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08 June 2020

Charting High-Resolution Oscilloscope Performance

Eric Bogatin, Signal Integrity Evangelist, Teledyne LeCroy

Signal measured by 8-bit and 12-bit oscilloscope.
Figure 1. 8-bit vs. 12-bit oscilloscope waveforms.
There are hundreds of different oscilloscope models from a dozen different vendors. How do you make sense of all of their different features to pick the one instrument right for your application?  Here is a simple way of comparing high-resolution oscilloscopes.

High-resolution usually refers to the vertical resolution, the quantization of a waveform into a fixed number of vertical levels measured by the oscilloscope's Analog-to-Digital Converter (ADC). The more bits of vertical resolution, the more levels and the more detailed the waveform rendered. The earliest digital storage oscilloscopes (DSO) started as 8-bit resolution, or 256 vertical levels. But in the last ten years, as ADC chip technology got faster, higher resolution appeared in the industry, pioneered by Teledyne LeCroy.  Now, many oscilloscopes come with 10-bit and 12-bit vertical resolution. Why would you want higher resolution? The answer: the ability to see lower level signals and better dynamic range. An example of the difference between the same signal measured with an 8-bit and 12-bit oscilloscope is shown in Figure 1. 

Figure 2. Oscilloscope radar chart.
But not all 12-bit oscilloscopes are equal. With some oscilloscopes, the higher vertical resolution comes at a reduced bandwidth or reduced sample rate. Or maybe you get 12-bit resolution, but fewer channels. When selecting an oscilloscope, it's not about the maximum values possible for each individual specification, it’s about the performance metrics you get for all specifications at the same time.

When comparing different high-resolution oscilloscopes, one useful method for visualizing the tradeoffs in performance is the radar chart. On this special chart, for each specific configuration of an oscilloscope, we plot four metrics simultaneously on four different axes: maximum sample rate, vertical resolution, bandwidth and number of channels. These are the performance metrics you get, at the same time, when using that oscilloscope. An example of these axes is shown in Figure 2.

Figure 3. WavePro HD vs. DSOS804A.
Any high-resolution oscilloscope's properties can be plotted on these axes, and you can tell at a glance what the tradeoffs are. For example, when evaluating 8 GHz, 4-channel oscilloscopes of comparable performance, the Teledyne LeCroy WavePro HD and the Keysight DSOS804A, even though they both are rated at 20 GS/s and 4 channels, you don’t get all 4 channels and 20 GS/s at the same time when using them. At 20 GS/s, you only get 2 channels. A radar chart plotting this comparison is shown in Figure 3. Generally speaking, the more area of the chart a particular oscilloscope's metrics cover, the better its performance. The difference in performance between the 12-bit Teledyne LeCroy HDO and the 10-bit Keysight DSO is obvious.

Sometimes the differences and tradeoffs are harder to balance. For example, here is the comparison between the Teledyne LeCroy WavePro HD and the Tektronix 6 Series, displayed in Figure 4.

Figure 4. WavePro HD vs. Tek 6 Series.
If your application requires 8 GHz bandwidth and 25 GS/s on 4 channels simultaneously, but 8-bit resolution is OK, the Tek scope's performance metrics may better satisfy your needs. But, if you need 12-bit resolution, maybe you are willing to trade off the number of channels and the slightly reduce sample rate of 20 GS/s, and the Teledyne LeCroy WavePro HD might give you better overall performance.

The next time you are evaluating a new oscilloscope, think about where your requirements fit on the radar chart, then carefully evaluate your options to see which one's performance metrics give the best value for your application. 

If you would like to learn more about high-resolution oscilloscope options, check out the two-part webinar series:

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