A direct digital synthesis system for acoustic wave sensors

Current designs for acoustic wave sensor system electronics are typically based on surface acoustic wave (SAW) oscillators, phase detectors, or phase-locked loops to measure changes in SAW velocity. The advantage of oscillators is a high resolution frequency output, as compared to phase detection systems which are more stable and can more easily provide amplitude information. Phase-locked loops (PLL) offer advantages of both the oscillator and phase detection systems but have the disadvantages of a fixed frequency range and the need for frequency counting circuitry. The objectives of this work were to study the performance of a direct digital synthesis (DDS) based PLL system with the advantages of a programmable frequency range, elimination of the need for frequency counting circuitry, and tolerance of large SAW sensor insertion losses. The DDS system tested had a resolution of 4 Hz and a range of 80 to 120 MHz in SAW humidity and temperature sensing applications indicating that the DDS based PLL is a practical electronic system for SAW sensors.     

Spurious signals in direct digital frequency synthesizers due to the phase truncation

Direct digital frequency synthesizers (DDS or DDFS) are widely used in modern communications and measurement devices. Their advantages are small size and power consumption together with excellent frequency stability, high frequency resolution, and short switching times. The difficulties are rather low output frequencies (500 MHz at the present state of the art) and a large set of the spurious signals very often above the -80 dB level. One source of spurious signals in DDS is the use of smaller number, W, of the most significant bits (MSB) applied for the output sine wave reconstruction from all R bits stored in the accumulator. The result is a phase modulation of the output signal. The problem was first solved in a rather complicated way with the result that the level of the largest spurious signal is about -6 W dB below the carrier with an increase of 3.9 dB in some instances. A simpler solution of the problem of spurious signal level due to the phase truncation in DDS was found earlier. However, no attention was paid to the validity of the corrections suggested. In this paper we will be concerned with this problem and investigate the validity and correctness of these generally cited results and provide a simple way for finding positions, levels, and numbers of these spurious signals generated by truncation to W bits of the phase information stored in the DDS accumulator memory of R bits (W相似文献   

A novel frequency discriminated phase controller for high-speedoptical clocks generated from laser diodes

We demonstrate the in-situ frequency discrimination and continuous phase tuning of high-speed optical clocks generated from a directly modulated/encoded laser diode with respect to a free-running microwave clock. This is performed by using a DC-voltage controlled, frequency-discriminated optoelectronic phase shifter (OEPS). The transfer function of the phase shifter versus controlled voltage is linear with a maximum phase-tuning range of up to 1.957π (340°). The tuning responsivity of the OEPS can be adjusted from 50 to 90°/V. The fluctuation and drift in phase of the controlled signal are about 0.05° and 0.003°/min. The tuning resolution of 0.2° at a 3-mV increment of controlling voltage is achieved by using a high-precision voltage regulator. Relative timing jitter of the controlled optical microwave clock is less than 5 ps     

Chirp信号发生器的设计与实现

本文利用DDS技术在FPGA平台上设计实现了一种Chirp信号发生器,该信号发生器能产生标准的Chirp波形和正余弦波形,通过PC机软件可调整波形的类型、频率和相位。该信号发生器输出频率范围为0．01Hz-25MHz,频率分辨率为0．01Hz,具有控制灵活,输出稳定的优点。     

Distributed Brillouin sensor system based on offset locking of two distributed feedback lasers

An offset locking technique, which uses an external optical delay line to tune the distributed feedback (DFB) laser frequency and a proportional-integral-derivative (PID) controller to lock the tuned frequency, is proposed for the first time, to the best of our knowledge, in the distributed Brillouin sensor system. This method provides large tuning range (greater than 1 GHz), high tuning speed (less than 100 mus per frequency step), and frequency tuning is independent of the laser frequency and power. The two DFB lasers are phase locked at the Brillouin frequency using a hardware PID controller. Using this offset locking with optical delay line, we demonstrated a high signal-to-noise ratio of 32 dB, which allows 1 m spatial resolution and better than 0.6 MHz frequency measurement accuracy (equivalent to 0.5 degrees C temperature resolution or 8 microepsilon strain resolution) over kilometers sensing length. The bias of the electro-optic modulator is controlled by a lock-in amplifier to provide high temperature or strain measurement accuracy.     

