A Design and Implementation of Simple Spectrum Analyzer Based on DDS Hang Yu1, Sun Kexue1,2,a, Liu Yanming1, Zhu Siqing1 and Cheng Xiefeng1,2,b

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1 3rd International Conerence on Machinery, Materials and Inormation Technology Applications (ICMMITA 015) A Design and Implementation o Simple Spectrum Analyzer Based on DDS Hang Yu1, Sun Kexue1,,a, Liu Yanming1, Zhu Siqing1 and Cheng Xieeng1,,b 1 School o Electronic Science and Engineering, Nanjing University o Posts and Telecommunications, Nanjing, 1003, China Jiangsu Province Engineering Lab o RF integration & Micropackage, Nanjing, 1003, China a sunkx@njupt.edu.cn, b chengx@njupt.edu.cn Keywords: Spectrum analyzer; DDS; STM3; Swept-requency. Abstract. In this paper, we design a basic spectrum analyzer which can analyze the signal with the bandwidth in 10 MHz~100MHz and the minimum resolution in 100 KHz. We utilize the AD9854 DDS (Direct Digital Synthesizer), STM3F407, AD831, AD835 and the TFT LCD to design and implement a system that has menu-driven unction selection mode and better man-machine interace. This system and actual test results showed that this spectrum analyzer can achieve the design goal. It reduces the cost and is portable with low power consumption. Introduction Nowadays a spectrum analyzer is widely used in a lot o ields but dedicated equipment is quite expensive. We need to ind a low-cost and general design to achieve spectrum analyzer which engineers and scientists can implement by themselves easily. Some designs o spectrum analyzer are based on high speed ADC and FPGA to get spectrum by FFT. But the bandwidth limited by the speed o ADC is not very wide, so it is expensive without generality. Some designs use PLL based on VCO, but it is hard or individual to achieve and expensive. We need to ind a way to solve these problems [1, ]. So we chose DDS to realize this system. The direct digital requency synthesizer (DDS) has characteristics o strong resolving power, quickly switches and abroad applications. This technology can help us to design with low cost and easy implementation or individual.dds AD9854 can generate sine signal rom 0 to 10MHz. However, when the requency reaching 60 MHz, the amplitude o the signal will decline sharply. Thereore, we just use the requency rom 0.5 MHz to 50MHz. Then, we can utilize an AD835 as multiplier which can double requency. Local oscillator made o DDS combined with multiplier will be controlled by STM3 to sweep rom 1 to 100 MHz, with the step o 100 KHz [3-5]. In this paper DDS which is controlled by STM3F407 will sweep requency to detect other signals. It's inexpensive and easy to realize or scientists and engineers. Overall design We use a touch screen as a console to operate the whole system. STM3 will execute the command rom screen to control DDS to sweep. The swept-requency signal will be mixed with detection signal by AD831.The mixed signal will pass a 100 KHz low-pass ilter to get the low requency component. Then we use the ADC o STM3 to detect the low requency signal. STM3 can analyze the requency o detection signals according to the eedback signal measured by the ADC o STM The authors - Published by Atlantis Press 157

2 Screen STM3 DDS Filter Multiplier Signals Mixer Ampliier Fig.1. The system s general design Theoretical analysis and calculation The theory o ADC To measured accurately, we decide give ADC 1ms to detect low-requency signal. Now we need to calculate the lowest requency that the ADC could measured accurately. We use 36 MHz ADC clock and 1bits sampling. It's sampling requency is adc sam = =.4Mhz 15 (1) So the ADC will sample 400 times in a 1ms. To get more accurate data, we need to sample at least 10 cycles. 10 min = = 10khz T () So the lowest requency we could measure is 10khz.We had to abandon the ormal plan which uses a 100 KHz low-pass ilter. We replace it with a 10 KHz~110 KHz band-pass ilter. The design o ampliier The amplitude o signal generated by AD9854 is about 500mV, the output signal o multiplier will decline to 15mV, so we need to design a high requency ampliier. It's about 4 times gain. Multiplier The multiplier depends on AD835. I input signal X and Y are sine wave [3], A W = XY = ( Asinωt)( Asinωt) = (1 cos ωt) (3) So at the rear o multiplier, we should attach a capacitor to ilter DC component and an ampliier to Ampliy signal. And we ll get double requency signal that we need. The mixer depends on AD831. Functional block diagram o AD831 is shown in Fig.. Fig..Functional block diagram o AD831 When the integral output ampliier is used, pins IFN and IFP are connected directly to pins AFN and AFP; the on-chip load resistors convert the output current into a voltage that drives the output ampliier. The ratio o these load resistors to resistors R1, R provides nominal unity gain (0 db) 158

