What is the relationship between RF chip and baseband chip? How does it work?

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What is the relationship between RF chip and baseband chip? How does it work? [Copy link]

Traditionally, a mobile phone that supports making calls, sending text messages, network services, and APP applications generally consists of five parts: radio frequency, baseband, power management, peripherals, and software.
Radio frequency: generally the part that sends and receives information;
baseband: generally the part that processes information;
power management: generally the part that saves power. Since mobile phones are devices with limited energy, power management is very important;
peripherals: generally include LCD, keyboard, casing, etc.;
software: generally includes system, driver, middleware, and application.
In mobile phone terminals, the most important core is the radio frequency chip and baseband chip. The radio frequency chip is responsible for radio frequency transmission and reception, frequency synthesis, and power amplification; the baseband chip is responsible for signal processing and protocol processing. So what is the relationship between the radio frequency chip and the baseband chip? Let's talk about the history
of the relationship between the radio frequency chip and the baseband chip.
Radio frequency (Radio Frequency) and baseband (Base Band) are both translated from English. Among them, the earliest application of radio frequency is Radio - wireless broadcasting (FM/AM), which is still the most classic application of radio frequency technology and even the radio field. The
baseband is a signal with the center point of the band at 0Hz, so the baseband is the most basic signal. Some people also call the baseband "unmodulated signal". This concept was once correct. For example, AM is a modulated signal (no modulation is required, and the content can be read through the sound-emitting components after receiving).
But for the modern communication field, baseband signals usually refer to signals that are digitally modulated and the center point of the spectrum is 0Hz. And there is no clear concept that the baseband must be analog or digital. It depends entirely on the specific implementation mechanism.
Back to the point, the baseband chip can be considered to include a modem, but it is not limited to a modem. It also includes channel encoding and decoding, source encoding and decoding, and some signaling processing. The RF chip can be regarded as the simplest up-conversion and down-conversion of the baseband modulated signal.
The so-called modulation is to modulate the signal to be transmitted onto the carrier according to certain rules and then send it out through the wireless transceiver (RF Transceiver). Demodulation is the opposite process.
Working principle and circuit analysis
RF is abbreviated as RF. RF is radio frequency current, which is a high-frequency alternating electromagnetic wave. It is the abbreviation of Radio Frequency, which means the electromagnetic frequency that can be radiated into space. The frequency range is between 300KHz and 300GHz. Alternating current that changes less than 1000 times per second is called low-frequency current, and that greater than 10,000 times is called high-frequency current, and radio frequency is such a high-frequency current. High frequency (greater than 10K); radio frequency (300K-300G) is the higher frequency band of high frequency; microwave frequency band (300M-300G) is the higher frequency band of radio frequency. Radio frequency technology is widely used in the field of wireless communications, and cable TV systems use radio frequency transmission.
Radio frequency chip refers to an electronic component that converts radio signal communication into a certain radio signal waveform and sends it out through antenna resonance. It includes a power amplifier, a low-noise amplifier, and an antenna switch. The radio frequency chip architecture includes two major parts: the receiving channel and the transmitting channel.

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The structure and working principle of the receiving circuit
When receiving, the antenna converts the electromagnetic waves sent by the base station into weak AC current signals, which are filtered and amplified at high frequency, and then sent to the intermediate frequency for demodulation to obtain the receiving baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); and sent to the logic audio circuit for further processing.
The key points of this circuit are: 1. The structure of the receiving circuit; 2. The functions and effects of each component; 3. The flow of receiving signals.
1. Circuit structure
The receiving circuit consists of antenna, antenna switch, filter, high-power amplifier (low-noise amplifier), intermediate frequency integrated block (receiving demodulator) and other circuits. Early mobile phones had primary and secondary mixing circuits, the purpose of which was to reduce the receiving frequency before demodulation (as shown in the figure below).

2. Functions and effects of each component
1) Mobile phone antenna:
Structure: (as shown below)
Mobile phone antennas are divided into external and internal antennas; they are composed of an antenna base, a solenoid, and a plastic cover.

Function: a) When receiving, convert the electromagnetic waves sent by the base station into weak AC current signals. b) When transmitting, convert the AC current amplified by the power amplifier into electromagnetic wave signals.
2) Antenna switch:
Structure: (as shown below)
The mobile phone antenna switch (combiner, duplex filter) consists of four electronic switches.

