Showing papers on "Automatic frequency control published in 1987"

Frequency stabilization of semiconductor lasers by resonant optical feedback

Abstract: With simple optical geometries a separate resonant Fabry-Perot cavity can serve as an optical feedback element that forces a semiconductor laser automatically to lock its frequency optically to the cavity resonance. This method is used to stabilize laser frequencies and reduce linewidths by a factor of 1000 from 20 MHz to approximately 20 kHz.

Optimal Decentralized Load Frequency Control

Abstract: The paper presents a novel methodology for feasibility analysis and design of optimal decentralized load frequency control for multi-area interconnected electric power systems. Feasibility of the proposed decentralized control scheme is determined with a fixed mode evaluation algorithm based on eigenvalue dynamics. The eigenvalue sensitivity expressions are also used to determine decentralized feedback gains that will result in system transient performance similar to the one obtained with a centralized optimal control law. The methodology is illustrated with a two-area power system example.

A versatile power converter for high frequency link systems

Abstract: The single-phase high frequency link appears to be an attractive alternative to the dc link commonly employed in power conversion systems. This paper proposes a power converter suitable for one-step conversion of the single-phase high frequency link voltage to the three-phase low frequency voltages typically required for interfacing with system sources and loads. The converter is utilizes zero voltage switching principles to minimize switching losses and uses an easy to implement technique of pulse density modulation for the control of the amplitude, frequency and the waveshape of the synthesized low frequency signals. Adaptation of the proposed topology for power conversion to single-phase ac and dc voltage or currents outputs is shown to be straight forward. The feasibility of the proposed power circuit and the control technique have been experimentally verified.

Continuously-tunable single-frequency semiconductor lasers

Abstract: The design of "ideal" tunable laser sources that are capable of being continuously tuned over the entire gain bandwidth with maximum mode suppression are considered. It is shown that certain extended-cavity three-section configurations satisfy the necessary conditions for such electronic tunability, but that phase shifters with advanced capabilities are necessary. It is also shown that it is not possible to achieve this goal in a two-section configuration, although continuous tuning up to about one longitudinal mode spacing should be possible with some compromise in spurious mode suppression.

Coherent Lightwave Communications

Abstract: In this talk we will present fundamental theoretical considerations affecting the performance of coherent lightwave communication systems. These systems include a single-mode laser and a modulator at the source and a coherent receiver which is the optical onalog of a superheterodyne radio set. Instead of detecting photons directly, the coherent receiver first converts the incoming signal from the optical regime down to the radio regime, and then uses conventional electronic circuitry to perform various signal processing operations, such as amplification and demodulation. In principle, this technique can yield large increases (~ 20 dB) in receiver sensitivity compared with direct detection using avalanche photodiodes. One of the prime causes of this degradation has been identified as laser phase noise, a phenomenon that is known to be particularly serious in semiconductor devices.

Stabilizing the Frequency of Hysteretic Current-Mode DC/DC Converters

Abstract: A systematic investigation of methods to stabilize the operating frequency of hysteretic current-mode dc/dc converters through control of the current hysteresis is presented. The control laws for every power stage are derived, and two open-loop and two closed-loop circuits are shown and analized in detail. The interaction of the major voltage control loop and the frequency control circuitry is also investigated for buck converters. It is shown that, if the average inductor current is programmed, the two mechanisms are independent, while if the peak inductor current is programmed, they are not. The frequency control circuitry in the latter case decreases the phase margin of the voltage control loop and can lead to instability. Simple circuits are developed to implement the superior average inductor-current programming. As a consequence of being controlled via the current hysteresis, the operating frequency cannot be stabilized in the light mode, where VO/RL < IH/2. In the special case, where the frequency of the converter is stabilized by a phase-locked loop, a method and circuit are shown to solve this problem and achieve constant frequency operation at any load.

Dynamic brake control

Abstract: A dynamic brake control system for an alternating current motor powered by a variable voltage, variable frequency inverter which regulates the speed of the motor An AC to DC converter converts AC line power to DC power with the DC power being transferred to the inverter via a DC link During electrical dynamic braking, the inverter functions to transfer power from the motor to the DC link The torque available to the motor during braking is a function of the receptivity of the DC link Link receptivity controlled by a resistance connected to the link in which the resistance is modulated at a frequency dependent upon the incoming AC line frequency at the converter The modulation is binarily controlled so as to minimize ripple current on the DC link without the necessity of providing high frequency control of the modulating electronics

Principles of optical phase-locking: Application to internal mirror He-Ne lasers phase-locked via fast control of the discharge current

Abstract: While phase-locking techniques have long been in routine use in the microwave frequency domain, the ten-thousand-fold increase of the carrier frequency in the visible domain brings new problems for effective optical phase control. Particularly robust locks can be based on combining phase-locking under quiescent conditions with an outer frequency-control loop which takes control when phase lock is lost. Electronic strategies for implementing this process are discussed. Diagnostic techniques to allow characterization of phase-lock performance of optical oscillators are presented and discussed using as an example the phase-locking of a low-cost internal mirror He-Ne laser tube. Effectively complete phase lock was possible using small variations of the discharge current to provide fast frequency control.

