RF Microwave Circuit 01

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RF Microwave Circuit

• Microwave Passive Circuits

Microwave passive circuits are usually composed of various transmission lines, various passive devices, various semiconductor control devices, such as filters, limiters, phase shifters, switches, attenuators, isolators, power dividers and couplers.

Various microwave passive circuits also have different forms (different device packages, different circuit types, etc.) according to different usage frequencies (low frequency, high frequency, millimeter wave, etc.), powers, and different implementation methods (microwave, waveguide, cavity, dielectric, etc.).

Passive circuits will not produce self-oscillation, and the perspective of passive circuits does not have to be that broad, considering only the technical indicators of the working frequency band. However, when passive circuits are used in conjunction with active systems, the design needs to consider the corresponding technical bandwidth and technical guarantees, and cannot be limited to passive indicators, otherwise the entire system will become unstable.

• Microwave Active Circuit

Microwave active circuits are generally composed of transmission lines, passive devices, various semiconductor active devices, such as low noise amplifiers, power amplifiers, frequency sources, mixers, frequency multipliers and detectors. With the rapid development of microwave monolithic integrated circuits (MMIC) and digital technology, microwave circuits have also introduced various microwave monolithic integrated circuits, A/D, D/A converters, DDS devices, etc.

The reason why some passive circuits are classified as active circuits is that these passive circuits are basically used in active systems. Active circuits also have different implementation methods depending on external conditions such as the frequency they are used in.

Active circuit designers must have a broad design perspective, that is, when designing, they cannot only consider the technical indicators within the operating frequency, but also take into account the entire spectrum range to ensure that the circuit can work stably.

RF Microwave Systems

• Microwave Passive System

Microwave passive systems mainly realize the functions of transmission, distribution, synthesis, etc. of the signals required by the system, and are usually composed of a number of passive microwave circuits. For example, feeder systems in radars, passive array systems, radio and television feeder systems, feeder systems in communication equipment, etc.

In the radar system, there are passive systems such as antennas, row feeds, column feeds, sum and difference networks, filters, hinges (rotating joints), directional couplers, phase shifters, attenuators, etc.

The antennas, power dividers, low-loss high-power cables, filters, lightning arresters, etc. of radio and television also constitute their passive feeder systems; the antennas, power dividers, low-loss high-power cables, duplexers, etc. of communication base stations constitute the base station passive feeder systems.

Of course, with the improvement of system architecture, active circuit modules are also integrated into passive systems, such as embedding the receiving low noise amplifier into the duplexer in the communication base station...

The main technical indicators of passive systems include bandwidth, loss, isolation, matching (standing wave coefficient), power handling, etc. In the field of communications, there are also technical indicator requirements for third-order intermodulation suppression.

• Microwave transceiver system

The microwave transceiver system mainly realizes the functions of amplifying, filtering and transforming the received and transmitted signals. It is generally composed of a transmitting channel and a receiving channel. Each channel can be frequency conversion type, direct type, or hybrid type. Generally speaking, it does not include the functions of signal generation and baseband demodulation.

Such as conventional transceiver components in phased array radars, repeaters for mobile communication blind spots, transceiver channels for communications, etc. The common feature is that they work at radio frequency or microwave frequency and may contain one or two frequency conversion unit circuits.

The main technical indicators of the transceiver system include gain, noise figure, dynamic range, output power, sensitivity, stability, etc.

• Microwave Receiver

The receiver mainly realizes the amplification, filtering and demodulation functions of weak signals. It is generally composed of receiving channel, demodulation circuit and control circuit, and can be divided into direct type, superheterodyne type and other forms. In addition, with the development of digital technology, it can also be divided into analog demodulation and digital down-conversion (demodulation) receivers.

The advantages of the direct-type receiver are simple structure, low cost, easy design, etc., but its disadvantages are poor sensitivity, small dynamic range, and weak anti-interference ability.

The disadvantages of superheterodyne receivers are relatively complex structure, high cost and complex design, but they have advantages of high sensitivity, relatively large dynamic range and strong anti-interference ability. The main difference between analog and digital receivers is the demodulation circuit. Digital receivers have good temperature stability and can achieve higher design indicators.

The main technical indicators of the receiver include noise figure, receiving sensitivity, RF bandwidth, dynamic range, intermediate frequency bandwidth, spurious suppression, amplitude and phase balance, etc.

• Microwave transmitter

Microwave transmitters mainly realize the functions of modulation, frequency conversion, filtering and amplification of baseband signals. They are generally composed of transmission channels, modulation circuits, control circuits, etc. In addition, with the development of digital technology (digital-to-analog converter D/A, direct digital synthesizer DDS), functions such as up-conversion and digital signal generation can also be realized in digital ways.

The transmitter usually modulates the baseband signal required by the system through modulation (up-conversion unit is required if necessary), filtering and power amplification circuits, and then transmits or scans the carrier through the antenna to the predetermined target. Now, digital technology (DDS, D/A) can be used to generate a variety of signals (software radio). In addition, its power amplification, system cooling (such as radar power transmitter), reliability, and cost (linearity) will face challenges.

The main technical indicators of the transmitter include transmission power, spurious, harmonic suppression, intermodulation suppression, efficiency, bandwidth, etc.

• Frequency Source System

The main function of the frequency source is to generate various frequencies required by the system and provide the system clock reference. Frequency sources can be divided into two categories: single-point and multi-point frequency hopping according to different system requirements. The implementation methods include distributed frequency source, direct synthesis frequency source, phase-locked synthesis frequency source and digital frequency source.

The frequency source usually provides the local oscillator signal for the mixing unit (up-conversion and down-conversion) in the microwave system, and also provides the carrier signal for the modulation and demodulation circuit. Usually, different systems have different requirements for the frequency source. The advantages of the distributed frequency source are simple design and high system reliability, but the disadvantages are large size, high cost, poor coherence, and the jump speed is determined by the selection switch. The direct synthesis frequency source is complex in design and high in cost, but it has the advantages of medium size, fast jump speed and good coherence. The advantages of the phase-locked synthesis frequency source are medium design, small size and low cost, but the disadvantages are slow jump speed and reduced reliability. The advantages of the digital frequency source are simple design (mainly relying on DDS, D/A devices and related programming software), small size, good temperature characteristics, low cost, and the fastest jump speed, but the disadvantage is that the output frequency is limited, and measures need to be taken to increase the frequency.

The main technical indicators of the frequency source include output power, frequency accuracy and precision, long-term stability, instantaneous stability (phase noise), spurious and harmonic suppression, frequency hopping time, load-pull effect, etc.