854611203数据采集同步滤波器在锁定放大器中的应用中英文.doc

数据采集同步滤波器在锁定放大器中的应用摘要：锁定放大器是用同步滤波器来代替相敏检测器，同步滤波器是基于信号采样的方法提供了一个极其窄的带通滤波器的特性，它的中心频率能够通过调整频率很容易的控制，因为滤波器比相敏检测器在谐波时有更小的响应，误差所引起的和声变得也很小，并且对滤波器的性能的要求也比较灵活。通过锁定放大器噪声中的微小信号能够准确的检测出来，而不像在传统锁定放大器需要通过繁琐的相位调整。 1引言锁定放大器运用了相敏检测器和低通滤波器，不仅提供了对频率的高选择性，而且即使在中心频率，也有一个拒绝相外噪声性能。在测量相位和分布求积分中，相敏检测是不可或缺的。当检测大量的某一频段的信号时，它的性能使操作繁琐乏味，参数的变化使相位发生漂移，例如，一个滤波网络检测它的频率响应。对每一个参数都必须进行相位调整。在许多锁定放大器中相位灵敏调解器被设置是为模拟开关式相位检测器，它巨大动力范围，对被检测信号的奇和声产生响应，有害信号的初步滤波就能够避免误差。一个调谐滤波器，经常用来做为预滤波，然而相频检测依赖于通频带，在PSD输出信号的微小变化就会引起大量的误差。于PSD相比，新式的锁定放大器借助同步滤波应用了参考频率。这个滤波器所提供的定相频率选择性与一般的锁定放大器一样高，除此之外滤波器对和声的响应也比PSD小得多。2锁定放大器概述它的结构分为两个部分；信号通道，用来处理被测信号。参考通道，提供同步滤波器的驱动信号来调谐参考频率。参考通道能够同时工作在内部模式和外部模式，这和传统锁定放大器一样。下面就详细介绍它的工作性能。 （1）信号通道信号通道主要由前置放大器，预滤波，同步滤波器升压放大器，RMS检测器及指示器。前置放大器把输入信号增益达到100分贝。通过两个通道所形成的接地环路中差分输入，减少了噪声的生成。预滤波消除了信号的残留边带，有害通频带；它们是5fc,7fc,等等。这些对频率的要求和传统锁定放大器的预滤波的衰减斜率对频率的要求是完全不同的，很容易解决预滤波的相位特性问题。当锁定放大器与外部参考信号一起工作时，预滤波必须被省却或者通频带必须被展宽以免限制扫描范围。在这种情况下比起一般的锁定放大器，运用同步滤波来测量受噪声影响的微弱信号减少误差。（2）参考通道同步滤波需要6相位驱动信号。通过工作在6fc的时钟频率的6-bit环计算器，信号很容易被产生。在内部模式操作中，一个可变电压源E，为航向控制振荡器提供控制电压产生时钟信号。环状计算器的两个输出让一个R-S重复设置产生方波信号。这个方波为后继同步滤波器提供了与信号通道相同频率的波形。和低通滤波器的边带限制相联系，同步滤波器只提取方波信号中的基波成分。正弦信号的获得以用来作为参考输出信号。信号通道中的低通滤波器和预滤波器的斩波是通过在电源E和控制面板交汇处相同方法设置的。参考通道的功能是为相敏检测器（PSD）提供与被测信号相干的控制信号。参考通道由方波发生电路和移相电路组成,其作用是提供一个与输入信号同相的方波,调整移相电路使参考信号与有用信号相位一致,从而信噪比改善为最佳.参考输入可以是正弦波、方波、三角波、脉冲波或其他不规则形状的周期信号，其频率也是载波频率w。，由触发电路将其变换成为规则的同步脉冲波。参考通道的输出r(t)可以是正弦波，也可以是方波。为了防止r(t)的幅度漂移影响锁定放大器的输出精度，r(t)最好采用方波开关信号，其正负半周之比为11，在下节介绍锁定放大原理时我们分别以正弦波和方波为例，来进行比较。移相电路是参考通道的中心部件，它可以实现按级跳变的相移（，）和连续可调的相移（0）。移相电路可以是模拟门积分比较器，也可以用锁相环（PLL）实现，或用集成化的数字式鉴相器、环路滤波和压控振荡器（VCO）组成。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 amplifier 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 responses 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 without 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 centre 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 performance 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 implemented 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-frequency 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 insensitive 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 the 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 performances 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 ground 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 of 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 has 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 controlled oscillator) to generate the clock signal. Two outputs $1 and $4 of the ring counter let an R-S flip-flop set and reset respectively to form a square wave offc. This square wave is applied to the following synchronous filter of the same type as in the signal channel. Associating with the band restriction of a low-pass filter (LPF l), the synchronous filter picks only the fundamental component out of the square wave. The sinusoidal wave of fc obtained is employed as a reference output signal. Cutoff frequencies of the low-pass filter and the prefilter in the signal channel are set by means of the same front panel dial in conjunction with the setting of E.