Explain the working principle of RF chips

The relationship between RF chips and baseband chips

Let's talk about history first. Radio Frequency and Base Band both come from English literal translation. The earliest application of radio frequency is Radio - Radio Broadcast (FM/AM), which is still the most classic application of radio frequency technology and even in the field of radio.

The baseband is the signal with the center point of the band at 0Hz, so it is the most fundamental signal. Some people also refer to baseband as "unmodulated signal", and this concept was once correct. For example, AM is a modulated signal (no modulation required, content can be read through sound emitting components after reception).

But for the modern communication field, baseband signals usually refer to signals that have been digitally modulated and have a spectral center point at 0Hz. And there is no clear concept indicating that the baseband must be analog or digital, it all depends on the specific implementation mechanism.

To get back to the point, baseband chips can be considered to include modems, but not just modems. They also include channel encoding and decoding, source encoding and decoding, and some signaling processing. RF chips can be seen as the simplest up and down conversion of baseband modulation signals.

The so-called modulation refers to the process of modulating the signal that needs to be transmitted onto a carrier wave according to certain rules, and then sending it out through a wireless transceiver (RF transceiver). demodulation is the opposite process.

Working principle and circuit analysis

Radio frequency, abbreviated as RF, refers to radio frequency current, which is a high-frequency alternating current electromagnetic wave. It stands for Radio Frequency and represents the electromagnetic frequency that can radiate into space, with a frequency range between 300KHz and 300GHz. AC current that changes less than 1000 times per second is called low-frequency current, while AC current that changes more than 10000 times per second is called high-frequency current, and RF is such a high-frequency current. High frequency (greater than 10K); Radio frequency (300K-300G) is a higher frequency band of high frequency; The microwave frequency band (300M-300G) is also a higher frequency band of radio frequency. Radio frequency technology is widely used in the field of wireless communication, and cable television systems adopt radio frequency transmission.

RF 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 RF chip architecture consists of two main parts: the receiving channel and the transmitting channel.

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, filters them, amplifies them at high frequencies, and sends them to the intermediate frequency for demodulation to obtain the received baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); Send it to the logic audio circuit for further processing.

The key points to master in this circuit are: 1. The structure of the receiving circuit; 2. The functions and roles of each component; 3. Receiving signal process.

1. Circuit structure

The receiving circuit consists of an antenna, antenna switch, filter, high voltage amplifier (low noise amplifier), intermediate frequency integrated block (receiving demodulator) and other circuits. Early mobile phones had primary and secondary mixing circuits, which aimed to lower the receiving frequency before demodulation.

2. Functions and functions of each component

1) Mobile phone antenna:

There are two types of mobile phone antennas: external and internal antennas; Composed of antenna base, solenoid, and plastic cover.

effect: a)、 When receiving, convert the electromagnetic waves sent by the base station into weak AC current signals. b)、 Convert the amplified AC current from the amplifier into an electromagnetic wave signal during transmission.

2) Antenna switch:

The mobile phone antenna switch (combiner, duplex filter) consists of four electronic switches.

effect: a)、 Complete the switching between receiving and transmitting; b)、 Complete the switching of 900M/1800M signal reception.

The logic circuit sends control signals (GSM-RX-EN; DCS-RX-EN; GSM-TX-EN; DCS-TX-EN) according to the working status of the mobile phone, making each path conductive, so that the received and transmitted signals go their own way without interfering with each other.

Due to the inability of mobile phones to receive and transmit simultaneously in the same time slot during operation (i.e. not transmitting during reception and not receiving during transmission). Therefore, in the later stage, the new mobile phones removed the two switches of the receiving channel, leaving only two transmission conversion switches; The task of receiving and switching is entrusted to the high-level tube for completion.

3) Filter:

Structure: The mobile phone has a high-frequency filter and an intermediate frequency filter.

Function: Filter out other useless signals and obtain pure received signals. The later new mobile phones are all zero mid-range phones; Therefore, there is no intermediate frequency filter in mobile phones anymore.

4) High frequency amplifier tube (high-frequency amplifier tube, low-noise amplifier):

Structure: There are two high-level tubes in the mobile phone: 900M high-level tube and 1800M high-level tube. All are transistor common emitter amplification circuits; In the later stage, the new type of mobile phone integrates the high amplifier tube inside the intermediate frequency.

effect: a)、 Amplify the weak current sensed by the antenna to meet the signal amplitude requirements of the subsequent circuit. b)、 Complete the switching of 900M/1800M receiving signals.

Reason: a)、 Power supply: The base bias of two high voltage transistors, 900M and 1800M, share one circuit and are provided by the intermediate frequency synchronous circuit; The bias voltage of the collectors of the two tubes is split into two channels by the intermediate frequency CPU according to the receiving status command of the mobile phone; Its purpose is to complete the switching of 900M/1800M received signals.

b)、 Principle: After filtering out other clutter through a filter, a pure 935M-960M received signal is obtained, which is coupled by a capacitor and sent to the corresponding high amplifier tube for amplification. Then, it is coupled by a capacitor and sent to the intermediate frequency for further processing.

