Digital oscilloscope use must pay attention to the problem
1. Introduction
The use of digital oscilloscopes is becoming increasingly popular due to their unique advantages such as waveform triggering, storage, display, measurement, waveform data analysis and processing. Due to the large performance differences between digital oscilloscopes and analogue oscilloscopes, if they are not used properly, they will produce large measurement errors, thus affecting the test task.
2, distinguish between analogue bandwidth and digital real-time bandwidth
Bandwidth is one of the most important indicators of oscilloscopes. The bandwidth of an analogue oscilloscope is a fixed value, while the bandwidth of a digital oscilloscope has two types of analogue bandwidth and digital real-time bandwidth. The highest bandwidth that can be achieved by a digital oscilloscope using sequential or random sampling techniques for repetitive signals is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digitising frequency and the waveform reconstruction technique factor K (digital real-time bandwidth = the highest digitising rate / K), which is generally not given directly as an indicator.
From the definitions of the two bandwidths, it can be seen that the analogue bandwidth is only suitable for the measurement of repetitive periodic signals, while the digital real-time bandwidth is suitable for both repetitive signals and single-shot signals. Manufacturers claim that the bandwidth of oscilloscopes can reach how many megabytes, in fact, refers to the analogue bandwidth, digital real-time bandwidth is lower than this value. For example, the bandwidth of TEK's TES520B is 500MHz, which actually refers to its analogue bandwidth of 500MHz, while the maximum digital real-time bandwidth can only reach 400MHz, which is far below the analogue bandwidth. Therefore, when measuring a single signal, be sure to refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement.
3, about the sampling rate
Sampling rate, also known as the digitising rate, refers to the unit of time, the number of samples of the analogue input signal, often expressed in MS/s. Sampling rate is an important indicator of digital oscilloscopes.
(1) If the sampling rate is not enough, the mixing phenomenon is easy to occur.
If the input signal of the oscilloscope is a 100KHz sinusoidal signal, the oscilloscope displays a signal frequency of 50KHz, how is this? This is because the sampling rate of the oscilloscope is too slow, resulting in the phenomenon of aliasing. Mixed is the frequency of the waveform displayed on the screen is lower than the actual frequency of the signal, or even if the oscilloscope on the trigger indicator has been lit, and the display of the waveform is still not stable. The generation of mixing is shown in Figure 1.
So, for a waveform of unknown frequency, how to determine whether the displayed waveform has generated a mixing? It can be done by slowly changing the sweep speed t/div to a faster time base, to see if the frequency parameter of the waveform changes sharply, if yes, it means that the waveform mixing has already occurred; or the wobbling waveform stabilises at a faster time base, which also means that the waveform mixing has already occurred. According to Nyquist's theorem, the sampling rate should be at least 2 times higher than the high-frequency component of the signal in order to avoid mixing, for example, a 500MHz signal needs at least 1GS/s sampling rate. There are several ways to prevent mixing from occurring in a simple way:
a. Adjust the sweep rate;
b. Use Autoset;
c. Try switching the collection method to Envelope or Peak Detection, since Envelope looks for extreme values in multiple collection records and Peak Detection looks for maximum and minimum values in a single collection record, both of which can detect faster signal changes.
If the oscilloscope has an InstaVu collection method, it can be used because this method collects waveforms quickly, and the waveforms displayed with this method are similar to those displayed with an analogue oscilloscope.
(2) Relationship between sampling rate and t/div
The maximum sampling rate of each digital oscilloscope is a fixed value. However, at any one scan time t/div, the sampling rate fs is given by the following formula: fs=N/(t/div) N is the sampling points per frame.
When the number of sampling points N is a certain value, fs is inversely proportional to t/div, the larger the sweep speed, the lower the sampling rate.
In summary, when using a digital oscilloscope, in order to avoid mixing, it is best to place the sweep speed gear in a faster position. If you want to capture fleeting burrs, the sweep speed is best placed in the slower position of the main sweep speed.
