How to use LOTO oscilloscope to draw frequency response characteristic curve?

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How to use LOTO oscilloscope to draw frequency response characteristic curve? [Copy link]

In work and projects, we often encounter a functional circuit module to condition the signal, or filter, or amplify, or attenuate, or transform impedance. These functional circuit modules may be passive resistors and capacitors, or active op amp circuits, or more complex systems. However, the most important characteristic of their signal conditioning is the frequency response characteristic curve. Most of the time, we need to understand its frequency response curve to design and verify the system.

The host computer software of the LOTO oscilloscope has the functions of frequency sweep and frequency response characteristic curve plotting. If you purchase a combination of an oscilloscope + signal source module, such as OSCA02S, you can plot the frequency response characteristic curve of the circuit under test without the need for additional products.

The following figure shows the typical wiring for frequency response characteristic curve test:

In the figure above, we have prepared an op amp module as the circuit under test. We connect the output of the signal source module to the input of the op amp. The software will control the frequency sweep of a sine wave signal output to stimulate the op amp circuit. This input terminal is also connected in parallel with an oscilloscope probe, and this signal is input to the channel B of the oscilloscope. In this way, we can see the frequency sweep output of the signal source on the oscilloscope, which is the input signal waveform of the circuit under test. The op amp circuit under test amplifies the input signal and outputs it. We directly connect the output signal to the channel A of the oscilloscope.

Now the hardware equipment and wiring are ready. If you do not have a suitable circuit to be tested in order to get familiar with the function, you can also manually adjust the amplitude attenuation of the signal source during the frequency sweep to simulate. We can measure the curve of the amplitude of the output signal of the op amp as the frequency changes, and we can also measure the curve of the amplification factor of the op amp as the signal frequency changes.

In the software, we first need to set the parameters of the frequency sweep, as shown in the figure below, including the output sine wave, the starting frequency of the frequency sweep, the final frequency, the step amount, the step time interval, etc. One thing that needs special attention is that we must select "Automatic scanning of frequency response curve". This option will help us automatically set the time gear of the oscilloscope during the frequency sweep process, so that we do not need to manually adjust the time gear continuously to prevent the waveform from being too dense or too sparse.

Then, we click this button to open the frequency response curve function interface:

We will open the control panel and graphical interface of the frequency response characteristic curve:

In the curve fitting setting area, we have to select "None", which means no curve fitting. We use real measurement points to represent the entire frequency response curve. After completion, we can do curve fitting.

For example, if we want to measure the response curve of the amplitude of the op amp output as the frequency changes, then we can choose the amplitude of the channel corresponding to the oscilloscope connected to the op amp output signal as the ordinate of the frequency response curve:

After the settings are completed, we switch back to the interface of the oscilloscope and signal source, and click the sweep button to start the sweep:

The frequency sweep will be performed according to the set parameters, but there will be a switching time of one or two seconds at the beginning. The signal output during this period may not be stable, so after the waveform is stable, we click the start button on the frequency response curve interface to start drawing the frequency response curve, as shown in the figure below:

Next, we don’t need to do anything. We just need to observe the drawing of the frequency response curve and wait for it to end. The following figure shows the frequency response curve composed of the data points we obtained by frequency sweeping:

When we have finished sweeping the frequency or observed that we have obtained enough curves, we can click the pause button in the lower right corner to end the drawing. At this time, even if the oscilloscope and signal source software are still sweeping the frequency, the interface of the frequency response curve will no longer be updated.

At this time, we can click the "validation" button. The function of this button is to normalize the data points in the frequency sweep process and remove some illegal data points caused by interference or misoperation, so as to better perform curve fitting, as shown in the following figure:

Above we see the frequency response characteristic curve of logarithmic coordinates obtained by frequency sweeping. This is because when we sweep the frequency, the logarithmic coordinate option is used by default. We can also select the linear coordinate option to display the frequency response curve of the linear coordinate system:

We can see that the frequency response curve has automatically marked the -3DB position and the corresponding cutoff frequency. The cutoff frequency is around 64K Hz. We can choose a variety of curve fitting methods: linear, quadratic polynomial, cubic polynomial, exponential fitting, logarithmic fitting.

Taking the logarithmic coordinate system as an example, quadratic polynomial fitting:

Taking the logarithmic coordinate system as an example, quadratic and cubic polynomial fitting:

Take the logarithmic coordinate system as an example, all fitting options are turned on:

Take the linear coordinate system as an example, all fitting options are turned on:

We have made a video to record and demonstrate the entire process. You can refer to the following video link:

https://www.ixigua.com/7135738415382790663?utm_source=xiguastudio

Jacktang

The combination of LOTO oscilloscope and host computer software is very powerful. It can sweep frequency and plot frequency response characteristic curves. This is useful.