What Is a Digital Storage Oscilloscope?
A digital storage oscilloscope is a measuring instrument that converts changes in electrical signals over time into digital data, records them in its internal memory, and presents them on a display.
Unlike testers, which only measure voltage or current at a single point in time, oscilloscopes can observe the period/frequency of an electrical signal, the rising edge of a signal, and the time/phase difference between multiple signals.
Oscilloscopes are broadly classified into digital storage oscilloscopes and analog oscilloscopes. While analog oscilloscopes project electrical signals onto a cathode-ray tube in real-time, digital storage oscilloscopes, which are used here, collect information as “points” obtained by sampling electrical signals at discrete time intervals and display pseudo waveforms.
Uses of Digital Storage Oscilloscopes
Oscilloscopes are measuring instruments used to observe electrical signals, such as voltage and current, in electrical circuits as waveforms with the horizontal axis of time. They are used to verify and debug the operation of industrial and consumer equipment during development, and to analyze the cause of product failure.
Digital storage oscilloscopes convert electrical signals into waveforms after data processing, such as A/D conversion. As a result, they used to have a disadvantage in terms of real-time performance compared to analog oscilloscopes.
However, since 2000, the screen update rate has been improved to a level that does not interfere with practical use. In addition, since the price of digital storage oscilloscopes has gradually decreased, they are primarily used nowadays.
Principle of Digital Storage Oscilloscopes
1. Input Signal Processing
Digital storage oscilloscopes use an attenuator to adjust the sensitivity of the input signal, an amplifier to optimize the amplitude, and an A/D converter to convert the signal to a digital value. The digital value is recorded in the recording memory as the data of a single point in the waveform.
2. Recording Memory
The recording memory has a FIFO (first-in-first-out) memory structure. When the recording memory is full, the oldest data is discarded and new data is written. As a result, the recording memory is always kept up-to-date with the latest data.
3. Signal Waveform
Writing from the A/D converter to the recording memory is controlled by the trigger circuit. When writing to the recording memory is stopped by a signal from the trigger circuit, the waveform record, which is a collection of data for each point stored there, is transferred to the display memory. Based on the data in the display memory, the signal waveform is displayed on the oscilloscope display.
4. Pre-trigger
When the trigger circuit signal immediately stops the acquisition of a new signal, the waveform record stored in the recording memory is that before the trigger signal. This ability to observe input signals prior to the trigger signal is one of the features of digital storage oscilloscopes and is called a pre-trigger. With analog oscilloscopes, it is difficult to capture waveforms before the trigger signal because the sweep of the bright line starts after the trigger signal is received.
How to Select Digital Storage Oscilloscopes
When selecting a model, it is important to use an oscilloscope with sufficient specifications for the measurement contents. Specifically, the following points should be considered:
- Frequency response
The wider the frequency band, the better - Sampling frequency
The faster the sampling frequency, the better - Number of channels
The higher the number of channels, the more advantageous - Memory length
The larger the memory capacity, the greater the advantage - Available probe types
The more probes available, the better - Trigger function
It is advantageous if various trigger conditions can be set.
In addition to the basic use of digital storage oscilloscopes for waveform observation, their applications are expanding to include timing verification, waveform analysis, and compliance testing. On the other hand, the higher the performance required, the higher the price will inevitably be. Therefore, there is an increasing need to select a model with functions that match the intended use.
Other Information on Digital Storage Oscilloscopes
1. Use of Waveform Recording
Since digital storage oscilloscopes record input signals as digital data in the recording memory, waveform analysis using the data in the recording memory, for example, frequency analysis of signals by FFT operations, can also be performed. Furthermore, the data can be output to an external memory device (USB memory, etc.) for analysis and data storage using a PC.
2. Anti-aliasing Measures
Digital storage oscilloscopes may observe incorrect waveforms if the sampling interval is too long compared to the frequency of the input signal. This is called “aliasing.” To prevent this, it is necessary to acquire waveform data at a sampling frequency that is more than twice the maximum frequency of the input signal.