Methods of mapping from phase to sine amplitude in direct digital synthesis

There are many methods for performing functional mapping from phase to sine amplitude (e.g., ROM look-up, coarse/fine segmentation into multiple ROM's, Taylor series, CORDIC algorithm). The spectral purity of the conventional direct digital synthesizer (DDS) is also determined by the resolution of the values stored in the sine table ROM. Therefore, it is desirable to increase the resolution of the ROM. Unfortunately, larger ROM storage means higher power consumption, lower reliability, lower speed, and greatly increased costs. Different memory compression and algorithmic techniques and their effect on distortion and trade-offs are investigated in detail. A computer program has been created to simulate the effects of the memory compression and algorithmic techniques on the output spectrum of the DDS. For each memory compression and algorithmic technique, the worst case spurious response is calculated using the computer program.     

Direct digital synthesizer with jittered clock

Since the direct digital synthesizer (DDS) can potentially be used as a flexible clock source, it is of interest to study its spectrum purity as well as jitter characteristic. In this paper, we investigate the jitter transfer characteristic of the DDS clock driven by a jittered digital-to-analog converter (DAC) clock. We first derive the dosed form expressions of the spectrum of the DAC output signal with jittered driving clock. These expressions are then used to investigate the spectral structure of the DDS clock. Equations are derived for the calculation of the SNR. For a small phase noise power in the driving clock, the DDS clock SNR is obtained in a simple closed form and is shown to be lower than that of the input driving clock by the amount of 20 log(f_s/d_d) dB, where f_s is the nominal driving clock frequency and f_d is the desirable DDS output clock frequency     

Transmission of asymmetric coupling double-ring resonator

Based on the asymmetry between waveguide and double ring, the transmission and phase characteristics of coupled double-ring resonators are analyzed systemically. It is shown that the initial detuning determines the shape of transmission spectrum. The transmission spectrum of all-optical analog to electromagnetic inducted transparency (EIT) is controlled by tuning the asymmetric coupled parameter and loss. With the increasing of asymmetric coupled parameter, the transmission spectrum changes from EIT-like profile to Lorenz profile. The EIT-like transmission spectrum results from the interference between two Lorenz profiles. With the increasing of the loss, the transmission spectrum full frequency width at half-maximum broadens and its peak declines. The detuning and loss also make significant influences on the phase profile.     

Design of a vector-sum integrated microwave photonic phase shifter in silicon-on-insulator waveguides

An orthogonal vector-sum integrated microwave photonic phase shifter (IMWPPS), consisting of mode-order converter multiplexers (MOCMs), a variable optical power splitter (VOPS), an optical switch (OS) and fixed time delay lines (FTDLs), was theoretically demonstrated in a silicon-on-insulator wafer. MOCMs, as a key element of our device, were employed to generate orthogonal vector signals and served as lossless optical combiners. Combining with the thermo-optical VOPS, OS and FTDLs, the microwave phase shift of 0～2π could be achieved by a refractive index variation of 0～15×10(-3) in the millimeter wave band. The corresponding tuning resolution was about 1.64°/°C. This work, for the first time to our knowledge, provides an attractive solution to transferring a vector-sum method based bulk MWPPS into a integrated one, which is very important for large-scale optically controlled phase array antenna.     

多带超宽带OFDM系统射频频率合成器的研制

针对通信系统中使用的传统锁相式频率合成器难同步的缺点,提出了采用直接数字频率合成器(DDS)和锁相环(PLL)技术设计应用于多带.正交频分复用超宽带(MB-OFDM UWB)系统的射频频率合成器方案,并研制了用于试验的合成器.该方案可有效解决系统射频电路的同步问题,提高频率分辨率,降低相位噪声,改善载波频偏对 OFDM 系统影响,提高系统性能,同时还可以降低基带算法的复杂度.仿真与测试结果表明,该频率合成器指标满足 MB-OFDM UWB 系统要求,可应用于实用系统中.     

