| Nowadays,the accurate detection of weak signal is of extreme importance in many different research fields such as medicine,chemistry,environment and marine analysis,and industrial production monitoring.But high noise level environments make the right acquisition of accurate measurements difficult,especially when the output signal are very small compared to the noise level.In this direction,an interesting possibility is the lock-in amplifier,which could take out the data signal at a specific reference frequency and reject noise signals at different frequencies without affecting the measurement significantly.However,the commercial lock-in system in the market,which are expensive,heavy,and power-hungry,are typically appropriate for multi-frequency operation and not considered suitable used in the portable sensing system.Therefore,a portable lock-in amplifier based on the AD630 has been designed and produced,with the theories of phase detection and relative detection,which could detect the amplitude and phase of the input signal.The dedicated lock-in amplifier based on AD630 has been cheaper,flexible,designed simply,even it can work independently,which means it doesn't need to rely on special facilities.On the other hand,another advantage of a dedicated lock-in amplifier is that generally the circuit can be much smaller than a complete instrument and can be placed much closer to the sensor.This can dramatically decrease parasitic effects,noise and disturbance levels in the system.Considering the different demands,two kinds of lock-in amplifiers based on AD630 have been designed,and are used in two distinguished optical fiber sensing gas concentration detection system.One is the duel-channel lock-in amplifier based on AD630 for accurate phase measurement,which is used in the high precision optical oxygen concentration detection system.The other is the single-channel lock-in amplifier for amplitude measurement in water vapor concentration system.The main contents of this paper are as follows:1.The paper introduces the working principle and basic composition of the lock-in amplifier,especially the theory of phase detection and the relative detection.According to the different detection principles of the oxygen concentration detection system and the water vapor concentration detection,the duel-channel AD630 lock-in amplifier and a single AD630 lock-in amplifier are designed and fabricated respectively.2.The AD630 is selected as the core chip of the lock-in amplifier,and design each part of the lock-in amplifier,analyzing their function and giving the specific circuit design drawings.3.The performance of the AD630 phase locked amplifier circuit is tested,which is divided into two parts:duel-channel and single-channel.After testing,the dual AD630 lock-in amplifier can accurately detect the phase difference between the input signal and the reference signal,the relative error is 5.9%,the resolution is up to 0.014 degrees,and circuit fluctuations is 0.12%.Single-channel AD630 lock-in amplifier is used for amplitude detection.The results show that the linearity of the output signal is good,the circuit fluctuation is 0.24%,and the variance is 0.00207.4.As is displayed from the oxygen concentration detection system,in the measurement range of 0.99%-8%,the phase changes every 1 degree,smallest change of the oxygen concentration is 0.285%,the measured system fluctuations is 0.0263%.From the measurement data of several days,the average value of each concentration gradient of oxygen concentration corresponds to the maximum difference of 0.032%.5.As far as the water vapor concentration detection system,at 0-1200ppm concentration,each 1mV corresponds to 0.2ppm.In order to improve the signal to noise ratio(SNR)of the system at low concentration detection,we increases range automatic control block.After that,the SNR is improved dramatically.In 5ppm,the system resolution is improved;each 1mV corresponds to 0.002ppm,increasing two orders of magnitude compared with before. |
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