【正文】 sed in the performance , Therefore, highquality signal source was described as a large number of electronic systems heart. With munication, the continuous development of radar, multisignal sources, frequency stability, spectral purity, frequency range and the number of output frequencies, as well as the shape of the signal waveform to a growing number of requests. In order to improve the signal source output frequency stability, such as crystal oscillator can be used to resolve this issue. In order to meet the requirements of multifrequency number, frequency synthesis technology can be used, that is, addition and subtraction through multiplication and division of the frequency of operation can be seen from a high stability and high accuracy of the standard frequency source, resulting in a large number of have the same stability and accuracy of different frequencies. The use of DDS technology is the design of a mon waveform generator means, DDS not only can produce sine wave at the same time can generate arbitrary wave, this is the manner in which the other frequency synthesizer does not have the characteristics of any wave in all fields have a wide range of applications. Through the DDS arbitrary wave of such methods is a simple, lowcost way to increase the output waveform points can be achieving a high degree of accuracy, by other means this is second to none. Since the 8039。s products more representative. Such as the AD7008, AD9850, AD9851, AD9852, AD9858 and so on. The system clock frequency ranging from 30MHz to 300MHz, in which the system clock of the AD9858 is reached lGHz. The chips also have the modem function. Such as the AD7008 quadrature modulation signal can be generated, while the AD9852 can also be generated FSK, PSK, amplitude modulation and linear frequency modulation signal. These highly integrated chips, are integrated within the D / A converter, the accuracy of up to 12bit. At the same time have adopted a number of optimization designed to improve performance. because these chips are mostly used in water technology, water technology through the use of the phase accumulator to increase the operating frequency, allowing the output frequency of DDS chip can be further improved. Water through the use of technology in the phase accumulator to ensure that the premise of the operating frequency, the phase accumulator word length can be designed for longer, such as the AD9852 has reached the phase accumulator 48. Models rather than 32 before, so that the output signal of the frequency resolution significantly improved. The use of DDS technology to produce the DDS arbitrary waveform signal generator is a relatively new type of signal source, and has been widely put into use. It is not only capable of producing the traditional function of the signal generator can produce sine, square, triangle wave, sawtooth wave, but also can