【 Weak current College 】 digital oscilloscope use you must be aware of the problem 】

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.

Distinguish between analog bandwidth and digital real-time bandwidth

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 TES520B 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.

The sample rate

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.

1. If the sampling rate is prone to the phenomenon of mixtures

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:

Sweep speed adjustment;

? Use of automatic settings (Autoset);

? 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.

? 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.

2. the sampling rate and the relationship between t/div

Each digital oscilloscope maximum sample rate is a fixed value. However, in either the scan time t/div, sample rate fs from next-gives:

Fs = N/(t/div) of N sampling points for each cell

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 TDS520B speed and sample rate of the data:

Table 1-scan rate and sample rate

t/div(ns)1252550100200fs(GS/s)502510210.50.25

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 TDS520B measurement of a group of sweep speed and rise time data:

t/div(ms)502010521tr(μs)800320160803216

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.

Oscilloscope use considerations

(1) hot electronics generally avoid frequent power on, power off, so it is with an oscilloscope.

(2) if it is found that the waveform from outside interference, oscilloscope shell grounding.

(3) enter "Y" must not be too high voltage to avoid damage to the instrument, inmaximum attenuation also cannot be more than 400V.“ Y enter the "conductor suspension, subject to external electromagnetic interference interference wave occurs, you should avoid this phenomenon.

(4) shutdown before-degree adjustable knob counter-clockwise turn, the brightness is reduced to a minimum, and then disconnect the power switch.

(5) in the observation of the bright spot on the screen and adjust the brightness of the bright spot to moderate, not too light.