# A detailed explanation of the principles of radio frequency circuits

#### FSPCBA - FSPCBA

Dec 7, 2022

The radio frequency circuit is the RF circuit, which refers to the radio frequency current, which is the abbreviation of a high-frequency alternating electromagnetic wave. When the alternating current of the circuit changes less than 1000 times per second, it is called low-frequency current, and when it is greater than 1000 times, it is called high-frequency current, and radio frequency refers to high-frequency current.

A radio frequency circuit refers to a circuit in which the electromagnetic wavelength of the processing signal is on the same order of magnitude as the circuit or device size. At this time, due to the relationship between the size of the electronic device and the size of the wire, the circuit needs to be processed with the relevant theory of distribution parameters. This type of circuit can be considered as a radio frequency circuit, and there is no strict requirement for its frequency, such as the AC transmission line for long-distance transmission (50Hz Or 60Hz) Sometimes it is also dealt with by the relevant theory of RF.

Next, FS PCBA takes the mobile phone RF circuit as an example to introduce the principle of the RF circuit in detail.

The structure and working principle of the radio frequency receiving circuit:

When the mobile phone is receiving the signal, the antenna converts the electromagnetic wave sent by the base station into a weak alternating current signal. After filtering and high-frequency amplification, it is sent to the intermediate frequency for demodulation, and the received baseband information (RXI-P, RXI-N, RXQ-P, RXQ-N) is obtained, which is sent to the logic audio circuit for further processing.

Circuit Analysis - Circuit Structure:

The receiving circuit is composed of antenna, antenna switch, filter, high-amplification tube (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 lower the receiving frequency before demodulation.

The 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 frequency of the transmitting intermediate frequency signal is changed to 890M-915M (GSM) frequency signal by TX-VCO. After being amplified by the power amplifier, it is converted into electromagnetic waves by the antenna and radiated out.

Circuit Analysis - Circuit Structure:

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

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

The circuit generates four sections of local oscillator frequency signals (GSM-RX, GSM-TX, DCS-RX, DCS-TX) without any information, and sends them to the IF. The baseband information is modulated and transmitted phase-detected.

Circuit Analysis - Circuit Structure: There are four circuit structures in the mobile phone local oscillator circuit:

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)

b) Integrate the frequency synthesis integrated block inside the intermediate frequency, combined with an external RX-VCO (multi-purpose for mid-term phones and Nokia phones)

c) Integrate the frequency synthesis integrated block and the receiving voltage-controlled oscillator (RX-VCO), called the local oscillator integrated block (multi-purpose for mid-term and Samsung phones)

d) Integrate the frequency synthesis integrated block and the receiving voltage-controlled oscillator (RX-VCO) inside the intermediate frequency (multi-use for new models and miscellaneous brands).

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