外文翻譯---壓控振蕩型精密波形發(fā)生器

1、<p>　　壓控振蕩型精密波形發(fā)生器</p><p><b>　　引言</b></p><p>　　單片集成電路ICL 8038波形發(fā)生器聯(lián)結(jié)少量外部元件就能夠產(chǎn)生高精度正弦波、方波、三角波、鋸齒波和脈沖波。選擇不同的外部電阻或電容可得到0.001HZ到300HZ內(nèi)的任何頻率的信號(hào)，頻率調(diào)制和掃描可由一個(gè)外接電壓來完成。ICL 8038內(nèi)部采用肖特基勢壘二極

2、管和薄膜電阻，使用先進(jìn)的單片集成技術(shù)制造，所以輸出信號(hào)可以在很大的溫度范圍和電源變化內(nèi)保持穩(wěn)定。它可以與鎖相環(huán)共同使用，使溫度漂移低于250ppm/℃。</p><p><b>　　應(yīng)用信息</b></p><p>　　外接定時(shí)電容C的充放電受可控電流源控制，電流源#2由觸發(fā)電路控制其與電容C的通路的打開與關(guān)閉，而電流源#1則一直與電容C連通著。假設(shè)一開始觸發(fā)電路使電

3、流源#2與電容C斷開，外接電容由電流源#1進(jìn)行充電，充電電流為I，電容兩邊的電壓將隨時(shí)間線性上升。當(dāng)電壓到達(dá)比較電路的參考電平(一般設(shè)置為電源電壓的三分之一)觸發(fā)電路被觸發(fā)而改變其狀態(tài)，使電流源#2與電容C接通。電流源#2的電流一般設(shè)為2I,因此外接電容C以初電流I進(jìn)行放電，其兩端電壓隨時(shí)間線性下降。當(dāng)電壓下降到比較電路#2的參考電平時(shí)，觸發(fā)電路再次翻轉(zhuǎn)，使電流源#2與電容C的通路斷開，從而結(jié)束此次電容電壓上升和下降的過程，然后開始一個(gè)

4、新的循環(huán)。</p><p>　　從這個(gè)基本發(fā)生器電路中可同時(shí)直接得到四種波形。當(dāng)電流源#1和#2的電流分別設(shè)置在I和2I時(shí)，外接電容C的充電時(shí)間和放電時(shí)間是一樣的。因此電容C兩端產(chǎn)生一個(gè)三角波，觸發(fā)電路產(chǎn)生一個(gè)三角波，觸發(fā)電路產(chǎn)生一個(gè)方波，兩種波形送到緩沖電路分別從引腳(3)和引腳(9)輸出。</p><p>　　通過改變兩個(gè)外部電阻的阻值，可以使電流源的電平發(fā)生變化。當(dāng)兩個(gè)電流源的電流值

5、分別偏離I和2I時(shí)，將產(chǎn)生非對(duì)稱的鋸齒波從引腳(3)輸出同時(shí)脈沖波的占空比也可以相應(yīng)地由1%變?yōu)?9%由引腳(9)輸出。將三角波送到正弦波轉(zhuǎn)換電路可產(chǎn)生正弦波。該網(wǎng)絡(luò)有一個(gè)隨三角波的電壓趨向其極值點(diǎn)而逐漸下降的分流電阻。</p><p><b>　　波形設(shè)定</b></p><p>　　所有這些波形都可以通過兩個(gè)外接電阻元件來設(shè)定。電路連接方式有兩種如圖3。保持可調(diào)電

6、阻Ra和Rb分立，可以得到較好的波形輸出。Ra控制著三角波的上升段、正弦波的上升段和方波的高電平狀態(tài)。三角波的幅度設(shè)置為電源電壓的1/3，因此三角波的上升時(shí)間為三角波、正弦波的下降部分及方波的低電平部分的時(shí)間。</p><p>　　當(dāng)Ra=Rb時(shí)，占空比為50%。如果占空比只在50%左右的一個(gè)很小的范圍內(nèi)變化，那么圖3B中的線路連接方法較方便一點(diǎn)。1k的可變電阻器不能使得占空比滿足50%。2k或5k的電阻器可以使

7、得占空比滿足50%。對(duì)于兩個(gè)分開的可調(diào)電阻 </p><p>　　盡管沒有電壓在內(nèi)部觸發(fā)電路中被調(diào)節(jié)，其時(shí)間和輸出波形頻率均與電源電壓無關(guān)。這是由于輸入的電壓信號(hào)與其電源呈線性函數(shù)關(guān)系。</p><p><b>　　減小失真</b></p><p>　　為了減小正弦波的失真，接在引腳(11)與(12)之間的電阻最好是可調(diào)的。通過細(xì)調(diào)這一電阻，可

