As mentioned earlier, an oscilloscope is similar to a camera and can capture the image of the signal we perceive. According to the shutter speed, lighting conditions, aperture and film ASA level will affect the clarity and accuracy of the image captured by the camera. The basic architecture of an oscilloscope is similar, and the performance considerations of the oscilloscope will greatly affect its ability to achieve the required signal integrity.
Mastering a new technology usually involves learning new vocabulary, as well as learning to use an oscilloscope. This section will describe some commonly used metrics and oscilloscope performance terms. These terms are used to describe some basic guidelines, and these guidelines are the basis for the correct choice of oscilloscope for operation. Understanding and mastering these terms will help to evaluate and compare different oscilloscopes.
bandwidth
The bandwidth determines the basic oscilloscope's ability to measure the signal. As the signal frequency increases, the oscilloscope's ability to accurately display the signal will decrease. This specification indicates the frequency range that the oscilloscope can accurately measure.
The oscilloscope bandwidth refers to the frequency at which the sinusoidal input signal is attenuated to 70.7% of its actual amplitude, which is the -3dB point, based on a logarithmic scale (see Figure 46).
If there is not enough bandwidth, the oscilloscope will not be able to distinguish high frequency changes. The amplitude will appear distorted, the edges will disappear, and detailed data will be lost. If there is not enough bandwidth, all the characteristics about the signal, ringing and ringing are meaningless.
The method of measuring the bandwidth of the oscilloscope: accurately characterize the signal amplitude in the specific operation, and apply the 5 times criterion. The measurement error of the oscilloscope selected using the five-fold criterion will not exceed +/- 2%, which is sufficient for today's operation. However, as the signal rate increases, this empirical criterion will no longer apply. Remember, the higher the bandwidth, the more accurate the reproduced signal (see Figure 47).
Rise Time
In the digital world, time measurement is crucial. When measuring digital signals, such as pulses and step waves, it may be more necessary to consider the performance of the rise time. The oscilloscope must have a sufficiently long rise time to accurately capture the details of the rapidly changing signal.
Rise time describes the effective frequency range of the oscilloscope. The following formula is generally used to calculate the rise time of an oscilloscope for a specific signal type:
Please note that the basis for selecting the oscilloscope rise time is similar to the basis for selecting the bandwidth. With regard to bandwidth, considering the extreme case of signal rate, this empirical criterion is not always applicable. Remember, the faster the oscilloscope's rise time, the more accurate the capture of the rapid transition of the signal. In some applications, only the rise time of the signal may be known. Bandwidth and rise time are related by a constant:
Among them, k is a constant between 0.35 and 0.45, its value depends on the oscilloscope's frequency response characteristic curve and pulse rise time response. For an oscilloscope with a bandwidth less than 1 GHz, the typical value of the constant k is 0.35, and for an oscilloscope with a bandwidth greater than 1 GHz, the value of the constant k is usually between 0.40 and 0.45.
Sampling rate
Sampling rate: expressed as the number of samples per second (S / s), which refers to the frequency at which the digital oscilloscope samples the signal, similar to the concept of frames in a movie camera. The faster the oscilloscope's sampling rate, the higher the resolution and clarity of the displayed waveform, and the lower the probability of losing important information and events, as shown in Figure 50. If you need to observe slow-changing signals over a long period of time, the minimum sampling rate becomes more important.
Typically, in order to maintain a fixed number of waveforms in the displayed waveform record, the horizontal control button needs to be adjusted, and the displayed sampling rate will also change with the adjustment of the horizontal adjustment button.
How to calculate the sampling rate? The calculation method depends on the type of waveform being measured and the signal reconstruction method used by the oscilloscope. In order to accurately reproduce the signal and avoid confusion, Nyquist's theorem stipulates that the sampling rate of the signal must not be less than twice its highest frequency component. However, the premise of this theorem is based on infinitely long and continuous signals. Since no oscilloscope can provide an unlimited time record length, and, by definition, low-frequency interference is discontinuous, so use twice the highest frequency
The sampling rate of the component is usually not sufficient.
In fact, the accurate reproduction of the signal depends on its sampling rate and the interpolation method adopted by the signal sampling point gap. Some oscilloscopes provide operators with the following options: sinusoidal interpolation for measuring sinusoidal signals, and linear interpolation for measuring rectangular waves, pulses, and other signal types.
When using sinusoidal interpolation, in order to accurately reproduce the signal, the sampling rate of the oscilloscope must be at least 2.5 times the highest frequency component of the signal. When using linear interpolation, the sampling rate of the oscilloscope should be at least 10 times the highest frequency component of the signal.
