Third-generation mobile phone RF architecture

Most telecommunications companies have invested huge amounts of money to obtain 3G licenses, and now they also need to invest in infrastructure to support this long-awaited technology. The total cost will reach billions of pounds, so telecommunications companies are under great pressure to repay their debts and earn profits quickly. The fastest way to achieve this goal is to start the 3G consumer market as soon as possible.
For services such as multimedia functions with fierce price competition, the key to winning the favor of consumers and business customers and quickly recovering their investment is to provide affordable mobile phones. The internal circuits of mobile phones, including transmitters and receivers, account for a considerable portion of the cost of mobile phones. In order to reduce the overall cost of 3G mobile phone RF solutions, three main goals must be met: namely, to reduce chip costs and reduce external device requirements, and at the same time integrate the external devices that must be used into a solution similar to a system on chip (SoC) as much as possible .

The standard range involved Wideband Code Division Multiple Access (W-CDMA) will become the main standard for 3G. However, in Europe, mobile phones also need to be able to support the existing GSM 900 and GSM 1800 standards. There is a similar situation in the US market, where high-end mobile phones still need to support the GSM 900 and GSM 1800 standards for several years.
For RF designers who need to support the GSM Enhanced Data Rate Improvement (EDGE) standard, the problem is more complicated. EDGE will be used in suburban or rural areas to ensure that residents there can also enjoy the high data rate services provided by 3G, rather than just restricting 3G to the main urban areas covered by W-CDMA. EDGE will complement W-CDMA and provide data transmission rates of up to 384 kbps to the wider population. The evolution of 2G-3G is shown in Figure 1.

Integration in the design of mobile phones has always required the further reduction of PCB circuit board space. However, due to technological advances in recent years, it has become easier for consumers to provide smaller and lighter mobile phones in some ways. For example, a circuit with lower power consumption means that a smaller battery can be used to achieve the same talk and standby time.
The emergence of 3G brings new pressure on further reducing the size of PCBs. Because this requires the integration of larger multimedia displays and other features, including cameras, in the design of 3G phones that are not larger than existing 2G models.
There is also great pressure to reduce the overall cost of mobile phones. From a circuit perspective, the best way to reduce costs is to integrate as many components as possible into the SoC solution using smaller geometries and low-cost processes. Integrating external components can also bring additional benefits, including improved reliability, reduced production and assembly costs, and smaller EMC shielding issues.
Although it is possible to integrate multiple components in a single chip, in order to achieve duplexing, it is necessary to isolate between the transmit and receive channels of W-CDMA, which is not feasible for GSM. So the 3G solution must also use two chips at this stage. The transmitter and receiver chips designed by Zarlink Semiconductor use micro-lead frame (MLF) surface mount packages, which only require a small amount of space on a valuable PCB circuit board. The transmitting chip has 40 pins and measures 6mm x 6mm, while the receiving chip has 56 pins and measures 8mm x 8mm. SiGe BiCMOS technology is used in the manufacturing process to obtain devices with smaller geometry, lower cost and lower power consumption. The structure used can save expensive external components, such as SAW filters. At the same time also built-in VCO (including oscillation circuit), PLL, LNA and HPA pre-driver circuit.

The receiver structure superheterodyne is the traditional and mature receiving channel structure in radio transmission. However, because this architecture requires a large number of external components that are costly and take up a lot of PCB space, this solution is not suitable for 3G applications.
Direct conversion is an optional architecture that can greatly reduce the number of external filters required. However, the direct conversion (zero intermediate frequency) structure also has disadvantages. In order to compensate for the baseband DC offset, the control circuit becomes very complicated. For 3G radio units that need to support both EDGE and W-CDMA, this compensation becomes more difficult. This is because the control circuit must perform DC offset correction on the basis of the amplitude modulation signal.
These problems have led to the need for a new 3G mobile phone structure supporting EDGE that can effectively meet the requirements of 3G applications in terms of cost and functionality. Near zero intermediate frequency (NZIF) technology provides a compromise between superheterodyne and direct conversion technology. In principle, it looks like a superheterodyne structure, but the difference is that the intermediate frequency it uses is on a channel slightly higher than DC. By transferring the intermediate frequency to such a low frequency, it is possible to integrate the entire filtering function on one chip. This means that external IF SAW filters and DC bias control circuits operating at hundreds of MHz are no longer necessary. For W-CDMA, there is not much problem with the DC offset, and the DC component can be filtered without too much impact on the sensitivity. The only external filter required is the radio frequency filter before the LNA.

Figure 1 2G to 3G evolution process

The transmitter structure is on the transmit channel, and the "up-conversion" structure provides the simplest solution. However, due to the need for intermediate frequencies and expensive external filters, there are significant cost issues. Polar loop, offset PLL and direct modulation are all optional structures for 3G transmitters.
The offset PLL structure is widely used in GSM in recent years. This method removes all amplitude modulation (AM) components of the signal and provides a very clean spectrum. Unfortunately, this structure is not suitable for signals with AM content (such as EDGE).
Polar loop technology is similar to biased PLL, but allows AM components to be added to the RF signal. But for W-CDMA, the application of this technology is also problematic, because it is extremely difficult to achieve synchronization between phase and amplitude, and in some cases it is impossible.
At first glance, the last technique, direct modulation, does not seem to work. Because the required frequency of the local oscillator (LO) needs to be the same as the transmitted signal, this will result in a very large output signal and thus lead to VCO oscillator frequency pulling problems. However, by doubling the LO frequency and then dividing the frequency, the LO will no longer work at the same frequency as the transmitted signal, thereby avoiding frequency traction problems. This structure is suitable for both W-CDMA and EDGE. Since no external components are required, the cost of this structure is also low.
The current process (such as SiGe BiCMOS) can achieve a very clean and low noise VCO. For W-CDMA, the VCO will work above 4GHz. At this frequency, it is possible to integrate all the active components of the VCO, including the bias circuit and all the oscillation circuits. This means that the entire VCO can be integrated on the chip and then divided to the required frequency band.

Conclusion The latest manufacturing process and structural technology can meet the key technical requirements of 3G mobile phones: SiGe BiCMOS process enables the transmitter and receiver to maintain low power consumption while keeping the physical size as small as possible. The new system structure design method allows designers to reduce costs by eliminating external components or integrating them into the chip. In the future, in addition to further solving the challenges faced, it is also necessary to find a method for integrating the transmitter and receiver on a fully integrated single chip. â– 

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