Function: a) Complete the switching between receiving and transmitting; b) Complete the switching between 900M/1800M signal receiving.

The logic circuit sends out control signals (GSM-RX-EN; DCS-RX-EN; GSM-TX-EN; DCS-TX-EN) according to the working status of the mobile phone, so that each path is turned on, so that the receiving and transmitting signals go their own way and do not interfere with each other.
Since the receiving and transmitting cannot work in the same time slot when the mobile phone is working (that is, no transmission when receiving, no reception when transmitting). Therefore, the two switches of the receiving path are removed in the later new mobile phones, leaving only two transmitting conversion switches; the receiving switching task is completed by the high-frequency tube.
3) Filter:
Structure: There are high-frequency filters and intermediate-frequency filters in the mobile phone.
Function: Filter out other useless signals and get pure receiving signals. Later new mobile phones are all zero intermediate-frequency mobile phones; therefore, there is no intermediate-frequency filter in the mobile phone.
4) High-frequency tube (high-frequency amplifier, low-noise amplifier):
Structure: There are two high-frequency tubes in the mobile phone: 900M high-frequency tube and 1800M high-frequency tube. They are all triode common emitter amplifier circuits; later new mobile phones integrate high-frequency tubes inside the intermediate frequency.

Function: a) Amplify the weak current induced by the antenna to meet the signal amplitude requirements of the subsequent circuit. b) Complete the 900M/1800M receiving signal switching.
Principle: a) Power supply: The base bias voltages of the two high-power amplifier tubes of 900M/1800M share one path, which is provided by the intermediate frequency circuit at the same time; and the collector bias voltages of the two tubes are sent out by the intermediate frequency CPU according to the receiving status command of the mobile phone in two paths; its purpose is to complete the 900M/1800M receiving signal switching.
b) Principle: After filtering out other clutters, the pure 935M-960M receiving signal is sent to the corresponding high-power amplifier tube for amplification by capacitor coupling, and then sent to the intermediate frequency for the next level of processing through capacitor coupling.

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5) Intermediate frequency (RF interface, RF signal processor):
Structure: It is composed of receiving demodulator, transmitting modulator, transmitting phase detector and other circuits; new mobile phones also integrate high-power tubes, frequency synthesis, 26M oscillation and frequency division circuits inside (as shown in the figure below).

Function:
a) The internal high-power amplifier tube amplifies the weak current induced by the antenna;
b) When receiving, the 935M-960M (GSM) receiving carrier signal (with the other party's information) and the local oscillator signal (without information) are demodulated to obtain the receiving baseband information of 67.707KHZ;
c) When transmitting, the transmission information processed by the logic circuit and the local oscillator signal are modulated into the transmission intermediate frequency;
d) Combined with the 13M/26M crystal to generate a 13M clock (reference clock circuit);
e) According to the reference signal sent by the CPU, a local oscillator signal that meets the working channel of the mobile phone is generated.
3. Signal receiving process
When the mobile phone receives, the antenna converts the electromagnetic wave sent by the base station into a weak AC current signal, which passes through the antenna switch receiving path and is sent to the high-frequency filter to filter out other useless noise, obtaining a pure 935M-960M (GSM) receiving signal. After being coupled by the capacitor, it is sent to the corresponding high-frequency tube inside the intermediate frequency for amplification, and then sent to the demodulator for demodulation with the local oscillator signal (without information) to obtain 67.707KHZ receiving baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); it is sent to the logic audio circuit for further processing.
Structure and working principle of the transmitting circuit
When transmitting, the transmitting baseband information processed by the logic circuit is modulated into the transmitting intermediate frequency, and the transmitting intermediate frequency signal is frequency-upgraded to the frequency signal of 890M-915M (GSM) by the TX-VCO. After being amplified by the power amplifier, it is converted into electromagnetic waves by the antenna and radiated out.
The key points of this circuit are: (1) circuit structure; (2) functions and effects of each component; (3) transmitting signal process.
1. Circuit structure
The transmitting circuit consists of the transmitting modulator and phase detector inside the intermediate frequency; transmitting voltage-controlled oscillator (TX-VCO), power amplifier (PA), power controller (PC), transmitting mutual inductor and other circuits. (As shown below)