8、使正弦波失真小于1%。如果還要減小失真，可以用兩個(gè)電位器連接成如圖4所示方式，這樣的線路可通過調(diào)節(jié)使失真率減小到0.5%。</p><p>　　Ra、Rb和電容C的選擇</p><p>　　對(duì)于任一給定的輸出頻率，可以有很多種滿足這一頻率要求的RC組合，但是，最佳工作狀態(tài)對(duì)充放電電流的要求又為這些組合提出了一定的限制條件。放電電流大小，電路中的漏電流在高溫時(shí)將帶來顯著的誤差。因此電流不能小

9、于1uA;充電電流太大，晶體管的β值和飽和電壓將隨著電流的增加而增加帶來誤差。最佳工作范圍為10uA-1mA。如果引腳(7)和(8)短接，根據(jù)Ra的取值可以計(jì)算出I的取值：Rb用相似的方法可求得。電容C應(yīng)該取允許值的上限。</p><p><b>　　電源和輸出電平控制</b></p><p>　　波形信號(hào)發(fā)生器既可以接電壓為10V 到30V范圍的單電源，又可接

10、77;5V 到±15V的雙電源。接單電源時(shí)，輸出三角波和正弦波的平均線的電平正好是電源電壓的一半，輸出波形在電源電壓與地之間交替變化，接電壓對(duì)稱的雙電源時(shí)，所有輸出波形都以地對(duì)稱擺動(dòng)。</p><p>　　當(dāng)輸出方波時(shí)，其幅度不是限制死的。一個(gè)負(fù)載電阻可以接到任一電源上，只有此電源的輸出電壓小于波形信號(hào)發(fā)生器的擊穿電壓(30V)。用這種方式，當(dāng)波形信號(hào)發(fā)生器本身接高壓電源時(shí)，用負(fù)載電阻接+5V電源，就可

11、以使ICL8038的輸出方波與TTL電路兼容。</p><p><b>　　調(diào)頻和掃頻</b></p><p>　　波形發(fā)生器的頻率是其引腳(8)出的直流電壓的線性函數(shù)。改變引腳(8)處的電壓就可以對(duì)ICL8038的輸出頻率進(jìn)行調(diào)制。對(duì)于頻率±10%的改變，可以采用如圖5A所示的接法，調(diào)制信號(hào)通過一個(gè)隔直電容加在引腳(8)上。引腳(7)與(8)之間的電阻不是

12、必須的，但它可以將輸入阻抗從8k增加到R+8k。</p><p>　　對(duì)于大范圍的頻率改變或掃頻，可采用如圖5B所示接法，調(diào)制電壓加在正電源與引腳(8)之間。在這種方式下，整個(gè)電流源的偏置由調(diào)制信號(hào)提供，可以產(chǎn)生非常大的頻率掃描范圍。但在這種連接方式下，選擇電源時(shí)必須注意到：此時(shí)電流源的充放電電流不再只是電源電壓的函數(shù)，因此輸出頻率與電源電壓有關(guān)。引腳(8)處的點(diǎn)位從V+變化到(1/3Vsup-2V)。</

13、p><p><b>　　典型應(yīng)用</b></p><p>　　輸出的正弦波有一個(gè)相當(dāng)高的輸入阻抗。圖6的線路提供了緩沖及幅度調(diào)節(jié)作用。一個(gè)簡單的放大裝置可以被使用。</p><p>　　ICL8038極性電源工作，通過一個(gè)場效應(yīng)管開關(guān)的通斷使ICL8038引腳(10)的定時(shí)電容C短路停止ICL8038的振蕩。圖7所示一個(gè)FET選擇，有一個(gè)二極管相連

14、的選通信號(hào)輸入端允許輸出與它有相同的傾斜。</p><p>　　為了獲得1000：1的輸出頻率范圍，要求ICL8038的外接電阻Ra和Rb上的電壓減小接近于零。這要求引腳(8)的控制電壓比Ra或Rb上的峰值電壓高幾百毫伏。圖8用二極管來降低有效電源電壓。接于引腳(5)的大電阻電壓變化對(duì)輸出波形占空比的影響。</p><p>　　通過使用一個(gè)放大器使得輸出頻率與輸入掃描電壓的線性關(guān)系有顯著提

