Problems that should be noted when using virtual oscilloscopes
Bandwidth is one of the most important indicators of oscilloscopes. The bandwidth of a virtual oscilloscope is a fixed value, while the bandwidth of a virtual oscilloscope has two kinds of analogue bandwidth and digital real-time bandwidth. Virtual oscilloscopes for repeated signals using sequential sampling or random sampling techniques can achieve the highest bandwidth for the oscilloscope's digital real-time bandwidth, digital real-time bandwidth and the highest digitising frequency and waveform reconstruction technology factor K related to the (digital real-time bandwidth = the highest digitising rate / K), and is generally not given directly as an indicator. As can be seen from the definitions of the two bandwidths, 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 oscilloscope bandwidth 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 highest digital real-time bandwidth can only reach 400MHz far below the analogue bandwidth. So when measuring a single signal, be sure to refer to the digital real-time bandwidth of the virtual oscilloscope, otherwise it will bring unexpected errors to the measurement.
Sampling rate: Sampling rate, also known as the digitising rate, is the number of samples of an analogue input signal per unit of time, often expressed in MS/s. Sampling rate is an important indicator of the virtual oscilloscope. If the sampling rate is not enough, it is easy to mix the overlapping phenomenon.
If the oscilloscope's input signal is a 100KHz sinusoidal signal, while the oscilloscope shows the signal frequency is 50KHz, this is because the oscilloscope's sampling rate is too slow, resulting in the phenomenon of mixing. Mixed is the frequency of the waveform displayed on the screen is lower than the actual frequency of the signal, or even if the trigger on the oscilloscope has been lit, and the display of the waveform is still not stable. The generation of mixing is shown in Figure 1. Then, for an unknown frequency of the waveform, you can judge whether the displayed waveform has been generated by the mixing: slowly change the sweep speed t/div to a faster time base file, to see whether the frequency parameters of the waveform is a drastic change, if so, it shows that the waveform mixing has already occurred; or wobbling waveform stabilised in a faster time base file, it also shows 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, such as a 500MHz signal, at least 1GS/s sampling rate is required. There are several ways to simply prevent mixing from occurring:
? Using automatic settings
? Adjust the sweep rate;
? Try switching the collection method to Envelope or Peak Detection, as 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.
