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Agilent 54600 Scopes (54621A/D, 54622A/D, 54624A): Why is deep memory valuable?

The size of a digital oscilloscope's waveform acquisition memory (called the scope's memory depth or record length) is not usually listed up front along with analog bandwidth, sample rate, and vertical resolution in the scope's specs even though it can greatly affect the scope's performance. Unfortunately, except for an almost intuitive feeling that a deep memory (long record length) is probably better than a short one, the effects of memory depth on a scope's performance is complex particularly the relationship between the memory depth, sampling interval (sample rate), and the time span being viewed.

However mysterious its value is, deep memory is an absolute requirement for an oscilloscope that will be used to capture, view and measure mixed analog and digital designs or single-shot events. Its major advantage is that it allows higher sampling rates (and consequently shorter sampling intervals between digitized points) to be maintained which means higher horizontal resolutions. The bottom line is that every digitized sample point must be stored in the scope's memory, and if the memory is limited, then the sample rate must be decreased when long time spans are to be captured. Consider capturing 10 ms of time (1 ms/div timebase setting) with 3 different scopes with 3 different memory depths:

This means that even though a maximum sample rate may be specified, it will be maintained only if there is sufficient memory depth to store the digitized samples. The time span, the amount of time needed to capture the information about the signal, is determined by the application. The scope's memory depth and sampling rate must then be selected to meet the specific needs of the measurement.

With the new Agilent 54600 scope series 2MB of deep memory, it is possible to capture 10 ms of data (scope setting of 1 ms/div) and still capture and resolve 5 ns details. And with the new acquisition and display technology, all 2MB of deep memory are mapped to the high-resolution display. Intensity values are maintained on frequency of occurrence and are mapped to the 32 levels of intensity.

Thus, there are two benefits of deep memory with the new Agilent 54600 scopes. First, the long time spans can be captured and then zoomed in on for detailed analysis. Second, the 2MB deep memory is displayed to the high definition display with 32 levels of intensity for more insight into the signal. This is particularly valuable when dealing with mixed analog and digital designs where slow analog events required long time spans and fast digital control signals require the ability to maintain the sample rate so interactions may be viewed in detail.