15、高。</p><p><b>　　與鎖相環(huán)結(jié)合的應(yīng)用</b></p><p>　　由于ICL8O38縣有高度的頻率穩(wěn)定性，使其成為用于鎖相環(huán)系統(tǒng)中的理想積木式元件，其與相位檢測器及放大器共同使用將更增加其性能。</p><p>　　圖10給出了這種應(yīng)用的原理圖，為了使這些模塊相互匹配，兩個(gè)不同的電源電壓應(yīng)該被使用，圖10ICL8038的方波輸出

16、又返回到相位檢測器的輸入來確保相位檢測器的輸入電壓不超過相位檢測器的電源電壓，這里應(yīng)提醒注意的是如果相位檢測器需要的Vco．IN信號(hào)較小，則可通過電阻分壓的辦法來解決。</p><p>　　再之運(yùn)算放大器的輸入電平應(yīng)與ICL8038的頻率調(diào)制輸入相匹配(引腳(3)0．8V)，圖10中R及C構(gòu)成低通濾波環(huán)節(jié)，其用來濾掉高頻干擾的影響，該電路既可實(shí)現(xiàn)低溫度漂移的線性頻率輸出，而且還可確保輸出正弦波的相位與輸入波形完全

17、相同。</p><p>　　Precision Waveform Generator/Voltage Controlled Oscillator</p><p>　　Description</p><p>　　The ICL8038 waveform generator is a monolithic integrated circuit capable of pr

18、oducing high accuracy sine, square, triangular, sawtooth and pulse waveforms with a minimum of external components. The frequency(or repetition rate)can be selected externally from 0.001Hz to more than 300kHz using eithe

19、r resistors or capacitors, and frequency modulation and sweeping can be accomplished with an external voltage. The ICL8038 is fabricated with advanced monolithic technology, using Schottky barrier diod</p><p&g

20、t;　　Application Information</p><p>　　An external capacitor C is charged and discharged by two current sources. Current source#2 is switched on and off by a flip-flop, while current source#1 is on continuousl

21、y. Assuming that the flip-flop is in a state such that current source#2 is off, and</p><p>　　the capacitor is charged with a current I, the voltage across the capacitor rises linearly with time. When this vo

22、ltage reaches the level of comparator#1(set at 2/3 of the supply voltage),the flip-flop is triggered, changes states, and releases current source#2.This current source normally carries a current 2I,thus the capacitor is

23、discharged with a net-current I and the voltage across it drops linearly with time. When it has reached the level of comparator#2(set at 1/3 of the supply voltage),th</p><p>　　Four waveforms are readily obta

24、inable from this basic generator circuit. With the current sources set at I and 2I respectively, the charge and discharge times are equal. Thus a triangle waveform is created across the capacitor and the</p><p

25、>　　flip-flop produces a square wave. Both waveforms are fed to buffer stages and are available at pins 3 and 9. The levels of the current sources can, however, be selected over a wide range with two external resistors

26、. Therefore, with the two currents set at values different from I and 2I,an asymmetrical sawtooth appears at Terminal 3 and pulses with a duty cycle from less than 1%to greater than 99%are available at Terminal 9.The sin

27、e wave is created by feeding the triangle wave into a nonlinear ne</p><p>　　Waveform Timing</p><p>　　The symmetry of all waveforms can be adjusted with the external timing resistors.Two possible

28、 ways to accomplish this are shown in Figure 3.Best results are obtained by keeping the timing resistors RA and RB separate(A).RA controls the rising portion of the triangle and sine wave and the 1 state of the square wa

29、ve.</p><p>　　The magnitude of the triangle waveform is set at 1/3 VSUPPLY ;therefore the rising portion of the triangle.</p><p>　　The falling portion of the triangle and sine wave and the 0 stat

30、e of the square wave Thus a 50%duty cycle is achieved when RA=RB .If the duty cycle is to be varied over a small range about 50% only, the connection shown in Figure 3B is slightly more convenient.A 1k potentiometer may

31、not allow the duty cycle to be adjusted through 50%on all devices.If a 50%duty cycle is required,a 2k or 5k potentiometer should be used. With two separate timing resistors,the frequency is given by: </p><p>

32、;　　Neither time nor frequency are dependent on supply voltage, even though none of the voltages are regulated inside the integrated circuit.This is due to the fact that both currents and thresholds are direct,linear func

33、tions of the supply voltage and thus their effects cancel.</p><p>　　Reducing Distortion</p><p>　　To minimize sine wave distortion the 82k?resistor between pins 11 and 12 is best made variable.Wi

