Illustration of the design principles and applications of mobile phone radio frequency circuits

1. RF circuit composition and characteristics:

The radio frequency circuit of an ordinary mobile phone is composed of three major circuits: a receiving channel, a transmitting channel, and a local oscillator circuit. It is mainly responsible for receiving signal demodulation and transmitting information modulation. Early mobile phones used superheterodyne frequency conversion (mobile phones have primary and secondary mixing and primary and secondary local oscillator circuits) before demodulating the receiving baseband information; new mobile phones directly demodulate the receiving baseband information (zero intermediate frequency). Some mobile phones also integrate the frequency combination and receiving voltage-controlled oscillator (RX-VCO) into the IF.

(RF circuit block diagram)

(1) Structure and working principle of the receiving circuit:

When receiving, the antenna converts the electromagnetic wave sent by the base station into a weak AC current signal, which is filtered, amplified at high frequency, and sent into the intermediate frequency for demodulation to obtain the received baseband information (RXI-P, RXI-N, RXQ-P, RXQ- N); sent to the logic audio circuit for further processing.

1. Key points of this circuit:

(1). Receiver circuit structure.

(2) The functions and functions of each component.

(3), receiving signal process.

circuit analysis:

(1), circuit structure.

The receiving circuit is composed of antenna, antenna switch, filter, high-amplifier tube (low-noise amplifier), intermediate frequency integrated block (receiving demodulator) and other circuits. Early mobile phones had primary and secondary mixing circuits, whose purpose was to reduce the receiving frequency and then demodulate it (as shown in the figure below).

(receiving circuit block diagram)

(2) The functions and functions of each component.

1). Mobile phone antenna:

Structure: (as shown below)

Mobile phone antennas are divided into two types: external and internal antennas; they are composed of an antenna base, a solenoid, and a plastic envelope.

effect:

a) When receiving, the electromagnetic waves sent by the base station are converted into weak AC current signals.

b) When transmitting, the AC current amplified by the power amplifier is converted into an electromagnetic wave signal.

2), Antenna switch:

Structure: (as shown below)

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

(Picture 1)(Picture 2)

Function: Its main functions are two:

a) Complete receiving and transmitting switching;

b) Complete the 900M/1800M signal reception switching.

The logic circuit sends out control signals (GSM-RX-EN; DCS- RX-EN; GSM-TX-EN; DCS- TX-EN) respectively according to the working status of the mobile phone, so that the respective paths are turned on, so that the receiving and transmitting signals go their own way. Tao, do not interfere with each other.

Because the mobile phone cannot work in the same time slot when receiving and transmitting at the same time (that is, it does not transmit when receiving and does not receive when transmitting). Therefore, later new mobile phones removed the two switches in the receiving path, leaving only two transmitting switches; the receiving switching task was completed by the high-amplifier tube.

3), filter:

Structure: There are high-frequency filters and medium-frequency filters in mobile phones.

effect:

Its main function is to filter out other useless signals and obtain pure received signals. Later new mobile phones are all zero-IF mobile phones; therefore, there is no IF filter in the mobile phone.

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

Structure: There are two high-amplification tubes in mobile phones: 900M high-amplification tubes and 1800M high-amplification tubes. They are all triode common-emitter amplifier circuits; later new mobile phones integrated high-amplifier tubes inside the intermediate frequency.

(High frequency amplifier tube power supply diagram)

effect:

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-amplifier tubes of 900M/1800M share one channel and are provided by the IF simultaneous channel; and the bias voltages of the collectors of the two tubes are sent out by the IF CPU in two channels according to the reception status of the mobile phone. ; Its purpose is to complete the switching of 900M/1800M receiving signals.

b) Principle: After filtering out other clutter through the filter, the pure 935M-960M received signal is coupled by a capacitor and sent to the corresponding high-amplifier tube for amplification and then sent to the intermediate frequency through the capacitor coupling for post-stage processing.

5), IF (RF interface, RF signal processor):

structure:

It is composed of receiving demodulator, transmit modulator, transmit phase detector and other circuits; the new mobile phone also integrates high-amplifier tube, frequency synthesis, 26M oscillation and frequency division circuit inside (as shown below).

effect:

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

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

c). During transmission, the transmission information processed by the logic circuit and the local oscillator signal are modulated into a transmission intermediate frequency (described later).

d), combine with 13M/26M crystal to generate 13M clock (reference clock circuit).

e). According to the reference signal sent by the CPU, a local oscillator signal that conforms to the working channel of the mobile phone is generated (described later).

(2), receiving signal process. (Refer to zero-IF mobile phone)

When the mobile phone is receiving, 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 sends a high-frequency filter to filter out other useless clutter to obtain a pure 935M-960M (GSM) receiving signal, which is coupled by a capacitor. After being amplified by the corresponding high-amplifier tube inside the IF, it is sent to the demodulator to demodulate the local oscillator signal (without information) to obtain the 67.707KHZ receiving baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N); sent to the logic audio circuit for further processing.

