Application Research of ECL Power Switch

In the fiber optic communication system, information is carried by the light waves emitted by LED or LD, and the light waves are the carrier.

The process of loading information onto light waves is called modulation. Optical modulation can be divided into analog signal modulation and digital signal modulation according to the form of the modulation signal. At present, digital modulation is the main modulation method for optical fiber communication, which is the usual PCM coding modulation. The optical carrier is modulated on and off with a binary digital signal "1" or "0", and pulse coding (PCM) is performed. The advantage of digital modulation is that it has strong anti-interference ability, and the influence of noise and dispersion does not accumulate during interruption, so large-capacity and long-distance transmission can be achieved.

1 Optical transmitter

Simply put, a basic optical transmitter in an optical transmission system mainly includes an optical emitting device and its driving circuit. The optical emitting device includes a light emitting diode (LED), a laser diode (LD) or a laser modulator (LM); the driving circuit provides the appropriate "on" and "off" current for the system light source.

1.1 Basic Structure of Digital Optical Transmitter

In digital optical fiber communication, the main components of the laser transmitter are shown in Figure 1. The role of line coding is to convert digital signals into code types suitable for transmission in optical fibers. The modulation circuit completes the electrical-optical conversion of digital signals and loads the optical signal onto the emission beam of the light source, which is optical modulation. There are three types of optical modulation: direct intensity modulation, indirect intensity modulation, and coherent modulation. Direct intensity modulation (applicable to semiconductor lasers and light-emitting diodes) is often used in optical fiber communication, that is, by directly controlling the injection current of the light-emitting diode (LED) or laser diode (LD) to generate the required optical digital signal, changing the injection current of the LD or LED to adjust its output optical power, and realizing optical intensity modulation.

Theoretically, LED and LD are both current-controlled light-emitting devices, and the most important performance depends on their IP characteristics. Therefore, the most direct design method is to design the driver as a current source controlled by the input signal, and it must provide a current with a specified intensity and waveform. In practical applications, bipolar transistors or field-effect transistors (FETs) are connected to light-emitting devices as current output devices to form current drivers. Commonly used are single-ended current drivers and emitter-coupled current drivers. The speed of single-ended current drivers is affected by the cut-off process of transistors and LEDs or LDs, so they can only be used in low-bit-rate situations. High-bit-rate current drivers are designed using ECL (emitter-coupled logic) circuits, which are emitter-coupled current switches commonly used in digital modulation circuits. Their basic circuit form is shown in Figure 2.

1.2 Basic working principle of digital modulation circuit

The emitter-coupled current switch shown in Figure 2 is actually an emitter-coupled differential stage with a fixed input VBB on one side and a signal input on the other side. Its working principle is very similar to that of a single-input dual-end output differential amplifier, but it only transmits signals. Its working principle is: when Vin>VBB, Q1 tube is turned on, Q2 tube is cut off, and all current flows through the input tube; when Vin

2 Application of ECL current switch

At present, the author has applied ECL current driver in a high-speed digital system. Its basic principle is shown in Figure 3. Among them, Q1, Q2, and Q5 constitute the basic modulation circuit, Q3 and Q4 realize level shifting, and the integrated device MC10H124 completes the signal level conversion (TTL→ECL). The pin function of MC10H124 is shown in Figure 3. When using, it should be noted that the ECL level output pin (such as pin ② and ④ in the figure) must be connected to an external -2V voltage through a 50Ω resistor, and the unused output pin must be grounded through a 50Ω resistor. Output the ECL level signal from pin ② or ④, and the ⑥th pin is connected to the common selection voltage (here connected to +4V voltage).

2.1 Main features of ECL circuits

When high-speed pulse modulation is performed on the laser, ECL current switch is often used. It has a fast switching speed and can protect the current pulse waveform well. From the circuit structure point of view, ECL belongs to non-saturated digital logic. When working, the crystal effectively switches between the two upper states of amplification and cutoff, and does not enter the saturation region. It eliminates the "storage time" required to release the excess stored charge when the transistor switches from saturation to cutoff (that is, from "on" to "off") in the TTL circuit, and fundamentally eliminates the main obstacle to speed limitation - the saturation time of the transistor, which greatly improves the speed of the ECL circuit, and its average extension time reaches the nanosecond level. If a two-stage differential current switch is used in Figure 3 and driven on both sides, it can improve the waveform of the current pulse and increase the switching speed.

As an important part of the ECL circuit, the reference voltage VBB in Figure 3 is usually taken at the center of the ECL logic high and low levels, VBB=-1.3V (ECL), the logic high level VOH=-0.8V, and the low level VOL=-1.8V, so that the noise margins of the high and low levels are basically equal, and the noise margin of the circuit will not change too much within the full operating temperature range. VBB often shares the negative power supply with the ECL circuit, and is constructed by using diodes and emitter followers for level shifting based on the resistor divider.

2.2 System test data and analysis of its anti-interference capability

In the circuit shown in Figure 3, it is found through experiments that the values ​​of R1 and R2 in the circuit have an important influence on the anti-interference ability of the circuit. Within a certain range, if R2 remains unchanged, increasing R1 will increase the dynamic range of the signal at the base input of Q3, that is, the hysteresis voltage of the ECL current switch (similar to the Schmitt trigger) increases, ensuring that the current switch can work normally when VBB is within this range, thereby reducing the noise-induced slight fluctuations in the ECL signal input to the base of Q3 and causing the current switch to malfunction. The working principle of the switch is as described in 1.2 of this article to improve the anti-interference ability. Experiments have shown that if R1≈10R2 is taken, the ECL level signal input to the base of Q3 can be within an appropriate dynamic range, the ECL current switch has a more appropriate hysteresis voltage, and the reference voltage VBB of the base of Q4 is within this range, then the input signal of the base of Q3 can normally control the drive current of the laser LD.

If the chip MC10H124 is removed, the theoretical analysis shows that Q3 is in the cut-off state; but when the level converter (MC10H124) is connected, the base voltage of Q3 increases and works in the amplification area due to the external -2V voltage at the output pin. When the data signal is added to the signal input terminal ⑤ of MC10H124, the base signals of Q3 and Q1 are measured as follows: Vh3: -0.85V～-1.60V; Vb1: -2.20V～-3.00V, and the reference voltage of Q4 base VBB=-1.3V, which is within the dynamic range of Vb3, and the base voltage of Q2 is measured to be about -2.60V, which is also within the dynamic range of Vb1, so the ECL current switch can work normally.

From the above analysis, it can be seen that maintaining a stable VBB is a very important factor affecting the performance of the ECL circuit. It determines the threshold voltage, output logic level and anti-interference ability of the current switch. If VBB changes for some reason, the output logic may be confused and the anti-interference ability of the ECL circuit may be reduced. Therefore, as long as the ECL current switch has an appropriate hysteresis voltage and a stable switch threshold voltage VBB, the anti-interference ability of the system can be improved. Especially when the circuit works at ultra-high speed, these problems are particularly prominent.