5) Intermediate frequency (RF interface, RF signal processor):

Structure: Composed of receiving demodulator, transmitting modulator, transmitting phase detector and other circuits; The new mobile phone also integrates high amplifier tube, frequency synthesis, 26M oscillation, and frequency division circuit internally (as shown in the figure below).

effect:

a) The internal amplifier tube amplifies the weak current sensed by the antenna;

b) When receiving, demodulate the 935M-960M (GSM) receiving carrier frequency signal (with counterpart information) and the local oscillator signal (without information) to obtain the 67.707KHZ receiving baseband information;

c) When transmitting, modulate the transmission information processed by the logic circuit with the local oscillator signal into the transmission intermediate frequency;

d) Combining 13M/26M crystals to generate a 13M clock (reference clock circuit);

e) Generate a local oscillator signal that matches the working channel of the mobile phone based on the reference signal sent by the CPU.

3. Signal reception process

When receiving a mobile phone, the antenna converts the electromagnetic waves sent by the base station into weak AC current signals. After passing through the antenna switch receiving path, it sends a high-frequency filter to filter out other useless clutter, obtaining a pure 935M-960M (GSM) receiving signal. It is coupled by a capacitor and sent to the corresponding high-frequency amplifier tube inside the intermediate frequency for amplification. It is then sent to the demodulator and demodulated with the local oscillator signal (without information) to obtain 67.707KHZ receiving baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); Send it to the logic audio circuit for further processing.

The structure and working principle of the transmission circuit

When transmitting, modulate the baseband information processed by the logic circuit into the transmission intermediate frequency, and use TX-VCO to convert the frequency of the transmission intermediate frequency signal to 890M-915M (GSM) frequency signal. After being amplified by the power amplifier, it is converted into electromagnetic waves and radiated out by the antenna.

Key points to master in this circuit: (1) Circuit structure; (2) The functions and roles of each component; (3) The process of transmitting signals.

1. Circuit structure

The transmission circuit consists of an intermediate frequency internal transmission modulator and a transmission phase detector; The transmission voltage controlled oscillator (TX-VCO), power amplifier (power amplifier), power controller (power control), transmission transformer and other circuits are composed.

2. Functions and functions of each component

1) Transmission modulator:

Structure: The transmitting modulator is located inside the intermediate frequency, equivalent to the MOD in broadband networks.

Function: When transmitting, modulate the transmission baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) processed by the logic circuit with the local oscillator signal into the transmission intermediate frequency.

2) Transmission voltage controlled oscillator (TX-VCO):

Structure: The transmitting voltage controlled oscillator is a capacitor three-point oscillation circuit that controls the output frequency by voltage; During production and manufacturing, it is integrated into a small circuit board and leads out five pins: power supply pin, ground pin, output pin, control pin, and 900M/1800M frequency band switching pin. When there is a suitable working voltage, it oscillates to generate a corresponding frequency signal.

Function: Convert the transmitted intermediate frequency signal modulated by the intermediate frequency modulator into a frequency signal of 890M-915M (GSM) that can be received by the base station.

Principle: As is well known, base stations can only receive frequency signals of 890M-915M (GSM), while intermediate frequency signals modulated by intermediate frequency modulators (such as the 135M intermediate frequency signal transmitted by Samsung) cannot be received by base stations. Therefore, TX-VCO is used to increase the frequency of the transmitted intermediate frequency signal to 890M-915M (GSM).

When transmitting, the power supply sends out a voltage of 3VTX to operate TX-VCO, generating a frequency signal of 890M-915M (GSM) and splitting it into two paths: a)、 Sampling is sent back to the internal 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 oscillation frequency of TX-VCO 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 internal variable capacitance diode of TX-VCO, in order to adjust the frequency accuracy. b)、 After being amplified by the power amplifier, it is converted into electromagnetic waves and radiated out by the antenna.

From the above, it can be seen that TX-VCO generates frequency, samples and sends it back to the intermediate frequency, and then generates voltage to control the operation of TX-VCO; It precisely forms a closed loop and controls the frequency phase, so this circuit is also known as a transmit phase-locked loop circuit.

3) Power amplifier (power amplifier):

Structure: Currently, the power amplifier for mobile phones is a dual frequency amplifier (integrating 900M and 1800M amplifiers), divided into two types: black glue amplifier and iron shell amplifier; Different models of amplifiers cannot be interchanged.

Function: Amplify the frequency signal of TX-VCO oscillation to obtain sufficient power current, which is converted into electromagnetic waves and radiated out through the antenna.

It is worth noting that the amplifier amplifies the amplitude of the transmitted frequency signal and cannot amplify its frequency.

