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As the most widely used mobile communication standard in the world, GSM has been showing strong vitality since its large-scale commercial use around the world in the 1990s. Whether in Europe, the birthplace of GSM, or in Asia and Africa, the emerging markets for mobile communications, the GSM system has the most mature and complete industrial chain and user base, covering systems, terminals, equipment, software, testing and other fields. As GPRS, EDGE and other technologies that can further enrich GSM data applications begin to be applied, in a long time in the future, GSM will surely coexist with 3G standards for a win-win situation with extensive network coverage, reliable communication quality, and low tariffs. Pattern.
As the product that has the closest contact with users, mobile phone terminals have their price, performance, size, and energy-saving as their primary considerations when purchasing. Therefore, when mobile phone terminal designers compare various solutions, integration, performance, and power consumption are all important reference factors.
The most important cores in mobile terminals are the radio frequency chip and baseband chip. The radio frequency chip is responsible for radio frequency transceiver, frequency synthesis, and power amplification; the baseband chip is responsible for signal processing and protocol processing. In the earliest GSM terminals, the radio frequency part was a discrete component using bipolar technology, requiring more than 70 chips and components, with a large area and high cost. With the advancement and maturity of CMOS RF technology in the late 1990s, the use of CMOS technology compatible with digital technology to manufacture high-performance and highly integrated RF chips has gradually become a trend.
Basic characteristics of RDA6205 chip
RDA6205 is a transceiver chip developed by RDA Microelectronics. This is a high-performance, fully integrated CMOS GSM transceiver chip. The packaged chip size is only 5mm×5mm. The chip integrates LNA, mixer, filter, analog-to-digital converter (ADC), DSP, PLL frequency synthesizer and analog baseband interface. Only a few external components are needed to form a complete radio frequency subsystem, and it can communicate with a variety of mainstream baseband chips in the market through the built-in universal analog baseband interface.
As a fully integrated GSM radio frequency transceiver chip, RDA6205 has excellent radio frequency performance. Its GSM frequency band receiving sensitivity reaches -108dBm, far exceeding the -102dBm specified by the GSM standard; its transmission spectrum 400kHz ACPR reaches -70dBc, which is better than similar products Improved by 3~5dB.
The RDA6205 chip also supports digital compensation crystal oscillator (DCXO), so that the entire system can robustly resist the frequency drift caused by temperature changes, thereby meeting the demanding GSM standard requirements of transmission and reception frequency deviation <0.1ppm. In traditional solutions, users have to choose expensive temperature compensated crystal oscillators (TCXO) in order to achieve this goal. RDA6205 chip working principle
The principle block diagram of RDA6205 is shown as in Fig. 1.
When the RDA6205 chip is working in the receiving mode, it needs to change the radio frequency signal of any intensity into a baseband signal of basically constant intensity, and then send it to the baseband chip. The weak RF signal is input to the chip through the differential port and first enters the front-end LNA. The LNA can amplify the input signal by 20dB without introducing too much additional noise, which greatly reduces the requirements of other circuits in the receiving link for their own noise. The signal then enters the quadrature down-conversion mixer, where it is down-converted to a near-zero intermediate frequency of 100kHz. Because in the real complex GSM air network environment, we expect that there is often a large interference signal (block signal) near the received signal, so the signal after orthogonal downconversion needs to pass a complex band with a center frequency of 100kHz Pass filter, filter out the interference signal outside the band. The signal after the filtering operation is directly sent to the high-precision and high dynamic range delta-sigma analog-to-digital converter, where it is converted into a digital signal.
RDA6205 has built-in a powerful dedicated DSP unit. The digital signal sent from ADC will be used for digital down-conversion, DC cancel, frequency selection filtering and other functions. Subsequently, the processed signal is sent to the analog interface of the baseband chip through a flexibly programmable analog-to-digital converter (DAC).
The RDA6205’s entire receiving link gain is flexibly adjustable, and can provide a maximum gain range of more than 100dB. Under the AGC strategy of the baseband chip, the RDA6205 can easily handle various intensities from -102dBm to -15dBm while ensuring sufficient signal-to-noise ratio. RF input signal. When the RDA6205 chip is working in the transmit mode, the analog baseband signal sent by the baseband chip needs to be turned into a radio frequency signal to drive the RF front-end power amplifier (PA) to transmit.
Since the GSM standard uses a constant envelope modulation method such as GMSK, all useful information is carried in the phase domain of the radio frequency signal, so RDA6205 uses an advanced direct modulation transmission structure (Direct Modulation).
The analog baseband signal input by the baseband chip is first sampled and quantized by the ADC in the RDA6205 chip, and then the phase modulation information carried in it is determined, and the phase modulation information is differentiated, and then the modulation information is superimposed on the current through the delta-sigma PLL In this way, the local oscillator signal output by the PLL is the modulated radio frequency transmission signal. The RF modulated signal output by the PLL drives the module through the power amplifier, which can directly drive a 50Ω power amplifier.
RDA6205 has obtained excellent emission spectrum index by adopting advanced emission structure. Its ACPR index at 400kHz is 3~5dB higher than the traditional scheme.
RDA6205 chip application
As shown in Figure 2, RDA6205 can work with mainstream baseband chips in a variety of markets to achieve a GSM overall solution and design a GSM user terminal that is small in size, excellent in performance, and low in cost.
Thanks to the high integration of RDA6205, the radio frequency part of the entire terminal solution only includes radio frequency antennas, power amplifier switch modules, radio frequency surface acoustic wave filters (SAW) and a small number of sub-production components, which not only greatly reduces component costs, but also Reduce the design difficulty of RF PCB.
At the same time, compared with the traditional superheterodyne solution, the GSM user terminal using RDA6205 has fewer discrete components and high integration in the radio frequency part. The radio frequency signal is received from the antenna and passed through the front-end switch. It directly enters the RDA6205 chip. The analog signal processing process is completed inside the chip, avoiding the direct influence of various interference signals on the PCB on the radio frequency circuit, and greatly improving the anti-interference performance of the entire radio frequency system.
However, the traditional superheterodyne scheme has to use peripheral components such as intermediate frequency filters, which is not only limited in the area of the PCB, but also because the more sensitive intermediate frequency signals are easily interfered on the off-chip PCB, which affects the PCB Both design and manufacturing processes have higher requirements, which can easily cause a decline in the yield of the whole machine in mass production.
With the further popularization of multimedia applications such as FM, DVB, MP3, MP4, etc. in terminals, and the further increase of users’ demand for miniaturization and multi-functionalization of terminals, mobile phone applications using fully integrated RDA6205 will shine in the future market .
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