Oscilloscope knowledge: the respective characteristics of analog and digital oscilloscopes

An oscilloscope is a window for observing waveforms. It allows designers or maintenance personnel to see electronic waveforms in detail to achieve the effect of seeing is believing. Because the human eye is the most sensitive visual organ, it can clearly observe the end of the age and reflect objects to the brain very quickly, making comparisons and judgments. Therefore, the oscilloscope is also known as a waveform multimeter.

Early oscilloscopes only displayed changes in voltage over time and made qualitative observations. Subsequently, the improved oscilloscope has a quantitative function, measuring amplitude and time, and their changes. At the same time, in order to record and compare incidents, the long afterglow effect of cameras and oscilloscopes is used.

The frequency characteristics of the analog oscilloscope are determined by the vertical amplifier and the cathode oscilloscope. In the 1980s, oscilloscopes introduced digital processing and microprocessors, and digital oscilloscopes appeared. Now analog oscilloscopes are called analog real-time oscilloscopes (ART), and digital oscilloscopes are called digital storage oscilloscopes (DSO).

ART needs an amplifier and a cathode ray oscilloscope that are compatible with the bandwidth. As the frequency increases, the process requirements for the cathode ray oscilloscope are strict, the cost increases, and the bottleneck exists. As long as the DSO is compatible with the bandwidth of the high-speed A / D converter, other memories and D / A converters and displays are all lower-speed components. The displays can use LCD flat arrays and color screens.

DSO uses a microprocessor for control and data processing, so that DSO has advanced trigger, combination trigger, glitch capture, waveform processing, hard copy output, floppy disk recording, long-term waveform storage and other functions that ART does not have. The bandwidth of DSO currently exceeds 1GHz surpasses the performance of ART in many aspects. DSO also has shortcomings, the bandwidth depends on the sampling rate, the more common sampling rate is equal to 4 times the bandwidth. The reproduced waveform is filled by interpolation algorithm, and the waveform will be distorted; the A / D conversion speed is fast, but the D / A conversion speed is slow, so the waveform refresh rate is low, and occasional signals will be missed; the vertical resolution is generally 8 bits It is obviously lower; there are many panel knobs, complicated menus, and it is inconvenient to use; without brightness modulation, three-dimensional graphics cannot be observed; the waveform storage capacity is not enough to process the waveform, etc.

At present, the shortcomings of DSO have been basically overcome, but not all good performance is reflected in the same oscilloscope, that is, each DSO will have certain characteristics, and there are certain deficiencies. You should pay attention to comparison when choosing a model. Some models of DSO have the same waveform refresh rate as ART, but some models of DSO do not. One DSO has ART's three-dimensional graphic display capability on fluorescent screens, and most DSOs do not have this performance. Most DSOs have the same real-time bandwidth as single-pass bandwidth, but there are also DSOs that only guarantee real-time bandwidth.

The aforementioned DSOs include A / D converters and microprocessors. In this way, adding a card to the PC can also constitute a DSO, but generally the sampling rate is lower, the function is less, and the price is cheaper. There are also DSO modules using VXI bus and rack-mounted DSO plug-ins.

The DSO memory is the second-to-all A / D converter component in the oscilloscope. It stores the sample of the measured signal for the subsequent D / A converter to restore the waveform. Now the storage capacity can reach more than 1M.

Ordinary DSO has 8-bit vertical resolution, that is, there are 256 samples per scan, which requires 256 points of storage, which is equivalent to 256 bytes. If the resolution is increased and the horizontal axis is enlarged 10 times, it is equivalent to 20K bytes; the vertical axis is also enlarged 10 times, which is equivalent to 40K bytes. This shows that the DSO should have at least 2K bytes, and the medium DSO should have more than 40K bytes. If you want to record 10 times the above waveform, at least 400K bytes are required. Therefore, the storage capacity is very important.

Conversely, the storage capacity also affects the scanning speed. For example, the memory with only 50K points per scan records 100μs data, and the sampling interval is 2ns. At this time, the sampling rate is equivalent to 500MS / s, and the sampling rate is equal to 4 times the bandwidth. It is equal to 125MHz. Obviously, if you need to increase the sampling rate to 1000MS / s, then recording 100μs of data requires 100K points of memory.

In order to store a complete picture, suppose the picture element is 1024 × 512 = 0.5M bit, four pictures, must have 2M bit storage capacity. In the FFT analysis, additional storage is also required, and the components of the new waveform are compared with the reference waveform or the stored waveform. To facilitate waveform storage, some DSOs also provide floppy disks or hard disks for data recording. For more detailed indicators, please obtain technical indicators from the manufacturer, make a comprehensive comparison according to the purpose and funds, and consult to purchase an economical and applicable oscilloscope.

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