Working conditions of power amplifier:

a)、 Working voltage (VCC): The power supply for the mobile phone amplifier is directly provided by the battery (3.6V);

b)、 Ground terminal (GND): forms a circuit for the current;

c)、 Dual frequency power conversion signal (BANDSEL): controls the power amplifier to operate at 900M or 1800M;

d)、 Power control signal (PAC): controls the amplification amount (working current) of the power amplifier;

e)、 Input signal (IN); Output signal (OUT).

4) Transmission Transformer:

Structure: Two coils with equal wire diameter and number of turns are close to each other and composed using the principle of mutual inductance.

Function: Sample the transmission power current of the power amplifier and send it to the power control.

Principle: When the transmitting power current of 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, and after detection (high-frequency rectification), it is sent to the power control.

5) Power level signal:

The so-called power level refers to engineers dividing the received signal into eight levels when programming mobile phones, with each receiving level corresponding to one level of transmission power (as shown in the table below). When the phone is in operation, the CPU determines the distance between the phone and the base station based on the strength of the received signal, and sends out an appropriate transmission level signal to determine the amplification amount of the amplifier (i.e. when the reception is strong, the transmission is weak).

6) Power controller (power control):

Structure: It is an operational comparative 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 transmitting transformer during transmission, the secondary induced current is detected (high-frequency rectified) and sent to the power control; At the same time, the preset power level signal is also sent to the power control during programming; After comparing the two signals internally, a voltage signal is generated to control the amplification amount of the power amplifier, so that the working current of the power amplifier is moderate, which not only saves electricity but also prolongs the service life of the power amplifier (high power control voltage leads to high power amplifier).

3. Signal transmission process

When transmitting, the baseband information processed by the logic circuit (TXI-P; TXI-N; TXQ-P; TXQ-N) is sent to the transmission modulator inside the intermediate frequency and modulated with the local oscillator signal to form the transmission intermediate frequency. If the intermediate frequency signal cannot be received by the base station, TX-VCO must be used to raise the frequency of the transmitted intermediate frequency signal to 890M-915M (GSM) in order for the base station to receive it. When TX-VCO works, it generates a frequency signal of 890M-915M (GSM), which is split into two paths:

a)、 One sample is sent back to the internal 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 oscillation frequency of TX-VCO 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 internal variable capacitance diode of TX-VCO, achieving the purpose of adjusting the frequency.

b)、 Two channels are sent to the power amplifier for amplification, and then 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 transmission transformer during transmission, the secondary induced current is detected (high-frequency rectified) and sent to the power control; At the same time, the preset power level signal is also sent to the power control during programming; After comparing the two signals internally, a voltage signal is generated to control the amplification amount of the power amplifier, so that the working current of the power amplifier is moderate, which not only saves electricity but also prolongs the service life of the power amplifier.

The current situation of the domestic RF chip industry chain

In the field of RF chips, the market is mainly monopolized by overseas giants, including Qrovo, Skyworks, and Broadcom; In terms of domestic RF chips, there are no companies that can independently support the operation mode of IDM, mainly Fabless design companies; Domestic enterprises have formed a "soft IDM" operating model through collaboration in design, OEM, and packaging processes.

In terms of RF chip design, domestic companies have achieved some success in 5G chips and have certain shipping capabilities. RF chip design has a high threshold, and with RF development experience, it can accelerate the development of advanced RF chips in subsequent categories. At present, companies with RF chip design capabilities include Ziguang Spreadtrum, Weijie Chuangxin, Zhongpu Micro, ZTE, Rapoo Technology, Huahong Design, Jiangsu Juxin, Aistek, etc.

In terms of RF chip outsourcing, Taiwan has become the world's largest compound semiconductor chip outsourcing factory. The main outsourcing factories in Taiwan include Wenmao, Hongjieke, and Huanyu, while only Sanan Optoelectronics and Haiwei Huaxin have started to enter compound semiconductor outsourcing in China. Sanan Optoelectronics has the most complete layout in China, with GaAs HBT/pHEMT and GaNSBD/FET process layouts. Currently, it cooperates with more than 200 domestic enterprises and institutions, and more than 10 chips have passed performance verification and are about to enter mass production. Haiwei Huaxin is a subsidiary of Haite High tech Holdings and a joint venture with China Electronics Technology Group Corporation (CETC) 29th Institute. Currently, it has GaAs 0.25um PHEMT process capability.

In terms of RF chip packaging, the increase in frequency of 5G RF chips has a greater impact on circuit performance due to the connection lines in the circuit. Therefore, it is necessary to reduce the length of signal connection lines during packaging; On the other hand, it is necessary to package the power amplifier, low noise amplifier, switch, and filter into one module, which reduces the size and facilitates downstream terminal manufacturers to use. To reduce the parasitic effects 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 parasitic electrical effects caused by gold wire bonding wires and improves the RF performance of the chip; In the 5G era, high-performance Flip Chip/Fan In/Fan Out combined with Sip packaging technology will be the trend of future packaging.