Overview: 0 Introduction FM current source is a kind of current source. It needs to provide a stable current for the load. On the market, there are fewer frequency-adjustable current sources, mostly constant-current sources with fixed frequency. In some measurements, The use of a frequency-adjustable current source, such as measuring the body impedance network, requires the use of a frequency-adjustable current source. Conventional current source circuits are often implemented using analog circuits, and the output frequency stability and accuracy are not high. Nowadays, digital development is getting faster and faster, and the application of DDS technology is more and more extensive. The application of DDS technology can produce fast frequency conversion, high resolution and phase controllable signals. This has in the fields of electronic measurement, radar system, FM communication, etc. This is a very important role. This paper uses DDS technology to generate a frequency-adjustable sinusoidal signal to realize the design of the frequency-modulated current source. It can realize the output of 0~1 MHz signal, and realize it through signal amplification circuit, filter circuit and voltage-current circuit. The output of the alternating current improves the accuracy and stability of the frequency modulated current source. 1 Design of frequency modulation current source system The principle block diagram of the frequency modulation current source is shown in Figure 1. A sinusoidal AC signal is generated by the signal source, and an AC current signal is obtained through voltage-current conversion. The stability of the current signal is ensured through feedback control. . Sapphire is one of the hardest materials, which is virtually scratch proof. It helps to maintain its integrity in demanding physical environments. It has a melting point of over 2000 °C, making it ideal for high-temperature applications. Besides, sapphire can transmit ultraviolet, visible and infrared light was well as microwaves, a range broader than most materials. Zoolied can make one or multiply step sapphire glass. Sapphire Step Glass,Sapphirel Window With Step,Sapphire With Groove,Grooved Sapphire Windows Zoolied Inc. , https://www.zoolied.com
The FM current source is a current source whose frequency can be changed. The ARM control DDS outputs a sinusoidal voltage signal with adjustable frequency and amplitude. The DDS output signal is filtered and V/I converted to obtain a sinusoidal current signal. It is 0~20 mA, the frequency range is 0~1 MHz. The important technology of the FM current source implementation and the corresponding formula derivation are described in detail. The performance of the designed FM current source circuit is simulated. The simulation results show that The output frequency is 0~1 MHz. The amplitude is adjustable within 0~20 mA, which meets the design requirements. The circuit has high precision and good stability. 
The design of the FM current source system is shown in Figure 2. The schematic diagram shows that the entire FM current source includes ARM control DDS sine wave generation circuit, signal amplification circuit, low-pass filter circuit and voltage-current conversion circuit. 
The working principle of the circuit: input the current value and frequency value that need to be output by the FM current source through the keyboard input. The ARM writes the control word to the DDS, and the corresponding sinusoidal signal is output by the DDS. If the output needs to be DC, the corresponding sine does not need to be generated. The signal, the sinusoidal signal, obtains a large amplitude sinusoidal signal through the signal amplifying circuit, and the signal passes through a low-pass filter, and outputs a voltage-current conversion circuit to obtain a corresponding current signal.
In order to maintain the stability of the output current signal, the output signal is fed back to the ARM controller through the sampling resistor and the peak detection circuit. The A/D sampling is used to read the maximum value of the output signal, and the feedback is adjusted by the ARM to ensure the signal. Stability. The low-pass filter mainly filters out unwanted harmonics and reduces interference; RS 232 is the communication port.
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