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Achieving higher resolution in scopes

16 Jul 2012  | Jessy Cavazos

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However, featuring an ADC with more bits is useless to provide higher resolution without a low noise floor. With all oscilloscopes having noise associated with them (created from the oscilloscope front end, the ADC and other parts of the circuitry), it is imperative for oscilloscopes featuring higher bit ADCs to lower the noise floor so that the oscilloscope quantizes the signal as opposed to the noise of the oscilloscope itself. The whole signal chain from the input of the oscilloscope to the digitization process is critical to ensure customers receive the benefits from the extra bits of the ADC. Keeping the amplifiers very linear over wide ranges (essential as oscilloscopes handle a very wide dynamic range at their input) while keeping the noise floor low is the greatest technical challenge with 12bit scopes, and one of the reasons why there are not more of such oscilloscopes available in the market today. Noise issues can be further compounded by the use of probes in front of the oscilloscope input, which can add additional noise.

Other techniques available to reach higher resolution include averaging over multiple acquisitions and averaging across multiple points in a single acquisition. These techniques are available on all oscilloscopes including that of Tektronix, Agilent Technologies, and LeCroy. Averaging enables the user to take successive acquisitions and average all the values allowing the user to reduce the amount of noise generated by the oscilloscope. When the signal is averaged over time, the noise of the oscilloscope is relatively random. The more acquisitions taken, the more the noise averages out.

Averaging across multiple points in a single acquisition is a lesser-known averaging technique than the one mentioned above. However, it can be an effective method to reach higher resolution. All major vendors of oscilloscopes provide this capability. It is called HiRes mode at Agilent and Tektronix and ERES at LeCroy. In this mode, the oscilloscope oversample a signal for the sample rate that is required, take a number of points in a very small section and produce one resolving point by taking the average of all of those points. Similarly to the previous technique, averaging mitigates the oscilloscope noise by averaging out adjacent samples and eliminating the random noise associated with the oscilloscope.

The primary tradeoff with this method, however, is that the user may have to sacrifice significant bandwidth if the oscilloscope does not have adequate sample rate built into it. Also, while using filtering and averaging methods, users concerned about removing signal details, and therefore strive to keep as much of the real signal as possible. An example of an oscilloscope for which this method can prove to be effective to reach higher resolution would be Agilent's 9000 Series. While the user would need 2 times the bandwidth to the oscilloscope to accurately reconstruct the signal, Agilent's 1GHz oscilloscope provides 10 times that amount (20 GSa/s).

Therefore, when the user turns on the high resolution mode of the oscilloscope, the impact of noise from the oscilloscope is dramatically reduced, providing the user a clear understanding of what is happening on the signal. When the high-resolution mode is turned on, the bandwidth of the oscilloscope reduces from 1GHz to 558MHz. However, bandwidth reduction can be much more significant with oscilloscopes that do not feature excess sample rate capabilities.

Figure 3: The noise reduction by enabling the high-resolution mode on Agilent's 9000A oscilloscope. Source: Agilent Technologies

Final word
In summary, the desire among oscilloscope users for higher resolution is real, has been there for many years, and is here to stay. The good news is that various methods are available to engineers to achieve the higher resolution they are looking for, however each method should be evaluated carefully so as to make the best decision for the application at hand.

About the author
Jessy Cavazos is industry director at Frost & Sullivan.

To download the PDF version of this article, click here.

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