Discrete variable structure controller for load frequency control of multiarea interconnected power systems

Abstract: A discrete version of a variable structure controller (VSC) for load frequency control of a two-area thermal and multiarea interconnected power system is presented in this paper. System nonlinearities, such as generation-rate constraint and governor dead band, have been included in the simulation studies. Comparative studies between conventional integral control and variable structure control, conducted on a multiarea power system show the effectiveness of VSC.

Magnetron with frequency control for power regulation

Abstract: A microwave heating system uses a magnetron powered by a full wave full bridge inverter. The magnetron has its power output stabilized against variations which might be caused by fluctuations in the AC input line. The inverter powers the magnetron by way of a power transformer having a primary and a secondary. The current is sensed in the primary in order to change the switching frequency of the inverter. As the current in the primary will depend upon fluctuations in the AC input line voltage, sensing of the current in the primary may be used to sense variations in the voltage of the AC input line. Alternately, current in the secondary of the power transformer may be sensed and used to detect variations in the AC input line voltage. In either case, the sensed variations in AC input line voltage are used to change the inverter frequency. Because the secondary includes a series resonant tuned circuit, changes in the difference between the inverter switching frequency and the resonant frequency of the tuned circuit may be used to counteract AC input line variations and stabilize the magnetron output power.

Liquid level detecting device

Abstract: A liquid level detecting device comprises a vibrator for vibrating a sample vessel containing a sample solution, and a piezo-electric element provided on a suction member having an opening for sucking the sample solution. Changes in the output voltage of the piezo-electric element are read out as the suction member is moved vertically, contact of the lower end of the suction member with the sample solution is detected based on the changes in the output voltage, and the liquid level of the sample solution is detected. Or, the liquid level detecting device comprises a vibration transmitting plate for holding a nozzle member to a suction member, a vibrator for vibrating the nozzle member at a predetermined frequency, a frequency control circuit for changing the frequency of vibration, and a detector for detecting extreme values of the output voltage of the vibrator corresponding to changes in the frequency. The liquid level is detected based on changes in the extreme values.

Compensation circuitry for dual port phase-locked loops

Abstract: Phase port gain compensating circuitry is coupled to the phase modulation summing circuit and voltage controlled oscillator (VCO) gain compensating circuitry is coupled to the frequency control terminal of the VCO. Compensator control circuitry utilizes divider ratio control information to control the characteristics of the two compensators to compensate for otherwise undesirable effects on the phase-locked loop response parameters caused by changes in the divider ratio and in the VCO gain.

Microwave antenna and dielectric property change frequency compensation system in electrohydraulic servo with piston position control

Abstract: An electrohydraulic servo system which includes an actuator having a cylinder and a piston variably positioned therewithin, a servo valve responsive to valve control signals for coupling the actuator to a source of hydraulic fluid, and control electronics responsive to piston position for generating the valve control signals. A variable frequency rf generator is coupled through associated directional couplers to a pair of antennas which are positioned within the actuator cylinder and physically spaced from each other in the direction of piston motion by an odd multiple of quarter-wavelengths at a nominal generator output frequency. A phase detector receives the reflected signal outputs from the directional couplers, and provides an output through an integrator to the frequency control output of the generator to automatically compensate frequency of the rf energy radiated into the cylinder and thereby maintain electrical quarter-wavelength spacing between the antennas against variations in dielectric properties of the hydraulic fluid due to changes in fluid temperature, etc. A second phase detector is coupled to the generator and one antenna to generate a piston position signal.

The optical control of IMPATT oscillators

Abstract: This paper describes methods of controlling the frequency of IMPATT oscillators using optical rather than electrical signals. Analytic theories of optical tuning and injection locking are presented. Results from a comprehensive large signal computer model of the optically controlled IMPATT oscillator are given, illustrating the importance of the composition of the optically generated current on the optical control performance and demonstrating the capability of optical control for rapid frequency tuning. Finally, recent experimental work on optical tuning effects in W-Band IMPATT oscillators is presented.