2. Functions and effects of each component
1) Transmit modulator:
Structure: The transmit modulator is inside the intermediate frequency, which is equivalent to the MOD in the broadband network.
Function: When transmitting, the transmit baseband information (TXI-P;TXI-N;TXQ-P;TXQ-N) processed by the logic circuit and the local oscillator signal are modulated into the transmit intermediate frequency.
2) Transmit voltage-controlled oscillator (TX-VCO):
Structure: The transmit voltage-controlled oscillator is a capacitor three-point oscillation circuit with voltage-controlled output frequency; it is integrated into a small circuit board during manufacturing, and five pins are led out: power supply pin, ground pin, output pin, control pin, 900M/1800M frequency band switching pin. When there is a suitable working voltage, it oscillates to generate the corresponding frequency signal.
Function: Convert the transmit intermediate frequency signal modulated by the modulator in the intermediate frequency into the 890M-915M (GSM) frequency signal that the base station can receive.
Principle: As we all know, the base station can only receive the frequency signal of 890M-915M (GSM), while the intermediate frequency signal modulated by the intermediate frequency modulator (such as the intermediate frequency signal 135M transmitted by Samsung) cannot be received by the base station. Therefore, the TX-VCO is used to change the frequency of the transmitted intermediate frequency signal to the frequency signal of 890M-915M (GSM).
When transmitting, the power supply part sends out 3VTX voltage to make the TX-VCO work, and the generated frequency signal of 890M-915M (GSM) is divided into two paths: a) the sampling is sent back to the intermediate frequency, mixed with the local oscillator signal to generate a transmission frequency discrimination signal equal to the transmission intermediate frequency, and sent to the phase detector for comparison with the transmission intermediate frequency; if the TX-VCO oscillates the frequency that does not match the working channel of the mobile phone, the phase detector will generate a 1-4V jump voltage (DC voltage with AC transmission information) to control the capacitance of the variable capacitance diode inside the TX-VCO to achieve the purpose of adjusting the frequency accuracy. b) it is sent to the power amplifier and amplified, and then converted into electromagnetic waves by the antenna for radiation.
From the above, we can see that: the frequency generated by TX-VCO is sampled and sent back to the intermediate frequency, and then a voltage is generated to control the operation of TX-VCO; it just forms a closed loop, and it controls the frequency phase, so this circuit is also called a transmitting phase-locked loop circuit.
3) Power amplifier (power amplifier):
Structure: The power amplifier of the current mobile phone is a dual-frequency power amplifier (900M power amplifier and 1800M power amplifier integrated into one), which is divided into black glue power amplifier and iron shell power amplifier; different models of power amplifiers cannot be interchanged.
Function: Amplify the frequency signal oscillated by TX-VCO, obtain sufficient power current, and convert it into electromagnetic waves through the antenna for radiation.
It is worth noting that the power amplifier amplifies the amplitude of the transmitting frequency signal, and cannot amplify its frequency.
Working conditions of power amplifier:
a) Working voltage (VCC): The power supply of mobile phone power amplifier is directly provided by battery (3.6V);
b) Ground terminal (GND): Make the current form a loop;
c) Dual-frequency power conversion signal (BANDSEL): Control the power amplifier to work at 900M or 1800M;
d) Power control signal (PAC): Control the amplification amount (working current) of the power amplifier;
e) Input signal (IN); Output signal (OUT).
4) Transmitting transformer:
Structure: Two coils with equal wire diameter and number of turns are close to each other and are composed by mutual inductance principle.
Function: Sample the power current transmitted by the power amplifier and send it to the power control.
Principle: When the power current transmitted by the power amplifier passes through the transmitting transformer during transmission, a current of the same magnitude as the power current is induced in its secondary, which is sent to the power control after detection (high-frequency rectification).
5) Power level signal:
The so-called power level means that engineers divide the receiving signal into eight levels when programming the mobile phone. Each receiving level corresponds to a level of transmission power (as shown in the following table). When the mobile phone is working, the CPU determines the distance between the mobile phone and the base station based on the received signal strength and sends out the appropriate transmission level signal to determine the amplification of the power amplifier (that is, when the reception is strong, the transmission is weak).
Attached is the power level table:

