1、- 1 - 中文 1400 字 机械与电气工程学院 毕业设计（论文）外文翻译 所在学院： 机械与电气工程学院 班 级： 姓 名 ： 学 号： 指导教师 ： 合作导师 ： 2012 年 10 月 10 日 - 2 - 原文 : DESIGN OF AN INDIGENISED ELECTRONIC LOCK-IN AMPLIFIER UMESH KUMAR* Deptt. of Electrical Engineering, HT, New Delhi-llO016, India (Received 7 April 2000； In finalform 25 April 2000) An ind
2、igenised lock-in amplifier is designed that enables the accurate measurement of signals contaminated by broad-band noise, power-line pick-up, frequency drift, or other sources of interference. It does this by means of an extremely narrow band detector which has the centre of its passband locked to t
3、he frequency of the signal to be measured. Large improvements in signal-to-noise ratio are achieved. Keywords: Lock-in； Amplifier； Narrow band detector； Accurate signal measurement I. INTRODUCTION The lock-in Amplifier is analogous to a synchronous demodulator or a phase sensitive detector. It is a
4、specialized a.c. voltmeter that uses synchronous demodulation to measure signal strength or phase even under severe noise condition viz. where noise to signal ratio approaches 130dB. The instrument can be used wherever the signa of interest can be synchronised with or derived from a particular refer
5、ence signal. The output of a lock-in amplifier is a phase sensitive d.c. voltage proportional to the signal. It can be divided into four main sections: A signal channel； Reference Channel； Mixer (Phase sensitive detector) and a d.c. amplifier with a low-pass filter as shown in Figure 1. - 3 - II. DE
6、SCRIPTION OF THE VARIOUS PARTS In the signal channel, the input signal and noise is conditioned by the low-noise pre-amplifiers and post-amplifier with a filter sandwiched in between. The purpose of this filter is to reduce the possibility that the mixer will overload during severe noise conditions.
7、 The reference channel transforms the externally applied reference to a suitable square wave at reference frequency to drive or switch the mixer. The lock-in output is independent of the referenced amplitude. However, the output does depend on the phase difference between the signal and the referenc
8、e. The Mixer is the main part of the lock-in amplifier. It is like an electronic reversing switch whose sense or position is determined by the square drive polarity of the reference channel. The signal channel output is commutated at the reference frequency. The signal channel output consists of two
9、 equal amplitude sinusoids, one in and one out of phase with the reference. III. DESCRIPTION OF OPERATION During the first half cycle, the reference drive is positive and the switch in Figure 2 is in position A. The mixer output is a positive half-wave sinusoid during this interval. During the second half wave cycle, the switch is in position B and the mixer output is still positive. Therefore, when the reference and
