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Tradeoffs are at the root of engineering, and the most difficult decisions usually involve the balancing of features, performance, cost, time, and resources. These tradeoffs are driven by constraints. Constraints, in turn, can be relaxed by new technologies and techniques. Overcoming constraints is what leads to breakthroughs in performance and fundamental improvements in what engineers can accomplish.
In the last 35 years the industry has witnessed a transition of the most fundamental tool on the electrical engineers bench — the oscilloscope. Since 1971, the time of Hiro Moriyasu's first digital oscilloscope, there has been a steady transition from analog scope technology. While the benefits of digital technology were immediately apparent, the limitations of this technology required changes in debugging techniques that are in many cases not necessary today.
This article will focus on the "final few" tradeoffs and how new technologies have eliminated them. As a result, we'll rediscover a few older techniques that you can now use with confidence.
Why analog oscilloscopes were nice
There are many engineers who haven't fully embraced digital oscilloscopes, and there are many features of analog oscilloscopes that justify this feeling. First, it's much easier to trust the deflection of an electronic beam. Analog systems don't introduce aliased information. Next, the phosphorescent displays of analog scopes allow the display of subtle differences in repetitive signals. Audio and analog video engineers, in particular, appreciate this visibility. Finally, analog oscilloscopes are responsive — both to front panel knob changes as well as to changes in the signal.
Digital oscilloscopes change the game
Digital oscilloscopes fundamentally changed the engineering bench. In some cases, it simply made it more economical to implement features such as single-shot and delayed sweep measurements. Other features such as pre-trigger acquisition, parametric measurements, saving waveforms and data for later analysis, noise reduction, averaging, searching, zooms, and math measurements gave engineers a new set of troubleshooting and analysis tools.
However, these new features came at a price. The tradeoffs of digital technology included (in the order they were overcome):
- Low sampling rates
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Small memory buffers
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Slower waveform update rates
Low sampling rates
This was the first of the constraints to be relaxed. A low sample rate increases the risk of aliasing higher-frequency signal components. Early digital oscilloscopes had sample rates well below the Nyquist sampling frequency for its nominal bandwidth. Even with aggressive digital filtering, equivalent time (ET) sampling (increasing the sample rate of repetitive signals by shifting each acquisition minutely) was required to achieve the full bandwidth of the oscilloscope without aliasing.
This constraint was removed when sample rates reached three-to-four times the scope's bandwidth.
Implications for designers: If a scope is four times over sampled or more, in most cases, ET mode can be ignored. This frees you to make single-shot acquisitions.
Small memory buffers
This constraint was partly due to the expense of high-speed memory, and partly due to the architectural limitations of processing data through the acquisition system. Shallow memory limits the time viewed in single-shot measurements. But, more importantly, it reduces the effective sample rate of the scope (and thus its unaliased bandwidth) at higher time base (t/div) settings. With a limited number of samples available, each sample had to be spaced farther apart.
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