1、數(shù)據(jù)采集同步濾波器在鎖定放大器中的應(yīng)用摘要：鎖定放大器是用同步濾波器來代替相敏檢測器，同步濾波器是基于信號采樣的方法提供了一個極其窄的帶通濾波器的特性，它的中心頻率能夠通過調(diào)整頻率很容易的控制，因為濾波器比相敏檢測器在諧波時有更小的響應(yīng)，誤差所引起的和聲變得也很小，并且對濾波器的性能的要求也比較靈活。通過鎖定放大器噪聲中的微小信號能夠準確的檢測出來，而不像在傳統(tǒng)鎖定放大器需要通過繁瑣的相位調(diào)整。 1引言鎖定放大器運用了相敏檢測器和低通濾波器，不僅提供了對頻率的高選擇性，而且即使在中心頻率，也有一個拒絕相外噪聲性能。在測量相位和分布求積分中，相敏檢測是不可或缺的。當檢測大量的某一頻段的信號時，它

2、的性能使操作繁瑣乏味，參數(shù)的變化使相位發(fā)生漂移，例如，一個濾波網(wǎng)絡(luò)檢測它的頻率響應(yīng)。對每一個參數(shù)都必須進行相位調(diào)整。在許多鎖定放大器中相位靈敏調(diào)解器被設(shè)置是為模擬開關(guān)式相位檢測器，它巨大動力范圍，對被檢測信號的奇和聲產(chǎn)生響應(yīng)，有害信號的初步濾波就能夠避免誤差。一個調(diào)諧濾波器，經(jīng)常用來做為預(yù)濾波，然而相頻檢測依賴于通頻帶，在psd輸出信號的微小變化就會引起大量的誤差。于psd相比，新式的鎖定放大器借助同步濾波應(yīng)用了參考頻率。這個濾波器所提供的定相頻率選擇性與一般的鎖定放大器一樣高，除此之外濾波器對和聲的響應(yīng)也比psd小得多。2鎖定放大器概述它的結(jié)構(gòu)分為兩個部分；信號通道，用來處理被測信號。參考通

3、道，提供同步濾波器的驅(qū)動信號來調(diào)諧參考頻率。參考通道能夠同時工作在內(nèi)部模式和外部模式，這和傳統(tǒng)鎖定放大器一樣。下面就詳細介紹它的工作性能。 （1）信號通道信號通道主要由前置放大器，預(yù)濾波，同步濾波器升壓放大器，rms檢測器及指示器。前置放大器把輸入信號增益達到100分貝。通過兩個通道所形成的接地環(huán)路中差分輸入，減少了噪聲的生成。預(yù)濾波消除了信號的殘留邊帶，有害通頻帶；它們是5fc,7fc,等等。這些對頻率的要求和傳統(tǒng)鎖定放大器的預(yù)濾波的衰減斜率對頻率的要求是完全不同的，很容易解決預(yù)濾波的相位特性問題。當鎖定放大器與外部參考信號一起工作時，預(yù)濾波必須被省卻或者通頻帶必須被展寬以免限制掃描范圍。在

4、這種情況下比起一般的鎖定放大器，運用同步濾波來測量受噪聲影響的微弱信號減少誤差。（2）參考通道同步濾波需要6相位驅(qū)動信號。通過工作在6fc的時鐘頻率的6-bit環(huán)計算器，信號很容易被產(chǎn)生。在內(nèi)部模式操作中，一個可變電壓源e，為航向控制振蕩器提供控制電壓產(chǎn)生時鐘信號。環(huán)狀計算器的兩個輸出讓一個r-s重復(fù)設(shè)置產(chǎn)生方波信號。這個方波為后繼同步濾波器提供了與信號通道相同頻率的波形。和低通濾波器的邊帶限制相聯(lián)系，同步濾波器只提取方波信號中的基波成分。正弦信號的獲得以用來作為參考輸出信號。信號通道中的低通濾波器和預(yù)濾波器的斬波是通過在電源e和控制面板交匯處相同方法設(shè)置的。參考通道的功能是為相敏檢測器（ps

