Frequency response

About: Frequency response is a research topic. Over the lifetime, 25705 publications have been published within this topic receiving 332249 citations.

Detection of islanded behavior and anti-islanding protection of a generator in grid-connected mode

Abstract: A method of controlling a generator system connected to an electric power system in which the output frequency characteristic of the generator system is measured, a first phase angle and frequency of the measured frequency characteristic is estimated using a first phase locked loop having a first bandwidth, and a second phase angle and frequency of the measured frequency characteristic is estimated using a second phase locked loop having a second bandwidth greater than the first bandwidth. Further, the method calculates a frequency difference between the first and second estimated frequencies, and an angle variation that is proportional to the calculated frequency difference. The estimated second phase angle is then added to the calculated angle variation so as to form an output current phase angle reference, and an output current phase angle of the generator system is controlled to be aligned with the output current phase angle reference. The method also determines whether or not the generator system is within a generation island based on the measured frequency characteristic.

Method and apparatus for reducing DC offsets in a communication system

Abstract: Methods and apparatuses for reducing DC offsets in a communication system are described. In a first aspect, a feedback loop circuit reduces DC offset in a wireless local area network (WLAN) receiver channel. The frequency response of the feedback loop circuit can be variable. In a second aspect, a circuit provides gain control in a WLAN receiver channel. First and second automatic gain control (AGC) amplifiers are coupled in respective portions of the receiver channel. Circuits for monitoring DC offset, and for providing control signals for controlling the frequency response of the DC offset reducing circuits are also provided.

Frequency locking device

Abstract: An improved frequency locking circuit suitable for indirectly locking an optical frequency f 0 to a radio frequency f 1 or of locking the radio frequency to the optical frequency. A beam of optical frequency f 0 is modulated by a compound signal which is the sum of a signal at frequency f 2 and an FM subcarrier at frequency f 1 that is phase modulated at frequency f 3 to produce a phase modulated beam. The phase modulated beam is filtered by a filter having a transfer function having a characteristic frequency f f . A pair of control signals are generated that are proportional respectively to the amplitudes of two components of the filtered signal at frequencies f 2 , f 3 . These control signals are separately used in a pair of servo loops to separately establish fixed values of f 0 /f f and f 1 /f f . By using an optical cavity of the filter, the frequency locking circuit may be used for measuring the refractive index of a gas. The circuit may also be used for calibrating a multi-mode filter.

A technique for measuring frequency response of pressure, volume, and flow transducers

Abstract: A device and methodology is presented for testing the frequency response of pressure, volume, or flow transducers. Also reported are responses of selected transducers of all three types over the range of 2--120 Hz. Several pressure transducers tested had good frequency response when connected to the test system with a minimum of interconnecting fittings; others did not. Use of additional connectors degraded the response as did the addition of air-filled catheters. The frequency response of the pneumotachometers tested were influenced largely by the response characteristics of the associated pressure transducer and interconnecting fittings. These results emphasize the need to test the response characteristics of any transducer with specific connectors and fittings that are to be used to make the actual measurements of pressure, volume, or flow.

Theory of oscillator instability based upon structure functions

Abstract: Structure functions are apparently unfamiliar to most engineers and the unifying role they play in oscillator instability theory has largely gone unrecognized. This paper introduces and places into perspective the role which Kolmogorov structure functions have in theory. It is demonstrated that the rms fractional frequency deviation (phase accumulation) introduced by Cutler and Searle is related to the first phase structure function; the two-sample Allan variance is related to the second phase structure function. In addition, it is shown how the two-sample Allan variance is related to the rms fractional frequency deviation under suitable conditions. The L-sample Allan variance is also identified in terms of the first phase structure function; it is shown to be an asymptotically unbiased estimator of the rms fractional frequency deviation squared if the latter existL The utility of higher order structure functions of frequency and phase in the theory of instability is also demonstrated; in particular, how the frequency drift and "flicker"-type noise convergence problems can be overcome.