Radio frequency is abbreviated as RF, radio frequency is radio frequency current, which is the abbreviation of a high-frequency alternating current changing electromagnetic wave. The alternating current that changes less than one thousand times per second is called low-frequency current, and the one that changes more than one thousand times is called high-frequency current, and radio frequency is such a high-frequency current.
A radio frequency circuit refers to a circuit whose electromagnetic wavelength and the size of the circuit or device are in the same order of magnitude. At this time, due to the relationship between the size of the device and the size of the wire, the circuit needs to be dealt with by the related theory of distributed parameters. This type of circuit can be considered as a radio frequency circuit without strict requirements on its frequency, such as long-distance transmission AC power lines, sometimes also To deal with the relevant theory of RF.
Typical RF circuit block diagram
This is a system model of a wireless communication transceiver, which includes a transmitter circuit, a receiver circuit, and a communication antenna. This transceiver can be used in personal communications and wireless local area networks. In this system, the digital processing part is mainly to process digital signals, including sampling, compression, encoding, etc.; then it is converted into analog form through the AD converter and enters the analog signal circuit unit.
How to lay out the RF circuit1. Separate the high-power RF amplifier from the low-noise amplifier as much as possible. Simply put, keep the high-power RF transmitter circuit away from the low-power RF receiver circuit
2. Make sure that there is at least a whole piece of ground in the high-power area of ​​the PCB, preferably without vias. Of course, the larger the copper foil area, the better.
3. Circuit and power decoupling are also extremely important.
4. The RF output usually needs to be far away from the RF input
5. Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals.
RF circuit principleThe principle of the radio frequency circuit is divided into three parts. First, the structure and working principle of the receiving circuit, then the structure and working principle of the transmitting circuit, and finally the structure and working principle of the local oscillator circuit.
When receiving, the antenna converts the electromagnetic wave sent from the base station into a weak alternating current signal, filtered, amplified by high frequency, and sent to the intermediate frequency for demodulation to obtain the received baseband information. Send it to the logic audio circuit for further processing.
Circuit structure: The receiving circuit is composed of antenna, antenna switch, filter, high-frequency amplifier tube, intermediate frequency integrated block and other circuits. Early mobile phones had primary and secondary mixing circuits, whose purpose was to demodulate after lowering the receiving frequency.
When transmitting, the transmission baseband information processed by the logic circuit is modulated into a transmission intermediate frequency, and the transmission intermediate frequency signal is converted into a GSM frequency signal. After being amplified by the power amplifier, the antenna is converted into electromagnetic waves and radiated out.
Circuit structure: The transmitting circuit is composed of a transmitting modulator and a transmitting phase detector inside the intermediate frequency; a transmitting voltage controlled oscillator, a power amplifier, a power controller, a transmitting transformer and other circuits.
The local oscillator circuit generates four local oscillator frequency signals without any information; they are sent into the IF, and when receiving, the received signal is demodulated; when transmitting, the transmitted baseband information is modulated and transmitted phase discrimination.
The mobile phone local oscillator circuit has four circuit structures: A, which is composed of a frequency synthesis integrated block, a receiving voltage-controlled oscillator, a reference clock, and a preset frequency reference data.
B. Integrate the frequency synthesis integrated block into the intermediate frequency, combined with the external and receiving voltage-controlled oscillator.
C. Integrate the frequency synthesis integrated block and the receiving voltage-controlled oscillator into one, which is called the local oscillator integrated block or the local oscillator IC.
D. Integrate the frequency synthesis integrated block and the receiving voltage controlled oscillator into the intermediate frequency.
It is worth noting that no matter what structure mode is adopted, only the generated frequency is different; its working principle, the direction and function of the generated frequency signal are the same.
RF circuit applicationThe application fields of radio frequency circuits include ETC, railway locomotive and vehicle identification and tracking, container identification, entry and exit prohibition management, animal identification, tracking, automatic vehicle locking, etc., and there are many more, not to mention them all.
RF circuit designFour aspects teach you how to design radio frequency circuits: radio frequency interface of radio frequency circuit simulation, large interference signal of radio frequency circuit simulation, small expected signal of radio frequency circuit simulation, and adjacent channel interference of radio frequency circuit simulation.
