Monday, February 7, 2011
【 Weak current College 】 buy digital oscilloscope should pay attention to what issues 】
<br> <br> <br> Digital oscilloscope triggers for a waveform, storage, display, measurements, waveform data analysis processing, and other unique advantages, its use is becoming increasingly popular. Because digital oscilloscope with analog Oscilloscopes have a large performance difference ., if used improperly, can produce larger measurement error, thus affecting the test tasks. <br> Distinguish between analog bandwidth and digital real-time bandwidth <br> <br> Bandwidth is the most important indicator of the oscilloscope. Analog oscilloscope bandwidth is .a fixed value, and digital oscilloscope bandwidth analog bandwidth and two digital real-time bandwidth. Digital oscilloscope on repeating signal using sequential sampling or random sampling technology can achieve the highest bandwidth for digital real-time oscilloscope bandwidth digital real-time bandwidth and highest digital .frequency and waveform reconstruction techniques factor K related (digital real-time bandwidth = maximum digital rate / K), generally does not directly as an indicator. From the definition of two kinds of bandwidth can be seen, analog bandwidth only suitable for measurement of repeating .cycle signal, and digital real-time bandwidth at the same time suitable for repetitive signals and measurement of single signal. The manufacturers claim that the oscilloscope bandwidth can achieve much Siu, actually refers to the analog bandwidth, the bandwidth digital real-time is below .this value. For example say TEK Company TES3054B bandwidth for 500MHz, actually refers to its analog bandwidth, and the highest 500MHz digital real-time bandwidth can only achieve 400MHz far below analog bandwidth. Thus in the measurement of single signals, be sure to reference .the digital oscilloscope digital real-time bandwidth, otherwise the measurement will be to bring unexpected error. <br> <br> The sample rate <br> <br> Sample rate also known as digital rate refers to the unit of time, the analog .input signal sampling times, often expressed in MS / s. Sample rate is a digital oscilloscope, an important indicator. <br> <br> 1. If the sampling rate is prone to the phenomenon of mixtures <br> <br> If you .lose one signal oscilloscope as a sinusoidal signal, 100KHz oscilloscope displays the signal frequency is 50KHz, what's going on? this is because the oscilloscope sample rate is too slow, produces overlap. Overlap is the screen waveform frequency is lower than the actual frequency of the .signal, or even when the oscilloscope triggers the led on the light, and have displayed waveform is still unstable. Overlap of generated as shown in Figure 1. So, for an unknown frequency waveform, how to determine whether the displayed waveform has been producing mixtures .? you can slowly change the sweep speed t / div to a faster time base file, look for waveform frequency parameters are rapidly changing, if yes, describe waveform mixtures have occurred; or shaking of the waveform in a faster time base stalls stabilized, also .note the waveform mixtures has occurred. According to the Nyquist theorem, the sample rate is higher than the signal frequency components at least 2 times don't overlap occurs, such as a 500MHz signal, you need at least 1GS / s sample rate. There .are several ways you can easily prevent overlap occurs: <br> <br> Adjust the scanning speed; <br> <br> The use of automatic settings (Autoset); <br> <br> Try collection over to the envelope or peak .detect mode, because the envelope mode is multiple collection records for extreme peak detect mode, which is in a single collection records looking for maximum minimum value, both methods are detected faster signal changes. <br> <br> If the oscilloscope is InstaVu acquisition .mode, you can use, because this way acquisition waveform speed, use this method to display the waveform similar to use analog Oscilloscopes display waveform. <br> <br> 2. the sampling rate and the relationship between t / div <br> < .br> Each digital oscilloscope maximum sample rate is a fixed value. However, in either the scan time t / div, sample rate fs from next-gives: <br> <br> Fs = N / (t / div) of N sampling .points for each cell <br> <br> When sampling point n is a value, the fs and inversely t / div, so the larger the sample rate as soon as possible. The following is a set of scan TDS3054B speed and sample rate of .the data: <br> <br> Table 1-scan rate and sample rate <br> <br> t / div (ns) 1252550100200fs (GS / s) 502510210.50.25 <br> <br> In summary, when using a .digital oscilloscope, in order to avoid overlap, the sweep speed stalls preferably placed sweep speed faster. If you want to capture the moment dying Burr, sweep speed stalls is best placed in the main scan speed is slower. <br> <br> Digital .oscilloscope rise time <br> <br> In analog oscilloscope, oscilloscope rise time is a very important indicator. In DSO, rise time even as indicators clearly show. Because digital oscilloscope measurements of the reasons that the automatic measurement of rise time not only with .the location of sampling points, such as indicated in Figure 2, a rising edge just landed in the middle of two sampling point, then rise time interval for the digitalization of 0.8 times. Figure 2 b of the rising edge of the central one sampling point ., the same waveform, rise time interval for the digitalization of 1.6 times. In addition, the rise time is also relevant with sweep speed, here is when the same waveform TDS3054B measurement of a group of sweep speed and rise time data: <br> .<br> Table 2 sweep speed and rise time <br> <br> t / div (ms) 502010521tr (μs) 800320160803216 <br> <br> By the above set of data can be seen, although waveform rise time is a fixed .value, and digital oscilloscope measurement results are different because the scanning speed is far. Analog oscilloscope rise time has nothing to do with the sweep speed, and digital oscilloscope rise time not only with the sweep speed, also the location of sampling points, use digital .oscilloscope, we cannot use analog oscilloscope, according to the measured time to reverse launch signal rise time. <br> <br> <br>.