Design theory of electrically frequency-controlled narrow-linewidth semiconductor lasers for coherent optical communication systems

Abstract: We describe a design rule for the automatic frequency control of semiconductor lasers by electrical feedback for coherent optical communications. The linewidth achieved by the feedback control is not a good measure for evaluating the system performance, but the bandwidth of the feedback loop is the most important parameter. For example, a 500-MHz loop bandwidth is required for a heterodyne DPSK system when the free-running linewidth of the laser is 10 MHz.

Channel selection apparatus having automatic frequency compensation for transmission frequency error

Abstract: A channel selection apparatus having a double superhet tuner section (15) for converting one of a plurality of television channels contained in an input signal to a fixedly predetermined channel, includes an AFC circuit (11) for producing a frequency control voltage to execute closed-loop frequency control of the second local oscillator (5) of the tuner section while an AFC switch (16) is set in a first condition, whereas when the AFC switch is set in a second condition, a fixed frequency control voltage is applied to the second local oscillator and the frequency control voltage is applied to adjust the oscillation frequency of the first local oscillator (2) of the tuner section such as to compensate for any offset in the transmission frequency of a currently selected channel.

Cyclical, multiple frequency high-frequency induction heating apparatus

Abstract: A high-frequency induction heating apparatus for heating a material includes a voltage source, a high-frequency inverter coupled to the voltage source for feeding an alternating current, and a coil for applying the current to the material to induce eddy currents therein and heat the material. A frequency control device controls the inverter to cyclically feed a first current having a first frequency and a second current having a second frequency higher than the first current to substantially simultaneously heat an inner portion and an outer portion of the material, respectively. The control device also may cyclically switch the voltage level of the output of the inverter between a first voltage level and zero volts to change the power output thereof. The control device apparatus has a switching cycle time that is shorter than the thermal time constant of the material.

An implementation of current-mode control for a series-resonant DC-DC converter

Abstract: A new implementation of the (average) current mode control for a series resonant converter operated in the frequency range higher than the circuit resonant frequency will be introduced. The controller design is based on the ASDTIC (Analog-Signal-to-Discrete-Time- Interval Converter) concept but has some improvements over the original design which often suffered from static and dynamic instabilities. Some experimental results of a breadboarded converter with this new controller will be presented and the near-optimum fast response characteristics of the combination of this controller and the load current feedforward will be included to show the similarity between the current-mode controller for switching-mode converters and series-resonant converters.

Radar device for measuring the distance of the device to a surface

Abstract: This radar device for measuring the distance h of the device to a surface (2) includes transmission means (5) for transmitting to the surface a frequency-modulated wave E(t) provided by an oscillator (14) having a frequency control, receiving means (7) for receiving this wave R(t) reflected from the surface, a mixer circuit (10) for producing a beat wave between the transmitted and received waves and a processing circuit (20) including a beat-wave-digitizing circuit (25) and a time-frequency transform operator (27) for supplying frequency components from which the distance h is determined. This processing circuit (20) further includes an auto-correlation operator for effecting an auto-correlation of the beat wave digitized by the circuit (25) and also spectrum processing means (32) for finally determining the distance h.

Coordinated Frequency and Voltage Control of Synchronous Generators

Abstract: Three different types of controllers, i.e., the conventional controller, the linear optimal controller (LOC), and the variablestructure controller (VSC) are proposed for the coordinatedfrequency and voltage control of synchronous generators. Theinteraction between the frequency and voltage control loops is takeninto account by incorporating both the speed-governing system andthe excitation system into the mathematical model of thesynchronous generator. Results obtained from computer simulationindicate that the variable structure controller can yield the bestdynamic responses following a step load change.

All digital phase‐locked loop with a wide locking range

Abstract: The phase-locked loop (PLL) is used widely in communication engineering as one of the key functions Recently, some attempts have been made to construct a digital circuit for the phase-locked loop However, the common problems in those attempts is that there is a trade-off between the locking range and the output phase jitter To solve this problem, this paper proposes a new structure for the phase-locked loop with a wide locking range by combining the frequency control technique For this purpose, a new digital VCO is constructed, which can vary the central frequency of the system using the programmable divider and the adder Theoretical analyses are made for the transient behavior from the viewpoints of the locking range, frequency and phase, and the noise characteristics of the loop The result is compared with the results of experiment and simulation The theoretical value, experimental value and the result of simulation agreed well, indicating that the phase-locked loop has a wide locking range

