tailieunhanh - Atmospheric Acoustic Remote Sensing - Chapter 6

Trong chương trước, chúng tôi đã mô tả các tính chất khí quyển có thể truy cập SODARs, các yếu tố của SODAR thiết kế và hiệu chuẩn cụ. Trong một số trường hợp, chúng tôi cũng đã thảo luận tín hiệu-to-noise ratio một cách chung chung. Trong thực tế, tách tín hiệu có giá trị từ các nền tiếng ồn là một phần quan trọng của SODAR phần cứng và thiết kế phần mềm. Chúng tôi xem xét các tính năng này trong chương hiện hành. 6 Phân tích tín hiệu SODAR Tín hiệu MUA lấy mẫu . | 6 SODAR Signal Analysis In previous chapters we have described the atmospheric properties accessible to SODARs elements of SODAR design and instrument calibration. In a number of instances we have also discussed signal-to-noise ratio in general terms. In practice separating valid signals from the noise background is a major part of SODAR hardware and software design. We consider these features in the current chapter. SIGNAL ACQUISITION Sampling Although already discussed in Chapter 2 sampling will be briefly revisited. In the simplest case a SODAR transmits a signal Asin 2nfTt at a frequency fT. The received signal is continuous has reduced amplitude in general is Doppler shifted and has modified phase p t A sinpLjT Ạf t This signal can be sampled using an analog-to-digital converter ADC at times . m t mAt m 0 1 . m Js The sampling frequency is fs. The sampled signal has discrete values pm A Í sin 2. m I Jt AJ fs Ộ Aliasing For simplicity write J n 8 fs where n is an integer 0 1 . and Ỗ is a fraction. Then pm A sin I 2vmỉ Ộ since sin 0 2nmn sin 0 . As an example assume fs 960 Hz a signal component having frequency 960 960 3 1280 Hz gives the same digitized values as if it had 157 2008 by Taylor Francis Group LLC 158 Atmospheric Acoustic Remote Sensing frequency 960 3 320 Hz. The same is true for negative Ỗ. This means that higher frequency components can add into the lower frequency spectrum. This is called aliasing. This means that all frequency components outside of nfs fs 2 should be excluded from the signal before digitizing. This is called the Nyquist criterion. Usually this is interpreted as using anti-aliasing low-pass filters to remove all frequency components outside of fs 2 but in fact the criterion is satisfied if band-pass filters remove all components within a fs 2 bandwidth of nfs. Mixing For a SODAR the bandwidth of the Doppler spectrum is generally much smaller than fs. For example if the speed of sound is c 340 m s-1 fs .