How To Understand The Rfid Function?

- Jun 14, 2019-

Structure and working principle of the circuit

At the time of transmission, the transmission baseband information processed by the logic circuit is modulated into a transmission intermediate frequency, and the frequency of the transmitted intermediate frequency signal is changed to a frequency signal of 890M-915M (GSM) by the TX-VCO. After being amplified by the power amplifier, the antenna is converted into electromagnetic wave radiation.

The circuit masters the key points: (1), circuit structure; (2) the function and function of each component; (3) the process of transmitting signals.

1. Circuit structure

The transmitting circuit is composed of a transmitting modulator inside the intermediate frequency, a transmitting phase detector, a transmitting voltage controlled oscillator (TX-VCO), a power amplifier (power amplifier), a power controller (power control), a transmitting transformer, and the like.

2. The function and function of each component

1), transmitter modulator:

Structure: The transmit modulator is internal to the IF and is equivalent to the MOD in a broadband network.

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

2), launch voltage controlled oscillator (TX-VCO):

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

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

Principle: It is well known that the base station can only receive the frequency signal of 890M-915M (GSM), and the intermediate frequency signal modulated by the intermediate frequency modulator (such as Samsung transmits the intermediate frequency signal 135M) can not be received by the base station. Therefore, the TX-VCO should be used to transmit the intermediate frequency signal. The frequency becomes a frequency signal of 890M-915M (GSM).

When transmitting, the power supply part sends out the 3VTX voltage to make the TX-VCO work, and the 890M-915M (GSM) frequency signal is generated in two ways: a), the sample is sent back to the internal frequency, and the local oscillator signal is mixed to generate a transmitting intermediate frequency. The equal transmit and receive frequency signals are sent to the phase detector and compared with the transmit intermediate frequency; if the TX-VCO oscillation output frequency does not match the mobile phone's working channel, the phase detector will generate a 1-4V trip voltage (with AC transmission) The DC voltage of the information is used to control the capacitance of the internal varactor of the TX-VCO to achieve the purpose of adjusting the frequency accuracy. b), after being sent into the power amplifier, the antenna is converted into electromagnetic wave radiation after being amplified.

It can be seen from the above that the frequency generated by the TX-VCO is sent back to the internal frequency of the sample, and then the voltage is generated to control the TX-VCO operation; just forming a closed loop and controlling the frequency phase, the circuit is also called the transmit lock phase. Loop circuit.

3), power amplifier (amplifier):

Structure: At present, the power amplifier of the mobile phone is a dual-frequency power amplifier (integrated with 900M power amplifier and 1800M power amplifier), which is divided into two types: black rubber power amplifier and iron shell power amplifier; different types of power amplifiers cannot be interchanged.

Function: Amplify the TX-VCO oscillation frequency signal to obtain sufficient power current, which is converted into electromagnetic wave radiation through the antenna.

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

Power amplifier operating conditions:

a), working voltage (VCC): the power supply of the mobile phone amplifier is directly provided by the battery (3.6V);

b), ground (GND): make the current into a loop;

c), dual-frequency power conversion signal (BANDSEL): control power amplifier works at 900M or works at 1800M;

d), power control signal (PAC): control the amplification of the power amplifier (operating current);

e), input signal (IN); output signal (OUT).

4), launch transformer:

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

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

Principle: When the power amplifier emits power through the transmitting transformer when transmitting, the current of the same magnitude as the power current is induced in the secondary, after being detected (high-frequency rectification) and sent to the power control.

5), power level signal:

The so-called power level is that the engineers divide the received signal into eight levels during mobile phone programming. Each receiving level corresponds to the first-level transmitting 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 according to the received signal strength. At a distance, the appropriate emission level signal is sent to determine the amplification of the amplifier (ie, when the reception is strong, the transmission is weak).

With the power rating table:

6), power controller (power control):

Structure: Compare the amplifier for an operation.

Function: Compare the transmit 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 when transmitting, the current induced in the 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 internal comparison, a 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 prolong the service life of the power amplifier (high power control voltage and large power amplifier power).

3. Signal transmission process

When transmitting, the transmit baseband information (TXI-P; TXI-N; TXQ-P; TXQ-N) processed by the logic circuit is sent to the internal modulator of the intermediate frequency and modulated with the local oscillator signal to transmit the intermediate frequency. If the intermediate frequency signal base station cannot receive, the base station of the frequency signal that raises the frequency of the transmitted intermediate frequency signal to 890M-915M (GSM) can be received by the TX-VCO. When the TX-VCO is working, the frequency signal of 890M-915M (GSM) is generated in two ways:

a), sampling all the way back to the internal frequency, mixing with the local oscillator signal to generate a transmitting and discriminating signal equal to the transmitting intermediate frequency, and sending it to the phase detector to compare with the transmitting intermediate frequency; if the TX-VCO oscillation frequency does not match the mobile phone The working channel, the phase detector will generate a 1-4V trip voltage to control the capacitance of the TX-VCO internal varactor to achieve the purpose of adjusting the frequency.

b), the two-way power amplifier is amplified and converted into electromagnetic wave radiation by the antenna. In order to control the amplification of the power amplifier, when the power current passes through the transmitting transformer when transmitting, the current induced in the secondary is sent to the power control after the detection (high-frequency rectification); at the same time, the preset power level signal is also sent during programming. Power control; two signals generate a voltage signal after internal comparison to control the amplification of the power amplifier, so that the working current of the power amplifier is moderate, saving power and long power amplifier life.