Shear-mode magnetic force microscopy with a quartz tuning fork in ambient conditions

We have demonstrated high-resolution shear-mode magnetic force microscopy (MFM) using a quartz tuning fork in ambient conditions. A commercial magnetic cantilever tip was attached to one prong of the tuning fork to realize shear-mode MFM operation. We have obtained MFM images with a spatial resolution of less than 100?nm and demonstrated a frequency resolution of ～1?mHz, values which are achieved by phase shift detection methods.     

Spline-based high-accuracy piecewise-polynomial phase-to-sinusoid amplitude converters

We propose a method for direct digital frequency synthesis (DDS) using a cubic spline piecewise-polynomial model for a phase-to-sinusoid amplitude converter (PSAC). This method offers maximum smoothness of the output signal. Closed-form expressions for the cubic polynomial coefficients are derived in the spectral domain and the performance analysis of the model is given in the time and frequency domains. We derive the closed-form performance bounds of such DDS using conventional metrics: rms and maximum absolute errors (MAE) and maximum spurious free dynamic range (SFDR) measured in the discrete time domain. The main advantages of the proposed PSAC are its simplicity, analytical tractability, and inherent numerical stability for high table resolutions. Detailed guidelines for a fixed-point implementation are given, based on the algebraic analysis of all quantization effects. The results are verified on 81 PSAC configurations with the output resolutions from 5 to 41 bits by using a bit-exact simulation. The VHDL implementation of a high-accuracy DDS based on the proposed PSAC with 28-bit input phase word and 32-bit output value achieves SFDR of its digital output signal between 180 and 207 dB, with a signal-to-noise ratio of 192 dB. Its implementation requires only one 18 kB block RAM and three 18-bit embedded multipliers in a typical field-programmable gate array (FPGA) device.     

一种改善DDS电路系统杂散方法研究

针对直接数字频率合成器(DDS)电路系统输出杂散问题,分析DAC非线性对输出杂散的影响,在此基础上给出DDS电路系统杂散的来源模型。提出通过合理选择输出频段来减小DDS输出杂散的方案,并给出了仿真实例。实测结果验证了该方案可以有效地降低DDS输出杂散,对提高DDS电路输出信号的质量有很好的促进作用,在DDS电路设计方面具有指导意义。     

A synchronized two-phase sinewave generator for AC metrology systemcompensations

A slave audio two-phase sinewave generator, which can be directly synchronized to a master generator, is described here. It can be used in ac metrology setups to provide additional “compensation” signals, traditionally derived with expensive multidecade inductive voltage divider networks. The generator is based on two direct digital synthesis (DDS) chips, programmed via the parallel printer PC interface; their clock is provided by a phase-locked loop circuit, which ensures frequency and phase synchronization of the DDS outputs with the master generator. The synchronization reference signal can be provided via an optical link reference channel, which avoids interference and ground loops. In its present implementation, the output voltage is V_peak =10 V, frequency range 500 Hz-4 kHz. Total harmonic distortion is contained to -65 dB, and amplitude stability is better than 500 μV/V over 24 h     

环氧树脂改性固化工艺及配方对微观结构的影响

为了探讨固化工艺及配方对环氧树脂体系微观结构的影响,研究了用4,4′-二氨基二苯砜(DDS)作固化剂的环氧树脂与聚醚砜(PES)共混体系.采用差示扫描量热法(DSC)和扫描电镜(SEM)研究其相分离和微观结构,并通过改变聚醚砜的用量和固化工艺条件对其进行控制.结果表明,在环氧树脂与聚醚砜间形成了半互穿网络,对应于环氧态和聚醚砜态的玻璃化转变温度彼此靠近.通过控制反应进程和聚醚砜用量,可以获得不同的相结构,加入较多的聚醚砜或采用较高的固化温度,有利于提高聚醚砜增韧环氧树脂的冲击性能.     

