domingo, 10 de abril de 2011

Application of FPAA IC for Termostatting a DFB Laser Modules in Broadband Cable Communication Networks

1. INTRODUCTION


In present times Thermoelectrical elements of Peltier (TEP) are being used for cooling and hermostating of different fields in the science and techniques, where are set a strict requirements to the thermal regime of the elements and devices. Among them: microprocessor systems; HF, VHF, UHF and SHF amplifiers; CCD-detectors; optical receivers and emitting modules on the base of laser diodes [3, 5, 6, 10]. In the last group  of devices the TEP have found the widest-spreading, because it appears an opportunity for their integration in the emitting module and ensuring of highest accuracy at temperature stabilizing. This is very important for the high-speed systems transmitting data by wavelength division multiplexing (WDM), since the change of laser diode's temperature conduct an alternation in the length of the emitted wave. In systems of those type is important not only decreasing of the temperature of electric-optical system, but also for a precise control [1, 2, 7, 8]. The distance between frequency channels in lasers in multichannel module is in the order of several nanometers and the drift of the frequency during temperature changing, as a rule exceeds 0.1 nm/oC. For normal frequency dividing of channels is necessary to be ensured a stimulation of the laser with basic effective wavelength, which is impossible without precisely driving of the emitter's temperature.


2. STRUCTURE CIRCUITS AND CHARACTERISTICS OF THE SYSTEM FOR THERMO-ELECTRIC COOLERS (TEC) CONTROLLING


Simplified structure circuit for TEC controlling is shown on Fig.1. It contains: TEP, temperature detector, powerful linear or PWM-amplifier and controller, setting the function of regulation. For ensuring a thermostating TEP is used as a heater and as a cooler. That is achieved by an alternation in the polarity of the in it flowing current. As a rule the sensor (S) is assembled close to the most sensitive to temperature changes components. For S can be used thermocouples, platinum resistance temperature detectors (RTD), integral detectors and others. The standard thermoresistors in the examined application are rarely used, because they do not enough accuracy; have a big drift of resistance and nonlinear characteristic [4, 9].



Every circuit decision of the system for TEC controlling is realized on the one of the three methods: proportional (P), proportional-integral (PI) or proportional-integral- derivative (PID) controlling (Tab.1). From the signal received from the temperature sensor (S) by a feedback (FB) is brought out a setting signal uset, defining the system set point. The received signal of mistake ue is amplified and feed to the power amplifier (PA). He ensures the necessary value and polarity of the signal u(t) for controlling, defining the current through TEP.


Kp, Кi and Kd are coefficients of transferring respectively of the proportional, integral and the derivative channel. If the thermal inertness of TEC is bug, the speed of changing of the signal from the opposite connection will be relatively low and we can limit with using of a system for PI controlling.

Only a several producers offer completed solutions in the type of integral circuits for controlling of TEC. Among them are: Hytek Microsystems Inc. (HY-5640, HY5650 or HY56200), Analog Technologies Inc. (TEC-A1, TEC-4A, TEC6A101or ATEC24V10A1), MAXIM (MAX1637) and OKI (OLx109L-10), [12, 13, 14, 15]. Unfortunately they have some problems:


  • First one is that similar devices are oriented for solving of very tine circle of problems. Different characteristics of the used TEC are necessary in any case to be ensured unique electrical characteristics of the controller: accessible supply voltage; method for controlling the element of Peltier (continuous or PWM); admissible value of the pre-regulation; controller reaction rate; senescence of the temperature detectors; necessity of set point regulating; etc.

  • The second important problem, origin at using of ready controllers, is that most of them require a big number of outer discreet components, including a precise reference voltage unit and operational amplifiers.

  • Third is the problem of the controllers' prices. Because devices like those do not refer to the class of devices for mass usage, the expenses for developing and producing can not be compensated from the amount of sales. Besides more often TEC controllers are produced in a hybrid type, which increases its prime cost, too. Today the price of this controller for users is between 90 and 300 USA dollars [12, 13, 14]

  • Forth and most important is the problem related with the necessity of individual controllers adjustment. In every PID-controller is needed to be settled individual values for coefficients Kp. Ki, and Kd, to ensure not only the necessary speed of reaction, but also its stability in any conditions. In most of the traditional devices this is reached by a hand adjustment using potentiometers, which are put under a tear and senescence, decreasing the reliability and quality of the regulators.


