be either software recovery or hardware recovery) adapt to the clock frequencies evolving in the current serial standard? Some do, some don't, so it is important to understand the differences.
