Analysis and Research on GSM Mobile Phone Radio Frequency System (Part 2)

JasonYoo

Analysis and Research on GSM Mobile Phone Radio Frequency System (Part 2) [Copy link]

Introduction and analysis of traditional secondary frequency conversion

The traditional GSM cellular phone is a transceiver working in duplex mode. A mobile phone generally includes a radio frequency part and a baseband part; the radio frequency part includes the receiving and transmitting parts, and the baseband part includes digital logic, power management and display parts. The radio frequency system consists of two parts: radio frequency receiving and radio frequency transmitting. The radio frequency receiving circuit completes the functions of filtering, signal amplification, demodulation and other functions of the received signal; the radio frequency transmitting circuit mainly completes the functions of modulation, frequency conversion, power amplification and other functions of the voice baseband signal. Whether the radio frequency receiving system or the radio frequency transmitting system fails in the mobile phone circuit, it can cause the mobile phone to be unable to enter the GSM network. The radio frequency circuit includes the receiver radio frequency processing, the transmitter radio frequency processing and the frequency synthesis unit. Many traditional models use secondary frequency conversion. If there are two mixing circuits in the receiver radio frequency circuit, the receiver is a superheterodyne secondary frequency conversion receiver. The block diagram of the superheterodyne secondary frequency conversion receiver is shown in Figure 1.

The double-conversion receiver has two more mixers and a VCO, which is called IFVCO or VHFVCO in some circuits. The receiver circuits of Nokia, Ericsson, Samsung, Panasonic and Siemens mobile phones basically belong to this circuit structure. In this receiver circuit, if the RXI/Q demodulation is phase-locked demodulation, the reference signal for demodulation usually comes from the reference frequency signal. The double-conversion method uses 71MHz in the first mixing, that is, down-conversion. This mode has its own defects, high cost; requires many discrete components; has the problem of image interference; in the case of multi-mode and multi-frequency, such as China uses 900MHz and 1800MHz, if it is an exported mobile phone, more intermediate frequency filters are needed, because some countries use 850MHz and 1900MHz.