Application of Direct Downconversion Wideband Quadrature Demodulation Technology in Software Radio Handheld Platform

Pick    want:        This paper discusses the down-conversion technology in the handheld software radio platform, and compares the advantages and disadvantages of direct down-conversion and super-heterodyne structure. The direct down-conversion module with AD8347 as the core is introduced in detail , and its example application is given.

introduction

Software radio is a new type of radio processing technology developed with the rapid development of computer technology and high-speed digital signal processing technology. The basic idea is to bring the broadband A/D and D/A converters as close as possible to the antenna, and to implement the various functions of the radio as far as possible on the open, modular platform by software. It has great flexibility and Upgradeability. The emergence of software radio has made radio technology from hardware-based era to software-based era. It is the third generation of wireless communication technology after analog communication technology and digital communication technology.

An ideal software radio platform is to directly sample the RF signal, but due to the limitations of the technical level of broadband antenna, high-speed A / D and DSP , the conditions for realizing such an ideal platform are not yet available, only in the intermediate frequency or baseband. Sampling on. Therefore, the current research on software radio is more about studying the versatility and flexibility required by software radio under the existing technical conditions, and embodying the software design and generalization design ideas into concrete Application practice. FIG. 1 shows a spread spectrum communication handheld platform for baseband signal sampling processing based on a software radio idea.

Figure 1   Block diagram of software radio handheld platform for direct sequence spread spectrum receiver

 

Figure 2   Block diagram of RF direct down conversion module

Figure 3   DSP block diagram of the software radio platform

As can be seen from Figure 1 , the platform is mainly divided into two parts: the analog front end of the RF receiving and the DSP processing module. The spread spectrum signal bandwidth of the system is above 5MHz . For the RF module of the handheld platform, how to stably and accurately convert the wideband signal from the RF to the baseband for processing in the case of power consumption and volume limitation is a large difficulty. In response to this difficulty, the author used direct down-conversion technology to solve this problem better.

Down conversion implementation method

The basic principle of down-conversion is to mix and filter the RF modulated signal, and move it to a lower frequency band or baseband for processing. There are two main methods for implementation: (1) superheterodyne structure; (2) direct under frequency conversion. The superheterodyne structure has two-stage or multi-stage down-conversion process, and finally demodulates the signal on the low intermediate frequency or baseband; while the direct down-conversion technology eliminates the intermediate frequency conversion link and directly performs the received RF modulated signal once. The down-conversion is demodulated to obtain a baseband signal.

In direct comparison, direct down conversion is an ideal RF signal processing solution that has been expected by wireless communication engineers. Due to the limitation of manufacturing process and component size, this method has not been specifically promoted for a long time, but a superheterodyne structure scheme has been generally adopted. However, for handheld devices that emphasize volume and power consumption, the superheterodyne structure has significant problems compared to direct downconversion:

Compared with the superheterodyne structure, direct down conversion processing also has some weaknesses:

The weaknesses in these circuits can be effectively solved through careful design and debugging at a lower cost. In summary, the direct down-conversion technology directly converts the RF signal to the baseband for processing, which can eliminate multi-level IF circuits and reduce the number of peripheral components, cost, power consumption, printed circuit board area and design complexity. Suitable for handheld devices.      

      

Direct downconversion module for software radio platform

The platform's direct downconversion module is built from the AD8347 core and performs signal shifting from RF to baseband. The AD8347 is a single-chip, direct-conversion wideband quadrature demodulator IC from Analog Devices, Inc. that accepts RF inputs in the 800MHz to 2.7GHz range and downconverts the input signal to baseband in-phase / quadrature components. The cross component is sent to a high speed ADC for digitization. The AD8347 integrates an RF and baseband controllable gain amplifier on-chip , providing a 70dB variable gain range for easy complex wide dynamic range receive design. At the same time, it has a 50MHz output demodulation bandwidth to meet GSM , CDMA and the latest 3G communication bandwidth requirements. Its signal output has excellent phase and amplitude balance, the quadrature phase accuracy can reach 1 °, and the I/Q amplitude imbalance is controlled within 0.3dB ( ie 3.5%) , thus enabling wireless local loop and microwave radio connection. Provide an economical direct conversion solution.

In addition, the AD8347 integrates a buffered local oscillator driver that requires only -10dBm drive level, a baseband level detector, a DC offset nulling circuit, and a dual differential output amplifier with adjustable DC common-mode levels. Drive a variety of dual ADCs , such as the AD9201 , AD9218, and more. Figure 2 shows the block diagram of the RF direct downconversion module with AD8347 as the core. It is divided into four parts: AD8347 module, low noise amplifier, local oscillator frequency synthesis circuit and AGC circuit.

