Introducing oscilloscope frequency response, FFT function and probe attenuation comparison measurement

Oscilloscope measurement is something that many engineers need to understand. After all, it is the engineer's "eyes". If the "eyes" want to be bright, the alignment function needs to be well understood. Today, engineers from Boyu Xunming gave a brief introduction to the frequency response, FFT function and probe attenuation ratio of the oscilloscope, hoping to help engineers in need better understand the measurement functions of the oscilloscope.

Oscilloscope frequency response

System frequency response is a frequency-dependent function that represents how a reference signal (usually a sinusoidal waveform) of a specific frequency at the system input is transmitted through the system.

Use an oscilloscope to test the control loop response by controlling the built-in function generator or external function generator to sweep a sine wave within a certain frequency range while measuring the input and output of the device under test (DUT). Gain (G) and phase (P) are measured at each frequency and plotted on a frequency response Bode plot. When the loop response analysis is complete, you can move the markers on the graph to view the gain and phase values ​​measured at various frequency points.

Operation steps of FFT function of oscilloscope

Frequency domain analysis is a common method of signal analysis. Through spectrum analysis, many signal problems that are not clear or unclear in the time domain can be discovered. The oscilloscope is a traditional time domain analysis tool, but with the help of mathematical functions such as FFT, the oscilloscope can also implement the spectrum analyzer analysis function, thereby providing powerful time domain and frequency domain analysis functions.

Let’s use this oscilloscope to demonstrate the FFT function:

First, use an arbitrary wave generator to generate a wave signal with a frequency of 1KHz, an amplitude of 1Vp-p, and an impedance of 50 ohms. Input the arbitrary wave signal into channel one of the oscilloscope, click AUTOSETUP, and the waveform will be displayed normally.

Next, open the oscilloscope math function: Menu---Select function---Frequency analysis--click FFT

After turning on the FFT function, click F2 (fft) at the bottom of the screen, and the FFT configuration menu pops up on the right side of the screen - find the horizontal menu, enter it and set the frequency range to 20KHz. You can see the frequency distribution of the signal. In order to make the waveform more stable, adjust the level. The time base is 2ms per grid.

Then, you can open the tool under FFT configuration and open the display table, so that we can see the amplitude and frequency interval corresponding to different frequency points.

The influence of oscilloscope probe attenuation ratio on measurement

First, let’s understand the concept. The attenuation ratio is the attenuation ratio of the signal to the probe itself. In most cases, the oscilloscope can automatically identify the attenuation ratio, so that the oscilloscope can amplify the signal to the corresponding ratio display, so the user feels the Not affected by probe attenuation.

Figure 1, voltage probe 300V, 10:1

Figure 2. High voltage probe 10KV, 1000:1

However, when measuring small signals, if the oscilloscope uses a large attenuation ratio probe, the minimum range of the oscilloscope will become larger, which is very detrimental to the test.

For example:

The minimum range of the oscilloscope is 1mv/div. After connecting a 10:1 probe, the minimum range of the oscilloscope becomes 5mv/div, and the noise of the oscilloscope is amplified accordingly.

When using the 1:1 attenuation ratio, the minimum range of the oscilloscope is 1mv, and the trace noise is 467 microvolts;

When the attenuation ratio of the oscilloscope is 10:1, the noise of the oscilloscope is 5mv peak to peak; the noise of the oscilloscope becomes louder.

Therefore, in many situations where small signals are measured, it is recommended to use probes with small attenuation ratios while ensuring bandwidth.