3 rom RF-to-IF. The expression or the gain, in decibels, is π 1 4 G db = 0lg( )( )( ) π (4) Where: 4/π is the amplitude o the undamental component o a square wave. 1/ is the conversion loss, and π/ is the small signal dc gain o the AD831 when the LO input is driven ully positive or negative. The swept signal source based on DDS technique consists o a reerence requency source, a phase accumulator, and a RAM used to store the sine sampling points. We set the requency o the reerence requency source as clk and the counter capacity o the N phase accumulator as (N is the digit o the accumulator). I the requency control word is M, then the requency o the DDS system s output signal is[5][6] clk out = M N (5) clk that we use is 300Mhz.Then we can calculate the requency control word according to The equality(5). The detection cycle o the sotware is shown in Fig. 3. Start DDS generate signal ADC detect STM3 record Frequence lower than 50Mhz NO Display spectrum YES Increase 50Khz End Fig. 3. Detection cycle o sotware Circuit design The design o 10k~110khz band-pass ilter This ilter need to have a sharp edge,but it don t need to care about group delay,so we choose Chebyshev ilter. We use the ADC o STM3 with 1 bits Precision.Under the 3.3V reerence voltage,the minimum voltage could be detected is VRe Vmin = = 0.8mV 4096 (6) Vmax is about 500mV,so stopband Attenuation is A Vmin = 0lg = -56dB sb Vmax (7) 159

4 With the help o aided design sotware, we get a ilter as shown in Fig.4, and according to ADI s reerence design, we design the circuit as shown in Fig.5, Fig. 4. Circuits o band-pass ilter Fig. 5. Circuit o multiplier Results The test o point requency is listed in Table 1. We can see that the system is hard to identiy the signal like 1.99 MHz rom the Table 1.To improve the precision o this system, we must design a better ilter with a sharper edge. Because o the minimum resolution is 100 khz, so the test o point requency is not very accurate. The Table lists the AM wave test data. From this table we can see that, this spectrum Analyzer can identiy dierent requency o a complex signal. Table 1.Point requency test Point requency test Fre/MHz Measure1/MHz Measure/MHz Measure3/MHz Table.AM wave test AM wave test Fc Fw Lower sideband Upper sideband

5 To analyze the system s ability o identiy dierent requency component, we use AM signal to test it. The spectrum chart o AM wave is shown in Fig. 6. The test result as shown in Fig. 7. Fig. 6.AM wave s spectrum chart Fig. 7. spectrum chart in practical test Conclusion The paper ocuses on a design that realizes the DDS technology through the use o STM3. It achieves the goal that designs a spectrum analyzer which is inexpensive and convenient. This design can be applied on the a ew o ields such as communication systems and scientiic researches. Acknowledgement This research was inancially supported by the National Natural Science Foundation o China (Grant No ), Natural Science Foundation o the Jiangsu Higher Education Institutions o China (Grant No. 15KJD510001), and Natural Science Foundation o Nanjing University o Posts and Telecommunications (Grant No. NY14049). Reerence [1] Mingyu Gao. Yunei Liu. Jiye Huang. Zhiwei He, Design O The Automatic Jacquard Control System Based On STM3F407, International Conerence on Inormation Science, Electronics & Electrical Engineering, 014(), [] Wen-hui Chen. Fei Jia. Design o spectrum analyzer based on DDS, International Electronic Elements, 008(1), 4-5. [3] Chong Yao. Yueqiang Lu. Weilin Zhang, Communication Experiment Dedicated Digital Spectrum Analyzer, /WNIS.009.6, [4] Zeinert Martin and Makula Petr, Impact Assessment o Power Electronics on Experimental Aircrat Receiver, 015, /MILTECHS ,1-5. [5] Hongzhen YU. Communication electronic circuit. Tsinghau University Press, 01(9). 161