Noise detection by sampling digital baseband signal at eye openings

Abstract: A noise detection circuit for a digital radio receiver comprises a sampling circuit for sampling a digital baseband signal having a noise component when an eye opening of the baseband signal is largest. A waveform generator is connected to the output of the sampling circuit for generating a waveform which is a replica of the baseband signal and free from the noise component. The waveform generator comprises a threshold comparator having decision thresholds and a waveform recovery circuit which includes a resistor, a plurality of storage capacitors of equal values and a switch responsive to the output of the threshold comparator for selectively coupling the output of the sampling circuit through the resistor to the storage capacitors. A substractor is connected to the outputs of the sampling circuit and the waveform generator for generating a signal representative of the noise component of the baseband signal. The output of the subtractor circuit can be used for various purposes including noise measurement, squelch action and automatic local oscillator frequency control.

Variable frequency oscillator

Abstract: PURPOSE:To suppress the ununiformity between components by providing two frequency control terminals so as to form an automatic frequency control circuit and a PLL circuit without a voltage adder and adopting monolithic circuit integration for components in use. CONSTITUTION:The titled oscillator uses the circuit provided with two frequency control terminals 124, 125, voltage dependant varactor components 113 - 116 are isolated in terms of DC from power voltages 101, 102 to form the circuit suitable for monolithic circuit integration. Thus, the dispersion among components of the oscillator is suppressed and the effect of parasitic capacitance of wires of the substrate in use onto an inductor mounted on the substrate is also suppressed. Moreover, the components 113 - 116 are isolated from the power voltage to suppress the effect of the power voltage fluctuation only the oscillating frequency is suppressed. Furthermore, the two frequency control terminals 124, 125 are provided to attain the application of the circuit to a PLL circuit and an automatic frequency control circuit without any voltage adder.

Method and appartus for laser control

Abstract: The disclosure is directed to a method and apparatus for precisely controlling the frequency of a laser, and for precise control of distance. In a disclosed embodiment, a laser beam is split, modulated to obtain a frequency differential, and coupled to an interferometer which is designed to produce two processed beams having intensities that vary at the same ac frequency, but at a phase difference that is indicative of the laser frequency. The processed beams are converted to ac electrical signals which are coupled to phase control circuitry. The phase control circuitry determines the phase between the electrical signals and produces a control signal for controlling the laser frequency. Phase offset increments are introduced in the phase detection process, these phase offset increments resulting in controlled changes in the laser frequency. The laser frequency can be computed and read out as a function of the phase offset increments. In another embodiment, the control signal is used to accurately control a distance.

Implementation of optimal trajectory control of series resonant converter

Abstract: Due to the presence of a high-frequency LC tank circuit, dynamics of a resonant converter are unpredictable, often with large surge of tank energy during transients. Employing state-plane analysis technique, a novel “optimal trajectory control” utilizing the desired solution trajectory as the control law has been previously proposed for the series resonant converters. The method predicts the fastest response possible with minimum energy surge in the resonant tank. The principle of the control and its experimental implementation are described. The dynamics of the converter are shown to be close to time-optimal.

An oscillator frequency control arrangement for a stereo decoder

Abstract: A charge pump arrangement receives a digital error word representing an error signal and charges or discharges a capacitor in accordance with the sign of the error word by an amount that is proportional to the absolute value of the error word. The voltage produced in the capacitor is coupled to a frequency control terminal of a voltage controlled crystal oscillator that is used in, for example, a stereo decoder.

Resonant inverter having frequency control

Abstract: In a power-line-operated high-frequency electronic fluorescent lamp ballast, an inverter is powered from a DC supply voltage having a substantial amount of 120 Hz ripple. The fluorescent lamp is connected with the inverter's squarewave output voltage by way of a series-resonant L-C circuit. The amount of power supplied by the inverter to the series-resonant L-C circuit and/or to the fluorescent lamp at any given moment depends on three significant factors: (i) the instantaneous magnitude of the DC supply voltage, (ii) the instantaneous frequency of the inverter's squarewave output voltage, and (iii) the instantaneous operational characteristics of the fluorescent lamp. Arrangements are provided whereby the instantaneous inverter frequency is automatically adjusted so as to prevent inverter overload in case the fluorescent lamp is non-present or inoperative.

24 h frequency stabilisation of an SLM external-cavity semiconductor laser

Abstract: Long-term single-frequency stabilisation of an external-cavity semiconductor laser has been demonstrated using a multisegment composite-cavity configuration and an automatic frequency control loop with feedback to the external cavity. Mode-hopping free single-longitudinal-mode oscillation with a linewidth of about 200 kHz and a frequency shift within 28 MHz has been observed over 24 h.