】 【Weak current College of using fiber-optic solution to audio signal transmission problems --- Power By】 【China power house network.

<br> Summary: discusses using fiber-optic transmission of audio signal and optical transmitter works and its temperature and noise control.  <br> <br> Keywords: optical transmitter cable transmission signal quality <br> <br> 1 preface <br .>  <br> Puyang radio broadcasting center and transmitter room 200 metres apart, FM and the am-room transmitter Room 7 thousand meters apart, to the broadcast signal transmission has brought about tremendous inconvenience. In order to ensure the safety and transmit signal to .the transmitter of high quality, for several years, tried the method, because of the failure rate is high, the transmission is not good.  <br> In October 1998, I bought one 1kW FM transmitter and digital audio workstation, enables me to .stand for the first time enables audio signals from analog to digital conversion. The broadcasting center to launch the engine room of the audio signal using the audio cable transmission, but because of distance, the signal attenuation, promote the transmitter difficult, less than the technical .standards, therefore increasing the audio amplifier, mixer, and other peripheral equipment. But the noise problem occurred after repeated debugging and matching, the sound quality is still not reach the intended effect. Finally, in the central control room to increase the 50W small FM .transmitter, in increased FM transmitter room and am receiver, wireless transmission, wireless access methods replace the audio cable. But because of the increased number of intermediate device, the failure rate is as high, to the human and material resources have resulted in tremendous waste ., according to the rate is relatively high.  <br> In recent years, with the continuous development of the broadcasting industry, optical transmission system to its frequency bandwidth, large capacity, loss, anti-jamming ability, nonlinear distortion of small, stable ., easy maintenance, etc., are increasingly in the audio signal transmission. And fiber optic equipment and cables cost is very low, whether from the current or in the long run, all audio cable. Optical transmission has become the main radio and TV transmission .system. <br> <br> 2 fiber-optic lines in optical transmission network <br> <br> According to the transmission equipment, fibre optic lines in optical transmission network mainly has two applications: a digital transmission, typically a PDH, SDH .system. PDH is less used for radio and television transmission, used for data transmission. How to build trunk SDH transmission network, is currently broadcasting system used in legacy mode. Another for AM AM simulating analog transmission, CATV optical transmission system can be divided into .direct modulation amplitude modulation amplitude modulation system and outside. Direct modulation amplitude modulation system is semiconductor, Exciter, direct light intensity modulation signals will be converted to the FM modulation for transmission of optical signals. (DFB) actuators, its spectral lines narrow, linear, .high power output (up to 10 mW), 1310nm optical wavelength is currently using the most suitable for medium distance transmissions. <br>  <br> 3 optical transmitter <br> <br> Shanghai guangdian group production of optical transmitter KD-50A .type, its internal integrated with optical isolator, Thermistor, optical power monitoring and control of the photodiode, can provide high linearity and low noise, high power (up to 20mW) laser output. Thermistor resistance is 10 k su Tong "hard luck, laser .bias values static input current to 100mA, negative bias value voltage range is from-5 to 0V. Optical detection diodes (PD) anode current of 100-1600 mai. Cooler (TEC) normal temperature as 25 ¡æ. RF input impedance 25 Tong .positive electrode * ¤ <br> 3 1DFB actuator <br> DFB Exciter frequency characteristics and devices of partial value current relationship. When the offset value of the current exceeds the threshold, the optical power linear increase, Exciter frequency characteristics (such as noise frequency .distortion) and optical power of square root (or exceeds the threshold value for the current offset). Optical transmitter laser with optical output power is stable. Laser threshold current, partial value current and optical output power of laser working temperature. Laser internal heating makes .its performance dramatically lower, therefore, the optical transmitter for automatic power control (APC) circuits and automatic temperature control (ATC) circuits guarantee optical transmitter function plays a very important role.  <br> 3 2 automatic power control (APC) circuit < .br> When the laser current partial value is greater than the threshold current is added to the actuator in laser diode on partial values current and laser output power essentially proportional to the relationship. LED driving current through the change for direct modulation, therefore, automatic power .control (APC) circuit is the use of lasers in photoelectric detecting diodes (PD) measurement laser output optical power, and on the basis of the photodiode output current produces a voltage, and out-of-the-box to compare a reference voltage, .after feedback control circuit driving a steady current source, so as to achieve the automatic adjustment of the laser with optical output power, lasers work properly. When the optical power increase, led drive current control circuit to reduce the output power decrease; when the optical .power decreases, control circuit and laser control current increases, so that the output light power increase. Output optical power fluctuations not exceeding a certain range.  <br> Its work is: Exciter back to output light by photoelectric detecting diodes (PD) receive .back into light voltage, the N1 is enlarged to the comparator N2 reverse input; from DC power supply out of DC reference voltage sent to comparator N1 in-phase input. V1 and v2 components DC current source, the value of the laser provides partial, .the partial values for current current size by adjusting the DC reference voltage.  <br> In addition to the DC supply voltage, the light power control circuit also has two additional circuits: a slow startup circuit, when light transmitter power on, this circuit .allows the laser bias value current delay 2s before from 0 to set the value to remove the instantaneous current damage laser; the other is a current limiter, current limiting circuit control laser current maximum, even if the photoelectric detecting triode damaged, will not cause the .offset value of the current laser lost control while burning laser.  <br> 3 3 automatic temperature control (ATC) circuit <br> Laser threshold current, partial value current, output optical power and lasers operating temperature. Laser threshold current temperature-dependent ., as the temperature increases, the efficiency of the laser allows to reduce the output light power and actuator launch wave peak values change. In order to ensure that the work of the State ie laser threshold current, output power, must pass through the automatic temperature .control (ATC) circuit to control the cooler working state, elimination of temperature changes.  <br> Its work is when the laser temperature, thermistor resistance value changes, you can set a reference value and its comparison. Error voltage driven an amplifier that .electronic refrigerator provides current. Automatic temperature control circuits by changing to the actuator in the refrigerator on the current size and orientation, laser heating or cooling, thus controlling the actuators of working temperature, stable output power of laser. When the laser temperature rise, the .refrigerator refrigerant, laser temperature drop; when laser temperature decreases, the refrigerator heating, laser temperature rise. Automatic temperature control (ATC) circuit allows the laser work temperature is 25 ° c, to optical transmitter output power in a large temperature range remains stable. .4 fiber loss <br> <br> Fibre connections are divided into welding, and grinding, regardless of connection will produce connector loss. 1310nm optical attenuation coefficient (a) 0 35dB/km. Regardless of the distance signal transmission, optical fiber connections require .fibre splicing, its joint splicing loss generally 0 05dB following. So when calculating fibre automatic loss should also consider connector and other lossy (2km General from 1dB). L length fiber loss Am = aL + 1 0dB = 0.35 L + 1 0dB. I .-bed with cable length is 200 m, two welding head loss a 0 02dB, another 0 03dB. Two live connector connector loss are the technical requirements. <br>  <br> 5 cable noise <br> <br> Fiber optic cable .noise is mainly divided into laser noise and thermal noise and intrusive noise. Laser noise and thermal noise is determined by the equipment manufacturing process, intrusive noise from the outside world radio communication, broadcasting and television signals. <br>  <br> 6 conclusions .<br> <br> We use fiber-optic transmission of two programs: a comprehensive program of 100 1MHz, 1kW FM stereo, economic channels as 91MHz, 5kW stereo FM radio. Using optical transmission over two years to signal clearly, no noise, .it can be said that the optical signal instead of cable transmission, not only completely free from past noise, sound quality is poor, the signal is not clear, but also significantly reduces societal and inferior sowing phenomenon from occurring, better completion of the transmission of .radio and TV signals, and increase the frequency of task coverage expanded to hear rate, improving the economic and social benefits, to better serve the community. <br>.