Some measurement systems with sampling rates up to 20GS / s and bandwidths up to 4GHA capture high-speed, single-pulse, and transient events at 5 times the bandwidth.
Waveform capture rate
All oscilloscopes will flash. In other words, the oscilloscope captures the signal a certain number of times per second, and no more measurements will be taken between these measurement points. This is the waveform capture rate, expressed as the number of waveforms per second (wfms / s). Sampling rate means that the oscilloscope samples the frequency of the input signal in a waveform or period, and the waveform capture rate refers to the speed at which the oscilloscope acquires the waveform. The waveform capture rate depends on the type and performance level of the oscilloscope and has a wide range of changes. Oscilloscopes with a high waveform capture rate will provide more important signal characteristics and greatly increase the probability that the oscilloscope will quickly capture transient anomalies such as jitter, runt pulses, low frequency interference, and transient errors (see Figures 51 and 52).
The digital storage oscilloscope (DSO) uses a serial processing mechanism and can capture 10 to 5000 waveforms per second. Some DSOs provide a special mode, which can quickly store various captured information in mass storage, temporarily provide a higher waveform capture rate, and then a longer period of processing time, this period of processing time will not be reactivated , Reducing the possibility of capturing rare and intermittent events.
Most digital phosphor oscilloscopes (DPO) use a parallel processing mechanism to provide a higher waveform capture rate. Some DPOs can obtain millions of waveforms in a second, greatly increasing the possibility of capturing intermittent and difficult events, and allowing users to find problems in the signal faster. Moreover, the ability of DPO to capture and display three-dimensional signal characteristics in real time (such as amplitude, time, and time distribution characteristics of amplitude) enables it to obtain higher-level signal characteristics.
Record length
The record length is expressed as the number of points that make up a complete waveform record and determines the amount of data that can be captured in each channel. Since the oscilloscope can only store a limited number of waveform samples, the duration of the waveform is inversely proportional to the sampling rate of the oscilloscope.
Modern oscilloscopes allow users to select the record length in order to optimize details in some operations. Analyzing a very stable sinusoidal signal requires only a record length of 500 points; but if you want to parse a complex digital data stream, you need a record length of one million points or more.
Trigger ability
The trigger function of the oscilloscope scans horizontally and synchronously at the correct signal position, which determines whether the signal characteristics are clear. The trigger control button can stabilize the repeated waveform and capture the single pulse waveform. For more information about trigger performance, please refer to the Performance Terms and Triggers section of the application.
Valid bit
Effective bits are a measure of the oscilloscope's ability to accurately reproduce a sinusoidal signal waveform. This metric compares the actual error of an oscilloscope to a theoretically ideal digitizer. Since the actual number of errors includes noise and distortion, the frequency and amplitude of the signal must be specified.
Frequency response
Using bandwidth alone is not enough to ensure that the oscilloscope accurately captures high-frequency signals. The goal of oscilloscope design is a specific type of frequency response: maximum flat envelope delay (MFED). This type of frequency response provides excellent pulse fidelity with minimal overshoot and damped oscillation. Since the digital oscilloscope is composed of actual amplifiers, attenuators, analog-to-digital converters (ADCs), connectors, and relays, the MFED response is only an approximation of the target value. The pulse fidelity of products of different models and different manufacturers is very different (Figure 46 illustrates this concept).
Vertical sensitivity
Vertical sensitivity indicates the degree of amplification of a weak signal by a vertical amplifier, usually expressed in millivolts per scale (mv). The typical value of the minimum volts that a multi-purpose oscilloscope can detect is about 1mv per vertical display scale.
Scan speed
Scanning speed characterizes how fast the trajectory sweeps across the oscilloscope display, allowing you to discover finer details. The scanning speed of the oscilloscope is expressed in time (seconds) / div.
Gain accuracy
Gain accuracy is a measure of how accurately a vertical system attenuates or amplifies a signal. It is usually expressed as a percentage error.
Horizontal accuracy (time base)
Horizontal or time-base accuracy refers to the accuracy of the timing of the displayed signal in a horizontal system, usually expressed as a percentage error.
Vertical resolution (analog-to-digital converter)
The vertical resolution of an analog-to-digital converter, that is, the vertical resolution of a digital oscilloscope, refers to the accuracy with which the oscilloscope converts the input voltage to a digital value. The vertical resolution is measured by the number of bits. The calculation method can improve the effective resolution, such as high-resolution capture mode.
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