5、d）提供與被測信號相干的控制信號。參考通道由方波發(fā)生電路和移相電路組成,其作用是提供一個與輸入信號同相的方波,調(diào)整移相電路使參考信號與有用信號相位一致,從而信噪比改善為最佳.參考輸入可以是正弦波、方波、三角波、脈沖波或其他不規(guī)則形狀的周期信號，其頻率也是載波頻率w。，由觸發(fā)電路將其變換成為規(guī)則的同步脈沖波。參考通道的輸出r(t)可以是正弦波，也可以是方波。為了防止r(t)的幅度漂移影響鎖定放大器的輸出精度，r(t)最好采用方波開關(guān)信號，其正負半周之比為11，在下節(jié)介紹鎖定放大原理時我們分別以正弦波和方波為例，來進行比較。移相電路是參考通道的中心部件，它可以實現(xiàn)按級跳變的相移（，）和連續(xù)可調(diào)的相

6、移（0）。移相電路可以是模擬門積分比較器，也可以用鎖相環(huán)（pll）實現(xiàn)，或用集成化的數(shù)字式鑒相器、環(huán)路濾波和壓控振蕩器（vco）組成。lock-in amplifier using asampled-data synchronous filteryushi komachi and sueo tanakaschool of science and engineering, waseda university,shinjuku-ku, tokyoreceived 13 december 1974, infinalform 9 june 1975abstract ：a lock-in amplifi

7、er is constructed using a synchronous filter instead of a phase sensitive detector. the synchronous filter based on a signal sampling method offers an extremely narrow bandpass characteristic whose centre frequency can be easily controlled by its drive frequency. since the filter makes smaller respo

8、nses for harmonics of the signal at the tuned frequency than a phase sensitive detector, the error due to the harmonics becomes less and required performances of a prefilter are relaxed. by this lock-in amplifier the magnitude of a low-level signal affected by noise can be measured accurately withou

9、t annoying phase adjustments which are required in a conventional lock-in amplifier.1 introductionlock-in amplifier employing a phase sensitive detector (psd) and following low-pass filter, offers not only a high frequency selectivity but also an out-of-phase rejection even for the signals at the ce

10、ntre frequency (cox 1953, marton 1962, danby 1970). the phase sensitive performance is indispensable in the measurements where an in-phase component and a quadrature one must be detected separately. when only the magnitude of a signal at a certain frequency is to be measured, however, the performanc

11、e makes the operation annoying and tedious. this is particularly true where variations of a parameter give rise to phase shifts, e.g. a filter network under test of its frequency responses. phase adjustments must be made for each setting of the parameter. in most lock-in amplifiers the psd is implem

12、ented with analogue switches to achieve a large dynamic range, and so it responds to odd harmonics of the signal to be detected. thus preliminary filtering of these unwanted signals is required to avoid the error due to them. a tuned filter, often used as the prefilter, has however a marked phase-fr

13、equency dependence at the passband and can give a considerable error at the psd output by slight deviation from the correct tuning. instead of a psd, the novel lock-in amplifier described here employs a synchronous filter which tunes to an applied reference frequency. the filter offers a phase insen

14、sitive frequency selectivity as high as one obtained in usual lock-in amplifiers, and besides the responses of the filter to the harmonics are much smaller than those of a psd.plifier using 。2 description of the lock-in amplifierthe configuration is divided into two portions; signal channel where th

15、e signals to be measured are dealt with, and* reference channel which provides drive signals of the synchronous filter to tune it at reference frequencies. the referencechannel can work in both internal mode and external mode, as often seen in conventional lock-in amplifiers. the details of the perf

16、ormances are as follows.（1） signal channelthe signal channel consists of preamplifier, prefilter, synchronous filter, booster amplifier, rms detector and indicator. the preamplifier amplifies input signals with the gain up to 100 db. its differential input reduces common mode noise produced in the g

17、round loop formed by the two inputs, signal channel input and reference channel input. the prefilter eliminates signals at the frequencies in the remaining, unwanted passbands ; that is, 5fc, 7fc, etc. these frequencies are so different from the required frequency of f c that the attenuation slope o

18、f the prefilter need not be as steep as that of a prefilter required inconventional lock-in amplifiers. moreover the phase characteristics of the prefilter do not matter. when a lock-in amplifieris worked with a sweeping external reference signal, a prefilter must be dispensed with or its passband h

19、as to be widened in order not to restrict the sweep range. in such a case the lock-in amplifier using the synchronous filter can measure a lou-level signal affected by noises and by the harmonics with less error than a usual lock-in amplifier owing to the smaller harmonic responses.（2） reference channelthe synchronous filter requires such 6-phase drive signals as shown in figure 2. the signals can be easily produced by a 6-bit ring counter working at the clock frequency of 6 fc. in the internal mode operation, a variable voltage source e, supplies the control voltage of vco (voltage contro