6) Power controller (power control):
Structure: It is an operational comparison amplifier.
Function: Compare the transmission power current sampling signal with the power level signal to obtain a suitable voltage signal to control the amplification of the power amplifier.
Principle: When the power current passes through the transmission transformer during transmission, the current induced in its secondary is sent to the power control after detection (high-frequency rectification); at the same time, the preset power level signal is also sent to the power control during programming; after the two signals are compared internally, a voltage signal is generated to control the amplification of the power amplifier, so that the working current of the power amplifier is moderate, which saves power and prolongs the service life of the power amplifier (the higher the power control voltage, the greater the power of the power amplifier).
3. Transmitting signal flow
When transmitting, the transmission baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) processed by the logic circuit is sent to the transmission modulator inside the intermediate frequency and modulated with the local oscillator signal into the transmission intermediate frequency. The intermediate frequency signal base station cannot receive, and the TX-VCO must be used to increase the frequency of the transmitting intermediate frequency signal to the frequency signal of 890M-915M (GSM) so that the base station can receive it. When the TX-VCO is working, the frequency signal of 890M-915M (GSM) is generated and divided into two paths:
a) One path is sampled and sent back to the intermediate frequency, mixed with the local oscillator signal to generate a transmitting frequency discrimination signal equal to the transmitting intermediate frequency, and sent to the phase detector for comparison with the transmitting intermediate frequency; if the TX-VCO oscillation frequency does not match the working channel of the mobile phone, the phase detector will generate a 1-4V jump voltage to control the capacitance of the variable capacitance diode inside the TX-VCO to achieve the purpose of adjusting the frequency.
b) Two paths are sent to the power amplifier, amplified, and converted into electromagnetic waves by the antenna for radiation. In order to control the amplification of the power amplifier, when the power current passes through the transmitting transformer during transmission, the current induced on its secondary side is sent to the power control after detection (high-frequency rectification); at the same time, the preset power level signal is also sent to the power control during programming; after internal comparison of the two signals, a voltage signal is generated to control the amplification of the power amplifier, so that the operating current of the power amplifier is moderate, which saves electricity and prolongs the service life of the power amplifier.
Current status of the domestic RF chip industry chain
In the field of RF chips, the market is mainly monopolized by overseas giants. The main overseas companies are Qrovo, skyworks and Broadcom; in terms of domestic RF chips, no company can independently support the IDM operation model, mainly Fabless design companies; domestic enterprises have formed a "soft IDM" operation model through the collaboration of design, foundry and packaging.
In terms of RF chip design, domestic companies have achieved some results in 5G chips and have a certain shipment capacity. RF chip design has a high threshold. After having RF development experience, it can accelerate the development of subsequent advanced RF chips. At present, companies with RF chip design include Ziguang Zhanrui, Weige Chuangxin, Zhongpuwei, ZTE, Rapoo Technology, Huahong Design, Jiangsu Juxin, Aistek, etc.
In terms of RF chip foundry, Taiwan has become the world's largest compound semiconductor chip foundry. The main foundries in Taiwan are Win Semiconductors, Macronix and Huanyu. Only Sanan Optoelectronics and Hiwin Huaxin in China have begun to get involved in compound semiconductor foundry. Sanan Optoelectronics is currently the most complete domestic layout in China, with GaAs The HBT/pHEMT and GaNSBD/FET process layout is currently cooperating with more than 200 domestic enterprises and institutions. More than 10 chips have passed performance verification and are about to be mass-produced. Haiwei Huaxin is a subsidiary of Hitech High-tech Holdings and a joint venture with China Electronics Technology Group Corporation 29. It currently has GaAs 0.25um PHEMT process capabilities.
In terms of RF chip packaging, on the one hand, the increase in frequency of 5G RF chips causes the connection lines in the circuit to have a greater impact on the circuit performance, and the length of the signal connection lines needs to be reduced during packaging; on the other hand, the power amplifier, low-noise amplifier, switch and filter need to be packaged into a module, which not only reduces the volume but also facilitates the use of downstream terminal manufacturers. In order to reduce the parasitics of RF parameters, Flip-Chip, Fan-In and Fan-Out packaging technologies are required.
When packaging with Flip-Chip, Fan-In and Fan-Out processes, there is no need to use gold wire bonding wires for signal connection, which reduces the parasitic electrical effects caused by gold wire bonding wires and improves the RF performance of the chip; by 5G In this era, high-performance Flip-Chip/Fan-In/Fan-Out combined with SIP packaging technology will be the trend of future packaging.

Flip-Chip/Fan-In/Fan-Out and Sip packaging are advanced packaging, and their profitability is much higher than that of traditional packaging. After the domestic listed company, Changdian Technology acquired STATS ChipPAC, it formed a complete FlipChip+Sip technology packaging capability.