Design And Implementation Of Low Frequency Weak Signal Detection System Based On Quartz Crystal Filter

Weak signal detecting is necessary under a lot of circumstances. Weak signal detection has a special require for the design of detection system. As the development of science and technology, weak signal detecting system has been constantly improved and developed too, at present the widely used method in signal detecting system is lock-in amplifier.When it comes to the lock-in amplifier, the input signal’s frequency spectrum is migrated to direct-current by phase sensitive detector then goes through low pass filter, and the output signal will be the direct-current signal whose amplitude is proportional to the input signal. This paper proposes a low cost, and easy to implement weak signal detecting system after refer to the basic principle of lock-in amplifier in weak signal detection. Unlike lock-in amplifier, this system is implemented by using frequency mixer to shift the frequency of the input signal to a stationary frequency instead of DC. Then the post-stage circuits amplify the stationary frequency signal and reject others. In this way, amplification circuits adopt the alternating-current coupling, which can effectively overcome the direct-current drift of the circuits which is an important factor to influence the system performance when the amplifier gain is very high. In this design, a narrow band-pass filter is needed. The ordinary LC filter cannot satisfy this demand. Active band-pass filter is also difficult to achieve high frequency selectivity because of the influence of the stability. Crystal resonator can be used as narrow-band filter because of its high Q-factor. Building crystal filter by crystal resonator is common in RF signal process. The weak signal detection scheme proposed in this paper is similar to the principle of super-heterodyne receiver, but the crystal filter is consist of low frequency crystal resonator whose frequency is32.768KHz and the operational amplifier. In this way, narrow-band filtering is implemented in low frequency stage. The system has the advantages of narrow band, simple design and low cost etc.This article focuses on how to use low-frequency quartz crystal resonator design narrow-band filter. On this basis, proposed a new design scheme of weak signal detection system based on crystal filter. The new design scheme overcomes the main disadvantage that the crystal resonance frequency is fixed by using a frequency mixer and the multilevel filter is used to improve the frequency selectivity. The implement of the circuit and the test result are provided. The test results show that the system has very high frequency selectivity. Within the tested frequency range from10Hz to200KHz, the absolute bandwidth is less than2Hz, and the system can effectively detect the signal with0.5uV amplitude, at the same time the system has a good linearity at different frequency, and therefore it can meet the application requirements of most weak signal detection system.