Oscilloscope bandwidth refers to the frequency value of sinusoidal input signal source attenuated to 70.7% of its actual amplitude, i.e. - 3dB point, based on logarithmic scaling.
If there is not enough bandwidth, the oscilloscope will not be able to distinguish high-frequency changes. Amplitude will be distorted, edges will disappear, and details will be lost. Without sufficient bandwidth, all the characteristics of the signal source, ringing and ringing, etc. are meaningless.
The method of measuring the oscilloscope bandwidth: accurately characterize the signal source amplitude in the specific operation, and use the five-fold criterion. The measurement error of the selected oscilloscope using the five-fold criterion will not exceed +/-2%, which is enough for today's operation. However, with the increase of the signal source rate, this empirical criterion will no longer be applicable. Remember, the higher the bandwidth, the more accurate the reproduced signal source is.
rise time
In the digital world, the measurement of time is very important. When measuring digital signal sources, such as pulses and step waves, it may be more necessary to consider the performance of the rising time. The oscilloscope must have a long rise time to accurately capture the details of the fast-changing signal source.
Sampling rate
Sampling rate: Sampling point per second (S/s), the frequency at which an exponential digital oscilloscope samples a signal source, similar to the concept of a frame in a cinematographic camera. The faster the sampling rate of the oscilloscope, the higher the resolution and clarity of the waveform displayed, and the smaller the probability of loss of important information and events, as shown in Fig. 50. If a slowly varying signal source needs to be observed over a long time range, 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 sampling rate displayed will vary with the adjustment of the horizontal adjustment button.
Waveform capture rate
All oscilloscopes flicker. That is to say, the oscilloscope captures the signal source at a specific number of times per second, and no measurement will be made between these measuring points. This is the waveform capture rate, expressed as the number of waveforms per second (wfms/s). Sampling rate represents the frequency of the input signal source sampled by the oscilloscope in a waveform or period, and waveform acquisition rate is the speed of the waveform acquisition by the oscilloscope. Waveform acquisition rate depends on the type and performance level of the oscilloscope, and has a wide range of changes. The oscilloscope with high waveform acquisition rate will provide more important signal source characteristics and greatly increase the probability of oscilloscope capturing instantaneous anomalies such as jitter, short pulse, low frequency interference and instantaneous error (see Fig. 51 and 52).
Record length
Record length is expressed as the number of points constituting a complete waveform record, which determines the amount of data captured in each channel. Because 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 recording lengths in order to optimize the details of some operations. Analyzing a very stable sinusoidal signal source requires only 500 points of recording length, but if we want to parse a complex digital data stream, we need one million or more points of recording length.
Trigger capability
The trigger function of the oscilloscope synchronizes horizontal scanning at the correct signal source position, which determines whether the signal source characteristics are clear or not. The trigger control button can stabilize the repetitive waveform and capture the single pulse waveform. For more information on trigger performance, refer to the trigger section for performance terminology and applications.
Effective bit
Effective bit is a measure of the oscilloscope's ability to accurately reproduce sinusoidal source waveforms. This measure compares the actual errors of the oscilloscope with the ideal digitizer in theory. Since the actual number of errors includes noise and distortion, the frequency and amplitude of the signal source must be specified.
frequency response
Bandwidth alone is not enough to ensure that the oscilloscope accurately captures high-frequency signal sources. The objective 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. Because the digital oscilloscope is composed of an actual amplifier, attenuator, analog-to-digital converter (ADC), connector and relay, the MFED response is only an approximation of the target value. Pulse fidelity varies greatly between different models and products from different manufacturers (Figure 46 illustrates this concept).
Vertical sensitivity
Vertical sensitivity indicates the magnification of weak signal sources by vertical amplifiers, usually expressed in millivolts (mv) per scale. The typical minimum volt number detected by a multipurpose oscilloscope is about 1 MV per vertical display scale.