Digital storage oscilloscopes, or DSOs for short, today have to cover a wide range of different measurement scenarios. In addition to the traditional measurement task of an oscilloscope, measuring acquired voltages over time must be as precise and easy to use as analyzing serial data or specific power scenarios. In our three-part series of articles, we want to give you a quick and easy introduction to the measurement and functionality of modern DSOs. The first part is about the basic structure of an oscilloscope: how are the signals sampled, stored and displayed on the screen?
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Why a large acquisition memory is important
Acute best handheld oscilloscope has four analog inputs. The detected analog signals are first amplified by an amplifier to then digitized by the analog-to-digital converter. The acquired digital data must now be stored in a fast and sufficiently large acquisition memory.
For each of the channels, an amplifier, an A / D converter and an acquisition memory are available separately. This guarantees that the data collected in parallel is available without delay. The oscilloscope can use the data collected in memory for a variety of applications such as documentation, on-screen display, or for measurements and data analysis.
Analog and digital bandwidth of an oscilloscope
Oscilloscopes often talk about analog and digital bandwidth. The analog bandwidth corresponds to the bandwidth of the maximum detectable frequencies of the analog amplifier. Higher rates are either not or incorrectly passed to the A / D converter. It is always important that not only the analog bandwidth of the oscilloscope must be considered, but that of the sensing probe. The digital bandwidth of the oscilloscope, on the other hand, corresponds to the ” real-time ” sampling rate of the A / D converter. At a sampling rate of 40 GS / s, for example, 40 billion detection points per second can be recorded.
This corresponds to a temporal resolution of 25 ps between the individual detection points. The length of the maximum recording time depends strongly on the available memory. The larger the memory area, the larger the time windows can be detected with high resolution.
The memory size is specified in the number of measured values, which can be recorded per channel at most. With 256 Mpts and a digital bandwidth of 40 GS / s, a time window of 6.4 ms is recorded. The memory of two channels can also be cascaded if fewer channels are needed. A division of the memory into segments is also possible to detect signals with long, uninteresting pauses. The uninteresting data is not written to memory.
The three ways to sample a signal
LeCroy’s oscilloscopes have three different ways of sampling a signal. The most commonly used type is “real-time” signal sampling where the data is captured and stored in real-time. It must always be ensured that the digital sampling rate is high enough for the frequencies to be recorded. If this is not the case, so-called aliasing effects can occur. When sampling a signal, at least the Nyquist theorem must always be satisfied, which states that for sinusoidal signals, the sampling rate must be at least twice as large as the maximum frequency to be detected.
How to reduce the aliasing risk in a scope
In the case of pulses or square-wave signals, the sampling rate must even be ten times the maximum frequency. In the example, the blue curve is to be detected. Due to insufficient sampling, the red curve will be visible on the screen. This happens because the oscilloscope interpolates the curve between the detection points and thus represents the wrong curve.
High sampling rates and large memory depths can reduce the aliasing risk. At the beginning of measurement, it also makes sense to start with low “Time/div settings” and to increase them step by step.
The second type of signal sampling is Random Interleaved Sampling. In this method, the sampling rate is increased by multiple sampling of a signal and with each detection, the detection points are slightly shifted compared to the previous measurement. The superimposition of the individual measurements thus achieves a high effective sampling rate.
This method works only with periodically recurring signals. Roll mode is the third type of signal acquisition. It is often used in slow processes. The signal runs from the right to the left across the screen by simply adding newly acquired measurement points to the right, moving all the old ones to the left.
Editing signals on the scope
The detected signals are output by the oscilloscope on the screen. The signal can be displayed directly or enlarged as a zoom. Also, a variety of calculations, such as subtraction of two curves are possible. Manual measurements can now be made on the curves using the cursor. Even automatic parameter measurements including statistical evaluation are now standard on almost all devices. With many serial buses, decoders can also be connected to evaluate the very large number of serial data quickly and effectively.