The participants measured the voltage waveform of the output port of the signal generator

Problem analysis: through a signal displayed by the oscilloscope to ask whether the corresponding electromagnetic line signal generator works normally. In order to judge correctly, the questioner still lacks the following information:

The signal displayed by oscilloscope is the output signal of signal generator under what external load? Is the port open, or is it short circuited, or is it a resistive dummy load, or is it directly connected to the track electromagnetic line?

Signal generator is from which company to provide what model, which determines the basic form of signal generator and steady current.

For this reason, we assume that the signal source uses the same output current form and stable current form as published on the official website of the competition; the output load is directly connected to the electromagnetic line of the track. Is this signal source normal?

Continuous signal and discrete signal

## Basic principles

1、 Standard signal source, current standard and output circuit

In order to facilitate the students to make electromagnetic circuit power supply, the Organizing Committee of the competition stipulates a symmetrical and convenient current guidance signal which can be easily generated. The definition of signal is a 20 kHz symmetrical square wave current signal, and the current amplitude is 100 mA.

Current signal in electromagnetic wire

The Organizing Committee of the competition provides the reference design scheme and the finished circuit diagram of the signal source. According to the use of several competitions, this form of power supply basically meets the requirements of the competition. However, when the external electromagnetic wire is longer, the inductance value increases, which will cause the output current value to be too large. This paper analyzes the official account in the public number and gives the modification plan.

Circuit diagram of output stage of AC signal source

2、 Current signal measurement

The current signal is a basic physical quantity, the unit is ampere (a), which is defined as the current passing by two infinite parallel wires in vacuum with a distance of 1 meter when the force of 2 * 10 ^ – 7 Newton is generated per meter of length. It is very difficult to measure the current signal directly by using the defined method. There are several methods to measure the current signal in the circuit, especially the high frequency alternating signal

Series detection resistance in the loop: according to Ohm theorem I = u / R, the current signal can be calculated from the detected voltage signal;

Using AC transformer: This is to use the induced electromotive force generated by the change of current magnetic field;

Using Holt device;

Using the thermal power generated by the current to detect.

As the electromagnetic guidance signal is an alternating signal, the measurement of the signal not only needs to measure the amplitude of the signal, but also needs to measure the waveform of the signal. Because only the fundamental signal component plays a navigation role in the competition, as long as the amplitude of the fundamental signal can meet the definition. This is analyzed in the previous tweet.

problem analysis

1、 The input impedance of the circuit

In circuit theory, the ratio of current and voltage applied on the same port of linear circuit network is defined as the input impedance of the circuit, and the circuit with inductance or capacitance is defined as reactance. For dynamic circuits, the input current, voltage and impedance are also proportional in the s domain. We can find out the relationship between the third variable and the second variable.

The voltage, current and impedance of the circuit system corresponding to the same port

For example, if you know the input voltage and impedance, you can calculate the corresponding current.

In the previous question, because we only know the voltage signal observed by the oscilloscope, we don’t know the reactance of the electromagnetic line, so we can’t restore the input signal I (s), and then we can’t judge whether the output current waveform of the signal generator meets the requirements.

However, the period of the current signal can be calculated by the voltage waveform, which is about 50 μ m, so the frequency of the current signal is 20 kHz, which meets the definition of the rule.

2、 Current and voltage waveforms under inductance + resistance load

The input end of the track electromagnetic line can be approximated by the series connection of inductance and resistance. In general, the inductance ranges from tens to hundreds of microhens, and the resistance ranges from 0.0 to several ohms. Because of the inductance, the actual output current will not have a sudden change, so the standard square wave current defined above will be changed. According to the driving mode and current stabilizing mode of the signal source output circuit, the voltage and current waveforms of the signal generator on the L + R circuit are shown as follows:

The approximate waveform of output voltage and current signal under LR load

The output signal can be approximately divided into two stages

The first stage is constant voltage output stage

Constant voltage output stage is when the load current has not entered the constant current limit, the circuit output 2u0 voltage is applied to the load. U0 is the operating voltage of the output stage. If R is small, the output current will rise linearly.

