Research On The Design Of Weak Signal Processing Circuit Based On TDLAS

In recent years, the gas pollution is increasingly seriously, the detection of harmful gas concentration has been more and more attention under the specific environment. The Tunable Diode Laser Absorption Spectroscopy(TDLAS) detection technology as a representative method for the detection of gas concentrations has the high sensitivity, has become one of the preferred methods of the industry on-line measuring concentrations of trace gas in the atmosphere.This paper designed a lock-in amplifier which can realize phase shift and frequency multiplication used to extract the second harmonic signal in view of the TDLAS gas detection system. The whole block diagram of the detection system has been designed in this paper, mainly including photoelectric detector, I/V conversion circuit, lock-in amplifier circuit, low-pass filter circuit and A/D conversion circuit. The lock-in amplifier circuits designed for the TDLAS gas detection system mainly includes: pre-amplifier circuit, band-pass filter circuits, TTL level converter circuit, frequency multiplier and phase shifter circuit, phase-sensitive detector circuit, low-pass filter circuit, bipolar limiting circuit. According to the basic principle of PLL frequency synthesizer, the focus of the lock-in module design is to design the frequency multiplication and phase-shifting circuit, mainly using the 74HC4046 and 74HCT161.These two kinds of chips can achieve phase locked and frequency multiplier doubling selection, and also can be set up according to the actual situation to phase shift.In the lab environment, we use the known gas concentration to detect the correctness of the second harmonic. In the process of the circuit board test, the first thing is to use the oscilloscope to detect the TTL level and phase. If there is no problem and then to connect PC software, fed by a known concentration of gas to verify the second harmonic signal is correct or not. Through the experiment to verify the design scheme of lock-in amplifier and the circuit function, the experimental results show that TDLAS detection technology with second harmonic detection feasibility and accuracy of gas concentration.