Ⅰ Overview

A phone that can support phone calls, text messages, web services, and app applications usually contain five parts: RF, baseband, power management, peripherals, and software.

RF: it is the part that sends and receives information;

Baseband: it is the part of information processing;

Power management: it is the part of power-saving, because the phone is a limited energy device, so power management is very important;

Peripherals: they include LCD, keyboard, case, etc;

Software: it includes system, driver, middleware, application.

In the cell phone terminal, the RF chip and the baseband chip are the two most important cores. RF chip is responsible for RF transceiver, frequency synthesis, power amplification. Baseband chip is responsible for signal processing and protocol processing.

Ⅱ The relationship between RF chip and baseband chip

RF refers to Radio Frequency. The earliest application of RF is Radio - radio broadcasting (FM/AM), which is still the most classic application of RF technology and even the radio field.

The baseband is the band center point in the 0Hz signal, so the baseband is the most basic signal. Some people also call the baseband "unmodulated signal". Once this concept is right, for example, AM is a modulated signal (no modulation, after receiving the content can be read through the sound-emitting components).

But for modern communications, baseband signals are usually digitally modulated signals with a spectrum centered at 0Hz. And there is no clear concept that the baseband must be analog or digital, it all depends on the specific implementation mechanism.

The baseband chip can be thought of as including the modem, but not only the modem, but also the channel codec, source codec, and some signaling processing. The RF chip, on the other hand, can be seen as the simplest baseband modulation signal upconversion and downconversion.

The so-called modulation is a project that modulates the signal to be transmitted onto the carrier through a certain rule and then sends it out through the RF Transceiver.

Ⅲ Working principle and circuit analysis

RF is a high frequency AC change electromagnetic wave and is the abbreviation of Radio Frequency, which can be radiated into space electromagnetic frequency. The frequency range is between 300KHz ~ 300GHz. The alternating current that changes less than 1000 times per second is called low-frequency current, greater than 10000 times is called high-frequency current, and RF is such a high-frequency current. RF (300K-300G) is the higher frequency band of high frequency; microwave band (300M-300G) is also the higher frequency band of RF. RF technology is widely used in the field of wireless communication and cable TV system.

A radio frequency chip is 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, low noise amplifier, and antenna switch. The radio frequency chip architecture includes two parts: receiving channel and the transmitting channel.

Structure and working principle of the receiver circuit

The antenna converts the electromagnetic wave sent from the base station into a weak alternating current signal. After filtering and high frequency amplification, it is sent to the intermediate frequency for demodulation. Get the receiving baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); send it to the logic audio circuit for further processing.

The circuit focus on:

(1) receiving circuit structure;

(2) the function and role of each component;

(3) receiving signal flow.

1. Circuit structure

The receiver circuit consists of an antenna, antenna switch, filter, high amplifier (low noise amplifier), IF integrated block (receive demodulator), and other circuits. Early cell phones have a primary and secondary mixing circuit, the purpose of which is to reduce the received frequency and then demodulate (the following figure).

Receiver circuit block diagram

2. The function and role of each component

1) Cell phone antenna

Cell phone antenna structure

The cell phone antenna is divided into the external and built-in antenna. It is composed of the antenna seat, solenoid, and plastic envelope.

Role: a) The mobile phone antenna converts the electromagnetic wave sent from the base station into a weak alternating current signal. b) transmitting the amplified amplifier AC current into an electromagnetic wave signal.

2) Antenna switch

Antenna switch structure

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

Role:

• Complete receive and transmit switching;

• Complete the 900M/1800M signal reception switching.

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 cell phone so that the receiving and transmitting signals go their own way and do not interfere with each other.

3) Filter

Structure: cell phones have high-frequency filters, medium-frequency filters.

Function: Filter out other useless signals, to get a pure received signal. Later new cell phones are zero IF cell phones; therefore, there is no IF filter in the phone.

4) High frequency amplifier (low noise amplifier)

Structure: There are two high-frequency amplifiers in cell phones: 900M high-frequency amplifier, 1800M high-frequency amplifier. Both are triode common emitter amplifier circuits; later new cell phones integrated high amplifier in the IF internal.

Function:

• To amplify the weak current induced by the antenna to meet the demand for signal amplitude in the post-stage circuit.

• Complete 900M/1800M receive signal switching.

Principle:

Power supply: 900M/1800M base bias of two high-frequency amplifiers share one way, provided by the IF at the same time; and the bias voltage of the collector of the two tubes is sent out by the IF CPU in two ways according to the receiving state of the cell phone command IF. Its purpose is to complete the 900M/1800M receiving signal switching.

5) IF (RF receiver enclosure, RF signal processor)

Structure: IF consists of the receiver demodulator, transmit modulator, transmit phase discriminator, and other circuits. New cell phones also integrate the high amplifier, frequency synthesis, 26M oscillation, and frequency division circuit into the internal.

