Analysis and Research on GSM Mobile Phone RF System (Part 4)

JasonYoo

Analysis and Research on GSM Mobile Phone RF System (Part 4) [Copy link]

Digital low-IF

To solve the above problems, an isolated digital low intermediate frequency is used. As shown in Figure 3, it still ensures its high integration; there is also no intermediate frequency filter, intermediate frequency phase-locked loop, which eliminates image interference, and the 150KHz intermediate frequency filter has good bandwidth selectivity. Since each channel is 200KHz, it can well eliminate the problems of DC component and low-frequency interference. It can also prevent the self-coupling and mixing of the local oscillation from interfering with the received signal. However, he added a digital mixer.

This is very beneficial for the PCB layout of mobile phones, because many mobile phone motherboards use 6-layer boards, and a small number use 8-layer boards. Except for the ground wire and a few power lines from the LDO, most signal lines use a 4mil line width. Of course, the I/Q signal line received by the RF uses 8mil. There are many considerations for the wiring from the antenna switch to the coupler and antenna, such as line width and direction. Although there is a matching network to meet the power matching of 50 ohm power transmission, the initial direction has a great impact. Once it is laid, it is best not to change it frequently. If it is the same RF solution, it is best to pursue the best wiring and not change it frequently. Because it takes a lot of time in many RF tests and EMC tests, this greatly increases the development cycle. Therefore, if the peripheral circuit is not simplified, the RF performance is difficult to improve. At present, the antenna technology of many domestic manufacturers is not very good, and the impedance value and directivity are not particularly accurate. Even if the antenna above is normal and accurate, a big feature of mobile phones is that the layout space is very small and very limited, so there are many wiring rules, such as not routing under the local oscillator, separating the clock signal from the sensitive signal line, etc., and try to avoid the influence of FPC on the RF part. These cannot be taken into account at the same time. If enough space can be left in the RF part, more consideration can be given to the circuit effect of RF. In the past, it was only analyzed in power transmission, point measurement, and external field EMC test results. If more space can be left in the board for EMC analysis and microstrip line coupling considerations, there will be better RF effects, of course, the premise is that the indicators of these RF components are accurate. Currently, Silicon Company has similar ICs.