generate arbitrary waveform editor. Due to the characteristics of the DDS can also be very easy to produce some digital modulation signals, such as FSK, PSK, etc.. Some highend signal generator can even have a munication signal. At the same time, the frequency resolution of the output waveform, frequency and other indicators of accuracy are greatly improved. HP33120 panies such as HP can generate the lOmHz15MHz sine and square wave, but also can produce lOmHz5MHz of arbitrary waveform, but also have the modulation function, can be AM, FM, FSK, Burst, Sweep signal. HP39。各種波形曲線均可以用三角函數(shù)方程式來表示。 波形 發(fā)生器在電路實(shí)驗(yàn)和設(shè)備檢測(cè)中具有十分廣泛的用途。在工業(yè)、農(nóng)業(yè)、生物醫(yī)學(xué)等領(lǐng)域內(nèi)，如高頻感應(yīng)加熱、熔煉、淬火、超聲診斷、核磁共振成像等，都需要功率或大或小、頻率或高 或低的振蕩器。 程序控制輸出方式 計(jì)算機(jī)根據(jù)波形的函數(shù)表達(dá)式，計(jì)算出一系列波形數(shù)據(jù)瞬時(shí)值，并定時(shí)地逐個(gè)傳送給 D／ A 轉(zhuǎn)換器，合成出所需要的波形。但數(shù)據(jù)輸出定時(shí)不準(zhǔn)確，會(huì)影響信號(hào)的頻率和相位；波形數(shù)據(jù)輸出依靠指令的執(zhí)行來完成，當(dāng)需要同時(shí)輸出多個(gè)信號(hào)時(shí)，相鄰信號(hào)通道的輸出存在時(shí)間差；受計(jì)算機(jī)運(yùn)行速度的限制，輸出信號(hào)的頻率較低。原理框圖如圖 11 所示。計(jì)數(shù)器產(chǎn)生的地址碼提供出存儲(chǔ)器中波形數(shù)據(jù)所需要的地址信號(hào) ,波形數(shù)據(jù)被依次讀出后送至高速Ｄ /Ａ轉(zhuǎn)換器 ,將之轉(zhuǎn)變?yōu)槟M量 ,經(jīng)低通濾波器后輸出 所需的波形。但其取樣時(shí)頻率較高 ,對(duì)硬件的要求也較高 ,而且常需多級(jí)分頻或采用高性能的鎖相環(huán) ,其中分頻式的任意波形發(fā)生器頻率分辨率低 ,鎖相式的任意波形發(fā)生器頻率切換速度慢。由于用硬件電路取代了計(jì)算機(jī)的控制，信號(hào)輸出穩(wěn)定度高。更主要的是，可以將微處理器從信號(hào)輸出的負(fù)擔(dān)中解脫出來．如圖 12 為其工作流程圖。頻率合成技術(shù)是產(chǎn)生頻率源的一種現(xiàn)代化手段，已廣泛應(yīng)用于通信、導(dǎo)航、電子偵察、干擾與反干擾、遙控遙測(cè)及現(xiàn)代化儀器儀表中。頻率合成器既要產(chǎn)生所需要的頻率 ,又要獲得純凈的信號(hào)。 直接合成（ DirectFrequencySynthesis，簡(jiǎn)稱 DS）是通過倍頻器、分頻器、混頻器對(duì)頻率進(jìn)行加、減、乘、 除運(yùn)算，得到各種所需頻率。而且輸出端的諧波、噪聲及寄生頻率難以抑制。 間接合成（ IndirectFrequencySynthesis，簡(jiǎn)稱 IS ）又稱鎖相頻率合成PhaseLockedLoop Frequency Synthesis，簡(jiǎn)稱 PLLFS），是利用 鎖相環(huán)路的窄帶跟蹤特性來得到不同的頻率。目前在各種無線電臺(tái)中使用的頻率合成器普遍采用可變數(shù)字式鎖相環(huán)頻率合成器，通過 CPU 控制可獲得不同的頻點(diǎn)。但鎖相頻率合成器頻率轉(zhuǎn)換時(shí)間較長(zhǎng)，且合成的正弦波的參數(shù)，如幅度、頻率和相位較難控制。鎖相環(huán)式頻率合成器具有很好的窄帶跟蹤特性，可以很好地選擇所需頻率的信號(hào)，抑制雜散分量，并且避免了大量的濾波器，有利于集成化和小型化。除此之外，由模擬方法合成的正弦波的參數(shù)，如幅度、頻率和相位都很難控制。 直接數(shù)字式頻率合成（ DirectDigitalFrequencySynthesis，簡(jiǎn)稱 DDS 或 DDFS）是近年來發(fā)展起來的新的頻率合成技術(shù)。此方法是用隨機(jī)讀寫存儲(chǔ)器RAM 存儲(chǔ)一個(gè)波形周期的量化數(shù)據(jù)，按照不同頻率要求以頻率控制字為步進(jìn)對(duì)相位增量進(jìn)行累加，以累加相位值作為地址碼讀取存放在存儲(chǔ)器內(nèi)的波形數(shù)據(jù)，經(jīng) D/A 轉(zhuǎn)換和濾波可得所需波形輸出?；冢模模蛹夹g(shù)的頻率合成器有很高的頻率分辨率，可方便地實(shí)現(xiàn)頻率、相位調(diào)制功能 ,轉(zhuǎn)換速度快 ,且輸出波形的相位連續(xù)。 頻率合成器主要指標(biāo) 信號(hào)源的一個(gè)重要指標(biāo)就是能輸出頻率準(zhǔn)確可調(diào)的所需信號(hào)。利用頻 率合成技術(shù)制成的信號(hào)發(fā)生器，通常被稱為頻率合成器。頻率合成器的主要指標(biāo)如下： ：指的是輸出的最小頻率和最大頻率之間的變化范圍。 ：指的是輸出頻率的最小間隔。 ：頻譜純度以雜散分量和相位噪聲來衡量，雜散分量為諧波分量和非諧波分量?jī)煞N，主要由頻率合成過程中的非線性失真產(chǎn)生，相位噪聲是衡量輸出信號(hào)相位抖動(dòng)大小的參數(shù)。 DDS 的基本原理及性能特點(diǎn) 直接數(shù)字頻率合成 (Direct Digital Synthesis,簡(jiǎn)稱 DDS)技術(shù)是頻率合成領(lǐng)域中的一項(xiàng)新技術(shù)。 DDS 的工作原理是基于相位和幅度的對(duì)應(yīng)關(guān)系 ,通過改變頻率控制字來改變