(2) Structure and working principle of the transmitting circuit:

When transmitting, the transmit baseband information processed by the logic circuit is modulated into a transmit intermediate frequency, and the TX-VCO is used to change the frequency of the transmit intermediate frequency signal into a frequency signal of 890M-915M (GSM). After being amplified by the power amplifier, it is converted into electromagnetic waves and radiated by the antenna.

This circuit masters the key points:

(1), circuit structure.

(2) The functions and functions of each component.

(3). Transmitting signal process.

circuit analysis:

(1), circuit structure.

The transmit circuit is composed of the transmit modulator, transmit phase detector, transmit voltage controlled oscillator (TX-VCO), power amplifier (power amplifier), power controller (power control), transmit transformer and other circuits inside the intermediate frequency. (As shown below)

(Transmitter circuit block diagram)

(2) The functions and functions 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: During transmission, the transmission baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) processed by the logic circuit and the local oscillator signal are modulated into a transmission intermediate frequency.

2). Transmit voltage controlled oscillator (TX-VCO):

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

Function: Convert the transmitted IF signal modulated by the IF modulator into the 890M-915M (GSM) frequency signal that the base station can receive.

principle:

As we all know, the base station can only receive frequency signals of 890M-915M (GSM), but the base station cannot receive the intermediate frequency signal modulated by the intermediate frequency modulator (such as Samsung transmitting the intermediate frequency signal 135M). Therefore, the TX-VCO must be used to increase the frequency of the transmitted intermediate frequency signal. Change 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 frequency signal of 890M-915M (GSM) is generated in two ways: a), the sample is sent back to the inside of the intermediate frequency, and mixed with the local oscillator signal to generate an intermediate frequency with the transmitting intermediate frequency The equal transmit frequency discrimination signal is sent to the phase detector for comparison with the transmit 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 (with AC transmitter DC voltage of the information) to control the capacitance of the varactor diode inside the TX-VCO to achieve the purpose of adjusting the frequency accuracy. b) After being amplified by the power amplifier, it is converted into electromagnetic waves and radiated by the antenna.

It can be seen from the above: the frequency is generated by TX-VCO, the sample is sent back to the IF, and then the voltage is generated to control the operation of TX-VCO; ​​it just forms a closed loop and controls the frequency phase, so this circuit is also called transmit phase lock. loop circuit. ,

3), power amplifier (power amplifier):

Structure: The current power amplifier of mobile phones is a dual-band power amplifier (900M power amplifier and 1800M power amplifier integrated), which are divided into two types: vinyl power amplifier and iron case power amplifier; different models of power amplifiers cannot be interchanged.

Function: Amplify the frequency signal oscillated by the TX-VCO, obtain sufficient power current, and convert it into electromagnetic waves through the antenna for radiation.

Note: The power amplifier amplifies the amplitude of the transmitted frequency signal, not its frequency.

Working conditions of power amplifier:

a) Working voltage (VCC): The power supply of the mobile phone amplifier is directly provided by the 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): controls the amplification amount (operating 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 using the principle of mutual inductance.

Function: Send the power amplifier transmit power current sample to the power control.

Principle: When the power amplifier transmitting power current passes through the transmitting transformer during transmission, a current of the same size as the power current is induced on its secondary side, which is detected (high-frequency rectified) and sent to the power control.

5), power level signal:

The so-called power level means that engineers divide the received signal into eight levels when programming the mobile phone. Each receiving level corresponds to one level of transmit power (as shown in the table below). 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. Far and near, the appropriate emission level signal is sent to determine the amplification amount of the power amplifier (that is, when the reception is strong, the emission is weak).

Attached is the power level table:

6), power controller (power control):

Structure: It is an operational comparison amplifier.

Function: Compare the transmit power current sampling signal with the power level signal to obtain a suitable voltage signal to control the amplification amount 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 during programming is also sent to the power control; both The signals are compared internally to generate a voltage signal to control the amplification amount of the power amplifier, so that the working current of the power amplifier is moderate, which not only saves power but also 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 process.

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 intermediate frequency signal base station cannot receive it, the TX-VCO must be used to increase the frequency of the transmitted intermediate frequency signal to 890M-915M (GSM) in order for the base station to receive it. When the TX-VCO is working, the frequency signal generated from 890M-915M (GSM) goes in two ways:

a). All the samples are sent back to the IF, mixed with the local oscillator signal to generate a transmit frequency discrimination signal equal to the transmit IF, and sent to the phase detector for comparison with the transmit IF; if the TX-VCO oscillation frequency does not match the mobile phone The working channel, the phase detector will generate a 1-4V jump voltage to control the capacitance of the varactor diode inside the TX-VCO to adjust the frequency.

b). The two-way input power amplifier is amplified and converted into electromagnetic waves by the antenna for radiation. In order to control the amount of power amplifier amplification, 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 during programming is also sent to Power control: Two signals are compared internally to generate a voltage signal to control the amplification amount of the power amplifier, so that the operating current of the power amplifier is moderate, which not only saves power but also prolongs the service life of the power amplifier.