34、th this arrangement distortion of less than 1%is achievable.To reduce this even further,two potentiometers can be connected as shown in Figure 4;this configuration allows a typical reduction of sine wave distortion close

35、 to 0.5%.</p><p>　　Selecting RA,RB and C</p><p>　　For any given output frequency, there is a wide range of RC combinations that will work, however certain constraints are placed upon the magnitu

36、de of the charging current for optimum performance. At the low end, currents of less than 1μA are undesirable because circuit leakages will contribute significant errors at high temperatures. At higher currents (I>5mA

37、),transistor betas and saturation voltages will contribute increasingly larger errors. Optimum performance will, therefore, be obtained wit</p><p>　　Waveform Out Level Control and Power Supplies</p>&

38、lt;p>　　The waveform generator can be operated either from a single power supply(10V to 30V)or a dual power supply(±5V to±15V).With a single power supply the average levels of the triangle and sine wave are a

39、t exactly one-half of the supply voltage, while the square wave alternates between V+ nd ground. A split power supply has the advantage that all waveforms move symmetrically about ground.</p><p>　　The square

40、 wave output is not committed. A load resistor can be connected to a different power supply, as long as the applied voltage remains within the breakdown capability of the waveform generator(30V).In this way, the square w

41、ave output can be made TTL compatible(load resistor connected to+5V)while the waveform generator itself is powered from a much higher voltage.</p><p>　　Frequency Modulation and Sweeping</p><p>　

42、　The frequency of the waveform generator is a direct function of the DC voltage at Terminal 8(measured from V+).By altering this voltage, frequency modulation is performed. For small deviations(e.g.±10%)the modulati

43、ng signal can be applied directly to pin 8,merely providing DC decoupling with a capacitor as shown in Figure 5A.An external resistor between pins 7 and 8 is not necessary, but it can be used to increase input impedance

44、from about 8k?(pins 7 and 8 connected together),to about(R+8k).</p><p>　　For larger FM deviations or for frequency sweeping, the modulating signal is applied between the positive supply voltage and pin 8(Fig

45、ure 5B).In this way the entire bias for the current sources is created by the modulating signal, and a very large(e.g.1000:1)sweep range is created(f=0 at VSWEEP=0).Care must be taken,however,to regulate the supply volta

46、ge;in this configuration the charge current is no longer a function of the supply voltage(yet the trigger thresholds still are)and thus the freque</p><p>　　Typical Applications</p><p>　　The sine

47、 wave output has a relatively high output impedance (1k?Typ).The circuit of Figure 6 provides buffering,gain and amplitude adjustment.A simple op amp follower could also be used. </p><p>　　With a dual supply

48、 voltage the external capacitor on Pin 10 can be shorted to ground to halt the ICL8038 oscillation.Figure 7 shows a FET switch,diode ANDed with an input strobe signal to allow the output to always start on the same slope

49、.</p><p>　　To obtain a 1000:1 Sweep Range on the ICL8038 the voltage across external resistors RA and RB must decrease to nearly zero.This requires that the highest voltage on control Pin 8 exceed the voltag

50、e at the top of RA and RB by a few hundred mV.The Circuit of Figure 8 achieves this by using a diode to lower the effective supply voltage on the ICL8038.The large resistor on pin 5 helps reduce duty cycle variations wit

51、h sweep.</p><p>　　The linearity of input sweep voltage versus output frequency can be significantly improved by using an op amp as shown in Figure 9.</p><p>　　Use in Phase Locked Loops</p>

52、<p>　　Its high frequency stability makes the ICL8038 an ideal building block for a phase locked loop as shown in Figure 10.In this application the remaining functional blocks,the phase detector and the amplifier,c

53、an be formed by a number of available ICs(e.g.,MC4344,NE562,HA2800,HA2820).</p><p>　　In order to match these building blocks to each other,two steps must be taken.First,two different supply voltages are used

54、 and the square wave output is returned to the supply of the phase detector.This as sures that the VCO input voltage will not exceed the capabilities of the phase detector.If a smaller VCO signal is required,a simple res

55、istive voltage divider is connected between pin 9 of the waveform generator and the VCO input of the phase detector.</p><p>　　Second,the DC output level of the amplifier must be made compatible to the DC lev

56、el required at the FM input of the waveform generator(pin 8,0.8V+).The simplest solution here is to provide a voltage divider to V+(R1,R2 as shown)if the amplifier has a lower output level,or to ground if its level is hi

57、gher.The divider can be made part of the low-pass filter.</p><p>　　This application not only provides for a free-running frequency with very low temperature drift,but is also has the unique feature of produc