Explanation of Common Oscilloscope Terms

1. Bandwidth: refers to the frequency value when the sinusoidal input signal decays to 70.7% of its actual amplitude, that is, the -3dB point (based on a logarithmic scale). This specification indicates the frequency range that an oscilloscope can accurately measure. Bandwidth determines the oscilloscope's basic measurement capability of the signal. As the signal frequency increases, the oscilloscope's ability to accurately display the signal will decrease. Without sufficient bandwidth, the oscilloscope will not be able to distinguish high-frequency changes. The amplitude will be distorted, the edges will disappear, and the details will be lost. Without sufficient bandwidth, all the characteristics, ringing and humming obtained about the signal are meaningless.

▲5x rule (oscilloscope bandwidth required = highest frequency component of the measured signal x 5) The measurement error of an oscilloscope selected using the 5x rule will not exceed ±2%, which is generally sufficient. However, as the signal frequency increases, this rule of thumb is no longer applicable. The higher the bandwidth, the more accurate the reproduced signal.

2. Rise time: In the digital world, time measurement is crucial. When measuring digital signals, such as pulses and step waves, the rise time may be more important. The oscilloscope must have a long enough rise time to accurately capture the details of fast-changing signals.

▲Oscilloscope rise time = fastest rise time of the measured signal + 5. Rise time describes the effective frequency range of the oscilloscope. The basis for selecting the oscilloscope rise time is similar to the basis for selecting the bandwidth. The faster the oscilloscope's rise time, the more accurate it is in capturing the rapid changes of the signal.

3. Sampling rate: The sampling rate indicates the frequency at which the oscilloscope samples the input signal within a waveform or cycle. It is expressed as samples per second (S/S). The faster the sampling rate of the oscilloscope, the higher the resolution and clarity of the displayed waveform, and the lower the probability of losing important information and events. If you need to observe slow-changing signals over a longer time range, the minimum sampling rate becomes more important.

The method for calculating the sampling rate 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 aliasing, the Nyquist theorem stipulates that the signal must be sampled at a rate no less than twice its highest frequency component. However, this theorem is based on infinite time and continuous signals. Since no oscilloscope can provide an infinite time record length, and low-frequency interference is by definition discontinuous, it is not enough to use a sampling rate twice the highest frequency component. In fact, the accurate reproduction of the signal depends on its sampling rate and the interpolation method used to interpolate the gaps between the signal sampling points.

▲When using the sine difference method, in order to accurately reproduce the signal, the oscilloscope's sampling rate must be at least 2.5 times the highest frequency component of the signal. When using the linear interpolation method, the oscilloscope's sampling rate should be at least 10 times the highest frequency component of the signal.

4. Waveform capture rate: refers to the speed at which the oscilloscope captures waveforms. All oscilloscopes will flash. In other words, the oscilloscope captures the signal a certain number of times per second, and no measurements will be made between these measurement points. This is the waveform capture rate, expressed as waveforms per second (wfms/s). The waveform capture rate depends on the type and performance level of the oscilloscope and has a wide range of variation. An oscilloscope 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.

5. Record length: It is expressed as the number of points that constitute a complete waveform record, which 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.

6. Trigger capability: The trigger function of the oscilloscope synchronizes the horizontal scan at the correct signal position point, which determines whether the signal characteristics are clear. The trigger control button can stabilize the repetitive waveform and capture the single pulse waveform.

7. Effective bits: It is a measure of the oscilloscope's ability to accurately reproduce a sinusoidal signal waveform. This measure compares the actual error of the oscilloscope with the theoretically ideal digitizer. Since the actual error number includes noise and distortion, the frequency and amplitude of the signal must be specified.

8. Frequency response: Bandwidth alone is not enough to ensure that an oscilloscope accurately captures high-frequency signals. The target set for the oscilloscope 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 ringing. Since digital oscilloscopes are 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 from different manufacturers varies greatly.

9. Vertical sensitivity: Vertical sensitivity indicates the degree of amplification of weak signals by the vertical amplifier, usually expressed in millivolts per scale. The typical value of the minimum volt number that a multi-purpose oscilloscope can detect is about 1mv per vertical display scale.

10. Scan speed: Scan speed indicates how fast the trace sweeps across the oscilloscope display so that finer details can be found. The scan speed of the oscilloscope is expressed in time (seconds)/division.

11. Gain accuracy: Gain accuracy is a measure of how accurately the vertical system attenuates or amplifies the signal, and is usually expressed as a percentage error.

12. Horizontal accuracy: Horizontal or time base accuracy refers to the accuracy of the timing of the display signal in the horizontal system, usually expressed as a percentage error.

13. Vertical resolution: The vertical resolution of an analog-to-digital converter, or digital oscilloscope, refers to the accuracy with which the oscilloscope converts the input voltage into a digital value. Vertical resolution is measured in bits. Calculation methods can increase the effective resolution, such as high-resolution capture mode.