3. The structure and working principle of the local oscillator circuit: (local oscillation circuit, phase-locked loop circuit, frequency synthesis circuit)

This circuit generates four local oscillator frequency signals without any information (GSM-RX; GSM-TX; DCS-RX; DCS-TX); they are sent to the IF to demodulate the received signal when receiving; to transmit when transmitting The baseband information is modulated and transmitted phase detected.

This circuit masters the key points:

(1), circuit structure.

(2) The functions and functions of each component.

(3) Working principle of local oscillator circuit.

circuit analysis:

(1), circuit structure. There are four circuit structures of mobile phone local oscillator circuits:

a), composed of frequency synthesis integrated block, receiving voltage controlled oscillator (RX-VCO), 13M reference clock, and preset frequency reference data (SYN-DAT; SYN-CLK; SYN-RST; SIN-EN) (early Multi-purpose for mobile phones; as shown below).

b) Integrate the frequency synthesis integrated block into the IF, and combine it with an external RX-VCO (multipurpose for mid-range and Nokia phones; (as shown below)

c) Integrate the frequency synthesis integrated block and the receiving voltage controlled oscillator (RX-VCO) into one, called the local oscillator integrated block or local oscillator IC (used in mid-term and Samsung phones; as shown below).

d) Integrate the frequency synthesis integrated block and the receiving voltage controlled oscillator (RX-VCO) inside the intermediate frequency (multipurpose for new models and unknown brands; as shown below).

It is worth noting that no matter what structural mode is used, only the frequency generated is different; its working principle, the direction and function of the frequency signal generated are the same.

(2) The functions and functions of each component.

a), receiving voltage controlled oscillator (RX-VCO):

The structure and working principle are the same as TX-VCO; ​​the difference from TX-VCO is that TX-VCO generates two frequency bands and only participates in transmitting; while RX-VCO generates four frequency bands and participates in both receiving and transmitting; both VCOs are not interchangeable.

b), frequency synthesis integrated block:

It is a comparison operational amplifier; it compares the frequency sampling signal generated by RX-VCO and the preset frequency reference data internally, and uses the 13M reference clock as a reference to generate a 1-4V jump voltage (pure DC voltage) to control the RX-VCO The purpose of oscillating the accurate local oscillator frequency.

c). Preset frequency reference data:

That is, when engineers design mobile phones, they pre-set the local oscillator frequency standards required when the mobile phones work on different channels (124 for GSM mobile phones), list them in a data table, and store them in the font library. That is, the frequency combination clock (SYN-CLK) sent by the CPU; frequency combination data (SYN-DAT); frequency combination reset (SYN-RST); frequency combination start (SIN-EN).

(3) Working principle of local oscillator circuit.

After the mobile phone is turned on normally, the power supply part sends out the frequency combined power to make the local oscillator circuit work. At this time, the RX-VCO oscillates out the local oscillator frequency signal and goes in two ways:

1). Send the local oscillator frequency sample into the frequency synthesis integrated block and compare it internally with the preset frequency reference data; and use the 13M reference clock as a reference to generate a 1-4V jump voltage to control the internal variable of the RX-VCO. The capacitance of the capacitive diode is adjusted to adjust the output frequency so that the RX-VCO oscillates out the local oscillator frequency required for the mobile phone's working channel (commonly known as fine-tuning).

2). The local oscillator frequency is sent into the IF, and after frequency division, it is divided into three channels:

a) When receiving, the local oscillator frequency is sent to the receiving demodulator to demodulate the received signal (that is, the local oscillator frequency and the receiving frequency are equal in size and opposite in phase and the frequency signals are moved and offset; the remaining information sent by the other party) .

b) When transmitting, the local oscillator frequency is sent to the transmit modulator, and the transmit baseband information (TXI-P; at the local oscillator frequency).

C). When transmitting, the frequency sample generated by the TX-VCO is sent back to the IF, mixed with the local oscillator frequency, and generates a transmit frequency discrimination signal equal to the transmit IF frequency.

The 900M/1800M local oscillator frequency conversion is controlled by the CPU sending a dual-frequency power conversion signal (BANDSEL) (commonly known as coarse adjustment).

It can be seen from the above that the frequency generated by RX-VCO is fed into the frequency synthesis integrated block, and then the voltage is generated to control the operation of RX-VCO; ​​it just forms a closed loop and controls the frequency phase, so this circuit is also called Phase locked loop circuit. ,

From the working principle of the frequency combination circuit, the local oscillator frequency and the receiving frequency must be synchronized (the same working channel) before the mobile phone can have a signal. How does the CPU determine the working channel of the mobile phone? It turns out that when the mobile phone is turned on, the CPU sends the SYN-DAT, SYN-CLK, SYN-RST, and SIN-EN required for all working channels of the 900M/1800M system to make the RX-VCO generate all local oscillator frequencies. , then sent into the IF to connect with the receiving frequency until the logic circuit receives the baseband information. And it is locked on this channel. Therefore, it is a long process for mobile phones to find the network.