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We developed an array of signal processing systems (led lights), due to the array antenna must be placed in the surrounding open area, and array signal processing unit is located in a circular array antenna central location, which requires a computer outside of the array signal processing unit in 1 ~ 2km between the duplex communication. Proportion in the time index in the entire system in order to reduce the data transmission time, the data transfer rate should be not less than the rate of E1 (2.048Mb / s), and ask the communications link is safe, reliable. Analysis of various data communication technologies, the final choice of a fiber-optic key road, and achieved satisfactory results.

A copper media data link

Also requires a higher data transmission rates and longer communication distance in the copper media link coding and modulation is difficult to meet the index requirements, but the high-speed modem is not only expensive, but the handshake process time-consuming random real-time requirements field of the transmission data will obviously not apply. The most important thing is that the traditional copper media link produces both electromagnetic interference signals, and also easily affected by electromagnetic interference, and difficult to meet the requirements of the EMC (electromagnetic compatibility) and EMI (electromagnetic interference) standards.

Analysis of two fiber-optic link

The traditional image of the people, the fiber used in short-distance communication is not economical, but it has a copper media key road incomparable advantages: ① fiber link is neither emit electromagnetic wave, is not affected, there is no interference between the fiber error rate is greatly reduced. The designers do not consider the incoming ambient noise may be coupled; ② fiber to provide a communication link between the two electrically isolated to eliminate the problems caused by ground potential between the long-distance equipment. Designers no longer have to impedance matching and a headache; ③ fiber-optic transmitter can be digital modulation driving circuit. Digital baseband signal through a simple line coding, you can directly drive the fiber optic transmitter.

Now, the price of fiber optic devices than ever before has been a sharp decline, With the cable price increases and the fiber-optic device prices to decline further in the future, when people choose fiber is no longer first consider the price factor. Therefore, considering the speed and distance indicators, select the appropriate type of fiber transceiver and its drive circuit, high performance and low cost fiber-optic communication key road, this is the focus of this paper to.

The choice of optical fiber communication devices

A basic fiber-optic communication system is very simple: an LED panel emitter electrical signals into optical signals, and the coupling into the transmission fiber, the optical signal through the fiber to reach the optical receiver, the received optical signal back into the original the electrical signal output.
· Cable options: general, quartz glass fiber due to its low loss, high bandwidth for long-distance communications links, such as Ethernet and FDDI standard specifies multimode 62.5/125μm quartz glass fiber. Thin core fiber requires high-precision connectors to reduce the coupling loss, low-cost cables and connectors for industrial applications. 1mm POF (Plymer Optical Fibers) and 200μm HCS (Hard Clad Silica) fiber is the best choice, they are the step-index multimode fiber.
1mm POF typical loss value at 650nm wavelength is 0.2dB / m, 200μm HCS fiber to only 8dB/km 650mm wavelength typical loss value, less in the 820nm wavelength. HCS fiber is the core of the quartz glass cladding is a patented high-strength polymers, not only increased the intensity of the optical fiber, and moisture, pollution prevention and the outer sheath of PVC 2.2mm. HCS fiber can work in a 40 ℃ ~ +85 ℃ temperature range, the erection of a temperature range of 20 ℃ to +85 ° C, in both performance and price to meet the system requirements.
· Operating wavelength choice: fiber optic communication system design must take into account the impact of fiber loss and dispersion of the system, due to loss and dispersion are related to the operating wavelength of the system, the operating wavelength choose to become a major problem in system design. Consider the system specifications and selected fiber, select the 820nm wavelength enables HCS fiber loss as low as 6dB/km dispersion has reached the minimum.
· Choice of light source: LED can be selected in the 820m wavelength light compared with the semiconductor laser, the LED drive circuit is simple and low cost.

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In summary, the cable selection 200μm HCS fiber optical transceiver device selection of the 820nm wavelength of the HP’s HFBR-0400 series. In the 1500m distance in the series HFBR-14X2/HFBR- 24X2 reach 5MBd rate of the operating temperature range from 40 ℃ to +85 ° C, ST, SMA, SC and FC, multiple port models to choose from. HFBR-14XZ type AlGaAs LED tubes with 820nm wavelength HFBR-24XZ internal integration of an IC chip including PIN photo detector, the DC amplifier, and the open collector output Schottky transistor, its output directly with popular TTL and CMOS IC connected.

4 A practical fiber optic LED driver circuit

The fiber optic duplex communication system in which the role of the driver circuit is to convert electrical power into optical power, and modulation of the transmission of electrical signals to the output of the light source, its light while providing the bias current and changes with the digital signal modulation current.

Design LED driver circuit must first consider the peak of the LED drive current, and if more than the peak drive current produced by the optical signal will overshoot phenomenon. Chong, resulting in the receiving device to cause the signal coupled from the amplifier noise, the results may be beyond the detection threshold of the comparator, causing the error. LED has extinguished than the features lit difficulties, it will produce a smearing, This phenomenon is particularly evident when using the serial driver circuit. The parallel drive mode can provide a low resistance channel for the carrier in the LED, thereby reducing the pulse width distortion and slow smearing.

Resistance RSI in the drive circuit is used to adjust the optical output power, be careful not to exceed the peak drive current of the LED, or optical signal will overshoot. Another circuit of the SN75451 with the characteristics of low impedance, high current rate, to avoid the long tail phenomenon.

Pulse width distortion (PWD) is a major factor to limit the rate of fiber-optic link, which is caused by transmission delay between the input and output are not equal. Noted that PWD is always positive, so we can take advantage of the RC circuit to delay the LED is lit.

The optical receiver circuit is an important part of the optical fiber communication system, its performance is good or bad is a comprehensive reflection of the optical fiber communication system performance. Its role is to optical fiber transmission over optical signals into electrical signals, then amplified and balanced, the decision circuit and restore the original signal of the transmitter.

The presence or absence of the commonly used optical signal in optical fiber transmission line, “1” and “0” code in order to avoid the long stream even “0” or long even “one”, conducive to the extraction of the clock, the need for coding circuit. Array signal processor is sent to the parallel data rate and conversion circuits.