How The Digital Oscilloscopes Measure Small Signals

People who know about the principles of digital oscilloscopes can know they are sampling with high speed ADC

. In order to ensure get high sampling rate, so the ADC number of digital device is only 8 bit, so the noises of themselves is relatively high. Particularly when we testing the small signals, as the noise of small signals is not much higher than the noise of digital oscilloscopes, therefore, the signal noise ratio (SNR) is very bad. For example, a signal which is common in the communication, the frequency of it is low and the amplitude is small, so the noise of the digital oscilloscope above the signal will lead to the signal displayed unclear, therefore it is not easy card by cursor.

To improve the SNR, the common way we think is to use the function of waveform averaging on digital oscilloscopes. Averaging function is through multiple acquisitions, make the number of waveforms superimposed and average the noise, but only for repeatable signals. However, the signal is non-repeatable, make the signal waveforms superimposed then confuse itself, let alone to measure.

There are a lot of non-repeatable small signals need to be measured in practical applications, such as a power pulse, the signals from sensor, the ripple of power supply. As they are non-repeatable signals, so they can not use the function of average to be measured. But how to improve the accuracy of the measurement when measure these signals? The answer is to use the high-resolution mode of the digital oscilloscope.

The high resolution mode, that is to say average adjacent multiple points from a waveform which is collected by digital oscilloscope. We should notice the difference between high-resolution mode and the average mode. Average mode is made the points of the same position which on many waveforms averaged. And high-resolution mode is made the points which are adjacent on the same waveform averaged. Though the essence of high resolution is average, only one waveform is OK and not need many acquisitions.

By setting the high-resolution mode, we can see the effect of noise which on the digital oscilloscope is eliminated out much, the signals become more clearly, more accurate measurements will be.

However, the high-resolution mode is not omnipotent, the essence of averaging the adjacent sampling points is low pass filter of the signals, so if there are some high frequency components of the signal will be averaged out. From the waveform diagram, when the device is set to the high-resolution mode, the more points, the more clear signals, but the lower the bandwidth of the digital oscilloscope.

From that, we can see high-resolution mode improves the accuracy of measurement is to sacrifice for the bandwidth of digital oscilloscope. Therefore, high-resolution mode is not for the measurement of the high-frequency signals. The measurements for the high-frequency small signals are depending on the noisy indicators of the measuring instruments.

by: ellashining

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