Integration differences between low frequency and high frequency RF wireless systems

The integration of low-frequency and high-frequency RF wireless systems is quite different. In the high frequency band, because CMOS technology can achieve a higher bandwidth than bipolar technology, it is the preferred technology for RF circuits. Generally, RF-CMOS and digital CMOS are not integrated on the same chip. The most important system in the low frequency band is the cellular communication system. The focus of the RF function integration of this type of system is the integration of passive components. This article introduces a strategy for integrating passive components and RF active components through multiple packages or modules.

The radio frequency function plays an important role in the two-point transmission of information in the communication system. In this type of system, the RF function is usually physically separated from other functions, and RF transmission and reception are generally implemented by different ICs. In order to reduce system size and cost, people continue to explore ways to integrate RF with other functions of the system. Among them, the development of DSP technology has had a very important impact. In addition to this development trend of RF and non-RF integration, RF devices themselves have other integration development trends. These different development trends are because different systems require different technologies to achieve the required RF functions. For example, before passing the received signal to a low noise amplifier (LNA), some systems require effective filtering of the signal. This requires the use of ceramic filters or surface acoustic wave (SAW) filters to filter the received signal, but these filters Neither can be integrated into the receiver IC.

The difference between low frequency and high frequency systems

An important difference between low-frequency and high-frequency systems is that the latter can only achieve signal transmission when there is no barrier between the transmitter and receiver, while low-frequency systems do not have such requirements, so they can achieve greater Covered area. There is no obvious demarcation point between low frequency and high frequency, and its transition frequency is between 2-5GHz and depends on system characteristics, such as transmitter output power and receiver sensitivity. This article uses 2.4GHz as the conversion point for high and low frequencies. High-frequency systems can also be divided into long-distance systems and short-distance systems. Long-distance systems such as radar, satellite links, base station links, fixed wireless broadband access (FWBA), etc., these systems require higher transmission power than short-distance systems, such as Bluetooth and 802.11a/b.

High frequency RF integration

The target market of the short-range wireless communication system is the consumer electronics market, which requires small size and low cost, and as the application demand for transmitting video streams through data grows, the data transmission rate will continue to increase. These systems are basically portable battery-powered products, requiring long standby and talk time.

Since there are fewer transmitters working in high frequency bands, high frequency systems (above 2.4 GHz) can achieve high bandwidth and moderate receiver selection characteristics. Similarly, the signal-to-noise ratio (S/N) of the receiver is high, so the output power of the transmitter can be lower. For example, 802.11b has a bandwidth of 11 Mbps at 2.4 GHz, and 802.11a can reach up to 54 Mbps at 5 GHz. The use of wider bands or more complex modulation methods requires stricter signal linearity, and linearity is closely related to the transmitter.

Figure 1 shows the comparison of the operating frequency development that can be achieved by CMOS and BiCOS

The process technology adopted by the system is related to the achievable operating frequency. Figure 1 shows the comparison of the achievable operating frequency development of CMOS and BiCOS. Assuming that fmax is directly related to the available operating frequency, it is clear that CMOS is a better choice. In addition, CMOS can meet the non-strict selectivity, signal-to-noise ratio and output power requirements, but the dynamic performance is reduced due to the low operating voltage. However, since many systems work on open frequency bands, there may be many transmitting devices interfering with each other between the transmitter and the receiver. For example, a microwave oven interferes with Bluetooth communication as a typical example.

Although CMOS has these advantages at high frequencies, BiCMOS technology has the advantages of bipolar technology RF model, transistor parameter matching, and BiCMOS design experience is more abundant. The size is not a major consideration in process selection, because 0.18um CMOS or BiCMOS processes have similar chip sizes to achieve Bluetooth transceiver functions.

If you choose CMOS technology, standard digital CMOS will be a development trend. Because these digital CMOSs have already adopted a multi-layer mask process, there will be no additional options. Digital functions will occupy the largest chip area, so the main cost will be generated in these digital functions.

Does it make sense to integrate digital circuits and RF functions on a single chip using mainstream CMOS technology? This problem needs to be considered from two aspects: From a technical point of view, it is possible to use standard CMOS improved to achieve RF functions, such as a high-impedance substrate to reduce crosstalk through the substrate, and the use of thick dielectrics to achieve high-quality passive components Factors, etc.; from an integration perspective, applying standard CMOS to radio frequency and integrating digital and RF functions on a chip does not have many benefits, because digital and RF models and libraries are fundamentally different. Digital circuits are often designed in the VHDL/Verilog language. The digital libraries of CMOS technology are usually implemented before the emergence of new technologies. These digital libraries are used from generation to generation, so design engineers can carry out digital designs before the next-generation process is released.

For RF design, models and libraries are only possible after the process appears, so RF devices have their unique characteristics. Because RF functions generally do not have 1:1 reusable modules, every new device must be developed from scratch. The RF library usually lags behind the digital library by 1-2 years. The use of mainstream CMOS technology to implement RF functions means that it will be one generation behind in technology. Therefore, the integration of digital and RF functions on a chip means that the previous generation of CMOS technology will be used to implement digital functions, which are usually more expensive to implement. Moreover, passive components (inductors) and RF/analog functions cannot truly develop simultaneously with CMOS process technology. Therefore, the area occupied by the RF part relative to the digital part will increase with different technology generations.