What is the application principle of diodes in active antenna systems?

1. Overview of Active Antenna Systems
(1) Basic composition
The active antenna system mainly consists of antenna array, RF front-end module, digital signal processing module, and power module. The antenna array is responsible for receiving and transmitting electromagnetic wave signals; The RF front-end module includes low-noise amplifiers, power amplifiers, filters, etc., which are used to amplify, filter, and process signals; The digital signal processing module performs digital processing on analog signals to achieve functions such as modulation and demodulation, beamforming, etc; The power module provides stable power supply for the entire system.
(2) Working principle
In receiving mode, the antenna array receives electromagnetic wave signals in space and converts them into weak electrical signals. These electrical signals are amplified by the low-noise amplifier of the RF front-end module and transmitted to the digital signal processing module for further processing, ultimately restoring the original information. In transmission mode, the digital signal processing module modulates the information to be transmitted, amplifies it through the power amplifier of the RF front-end module, and then converts the electrical signal into an electromagnetic wave signal through the antenna array to be transmitted into space.
2. The application principle of diodes in active antenna systems
(1) Signal modulation and demodulation
In the signal modulation and demodulation process of active antenna systems, diodes play an important role. Taking the mixing diode as an example, in a superheterodyne receiver, the mixing diode is used to mix the received RF signal with the signal generated by the local oscillator to produce an intermediate frequency signal. Mixing diodes utilize their nonlinear characteristics to allow two input signals to interact within the diode, generating sum and difference frequency signals. By selecting an appropriate filter, the desired intermediate frequency signal can be extracted, thereby achieving down conversion of the signal. At the transmitting end, the mixing diode can also be used for up conversion to modulate the baseband signal onto the RF carrier.
(2) Power control
Power control is one of the key technologies in active antenna systems, which can dynamically adjust the transmission power based on factors such as communication distance and signal quality to improve the system's capacity and coverage range, while reducing interference to other users. Diodes can be used as variable attenuators in power control. For example, PIN diode is a commonly used power control diode, and its impedance can be adjusted by changing the DC bias voltage applied to it. When the forward bias voltage increases, the resistance of the PIN diode decreases and the signal attenuation decreases; On the contrary, when the forward bias voltage decreases, the resistance increases and the signal attenuation increases. By precisely controlling the bias voltage of the PIN diode, precise control of the transmission power can be achieved.
(3) Switch control
In active antenna systems, it is often necessary to switch between different signal paths to achieve functions such as beamforming and frequency reuse. Diodes can serve as switching elements to achieve rapid switching of signal paths. Schottky diodes have low forward voltage drop and fast switching speed, making them suitable for high-frequency switching applications. In the on/off state, the forward resistance of the Schottky diode is very small, equivalent to conduction; In the cut-off state, its reverse resistance is very large, equivalent to disconnection. By applying appropriate voltage in the control circuit, the conduction and cutoff of Schottky diodes can be controlled, thereby achieving signal path switching.
(4) Harmonic suppression and frequency selection
In active antenna systems, the presence of nonlinear devices can generate harmonic signals, which can have adverse effects on system performance, such as interfering with communication in other frequency bands. Diodes can be used for harmonic suppression and frequency selection. For example, by utilizing the nonlinear characteristics of diodes, harmonic suppression circuits can be designed. By selecting the appropriate working state and circuit parameters of diodes, harmonic signals can be suppressed. Meanwhile, diodes can also be combined with inductors, capacitors, and other components to form filters, enabling selective passage of specific frequency signals and improving the frequency selectivity of the system.
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