How to judge whether the signal displayed by the oscilloscope is normal

Contestants measure the voltage waveform at the output port of the signal generator

Problem analysis: Use a signal displayed on the oscilloscope to ask whether the corresponding electromagnetic wire signal generator is working properly. In order to make a correct judgment, the questioner still lacks the following information:

The signal displayed by the oscilloscope is the output signal of the signal generator under what external load? Is the port disconnected, short-circuited, resistor dummy load, or directly connected to the track electromagnetic wire?

The signal generator comes from which company and what model it provides, which determines the basic form of the signal generator's fundamental wave and the steady flow condition.

For this reason, we assume that this signal source adopts the same output current form and steady current form as announced on the official competition website; the output load is directly connected to the electromagnetic wire of the track. Let’s analyze whether this signal source is normal?

Continuous signals and discrete signals

Fundamental

1. Standard signal source current standard and output circuit

In order to facilitate the students participating in the production of electromagnetic track power supply, the competition organizing committee stipulated a symmetrical and convenient current guidance signal that can be easily generated. The definition of the signal is a 20kHz symmetrical square wave current signal with a current amplitude of 100mA.

Current signal in magnet wire

The competition organizing committee provided the reference design plan for making the signal source and the finished circuit diagram. 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 relatively long, the inductance component increases, which will cause the output current value to be too large. This problem has been analyzed in many tweets on the public account, and a modification plan has been given.

Circuit diagram of AC signal source output stage

2. Current signal measurement

The current signal is a basic physical quantity, and the unit is ampere (A). It is defined as the current flowing through two infinitely long parallel wires 1 meter apart in vacuum when a force of 2*10^-7 Newtons is generated per meter of length. size. It is very difficult to directly use the defined method to measure the current signal. The usual methods for measuring current signals in circuits, especially high-frequency alternating signals, are as follows:

Series detection resistor in the loop: According to Ohm's theorem I = U/R, the current signal can be deduced from the detected voltage signal;

Utilizing AC transformers: This is the use of induced electromotive force generated by changes in the current magnetic field;

Using Hall characteristic devices;

Detection of thermal power generated by current, etc.

Since the electromagnetic guidance signal is an alternating signal, measuring the signal requires not only measuring the amplitude of the signal, but also measuring the waveform of the signal. Since only the fundamental signal component plays a navigation role in the game, it is enough to ensure that the amplitude of the fundamental wave of the signal meets the definition. Regarding this, there is an analysis in the previous tweet.

problem analysis

1. Input impedance of the circuit

In circuit theory, the ratio of current and voltage applied to the same port of a linear circuit network is defined as the input impedance of the circuit, and a circuit with inductance or capacitance is defined as reactance. For dynamic circuits, the input current, voltage and input impedance of the circuit are also a linear proportional relationship in the transformation domain (s domain). Through this relationship, the third variable can be found by knowing two of the variables.

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

For example, if the input voltage and impedance are known, the corresponding current can be found.

In the previous question, since we only know the voltage signal observed by the oscilloscope and do not know the reactance of the electromagnetic wire, it is essentially impossible to restore the input signal I(s), and thus it is impossible to judge whether the output current waveform of the signal generator meets the requirements.

However, it can still be found from the voltage waveform that the period of the current signal is approximately 50 microseconds, so the frequency of the current signal is judged to be 20kHz, which meets the definition of the rule.

2. Current and voltage waveforms under inductive + resistive load

The input to the track magnet wire can be approximated using a series connection of an inductor and a resistor. Typically, the inductance ranges from tens to hundreds of microhenries, and the resistance ranges from a few tenths to several ohms. Due to the presence of inductance, the actual output current will not have a sudden change, so it will return to the standard square wave current defined previously. According to the signal source output circuit driving mode and steady current mode, the voltage and current waveforms of the signal generator on the L+R circuit are roughly as shown in the figure below:

The output voltage and current signal approximate waveforms under LR load

The output signal can be approximately divided into two stages:

The first stage is the constant voltage output stage:

In the constant voltage output stage, when the load current has not yet entered the constant current limit, the circuit outputs a voltage of 2U0 and is applied to the load. U0 is the operating voltage of the output stage. If R is relatively small, the output current will rise approximately linearly.

The second stage is the 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 resistance of the load.

Based on the above analysis and comparing the waveform sent by the student who asked the question, it can be seen that the shape of the voltage U(t) is generally similar to the waveform displayed by the oscilloscope, but there is still a large distortion.

Verification experiment

Next, we use a standard digital display power supply used in the competition, and use an electromagnetic coil simulated by resistance and inductance to observe the output voltage waveform to further verify the theoretical analysis of the above analysis and discuss it in comparison with the waveform of the question.

Signal source delay measurement method used in the experiment

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

Signal source output voltage waveform under 15 ohm load

It can be seen that under resistive 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 value of the voltage is 1.5V, the resistance is 15 ohms, and according to Ohm's law, the output current setting value is 100mA.

Another interesting observation about the above measurement waveform is that for the same signal, when the scanning speed of the oscilloscope is 200ms/div and 10us/div, the waveforms are almost the same. Please note that at this time, the corresponding waveform is a square waveform with a frequency of 1Hz. Wave signal, one corresponds to a 20kHz signal. Why does the same signal oscilloscope measure two different frequencies?

Experiment 2: Use a 60uH color ring inductor as the load.

The measurement results are shown in the figure below:

The output voltage waveform of the color ring resistor as a signal source under load

Since the equivalent series resistance Rs in the inductor is very small, at 100mA, the corresponding constant voltage is only 0.1V, so in the above measured waveform, the most obvious one is the constant voltage output pulse signal. This waveform is consistent with the waveform analyzed previously.

For specific measurements, the relationship between L, R0, U0, I0, etc. can also be verified separately. The measurement and analysis process is omitted.

During the measurement process, as the scanning speed of the oscilloscope is different, different voltage waveforms will be measured for the output voltage signal. The three consecutive pictures below show the voltage waveforms measured at scanning speeds of 200ms, 40ms, and 10us respectively.

Voltage waveform with scanning speed of 200ms/div

Voltage waveform with scanning speed of 40ms/div

Voltage waveform with scanning speed of 10us/div

Analyzed

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

Comparison of the voltage waveforms of the track electromagnetic wire load and the color ring inductor experiment

Therefore, it is not yet known with certainty whether the signal source used by the student meets the competition requirements.

This difference may be caused by the delay and current stabilization method of the output stage circuit of the signal source used by the students being different from the official reference design.

Please ask students to follow the process in the above experiment and conduct the experiment to check whether the signal source is normal. In addition, please enter the calibration in the official account? Search the tweet "Easy Calibration of Signal Source Current University" to test the current size of the signal source.

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

Enter the four characters ?qcy in the official account to get the answer and its explanation.