一种基于DDS的三频点标准超声功率源的设计

结合理论方法研究和具体硬件的设计,提出一种基于铌酸锂单晶基片的三频点标准超声换能器,由单片机控制DDS芯片产生3种频率的正弦信号,经过高频运算放大器电路,对标准超声换能器激励输出标准超声功率,从而形成三频点标准超声功率源,可作为一种超声功率传递标准。三频点标准超声功率源实现了18.9MHz高频超声波的输出。和英国NPL测量比对的结果显示,3个频率点上的超声功率值误差均不大于±5%,验证了三频点标准超声功率源输出数据的有效性。     

GPS卫星信号模拟器载波和码NCO研究与实现

论述了GPS(Global Positioning System)卫星信号模拟器载波NCO(数控振荡器)和码NCO的系统结构及作用,根据多速率信号处理理论建立DDS(直接数字频率合成器)模型,分析其信号频谱;根据泰勒级数压缩算法分析了载波NCO sine存储表压缩效率,提出了模拟器码NCO参数更新实现方式.利用verilog在Xilinx'ISE6.3中完成了载波NCO和码NCO的设计和仿真,并在FPGA中进行了实现.通过程序仿真与系统测试,证明本载波NCO和码NCO模块性能满足GPS卫星信号模拟器系统需求.     

A fiber optic phase modulator with an optical frequency shift of up to 20 GHz

A fiber optic phase modulator is described in which the phase of a light wave traveling in an optical fiber is modulated by a composite electromechanical resonator with Q≌1000. The instantaneous frequency of the wave at the modulator output varies according to a harmonic law. The interval of tuning of the instantaneous light wave frequency achieved in the experiment is 20 GHz, which corresponds to a modulation index of ≌10⁶. It is demonstrated that a maximum value of the frequency tuning range is determined by the elastic limit of the resonator material.     

Printing colour at the optical diffraction limit

The highest possible resolution for printed colour images is determined by the diffraction limit of visible light. To achieve this limit, individual colour elements (or pixels) with a pitch of 250?nm are required, translating into printed images at a resolution of ～100,000 dots per inch (d.p.i.). However, methods for dispensing multiple colourants or fabricating structural colour through plasmonic structures have insufficient resolution and limited scalability. Here, we present a non-colourant method that achieves bright-field colour prints with resolutions up to the optical diffraction limit. Colour information is encoded in the dimensional parameters of metal nanostructures, so that tuning their plasmon resonance determines the colours of the individual pixels. Our colour-mapping strategy produces images with both sharp colour changes and fine tonal variations, is amenable to large-volume colour printing via nanoimprint lithography, and could be useful in making microimages for security, steganography, nanoscale optical filters and high-density spectrally encoded optical data storage.     

Microdefocusing method for measuring acoustic properties using acoustic microscope

Many papers have been reporting on measuring acoustic properties of materials by acoustic microscopy. In a conventional method of V (z) curve analysis, the phase velocity and the propagation attenuation of a leaky surface acoustic wave (LSAW) are determined from the interference period Deltaz and the slope of the V(z) curve, respectively. For this method it is necessary to measure the V(z) curve for a period several times as long as the interference period Deltaz. Therefore, it is difficult to measure the acoustic properties of a sample with high resolution by the method. In order to overcome these problems, a method called the microdefocusing method is proposed. The method determines the acoustic properties of a sample by analyzing V (z) values measured in the microdefocusing region within an interference period Deltaz near a focal plane. An ultrasonic transducer called the butterfly transducer is proposed to be applied to this microdefocusing method and a digital signal processing procedure is developed to analyze the output of the ultrasonic transducer. Basic experiments are performed to confirm the principles of the new method.