The wireless transmitter and receiver are conceptually divided into two parts: base frequency and radio frequency. The fundamental frequency includes the frequency range of the input signal of the transmitter and the frequency range of the output signal of the receiver. The bandwidth of the fundamental frequency determines the fundamental rate at which data can flow in the system. The base frequency is used to improve the reliability of the data stream and reduce the load imposed by the transmitter on the transmission medium under a specific data transmission rate. Therefore, a lot of signal processing engineering knowledge is required when designing a fundamental frequency circuit on a PCB. The radio frequency circuit of the transmitter can convert and up-convert the processed baseband signal to a designated channel, and inject this signal into the transmission medium. On the contrary, the radio frequency circuit of the receiver can obtain the signal from the transmission medium, and convert and reduce the frequency to the base frequency.
Transmitter has two main PCB design goals: The first is that they must transmit a specific power while consuming the least power possible. The second is that they cannot interfere with the normal operation of transceivers in adjacent channels. As far as the receiver is concerned, there are three main PCB design goals: first, they must accurately restore small signals; second, they must be able to remove interfering signals outside the desired channel; and last, like the transmitter, they must consume power Very small.
The receiver must be very sensitive to small signals, even when large interference signals are present. This situation occurs when trying to receive a weak or long-distance transmission signal, and a powerful transmitter nearby is broadcasting in an adjacent channel. The interference signal may be sixty to seventy decibels larger than the expected signal and can be covered in a large amount during the input stage of the receiver, or the receiver may generate excessive noise during the input stage to block the reception of normal signals. If the receiver is driven into a non-linear region by the interference source during the input stage, the above two problems will occur. To avoid these problems, the front end of the receiver must be very linear. Therefore, "linearity" is also an important consideration when designing a receiver on a PCB. Since the receiver is a narrowband circuit, the nonlinearity is calculated by measuring "intermodulation distortion". This involves using two sine waves or cosine waves with similar frequencies and located in the center frequency band to drive the input signal, and then measuring the product of its intermodulation.
Distortion also plays an important role in the transmitter. The non-linearity generated by the transmitter in the output circuit may spread the bandwidth of the transmitted signal in adjacent channels. This phenomenon is called "re-growth of the spectrum". Before the signal reaches the transmitter's power amplifier, its bandwidth is limited; but the "intermodulation distortion" in the power amplifier will cause the bandwidth to increase again. If the bandwidth is increased too much, the transmitter will not be able to meet the power requirements of its adjacent channels. When transmitting digitally modulated signals, in fact, it is impossible to use spice to predict the further growth of the spectrum. Because the transmission of about 1,000 digital symbols must be simulated to obtain a representative spectrum, and high-frequency carriers are also required, which will make the transient analysis of spice impractical.
The receiver must detect small input signals very sensitively. The sensitivity of the receiver is limited by the noise generated by its input circuit. Therefore, noise is an important consideration in the PCB design of the receiver. Moreover, the ability to predict noise with simulation tools is indispensable.
Common problems in RF circuit design Interference between digital circuit modules and analog circuit modulesIf the analog circuit and the digital circuit work separately, each may work well. However, once the two are placed on the same circuit board and work together with the same power supply, the entire system is likely to be unstable. This is mainly because the digital signal frequently swings between the ground and the positive power supply, and the period is extremely short, often in the order of nanoseconds. Due to the larger amplitude and shorter switching time. This makes these digital signals contain a large amount of high-frequency components that are independent of the switching frequency. In the analog part, the signal transmitted from the wireless tuning loop to the receiving part of the wireless device is generally less than 1. Therefore, the difference between the digital signal and the radio frequency signal will reach 120 decibels. Obviously, if the digital signal cannot be separated from the radio frequency signal well. Weak radio frequency signals may be destroyed, and as a result, the performance of the wireless device will deteriorate or even fail to work at all.
Noise interference from power supplyRF circuits are quite sensitive to power supply noise, especially to glitch voltage and other high frequency harmonics. The microcontroller will suddenly draw most of the current for a short period of time within each internal clock cycle. This is because modern microcontrollers are manufactured using the C mos process. therefore. Assuming a microcontroller is running at an internal clock frequency of one megahertz, it will draw current from the power supply at this frequency. If you don't take proper power decoupling. Will cause voltage glitches on the power line. If these voltage glitches reach the power supply pins of the RF part of the circuit, they may cause work failure in severe cases.
Unreasonable groundIf the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit design, most digital circuit functions perform well even without a ground plane. In the RF frequency band, even a short ground wire acts like an inductor. If the ground wire layer is not used, most ground wires will be longer and the circuit will not have the characteristics of the design.
Radiation interference from antenna to other analog circuit partsIn PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have analog-to-digital conversion or digital-to-analog converters. If the analog-to-digital conversion input terminal is not processed properly, the RF signal may be self-excited in the electrostatic protection diode input to the converter. This causes converter deviation.
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