The second stage is constant current output stage

When the current reaches the current limit value, the output current is a constant set value; the output voltage is equal to the current set value multiplied by the load resistance.

According to the above analysis, compared with the waveform sent by the students, we can see that the shape of the voltage U (T) is similar to the waveform displayed by the oscilloscope, but there is still a great deformation.

## Verification experiment

Next, we use the standard digital display power supply used in the competition, and use the resistance and inductance analog electromagnetic coil to observe the output voltage waveform, so as to further verify the theoretical analysis of the above analysis.

Signal source delay measurement method used in the experiment

The figure below shows the output voltage waveform of the signal source.

Output voltage waveform of signal source under 15 ohm load

It can be seen that under the resistance load, the output voltage waveform is very close to the symmetrical square wave defined by the rules. Since there is no inductive reactance, there is a proportional relationship between voltage and current.

The peak voltage is 1.5V and the resistance is 15 ohm. According to Ohm theorem, the set value of output current is 100mA.

Another interesting appearance of the above measurement waveform is that the waveform of the same signal is almost the same when the scanning speed of the oscilloscope is 200ms / grid and 10us / grid respectively. Please note that at this time, a square wave signal with frequency of 1Hz and a 20kHz signal are corresponding. Why does the same signal oscilloscope measure two different frequencies?

Experiment 2: a 60 uh color loop inductor was used as the load.

The measurement results are shown in the figure below

Output voltage waveform of signal source with color ring resistor as load

Because the equivalent series resistance Rs in the inductor is very small, the corresponding constant voltage is only 0.1V at 100mA, so it is obvious that the constant voltage output pulse signal can be seen in the above measurement waveform. This waveform is consistent with the previous analysis.

The relationship between L, R0, U0 and I0 can also be verified by specific measurement. The process of measurement and analysis is omitted.

In the measurement process, with the different scanning speed of the oscilloscope, different voltage waveforms will be measured for the output voltage signal. The following three consecutive figures show the voltage waveforms measured at 200ms, 40ms and 10us respectively.

Voltage waveform with scanning speed of 200ms / grid

Voltage waveform with scanning speed of 40ms / grid

Voltage waveform with scanning speed of 10us / grid

## Summary and analysis

Careful students will find that the voltage waveform displayed in the question is quite different from that measured in the experiment using the color ring inductor as the load. The biggest difference is that the measurement waveform sent has a great fluctuation in the constant current stage, and has a large reverse voltage overshoot.

Comparison of voltage waveform between electromagnetic line load and color loop inductance experiment

Therefore, it is not clear whether the signal source used by the student can meet the requirements of the competition.

This difference may be caused by the difference between the delay current stabilizing mode of the output stage circuit of the signal source used by students and the official reference design.

Please ask the students according to the above experimental process, control experiment, to check whether the signal source is normal. In addition, please input the calibration in the official account. Search the tweet “simple calibration of signal source current” to test the current of signal source.

For the same signal in the experiment, why does the oscilloscope measure the signal waveforms of different frequencies?

In the official account, input qcy four characters, you can get the answer and its explanation.

Through the discussion of this problem, we can know that all our observed signals are actually converted from the signals to be sampled through a series of systems. For example, the measured current signal is converted into voltage signal by circuit system, and converted into acquisition value and displayed by oscilloscope system. The wrong understanding of any link will affect our judgment of the signal to be measured.

Cognitive bias

Similarly, our understanding of the external world is also transmitted through a series of external physical systems, coupled with sensory (eye, ear, touch, etc.) systems, and then comes to a conclusion through the innate and acquired cognitive systems. All these will distort the external reality. Therefore, in the face of their own conclusions, or the conclusions of others, we need to be able to enhance vigilance and cultivate more comprehensive analytical skills, so as to avoid more ridiculous and absurd cognitive results.

Editor: hfy