Design of upconverter in the hottest software radi

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The design of upconverter in software radio system

I. Introduction

the development of electronic warfare puts forward higher and higher requirements for the volume, accuracy, stability, flexibility and anti-interference performance of modern electronic systems. Accordingly, the requirements for the "heart" - frequency source technology [5] must also be higher and higher. In order to obtain higher frequency stability in the up/down converter of software radio system, PLL or high stability oscillation circuit was used in the past. However, no matter what method is adopted, its circuit and structure are relatively complex, and because PLL has the problem of acquisition time, its inherent characteristic of long frequency agility time makes it difficult to meet the technical requirements of high-speed and ultra-high speed. However, the development of electronic technology will inevitably require the future frequency source to have high accuracy, high frequency stability, ultra-high frequency agility speed and the function of generating a variety of signals flexibly and conveniently. Obviously, the traditional frequency synthesis method of up/down converter in software radio system is difficult to meet these requirements, and there are still shortcomings such as phase discontinuity in frequency agility. Therefore, it is necessary to adopt a more advanced frequency synthesis technology DDS (direct digital synthesis). This paper briefly introduces the development and principle of software radio and DDS technology, and emphatically analyzes the principle of a DDS chip AD9850 and its application in the upconverter of software radio system

II. Overview of software radio [3] [4]

software radio is a new stage in the development of radio system from analog to digital. It is a radio station that uses software control and digital signal processing (DSP) to complete the functions of traditional analog radio. Its goal is to realize the modulation and demodulation of working frequency band, modulation and demodulation mode Changes in wireless functions such as channel multiple access mode to form PCs terminals or base stations with different standards. Theoretically, the most "pure" software radio system should be completely completed in the form of digital signal processing, except for antenna, high-level amplifier, horn, ADC/DAC and its corresponding filter circuits (anti aliasingfilter and waveform forming filter). However, based on current technology, at the receiving end, if the RF signal from the antenna up and down needs to be analog mixed to the intermediate frequency (typical value is 70MHz or lower), and then (with a sampling rate of 65 m/70 m) into ADC digitization, the sampling here is generally under sampling (band-pass sampling), After that, the digital down converter (DC digital down converter with a slight decline in the D marine market) will down convert it to baseband. The method is to mix with the local NC quadrature oscillator to obtain the I & Q quadrature signal, and then filter and extract it for many times. After that, it may also need to decode the signal according to the source coding method to restore the required original signal. Accordingly, its sending mode and receiving mode are almost completely symmetrical

the actual software radio module composition is shown in Figure 1. 3、 Overview of DDS principle [2]

DDS system is composed of frequency control word K, N-bits phase accumulator, sine look-up table, D/a converter, low-pass filter, etc. DDS can be divided into two modules: numerical control oscillator (NCO) and digital to analog converter (DAC). As shown in Figure 2

module NCO realizes the digital waveform of the corresponding frequency from the digital frequency value input, and its working process is as follows:

(1) determine a frequency value with the frequency control word

(2) the frequency value is accumulated to the phase accumulator with a digital signal to generate real-time digital phase information

(3) the digital phase "word" is converted into the corresponding digital amplitude value "word" in the sine table. The digital amplitude value "word" is a kind of sampled digital signal, which can be expressed by coding, and usually adopts three formats: binary offset code, complement code and signed amplitude code

the module DAC converts the digital amplitude value generated by NCO into analog amplitude value at high speed and linearly, and the aliasing interference generated by DDS is filtered by the anti aliasing filter after the DAC

assuming that the reference clock frequency of the system is FC, then fo = KFC/2n. It can be seen that when n and FC are constant, the output frequency f0 only depends on K. Consider the special case when k = 1, and the frequency resolution △ f = FC/2n. The value n of the visible phase accumulator determines the frequency resolution. The main disadvantage of DDS system is that the maximum output frequency is limited. In practical engineering applications, in order to filter the primary image frequency of D/a output, it can only reach fo = 0.4 FC. At present, a series of DDS products have come out one after another. They have superior performance, high cost performance, and easier connection between bus and control and computer interface. They have been widely used in some electronic systems. The following is an example of a DDS chip AD9850 [3] recently developed by ad company

IV. working principle, characteristics and application of AD9850 1. Working principle and characteristics of AD9850 [4] AD9850 is a direct frequency synthesizer produced by ad company with a maximum clock of 125 MHz and advanced CMOS technology. AD9850 includes a high-speed DDS, a high-speed, high-performance DAC, comparator, etc. it constitutes a complete digital

word control programmable frequency synthesizer, and is a highly integrated chip with clock generation function. The functional block diagram of AD9850 is shown in Figure 3. The DDS system of AD9850 uses a 32 bits phase accumulator, which is truncated into 14bits before the data enters the sinusoidal look-up table. Finally, the analog sinusoidal signal output is generated by the internally integrated 10 bits DAC

ad9850 uses 5 bits to control the phase, and its output phase is 180 °, 90 °

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