Low noise amplifier

A low noise amplifier with a gain of 30dB and a noise figure of 1.5dB is selected, which has a large dynamic range and P-1dB of -3dBm . It is necessary to pay attention to the handling between it and other circuits in mechanical structure and circuit design. Mutual shielding problems, especially the isolation from the local oscillator generating circuit. The better the isolation is, the more it is possible to avoid the clogging of the RF signal amplifier due to the leakage of the local oscillator.

Local oscillator frequency synthesis circuit

The local oscillator signal is obtained by TCXO phase-locked multiplier, and adopts NS 's LMX2347 frequency integrated chip. This oscillation circuit is to be noted in the design size of the output level, the recommended control -8dBm about ADI recommended, while paying attention to the mask processing and design compatibility.

AGC circuit

The system adopts a spread spectrum system, which requires high receiving sensitivity, and the baseband signal bandwidth is about 5 MHz . The thermal noise input by the antenna is ideally:

-174 (dBm/Hz) * 5 (MHz) = -107 dBm

Under normal normal operating conditions, the signal strength is much smaller than the thermal noise level, and the signal-to-noise ratio is -20 dB . The non-correlated AGC is selected to control the final output signal amplitude to be 2VP-P . Care should be taken to control the AGC 's control point to cover the range of possible signal input strengths for specific operations.

DSP module for software radio platform

DSP Software Radio module as a data acquisition and processing core, including the ADC, FIFO and DSP, FIG. 3 is the thinning of the DSP core of the module of FIG. The bandwidth of the spread spectrum communication in the system is 5MHz . The AD8347 moves the wideband signal modulated on the 2.2GHz radio frequency to the near-zero intermediate frequency band, and is divided into two orthogonal signals of I and Q. Then the I and Q signals are sampled by A / D, the sampling rate of 20M / s. Finally, the A/D sampled data is passed to the DSP through the FIFO for demodulation and despreading.

The AD9201 in Figure 3 is a dual-channel, low-voltage, high-speed A/D from Analog Devices , with a sampling rate of 20M samples per second . Its accuracy is stable and reliable, and the accuracy is always maintained at 10 bits in the full sampling bandwidth . Its operating voltage setting is quite flexible, allowing it to vary from 2.7V to 5.5V , making it ideal for high-speed operation of portable devices at low voltages. The analog signal can be input differentially or single-ended to ground. The peak -to- peak value of the signal is usually set to 1Vp-p or 2Vp-p . VREF is the reference power supply output of the AD9201 and is stable at 1V , providing the AD8347 with an accurate voltage bias reference.

Processor core member is ADI's ADSP BF533, the DSP is ADI and Intel jointly launched the company in early 2003 of a fixed-point DSP, belonging to the new Blackfin family. The BF533 has a RISC command structure that is highly efficient and performs extremely well. It runs on a 600MHz frequency, having two 40 bit MAC and the two 32 bit ALU, 10 addressable unit address. The DSP integrates 148 k Bytes of full-speed RAM and has a rich set of external interfaces such as SPI , synchronous and asynchronous serial ports, watchdogs, and various timers. It is worth mentioning that the BF533 has good power control, consumes only 280mW at 600MHz , and can dynamically control the voltage input, adjust the operating frequency, and reduce the power consumption of the chip, which is very suitable for the design of mobile products.

The FIFO acts as an external high-speed data buffer to ensure proper system timing. The depth is 32K words and the response speed is above 100M Hz/s . The write clock WCLK is 40MHz , and the dual-channel high-speed data collected by the AD9201 is read in time . The read clock RCLK is controlled by BF533 for DMA transfer and stable operation above 50M words / sec. The FIFO provides a half full flag signal /HF to the DSP to ensure that the data is not filled and read.

      

System indicator analysis

The measured indexes of the RF direct downconversion module with the AD8347 as the core are shown in Table 1 .

From the test results in the table, the basic parameters are consistent with the technical parameters given by Analog Devices .

From the perspective of the DSP module, the zero-IF signal output from the AD8347 has a maximum signal-to-noise ratio of approximately 30 dB and a minimum signal-to-noise ratio of approximately -20 dB . In the strong SNR environment, the DSP module despreads the ideal effect for the baseband spread spectrum signal, and there is no error. In the lowest signal-to-noise ratio environment, the bit error rate after despreading of the DSP module is about 10-5 , which is the intended purpose of the system design.

Conclusion

The engineering practice proves that the software radio processing platform of the whole system has reasonable structure and stable performance. Since the integrated circuit of the micro package is used as much as possible, the size of the receiver is only slightly larger than that of the business card, and the power consumption is less than 1 watt, which satisfies the needs of the handheld. The software has been modularized, easy to upgrade, and has strong portability and compatibility. It can be applied to 3G test equipment and satellite communication. The platform has been successfully applied in a communication system, and the system operation results meet the technical requirements.

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