As is well known, electromagnetic fields are the most common type of force field. Combining electromagnetic field-based sensors with analog/digital circuits for signal analysis and processing, along with computers, creates an electromagnetic field-based non-destructive testing (NDT) system. Although significant progress has been made in electromagnetic field-based NDT, the results still have many shortcomings. One major contributing factor is that the signal of the testing system is susceptible to environmental interference, severely impacting its operation. The signal generation circuit was developed against this backdrop. [b]1 Basic Requirements for Signals in Electromagnetic Field NDT[/b] In electromagnetic field-based NDT systems, signal frequency has a significant impact on the testing results. Unstable signal frequencies can cause considerable difficulties for subsequent circuit processing, even hindering electromagnetic field detection. Therefore, a series of effective measures must be taken in circuit design to eliminate the influence of environmental factors such as temperature, ensuring a constant signal frequency during the testing process. Clearly, the design of the signal generation circuit is one of the most critical aspects of signal analysis and processing circuit design. Because the signal generation circuit is the signal source of the entire detection system, the stability of its signal frequency plays a crucial role in the operation of the entire detection system. [b]2 A New Signal Generation Circuit Design[/b] To meet the basic signal requirements of the electromagnetic field non-destructive testing system, a new signal generation circuit is proposed below. Its working principle is shown in Figure 1. [align=left] Figure 1 Signal Generation Circuit In the circuit diagram shown in Figure 1, an 8.192MHz signal is first generated by a crystal oscillator. This signal is then divided by a frequency divider (14 division) to become 500Hz (i.e., 8.192×10⁶/2¹⁴), which will be used as the standard frequency. The signal generator XR2206 will generate two signals with identical frequencies, one a sine wave and the other a square wave. The square wave signal is sent to the frequency divider circuit for division. To obtain a step-change frequency, the frequency division coefficient Kf of the frequency divider circuit has two values, namely 2 and 16. When the signal FRQA7 from the latch is low, the division factor Kf = 2; and when FRQA7 is high, the division factor Kf = 16. Let the frequency of the signal generated by the signal generator be f. After being divided by the frequency divider circuit, this signal becomes f/Kf. This signal is then sent to a synchronous subtractor, where it is processed with the lower 7 bits of the latch, FRQA0~6 (let's say this data is Ka), resulting in a signal frequency of f/KfKa. This output signal, along with a standard signal of 500Hz generated by the crystal oscillator and divided, is sent to a phase-locked loop for comparison. If they are equal (i.e., f/KfKa = 500), the frequency f of the signal generated by the XR2206 signal generator no longer changes. In this case, the frequency f can be calculated using the following formula: f = 500KfKa. If the frequency of the signal output from the synchronous subtractor is greater than 500Hz (i.e., f/KfKa > 500), the voltage Vc output by the phase-locked loop will decrease, thus causing the signal frequency f generated by the waveform generator XR2206 to decrease. This process continues until the signal frequency f/KfKa output by the synchronous subtractor equals 500Hz, thus ensuring f ≡ 500KfKa. If the frequency of the signal output from the synchronous subtractor is less than 500Hz (i.e., f/KfKa < 500), the voltage Vc output by the phase-locked loop will increase, thus causing the signal frequency f generated by the waveform generator XR2206 to increase. This process also continues until the signal frequency f/KfKa output by the synchronous subtractor equals 500Hz, thus always ensuring f ≡ 500KfKa. Therefore, the above circuit forms a closed-loop control. Regardless of changes in environmental factors (such as temperature), the signal frequency f generated by the signal generator XR2206 will not be affected. Thus, the signal frequency f generated by the signal generator XR2206 will always be constant (i.e., f ≡ 500KfKa, where Ka is a data sent by the computer to the latch and latched by the latch). This is the essence of the signal generation circuit design. Since the frequency of the sine wave generated by the signal generator XR2206 is exactly the same as the frequency of the square wave, the frequency of the sine wave will also always be constant at f, which is exactly what the test system needs. [b] Figure 1 will have 3 output signals with a frequency of f: [/b] (1) Sine wave Asin2πft, which will be applied to the inductor sensor to induce a magnetic field; (2) Square wave signal A90 that leads the above sine wave signal by 90°; (3) Square wave signal A0 that is synchronized with the above sine wave signal. When an inductive sensor is used for detection, the induced magnetic field generated by the sinusoidal signal changes, and the impedance of the inductive sensor also changes, thus altering the amplitude and phase of the inductive sensor's output signal. To calculate the amplitude, and especially the phase, of the inductive sensor's output signal at this time, the sensor's output signal needs to be demodulated using two subsequent square wave signals, that is, the output sinusoidal signal is divided into real and imaginary parts. The specific processing circuitry and analysis calculations will not be elaborated upon here. The XR2206 waveform generator is a relatively new signal generator. Its pin connections are shown in Figure 2, where Q5 is a field-effect transistor. The specific parameters of each component are shown in Tables 1 and 2. [ Figure 2 Waveform Generator] [Table 1 Resistance Parameter Values, kΩ] R1 R2 R4 R13 R14 R15 R16 R17 VR1 VR2 VR3 5.10​ 5.10 10.00 10.00 1.00 10.00 1.00 0.50 5.00 100.00 C[sub][size=2]1 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]2 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]3 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]4 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]6 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]7 [/sub][/size][/font] [/td][td][font=SimSun] C[sub][size=2]8 [/sub][/size][/font] [/td][/tr][tr][td][font=SimSun] 10.0 [/font] 1.0×10⁻³ 1.0 10.0 1.0 1.0 3.3 × 10⁻³ [align=left]Figure 3 Eddy current sensor detection Furthermore, various electronic instruments and meters have high requirements for signal frequency stability. Especially for electronic instruments and meters used in industrial control and testing, due to harsh working environments, strong anti-interference capabilities of the signal frequency are required. Therefore, the development of signal generation circuits provides an important way to improve the performance of such instruments and meters. [b]4 Conclusion[/b] In many engineering applications, there is a requirement to ensure stable signal frequency. The signal generation circuit proposed in this paper is designed to meet this requirement. This circuit uses a new type of signal generator, XR2206, and employs closed-loop control to eliminate temperature drift, ensuring stable signal frequency and thus meeting the signal frequency requirements of electromagnetic field non-destructive testing. Undoubtedly, this signal generation circuit is also fully suitable for other engineering applications requiring frequency stability. [/align]