How to use diodes for isolation in multi band communication devices?

1. The basic principle of diode isolation
(1) Unidirectional conductivity characteristic
A diode has unidirectional conductivity, which means it only allows current to pass in one direction. When the diode is forward biased, the potential barrier of its internal PN junction decreases, allowing current to pass smoothly; When the diode is reverse biased, the potential barrier rises and current can hardly pass through. This characteristic enables diodes to electrically isolate two circuits, preventing signals from being transmitted in unwanted directions. For example, in a circuit, if it is necessary to prevent reverse current from affecting other circuits, a diode can be connected in series in the circuit. When the current direction conforms to the forward bias, the diode conducts and the current flows normally; When the current direction is opposite, the diode cuts off, thereby achieving isolation of the circuit.
(2) Implementation method of isolation
There are two main ways for diodes to achieve isolation. One method is to directly connect them in series in the circuit and prevent reverse current through their unidirectional conductivity. For example, in a power circuit, in order to prevent damage to the circuit caused by reverse connection of the power supply, a diode can be connected in series at the input end of the power supply. When the positive and negative poles of the power supply are correctly connected, the diode conducts and the power supply is supplied normally; When the power supply is reversed, the diode cuts off, preventing current from passing through and protecting the subsequent circuit. Another way is to combine it with other components to form a specific isolation circuit. For example, in MOSFET driver circuits, in order to achieve slow on and fast off effects, the unidirectionality of diodes can be used for isolation. When the square wave input is at a high level, the Cgs junction capacitor of the MOSFET is charged. Due to the obstruction of the diode, the charging circuit charges Cgs through a resistor, resulting in a relatively slow charging speed; When the square wave input is at a low level, the two paths discharge, and the discharge circuit becomes two resistors in parallel, with a resistance smaller than that of a single resistor and a discharge speed greater than the charging speed, thus achieving the effect of slow opening and fast closing
2. The specific application of diodes in multi band communication equipment
(1) Power isolation
In multi band communication devices, external power supply and battery dual power supply are usually required for power supply. To prevent mutual interference between two power sources, diodes can be used for isolation. For example, a dual power supply scheme using two diodes in series is adopted, with a diode connected in series between each power source and the load. In this way, when one of the power sources is working, the diode conducts to supply power to the load; The other power supply, due to the isolation effect of the diode, will not affect the working power supply. Meanwhile, when the power supply voltage fluctuates or reverses, the diode can also provide protection to prevent damage to subsequent circuits. This power isolation method is simple, effective, cost-effective, and can ensure stable power supply for multi band communication equipment.
(2) Signal isolation
Preventing signal interference: In multi band communication devices, signals from different frequency bands may interfere with each other. For example, when a device processes both high-frequency and low-frequency signals simultaneously, the high-frequency signal may couple into the low-frequency signal circuit through the circuit, causing distortion of the low-frequency signal. To solve this problem, diodes can be added to the signal transmission path for isolation. Diodes can prevent the propagation of high-frequency signals to low-frequency signal circuits, thereby ensuring independent transmission of signals in each frequency band. For example, in a receiver circuit, in order to prevent leakage of the local oscillator signal to the RF input terminal, a diode isolation circuit can be added between the local oscillator signal output terminal and the RF input terminal. When the local oscillator signal is working normally, the diode conducts and the local oscillator signal passes through; When the local oscillator signal is abnormal or transmitted in reverse, the diode cuts off to prevent signal leakage and improve the performance of the receiver.
Protecting sensitive circuits: There are some signal sensitive circuits in multi band communication devices, such as microprocessors, sensors, etc. These circuits are susceptible to external signal interference, leading to unstable operation. By adding a diode isolation circuit at the input of the sensitive circuit, external interference signals can be effectively prevented from entering the sensitive circuit. For example, in a temperature sensor circuit, in order to prevent power noise or other high-frequency signals from interfering with the measurement accuracy of the sensor, diodes can be added to the power input and signal output terminals of the sensor for isolation. In this way, even if there is external interference signal, the diode can block it outside the isolation circuit, ensuring the normal operation of the sensor.
(3) Mixer port isolation
In the mixer of multi band communication equipment, the RF, LO, and IF ports need to be isolated from each other to prevent signal leakage and coupling. Diodes can be used to achieve isolation of mixer ports. For example, in a mixer circuit, the local oscillator signal and the RF signal need to be mixed in the mixer, but at the same time, it is necessary to prevent the local oscillator signal from leaking to the RF port or intermediate frequency port, as well as to prevent the RF signal from leaking to the local oscillator port. By adding diode isolation circuits between each port, the port isolation of the mixer can be improved. Isolation degree refers to the ratio of the power of the local oscillator or RF signal leaking to other ports to the input power, measured in dB. Generally speaking, the higher the isolation, the better the performance of the mixer. The diode isolation circuit can effectively reduce signal leakage, improve the isolation of the mixer, and thus enhance the overall performance of communication equipment.
3. Advantages of diode isolation applications
(1) Low cost
A diode is a common electronic component with a relatively low price. Using diodes for isolation in multi band communication devices does not require excessive cost increase. Compared with other complex isolation techniques, diode isolation schemes have significant cost advantages. For example, in some cost sensitive consumer communication devices, using diode isolation can effectively reduce the manufacturing cost of the product and improve its market competitiveness.
(2) High reliability
Diodes have the characteristics of simple structure and stable operation. Under normal working conditions, diodes can operate reliably for a long time and are not prone to malfunctions. In multi band communication devices, diode isolation circuits do not require complex control logic, only the appropriate diode model and parameters need to be selected according to circuit requirements. This makes the diode isolation circuit highly reliable, ensuring the stable operation of communication equipment.
(3) Easy to implement
The design and implementation of diode isolation circuits are relatively simple. Engineers only need to understand the characteristics and application methods of diodes to design suitable isolation circuits based on specific circuit requirements. Moreover, diodes come in various packaging forms, such as plug-in and surface mount, which can meet the installation requirements of different communication devices. In practical applications, diode isolation circuits can be easily integrated into the circuit boards of communication devices without the need for additional debugging and optimization, greatly shortening the product development cycle.
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