| SDARS MAX2140 XM wireless tuning solution |
| Abstract: This application note introduces the SDARS XM wireless tuning scheme built using the MAX2140, including the basic RF to demodulator, antenna to demodulator signal link, and introduces the characteristics of the MAX2140 tuner and some of the SDARS receiver Important RF specifications. Overview Satellite Digital Audio Wireless Service (SDARS) is a satellite-based direct broadcast wireless service system. In SDARS, the digitally encoded audio entertainment information is directly transmitted to the ground receiver through the orbiting synchronous satellite. When the ground receiver is not within the satellite coverage, it can also be sent to the receiver through the relay station. Maxim's MAX2140 provides a complete RF to baseband SDARS receiving solution. Complete SDARS receiver MAX2140 The MAX2140 is a receiver specially designed for SDARS and has the following features: Complete reception solution, only one SAW filter is required. Built-in RF AGC loop differential I / Q interface. Built-in complete frequency generator bias power supply. For low-power standby mode with external LNA overcurrent protection, Figure 1 shows the block diagram of the MAX2140.  Figure 1. MAX2140 block diagram / pin configuration Block diagram of XM wireless tuning module. The block diagram of RF input to demodulation channel is shown in Figure 2.  Figure 2. Block diagram of SDARS wireless tuner The audio signal is first received by the antenna. In order to meet the sensitivity requirements, an external low noise amplifier (LNA) needs to be connected. The LNA is controlled by the baseband controller; the tuner has its own RF AGC loop and IF AGC controlled by the baseband Loop, can effectively provide a total dynamic range of more than 92dB. The work of IF AGC and RF AGC is shown in Appendix A; SAW filters and on-chip integrated low-pass filters ensure the sensitivity of the channel. The back-end signal path also has a high-pass filter to eliminate DC offset; in order to achieve full image frequency suppression, the device uses an image frequency suppression mixer to ensure excellent half-IF suppression characteristics; N-frequency PLL can achieve very The small frequency step makes it easy to realize the AGC loop. In addition, the reference is provided by an external XTAL and an on-chip oscillator. The device also provides a reference buffer output. SDARS tuner specification 1. Frequency and channel bandwidth XM is allocated to the frequency band of 12.5MHz, the frequency range is between 2332.5MHz and 2345MHz, and the center frequency is 2338.75MHz. The spectrum can be roughly divided into 6 segments. Four of them are used to transmit satellite signals, and the other two are used for signal transmission of ground relay networks.  Figure 3. SDARS frequency bandwidth 2. The modulation satellite adopts QPSK modulation, and the relay adopts COFDM modulation to meet the requirements of terrestrial broadcasting. 3. RF Specifications of MAX2140 Table 1. RF Specifications of MAX2140 (For details, please refer to the data sheet.)
Appendix A: IF AGC and RF AGC  |
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