Fiber optic LED driver circuit design

We have developed an array signal processing systems, due to the antenna array must be placed in the four weeks open area, and array signal processing unit is located in the center of circular antenna on display, which requires the array signal processing unit and 1 ~ 2km away at the computer between duplex communication. In order to reduce data transfer time in the entire system processing time the proportion of indicators require data transfer rate should be not less than E1 (2.048Mb / s) rates, and to ask communications link safe and reliable. Through a variety of data communication technology of cold storage installation analysis, the final choice of fiber key way, and achieved satisfactory results.

1, the copper media analysis of the data link

Owing to the higher data rates and longer communication distance, in the copper link media encoding and modulation is not difficult to meet the target, but the high-speed modem is not only expensive and time-consuming process of both hands, in real-time requirements of random transmission of data field will obviously not apply. Most importantly, the traditional copper media links both produce electromagnetic interference signals, but also susceptible to electromagnetic interference, and difficult to meet the EMC (electromagnetic compatibility) and EMI (electromagnetic interference) standards.

2, the analysis of fiber-optic link

Impression in people's tradition, the fiber used in short-distance communication is not economical, but it has a copper media key road incomparable advantages: ① not emit electromagnetic wave fiber-optic link, not affected, there is no interference between the optical fiber , bit error rate is much lower. Designers do not take into account the ambient noise that may couple to come; ② fiber communication link provides electrical isolation between the parties, shop equipment, eliminating long distance between the potential due to the different problems caused. At the same time as the impedance matching designers no longer have a headache; ③ can be digitally modulated optical transmitter drive circuit. Digital baseband signal as long as the line through a simple code, you can directly drive the optical transmitter.

Now, the optical device prices have dropped significantly compared with the previous, and future price increases as cable and fiber optic devices further decline in prices, people will no longer first choice fiber consider the price factor. Therefore, we consider the speed and distance indicators integrated, select the appropriate type of fiber transceiver and its driver circuit, can achieve high performance and low cost of optical fiber communication key road, which is the focus of this paper to discuss.

3, the choice of optical fiber communication devices

A basic fiber optic communication system is very simple: an LED transmitter optical signals into electrical signals, and the coupling will enter the transmission fiber, the light signals through optical fiber to reach the receiver, which receives the optical signal to restore the original The electrical signal output.

Cable options: general, quartz glass fiber because of its low loss, high bandwidth for long distance communication links, such as Ethernet and FDDI standard specifies the use of multi-mode silica glass fiber 62.5/125μm. These small core fiber connectors to reduce the need for high-precision coupling loss, for industrial applications, require low-cost fiber optic cable and connectors.Therefore, 1mm of POF (PlymerOptical Fibers) and 200μm the HCS (Hard Clad Silica) fiber is the best choice, they belong to the step-index multimode fiber.

1mm POF loss of value in the 650nm typical wavelength of 0.2dB / m, and 200μm HCS fiber loss in the value of a typical wavelength of 650mm only 8dB/km, at 820nm wavelength less. HCS fiber core is sky quartz glass cladding is patented high-strength polymer, not only increased the strength of the fiber, moisture and pollution prevention, is the 2.2mm outer sheath of PVC. HCS fiber can work in a 40 ℃ ~ +85 ℃ temperature range, temperature range for the erection of a 20 ℃ +85 ℃, both in performance and price to meet the system requirements.

Wavelength selection: fiber-optic communication system design must take into account dispersion fiber loss and the impact on the system, due to loss and dispersion associated with the system operating wavelength, and therefore the choice of operating wavelength to become a major issue in system design.Considering the system requirements and selected indicators of fiber, choose 820nm wavelength enables HCS fiber loss as low as 6dB/km, while dispersion is minimized.

The choice of light source: the 820m wavelength, LED light source can be the best choice, compared with semiconductor lasers, LED drive circuit is simple and low cost.

In summary, the selection of 200μm HCS fiber cable, optical transceiver devices use HP's 820nm wavelength HFBR-0400 series. The series HFBR-14X2/HFBR- 24X2 can be reached at 1500m distance 5MBd rate, operating temperature range for a 40 ℃ ~ +85 ℃, with ST, SMA, SC and FC ports a variety of models to choose from. HFBR-14XZ using 820nm wavelength AlGaAs-based LED, HFBR-24XZ including integrated PIN photo detector, amplifier and DC open collector Schottky output transistor of an IC chip, the output can be directly with popular TTL and CMOS integrated circuits connected to.

4, a practical optical fiber LED driver circuit

Duplex fiber-optic communication system in which the drive circuit is the role of electric power into light power, and the modulation signal to be transmitted to the light output, it also provides the bias current source and digital signal changes with the modulation current.

LED driver circuit design must consider the LED drive current peak value, if more than the peak drive current will be produced optical signal overshoot. And the resulting electrical signal at the receiver due to the lower punch, plus to sky8888 from the amplifier noise, the result may be beyond the detection threshold comparator, resulting in error. LED goes out while there is a difficulty than the characteristics of light, it will produce smearing when using the serial driver circuit when this phenomenon is particularly evident. The parallel drive mode for the LED carrier to provide a low resistance channel, thereby reducing the pulse width distortion and slow smearing.

Driving circuit for regulating the resistance of RSI optical output power, be careful not to exceed the peak LED drive current, or optical signal will overshoot phenomenon. Another circuit SN75451 has a low impedance, high current rate, it avoid the long tail phenomenon.

Pulse width distortion (PWD) is a fiber optic link speed limit a major factor, it is due to propagation delay between input and output does not equal cause. Note that PWD is always positive, so we can use the RC circuit to delay the LED light.

Optical fiber communication system receiver circuit is an important component of the optical performance is good or bad is a comprehensive reflection of communications system performance. Its role is to be transmitted over fiber-optic light signals into electrical signals, then amplified and balanced decision circuit, a transmitter to restore the original signal.

In the optical transmission line, whether the optical signals used to represent "1" code and "0" code in order to avoid even the long stream of "0" or even longer, "1", will help to extract the clock, the need for coding the circuit. At the same time sent a signal processor array parallel data, the need for rate and conversion circuits.

5, PCB board design

Fiber-optic transceiver performance partly depends on the PCB board layout and routing technologies, and should therefore be subject to the following some basic rules:

① Design PCB board, recommended the formation to reduce the inductance of the power of public ground. If possible, use both a ground and a power layer, which will also reduce ground and power pins on the inductance.

② in the ground and power planes on the segmentation and openings should at least, 1314 which will reduce the additional inductance and increase the stability of transmit and receive circuits.

③ In the LED drive circuit and the connection between the length should be as short as possible, to reduce lead inductance.

④ 10μF tantalum electrolytic capacitor and a 0.1μF monolithic ceramic capacitor should be located near the LED drive signals in place, which will reduce the transmitter noise radiation and improve the response time of the LED light.

⑤ 0.1μF (or 0.01μF) bypass capacitors to meet at the receiver between pin 2 and 7, the distance between it and the receiver can not exceed 20mm.

⑥ Connect the receiver fiber optic line is the most critical part of the circuit in excess of the additional inductance and capacitance will reduce the optical receiver bandwidth and stability, reduce the sensitivity of the receiver. Therefore recommend the use of surface mount devices, not to use the socket.

After the actual test, confirmed that this circuit is fully functional requirements met, by shortening the communication time increases overall system performance.
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