The voltage frequency converter VFC (Voltage Frequency Converter) is a device that implements an analog-to-digital conversion function, which converts an analog voltage amount into a pulse signal whose frequency is proportional to the magnitude of the input voltage. The voltage-to-frequency converter is also called a voltage-controlled oscillating circuit (VCO), which is referred to as a voltage-controlled oscillating circuit . Voltage-to-frequency conversion is actually a conversion technique between analog and digital quantities. When an analog signal (voltage or current) is converted to a digital signal, the output of the converter is a series of rectangular waves whose frequency is proportional to the amplitude of the analog signal. Obviously the data is serial. This is different from the parallel output of the current general-purpose analog-to-digital converter, but its resolution can be very high. The analog-to-digital conversion of the serial output is useful in digital control systems, which can turn the analog error signal into a pulse signal proportional to it to drive the step servo for precise control. The VFC voltage-to-frequency converter (vfc) is a circuit produced by the Qingdao Transistor Research Institute. Voltage to frequency conversion can also be referred to as volt-frequency conversion. After converting the voltage signal into a pulse signal, the anti-interference ability of the signal can be obviously enhanced, which is also advantageous for long-distance transmission. AD conversion can be realized by the interface with the counter of the microcontroller. There are two common types of VFCs: (a) multivibrator VFCs; and (b) charge-balanced VFCs. Multivibrator VFC is simple, inexpensive, low power consumption and has a unit MS output (very easy to connect to some transmission media); charge balanced VFC is more accurate than multi-resonant VFC, and can integrate negative input signals . Voltage/frequency conversion, that is, v/f conversion, converts a certain input signal into a frequency signal in a linear proportional relationship. When the input voltage changes, the output frequency also responds to changes. Its function is to input the DC voltage conversion frequency proportional to its output voltage, so it is also called voltage control oscillation circuit. If any physical quantity is converted into an electrical signal by the sensor, the pre-processing is converted into a suitable voltage signal, and then the voltage-controlled oscillation circuit is controlled, and then the output of the voltage-controlled oscillation circuit is used to drive the counter to record the rectangle within a certain time interval. The number of waves, and digital display, can get the digital measuring instrument of the physical quantity. The voltage/frequency circuit is an analog/digital conversion circuit that is applied to various devices such as analog/digital conversion, frequency modulation, and remote telemetry. The task of the F/V conversion circuit is to convert the frequency change signal into a proportionally varying voltage signal. Such circuits mainly include circuits such as level comparators, monostable flip-flops, and low-pass filters. It has two types of general-purpose op amp F/V conversion circuits and integrated F/V converters. With the rapid advancement of electronic technology and computer technology, industrial automation has developed rapidly. In the field of industrial control, detecting sensor components play an increasingly important role, and various advanced sensors are being widely applied. However, many sensors only provide 4~20mA or 0~5V DC analog signal output, while most of the coal mine safety monitoring systems used in China's coal mines only allow access to analog signals of 1~5mA or 200~1000Hz, so they are used in general industrial sites. In order to realize the application in the coal mine, in addition to considering the explosion-proof factor, the output analog signal must be converted. The conversion of such an output signal, if a dedicated conversion device is purchased, is not only expensive but also convenient to use. In fact, designing and making some conversion circuits can also easily achieve the required performance. The design and principle of the practical voltage/frequency conversion circuit are introduced below. Voltage/frequency conversion circuit Voltage/frequency conversion, that is, V/F conversion, converts a certain input voltage signal into a frequency signal in a linear proportional relationship. When the input voltage changes, the output frequency also responds to changes. For the special requirements of coal mines, we only analyze how to convert the voltage into a frequency signal of 200 to 1000 Hz. There are many integrated chips available for V/F conversion. The LM331 is a high performance chip. It is produced by American NS. It is a very common voltage/frequency converter and can also be used as a precision frequency. Voltage converters, A/D converters, linear frequency modulation and demodulation, long-term integrators and other related devices. Because the LM331 uses a new temperature-compensated bandgap reference circuit, it has extremely high accuracy over the entire operating temperature range and down to 4.0V supply voltage. LM331 has a wide dynamic range of up to 100dB; good linearity, maximum nonlinear distortion less than 0.01%, low operating frequency down to 1Hz, good linearity, high conversion accuracy, digital resolution up to 12 bits, and simple external circuitry. It is easy to form a conversion circuit such as V/F or F/V with only a few external components, and it is easy to guarantee the conversion accuracy. The LM331 can be operated from dual or single supply, operates between 4.0 and 40V, can output up to 40V, and prevents Vs from shorting. Figure 2 is a typical voltage/frequency converter consisting of the LM331. The relationship between its output frequency and circuit parameters is: Fout= Vin·Rs/(2.09·R1·Rt·Ct) It can be seen that after the parameters Rs, R1, Rt, and Ct are determined, the output pulse frequency Fout is proportional to the input voltage Vin, thereby realizing a linear transformation of the voltage-frequency. By changing the value of Rs in the equation, the conversion gain of the circuit, that is, the linear proportional relationship between V and F, can be adjusted. Convert 1~5V voltage into 200~1000Hz frequency signal. The theoretical value of circuit parameters is R=18kΩ, Ct=0.022uF, R1=100kΩ, Rs=16.5528kΩ. After the parameters are basically determined, the required V/F linear transformation can be achieved by adjusting the potentiometer of Rs. It can be seen from Fout= Vin·Rs/(2.09·R1·Rt·Ct) that the resistances Rs, R1, Rt and the capacitance Ct directly affect the conversion result Fout, so there is a certain requirement on the accuracy of the component, which can be appropriately selected according to the conversion accuracy. Among them, Rt, Ct, Rs, and R1 should use stable components with low temperature drift, and Cin can choose 0.1uF or 1uF as needed. Capacitor C1 has no direct effect on the conversion result, but a capacitor with a small leakage current should be selected. The resistor R1 and the capacitor C1 form a low-pass filter, which can reduce the interference pulse in the input voltage, and is beneficial to improve the conversion accuracy. The 47Ω resistor in the circuit is necessary to ensure that the linear distortion of the circuit is less than 0.03%. The circuit of Figure 1 is a typical circuit and parameter for converting the voltage of 1~5V into a frequency signal of 200~1000Hz. To realize the conversion of 4~20mA or 0~5V into a frequency signal of 200~1000Hz, just add some auxiliary circuits. Other conversions are also analogous. The conversion circuit described above requires low cost, simple structure, convenient debugging, is very easy to implement, and has been used in many practical applications of mine monitoring equipment for its domestic coal mine safety monitoring system. The requirements are well suited to the application. 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