Function:

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

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

c) Transmitting the processed transmit information and the local oscillator signal modulated by the logic circuit into the transmitting IF;

d) Combine 13M/26M crystal to generate 13M clock;

e) According to the reference signal sent by the CPU, generate the local oscillation signal which meets the working channel of cell phone.

3. Receiving signal flow

When the mobile phone receives the signal, the antenna converts the electromagnetic wave sent from the base station into a weak AC current signal, passes through the antenna switch receiving path, sends a high-frequency filter to filter out other useless clutter, and obtains a pure 935M-960M (GSM) receiving signal. After the coupling is sent to the corresponding high-frequency amplifier tube in the intermediate frequency, it is sent to the demodulator and the local oscillator signal (without information) for demodulation to obtain 67.707KHZ receiving baseband information (RXI-P, RXI-N, RXQ-P) , RXQ-N); sent to the logic audio circuit for further processing.

Structure and working principle of the transmitter circuit

When transmitting signal, the transmitting IF processed by the logic circuit is modulated into the transmitting IF, and the TX-VCO is used to change the transmitting IF signal frequency to 890M-915M (GSM) frequency signal. After amplification by the amplifier, the antenna is converted into electromagnetic wave radiation out.

The circuit focus on:

(1) circuit structure;

(2) the function and role of each component;

(3) the transmit signal flow.

1. Circuit structure

The transmitter circuit consists of IF internal transmitter modulator, transmitting phase discriminator; transmitting voltage controlled oscillator (TX-VCO), power amplifier, power controller, transmitting mutual inductor, and other circuits. (The following figure)

Block diagram of the transmitter circuit

2. The function and role of each component

1) The transmitter modulator

Structure: The transmitter modulator is inside the IF, equivalent to the MOD in the broadband network.

Role: transmitting the logic circuit processed transmit baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) with the local oscillator signal modulation into the transmitting IF.

2) Transmit Voltage Controlled Oscillator (TX-VCO)

Structure: The transmit voltage-controlled oscillator is a capacitive three-point oscillation circuit that controls the output frequency by voltage. It is integrated into a small circuit board at the time of manufacture and leads to five pins: power supply pin, ground pin, output pin, control pin, 900M/1800M band switching pin. When there is a suitable working voltage, it oscillates to generate the corresponding frequency signal.

Function: To convert the transmitting IF signal modulated by the IF internal modulator into the 890M-915M (GSM) frequency signal that can be received by the base station.

3) Power amplifier

Structure: the current cell phone amplifier for dual-band amplifier (900M amplifier and 1800M amplifier integrated into one), divided into two types of vinyl amplifier and iron case amplifier; different models of amplifiers can not be interchanged.

Role: TX-VCO oscillation frequency signal amplification, to obtain sufficient power current, by the antenna into electromagnetic wave radiation out.

It is worth noting that the amplifier amplifies the amplitude of the emitted frequency signal, and cannot amplify his frequency.

4) Transmitting transformer

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

Function: Send the power amplifier transmit power current sampling 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, which is detected (high-frequency rectification) and sent to the power control.

5) Power level signal

The so-called power levels are the eight levels of received signals that the engineers programmed into the phone. Each reception level corresponds to one level of transmitting power (table below). When the phone is working, the CPU determines the distance between the phone and the base station according to the received signal strength and sends out the appropriate transmit level signal to decide the amplifier amplification amount (i.e. when the reception is strong, the transmit is weak).

Power level table

6) Power controller

Structure: It is an operational comparison amplifier.

Function: Compare the transmitted 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 current induced in its secondary is detected (high frequency rectification) and sent to the power control; at the same time, the preset power level signal is also sent to the power control during programming; two After the 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 electricity and can prolong the service life of the power amplifier (high power control voltage, high power amplifier power).

3. Transmitting signal flow

When transmitting, the transmit baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) processed by the logic circuit is sent to the transmit modulator inside the intermediate frequency and modulated with the local oscillator signal into the transmit intermediate frequency. If the IF signal base station cannot receive it, the TX-VCO must be used to increase the frequency of the IF signal to 890M-915M (GSM). When TX-VCO works, the frequency signal of 890M-915M (GSM) is generated in two ways:

a) A sampling is sent back to the IF, mixed with the local oscillator signal to produce a transmission frequency discrimination signal equal to the transmission IF, and sent to the phase detector for comparison with the transmission IF; if the TX-VCO oscillation frequency does not match the mobile phone The phase detector will generate a 1-4V jump voltage to control the capacitance of the TX-VCO's internal varactor diode to achieve the purpose of adjusting the frequency.

b) The two-way input power amplifier is amplified by the antenna and converted into electromagnetic waves to radiate out. In order to control the amplification of the power amplifier, when the power current passes through the transmitting transformer during transmission, the current induced in its secondary is detected (high-frequency rectification) and sent to the power control; at the same time, the preset power level signal is also sent to Power control. After the two signals are internally compared, 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 can extend the service life of the power amplifier.