Analysis of the design principle of fiber optic LED drive circuit

In order to reduce the proportion of data transmission time in the entire system processing time index, the data transmission rate should be no less than E1 (2.048Mb/s), and the communication link should be safe and reliable. After analyzing various data communication technologies, we finally chose the fiber optic link and achieved satisfactory results.

1. Copper medium data link analysis

Since both high data transmission rate and long communication distance are required, it is difficult to meet the index requirements without coding and modulation in copper medium links. However, high-speed modems are not only expensive, but also the handshake process between the two parties is time-consuming. They are obviously not applicable in scenarios requiring random real-time data transmission.

Most importantly, traditional copper media links not only generate electromagnetic interference signals, but are also easily affected by electromagnetic interference, and are not easy to meet the requirements of EMC (electromagnetic compatibility) and EMI (electromagnetic interference) standards.

2. Analysis of optical fiber links

In people's traditional impression, it is not economical to use optical fiber for short-distance communication, but it has incomparable advantages over copper medium links: ① Optical fiber links neither emit electromagnetic waves nor are they affected by them, there is no interference between optical fibers, and the bit error rate is greatly reduced. Designers do not need to consider the environmental noise that may be coupled in; ② Optical fiber provides electrical isolation between the two sides of the communication link, eliminating the problems caused by different ground potentials between long-distance devices. At the same time, designers no longer have to worry about impedance matching; ③ Optical fiber transmitters can use digital modulation drive circuits. Digital baseband signals can directly drive optical fiber transmitters as long as they are simply line-coded.

Nowadays, the price of optical fiber components has dropped significantly compared with the past. In the future, as the price of steel cables rises and the price of optical fiber components further drops, people will no longer consider the price factor first when choosing optical fiber. Therefore, we comprehensively consider the speed and distance indicators, select the appropriate type of optical fiber, transceiver and its driving circuit, and obtain a high-performance and cost-effective optical fiber communication link, which is the focus of this article.

3. Selection of fiber optic communication devices

A basic fiber-optic communication system is very simple: an LED transmitter converts an electrical signal into an optical signal and couples it into a transmission optical fiber. The optical signal travels through the optical fiber to the optical receiver, which restores the received optical signal to the original electrical signal for output.

Optical cable selection: Generally, silica glass optical fiber is used for long-distance communication links due to its low loss and high bandwidth. For example, Ethernet and FDDI standards specify the use of multimode 62.5/125μm silica glass optical fiber. These thin-core optical fibers require high-precision connectors to reduce coupling losses. For industrial applications, low-cost optical cables and connectors are required. Therefore, 1mm POF (Plymer Optical Fibers) and 200μm HCS (Hard Clad Silica) optical fibers are the best choices, both of which are step-index multimode optical fibers.

The typical loss value of 1mm POF is 0.2dB/m at 650nm wavelength, while the typical loss value of 200μm HCS fiber at 650nm wavelength is only 8dB/km, and even less at 820nm wavelength. The core of HCS fiber is quartz glass, and the cladding is a patented high-strength polymer, which not only increases the strength of the fiber, but also prevents moisture and pollution. The outer sheath is 2.2mm polyvinyl chloride. HCS fiber can operate in a temperature range of -40℃ to +85℃, and the installation temperature range is -20℃ to +85℃, meeting system requirements in terms of performance and price.

Selection of operating wavelength: The design of fiber-optic communication systems must take into account the impact of fiber loss and dispersion on the system. Since both loss and dispersion are related to the operating wavelength of the system, the selection of operating wavelength becomes a major issue in system design.

Taking into account the system requirements and the selected optical fiber, the 820nm wavelength can reduce the HCS optical fiber loss to 6dB/km, while also minimizing the dispersion.

Choice of light source: At a wavelength of 820m, LED is the best light source available. Compared with semiconductor lasers, LED driving circuits are simple and low in cost.

In summary, the optical cable uses 200μm HCS optical fiber, and the optical transceiver uses HP's 820nm wavelength HFBR-0400 series. The HFBR-14X2/HFBR-24X2 in this series can reach a rate of 5MBd within a distance of 1500m, and the operating temperature range is -40℃～+85℃. There are various port models of ST, SMA, SC and FC to choose from. HFBR-14XZ uses 820nm wavelength AlGaAs LED, and HFBR-24XZ integrates an IC chip including PIN photodetector, DC amplifier and collector open output Schottky transistor, and its output can be directly connected to popular TTL and CMOS integrated circuits.

4. A practical fiber optic LED driver circuit

The function of the driving circuit in the fiber-optic duplex communication system is to convert electrical power into optical power and modulate the electrical signal to be transmitted to the output of the light source. It provides the light source with both bias current and modulation current that varies with the digital signal.

When designing an LED drive circuit, the peak current of the LED must be considered first. If the peak drive current is exceeded, the light signal will overshoot. The undershoot of the electrical signal caused by this in the receiver, coupled with the influence of the amplifier noise, may cross the detection threshold of the comparator and cause bit errors. At the same time, LEDs are more difficult to turn off than to light up, which will produce tailing, which is particularly obvious when using a serial drive circuit. The parallel drive method can provide a low-impedance channel for the carriers in the LED, thereby reducing pulse width distortion and slow tailing.

The resistor RSI in the driving circuit is used to adjust the output power of the optical fiber. Note that it should not exceed the peak driving current of the LED, otherwise the optical signal will overshoot. In addition, the SN75451 in the circuit has the characteristics of low impedance and high current rate, which can avoid long tailing.

Pulse Width Distortion (PWD) is a major factor limiting the rate of a fiber optic link. It is caused by the unequal transmission delay between the input and output. Note that PWD is always positive, so we can use an RC circuit to delay the lighting of the LED.

The optical fiber receiving circuit is an important part of the optical fiber communication system. Its performance is a comprehensive reflection of the performance of the entire optical fiber communication system. Its function is to convert the optical signal transmitted by the optical fiber into an electrical signal, and then restore the original signal of the transmitting end through amplification, equalization, and judgment circuits.

In optical fiber transmission lines, the presence or absence of optical signals is often used to represent "1" and "0" codes. In order to avoid long "0" or "1" in the code stream and facilitate clock extraction, a coding circuit is required. At the same time, the array signal processor sends parallel data, which requires a rate conversion circuit.

5. PCB board design technology

The performance of the fiber optic transceiver is partly determined by the layout and wiring technology of the PCB board, so the following basic rules should be followed:

① When designing a PCB board, it is recommended to use a ground layer to reduce the inductance of the common ground line of the power supply. If possible, use a ground layer and a power layer at the same time, which will reduce the inductance on both the ground and power pins.

② The divisions and openings on the ground and power layers should be minimized, which will reduce the additional inductance and improve the stability of the transmitting and receiving circuits.

③The connection length between the driving circuit and the LED should be shortened as much as possible to reduce the wire inductance.

④10μF tantalum electrolytic capacitors and 0.1μF monolithic ceramic capacitors should be placed close to the driving LED signal, which will reduce the noise radiated by the emitter and improve the light response time of the LED.

⑤ A 0.1μF (or 0.01μF) bypass capacitor should be placed between pins 2 and 7 of the receiver. Its distance from the receiver should not exceed 20mm.

⑥ The receiver is the most critical part of the optical fiber connection line. Excessive additional inductance and capacitance in the circuit will reduce the bandwidth and stability of the optical fiber receiver and reduce the sensitivity of the receiver. Therefore, it is recommended to use surface mount devices and not use sockets.

After actual testing, it is confirmed that this circuit fully meets the practical requirements and improves the performance indicators of the entire system by shortening the communication time.