Lecroy: Oscilloscope Sampling Rate And Memory Depth (b)

Leaving the subject of sampling rate and Storage Depth of these two indicators related theory

, the next combination of common applications, we were more in-depth look at these two parameters on the real test of our. Power Measurement

the importance of long-stored As power Electronic The frequency is relatively low (mostly less than 1MHz), the use of high bandwidth used Oscilloscopes Do high-speed signal measurement engineers, often have a misconception, power measurement may be very simple fact is that for power measurement applications in the oscilloscope select number of engineers made a mistake, although the bandwidth of 500MHz oscilloscope with the power switch for a few hundred KHz frequency is adequate, but often we have ignored the sampling rate and memory depth of choice. For example, in common Switching Power Supply The test voltage is usually in the 200KHz switching frequency or faster, since there is often switching signal frequency modulation, engineers need to capture a quarter of cycle frequency signal, or half-cycle, or even more cycles. Switching signal rise time is about 100ns, we propose to ensure accurate reconstruction of the rising edge of the signal waveform on the need to have 5 or more sampling points, sampling rate that is at least 5/100ns = 50MS / s, that is, two samples time interval between points is less than 100 / 5 = 20ns, for at least capture the requirements of a frequency cycle, means that we need to capture a 20ms long wave, so that we can come up with the required storage oscilloscope per channel depth = 20ms/20ns = 1Mpts! ! ! Similarly, in analyzing the soft-start power supply during power devices to withstand the maximum voltage stress on the power you need to capture the whole process (more than ten milliseconds), the required storage oscilloscope sampling rate and depth of even higher!

I regret is that I often see engineers with only a 10K memory depth per channel oscilloscope to the top of the power supply test! ! ! Thus all the more felt as the oscilloscope manufacturers need to spend more time and energy to help engineers use the oscilloscope to establish a proper perspective. This is our Shenzhen office to write a series of articles in mind.

Storage depth on the results of FFT

In DSO, by fast Fourier transform (FFT) can be the signal spectrum, and thus a signal in the frequency domain analysis. Such as power harmonics measurement need to observe the FFT spectrum, high-speed serial data measurements are also often lead to system failure FFT to analyze the noise and interference. For the FFT operation, the oscilloscope acquisition memory available will determine the amount of signal components can be observed the maximum range (Nyquist frequency), while storage also determines the depth of frequency resolution f. If the Nyquist frequency of 500MHz, a resolution of 10kHz, consider the observation window to determine the length and the acquisition buffer size. To obtain a resolution of 10kHz, the acquisition time of at least: T = 1 / f = 1/10kHz = 100ms, 100kB memory for a digital oscilloscope, the maximum frequency can be analyzed:

f N / 2 = 10kHz 100kB / 2 = 500MHz

Figure 11 Oscilloscope FFT computation

Shown in Figure 12 the example, 266MHz 30kHz signal from the noise source by picking up noise. FFT (bottom of the track) shows to 266MHz for the middle of the range of 30kHz away from the peak. This distortion is very common, probably due to switch mode power supplies, DC-DC converters or other sources of crosstalk caused. It also may be the result of deliberate use of spread spectrum clock.

Lecroy: Oscilloscope Sampling Rate And Memory Depth (b)

By: xt

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