3. TEC CONTROLLING SYSTEM ON THE FPAA BASE


A simplified electrical circuit diagram for TEC controlling is shown on Fig.2. By the FPAA IC1 is realized a regulator ensuring the necessary method for regulating. It is forming a reference voltage for supplying of the thermo sensor and setting an operation point, too. Giving data for the respective configuration is being realized by EEPROM using SPI – interface, such as AT25080A (Atmel). For controlling thermoelectrically element's current is used a powerful operational amplifier IC2. For FPAA synchronization is used a built in generator with outer quartz resonator with frequency 16 MHz [11]. The main requirement to the operational amplifier IC2 is ensuring of the necessary value of output current. As a rule for most of the examined applications could be used device from the OPA548 type (Texas Instruments).



Using of circuit with PWM as a power amplifier in the systems for thermostabilizating of the laser module is not expedient, because circuits like those appears to be source of powerful disturbances and exert a lot of influence on the emitter's stability of operating. Besides the TEP experiences an impulse overload which decreases its resource.


As a temperature detector is used thermoresistor. Despite its foibles, main of which appears to be the major nonlinearity, in the offered circuit its application is justified, for FPAA permits that linearizing its characteristics is performed. This well decreases the price of the device in comparison with using of other detectors. A temperature detector from the other type can be connected directly to FPAA in necessity though, changing its structure at a respective way.


Unique priority of the given circuit is that she stays the same (unchanged) at realizing of every regulating characteristic, when practically using of any types of temperature sensors and TEP. Each alternation is made by a program way.


For realizing and adjustment the company Anadigm offers software for automatic designing Anadigm Designer® 2. It contents special surroundings for synthesis of PIDcontrollers in any combination – P, PI, PD, PID. On Fig.3, Fig.4 and Fig.5 are shown the structures of FPAA for P, PI and PID modes.


Along with the automatically synthesized standard circuits of the controller, in the structure is added a Voltage Reference (VR) for feeding of thermo sensor and settling initial bias of the output power amplifier. The level of voltage reference is determined by amplifiers with programmable gains (from 0,01 to 100). Operating point is settled by a program with an input programmable VR. However, we can put a signal setting on the input lead of FPAA and to make a hand adjustment, for example using a potentiometer.



On the base of FPAA by using a dynamic reconfiguration can be created an adaptive circuit. In this case "instantly" can be changed not only the parameters of the controller, but also the regulation type. That could be very useful for adapting of the circuit at changing of the TEP operating direction. Since the operation efficiency of a thermoelectrical element in a regime of cooling is lower then that in a regime of heating, at realizing of thermostating appears possibility of alternation in the parameters of the controller in dependence of the temperature changing direction.


Operation of the system in a regime of dynamic reconfiguration is illustrated on Fig.6. Shown are the ambient temperature alternations Tamb and the temperature of the laser module Tmod. TEP is used as for a cooling, such as for heating, during which is made an adaptation of the controller using a dynamic reconfiguration. In the temporary interval from 0 to t1 is realized PI-method of regulating. In a moment t1 is made a reconfiguration of FPAA in a regime of P – controller. At next alternation of Tamb is observed a sharply reducing of the system, which appears to be absolutely expected result.



4. CONCLUSION

By using of Field Programmable Analog Arrays (FPAA) is possible to be removed every up mentioned problems origin at the development and exploitation of the examined circuits. The dynamic reconfiguration of FPAA permits a creating of adaptive systems ensuring high accuracy at a minimal loss.


Repeated increasing of the life cycle of the devices: changing of every component from the system (TEP, thermo sensor, DFB laser or a module as one) do not leads to an alternation in the controller's circuit. The process of adjustment is being simplified at the expense of the possibility to decline the application of the regulating potentiometers. The system can be easily adapted to new developments. It is possible entering of new functions, those like defense from overloading of the output amplifier, thermal defense

and others.


Of course the examined systems can be realized by digital signal processors (DSP), ADC and DAC. Is it needed to mention what extra complications and problems arise and how many times is being increasing the price of the development and the a finished device? The conclusion could be just one: analog control of analog processes – what could be more simple and logical?


Wilmer J Sanchez V-19358601

CAF - Parcial 3

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