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With the rapid development of digital communications technology, direct sequence spread spectrum technology is widely used. Especially wartime use of spread spectrum communication confidentiality and anti survivability effective method. The spread spectrum communication refers to information data to be transmitted with a PN Pseudo code spreading, the modulated and then transmitted, at the receiving end, the phase encoding for despreading and related processing, to restore the original information data communication process. Spread spectrum communication with anti-jamming and anti-noise, resistance to multipath fading, high-security, low power spectral density, the high hidden, low interception rate can be multiple access multiplexing, and an arbitrary location, etc.. As a spread spectrum communication module, and despreading spread spectrum modulation demodulation module occupies an important position in a spread spectrum communication system. Spread spectrum modulation and demodulation technique despreading based on surface acoustic wave devices, as opposed to digital technology, with high processing speed, large processing bandwidth and strong processing capabilities, in particular, has an absolute advantage in the real-time processing is, therefore, at home and abroad are its special attention and put a lot of manpower and material resources for development. SAW spread spectrum letter machine based on this technology, has been widely used in ground-based, ship-based, airborne and spaceborne system. Firstly, starting from the principle to be discussed: the spread spectrum communications, spread spectrum modulation and demodulation technique despreading the principle in the first two chapters, the third chapter describes the basic design theory of SAW devices. Then, based on the above theory in the fourth chapter of the surface acoustic wave demodulation expansion module designed and developed a high sensitivity, dynamic SAW Despreading the demodulation module. The module has broken through the traditional the PSK demodulator despreading design, the SAW device applications despreading demodulator, its salient features are mainly the following two points. (1) the the despreading demodulation receiver structure is greatly simplified. Due to \Further, since the letter code rate of change in the spread spectrum modulation process is not a SAW tapped delay line (referred SAWTDL), can be implemented with a piezoelectric substrate and surface acoustic wave delay line (referred to as SAWDL) fusion design, and thus compared to the length of the SAW device with traditional, reduced by half, this design further reduces the receiver module of the structure. (2) high sensitivity, large dynamic range. In the design surface acoustic wave demodulation circuit expansion module design, the dynamic range of the receiver is a receiver preamp circuit decision in the design, we chose the AD's AD8309 logarithmic amplifier chip, dynamic range 100dB, a sensitivity of-78dBm; Further, we have chosen in the SAW despreading is 127 m spread spectrum PN sequences, additional nearly 20dB processing gain of the receiver module, which further improves the sensitivity of the receiver . Against the project, there are three major technical difficulties to be met: (1) large input dynamic range. The requirements of the dynamic range of 70dB. (2) high sensitivity. The sensitivity of-70dBm. (3) the use of the demodulated data signal of the fixed tap delay line. SAW despread the demodulation module to complete the function is more complex, involving circuit including digital, analog, low-frequency, intermediate frequency signal, so a specific design in addition to consider indicators to achieve, and at all levels of circuit impedance matching between the outer We also take into account the different characteristics of the circuit, its effective EMC design. Finally, in the fifth chapter, we present the results of surface acoustic wave despread demodulation module test. The test results showed that meet the test requirements of the module, the basic to meet spreading airborne radio performance and functional requirements, and superior to the traditional structure of the volume and the circuit composition module.
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