在水下無線通訊的環境中，聲波為最主要傳遞訊號的媒介，近年來由於國防的需求及海洋資源的探勘，水下通訊技術變成是各先進國家極力發展的重點。由於水下通訊系統包含發射端的調變及接收端的解調變技術，方能完成整個水下通訊。
本研究主要的目的是在於建立水下通訊數據機的基礎開發能力，並以適當的硬體設備與水聲感測器在實驗水槽下建立水下通訊基本之測試環境。水下通訊解調變技術藉由晶片XR2211-A的電路元件選用及設計，及晶片XR2212公式修正帶寬頻率問題，並以實際電路接線完成FSK(Frequency Shift Keying)解調變測試不同組合頻率的解調情況，藉此選定能辨識之較佳組合頻率34、38kHz實作電路板。
搭配TC-1026水聲傳感器，整合解調電路板，吾人實作一只水下接收聽音設備，並完成組合頻率34、38kHz及Baud Rate為4800bps的水下通訊之接收解調變測試，以建立未來能於海洋中應用之基礎能力。
未來若能結合電壓放大技術以驅動水聲傳遞的能力及單晶片處理發射及接收之訊號，將傳遞及接收解調訊號整合為一體的發射接收器，將可以使其運用於海洋下的傳遞及接收解調，並利用單晶片將High、Low的方波訊號轉換成有意義之0、1數位訊號，實現在海中傳遞聲音及圖片影像之水下通訊。

SUMMARY
In underwater wireless communication environment, sound waves are the most important medium for transmitting signals. In recent years, due to the demand for National Defense and the exploration of marine resources, underwater communication technology has become the focus of the development of advanced countries.
This study main goal is lies in the establishment underwater communication modem basic development ability, and in tests under the water tank trough by the suitable hardware equipment and the underwater acoustic sensor to establish the underwater communication basic test environment.
The underwater communication demodulation technology uses the circuit component selection and design of the chip XR2211-A, and the chip XR2212 formula to correct the bandwidth frequency problem and by the actual electric circuit wiring to completes FSK (Frequency Shift Keying) demodulate and test different combination frequencies the demodulation situation, hereby selecting a better combination frequency 34、38kHz that can be identified to make the electric circuit board.
Matching the TC-1026 transducer and integrated demodulation circuit board, we have implemented an underwater receiving listening device, and completes the combination frequencies 34, 38kHz and Baud Rate is the 4800bps underwater communication receive demodulation test to establish the basic capability for future application in the ocean.

INTRODUCTION
With the maturity of all aspects of science and technology, the exchange of various kinds of information in life is frequent, but also deepen the human desire to message. The current research and development of communications mainly focused on land communications, such as mobile phones. However, for the geographical advantage such as Taiwan,which be surrounded by the sea . In order to effectively use and develop, there is absolutely an important subject of underwater communication technology.
The main purpose of this study is to use the FSK demodulation technology to implement an underwater receiver to lay the foundation for the future development of the underwater acoustic modem.
MATERIALS AND METHODS
This research is mainly based on underwater communication as the main structure, with FSK modulation and demodulation technology as the core, integrating transducer and FSK demodulation electronic circuit hardware.
Use the existing underwater acoustic equipment of our laboratory and integrate Arbitrary Function Generator PXI-5401, Virtual Bench, PXI-8196 Controller produced by National Instruments and TC-1026 Transducer by RESON Corp and Type 8106 Hydrophone by B&K Corp establish a transmitting and receiving test system of underwater communications to understand how the underwater transducer's transceive performance and how to transmit and receive underwater sound.
Use the XR2211-A demodulation chip manufactured by Exar Corporation was used to realize the demodulation of FSK, The chip XR2212 formula corrected the bandwidth frequency and actually verified by circuit demodulation.Make the circuit into a circuit board, and integrated the TC-1026 transducer,power supply by Li-battery to produce an FSK demodulated Acrylic underwater receiver. Make circuit into a circuit board integrated TC-1026 transducer and power supply of Li-battery to produce a Acrylic FSK demodulation underwater receiver and the feasibility of the test is verified.

RESULTS AND DISCUSSION
This part is use the TC-1026 Transducer, circuit board made up of the logic of combination frequency 34, 38kHz, Baud rate=4800bps and the signal amplification circuit and Lithium battery powered to composed a demodulation underwater receiver, and placed in the underwater tank to measure. It is known from the test results that although the preliminary experimental demodulation is feasible, but the success rate is quite low. After the modified experimental test, the dB value of the power amplifier of the transmitting end is adjusted and matched with the amplifying circuit of the receiving end, so that the received signal is clearer and can be identified by the demodulation circuit to improve demodulation success rate.

CONCLUSION
The focus of this study is to implement an FSK demodulation underwater receiver. But the underwater communication is two-way for transmit and receive, In the future I want to be able to the direction of integration with the receive end and transmit end. Increase that the single-chip circuit processing input and output signals will be increased, and the power amplifier circuit will be added to the transmit end to promote signal transmission. And developed a self-designed underwater underwater acoustic modem.

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