Frequency drift

About: Frequency drift is a research topic. Over the lifetime, 5054 publications have been published within this topic receiving 56191 citations. The topic is also known as: chirp rate.

Arrangement for interference suppression in phase locked loop synchronized oscillators

Abstract: In a phase locked loop synchronized oscillator wherein a phase detector compares an oscillator frequency and a synchronizing frequency to derive a voltage for controlling the oscillator frequency there is provided a memory circuit between the phase detector and the oscillator for holding the instantaneous value of the detector output and a blocking circuit for sensing for interferences in the control voltage to prevent the memory from changing the value hold before the interference occurred.

Latitudinal shear instabilities during type I X-ray bursts

Abstract: Coherent oscillations have been observed during thermonuclear (type I) X-ray bursts from 14 accreting neutron stars in low-mass X-ray binaries, providing important information about their spin frequencies. However, the origin of the brightness asymmetry on the neutron star surface producing these oscillations is still not understood. We study the stability of a zonal shearing flow on the neutron star surface using a shallow-water model. We show that differential rotation of 2% between the pole and equator, with the equator spinning faster than the poles, is unstable to hydrodynamic shear instabilities. The unstable eigenmodes have a low azimuthal wavenumber m, wave speeds 1% or 2% below the equatorial spin rate, and e-folding times 1 s. Instability is related to low-frequency, buoyantly driven r-modes that have a mode frequency within the range of rotation frequencies in the differentially rotating shell. In addition, a modified Rayleigh's criterion based on potential vorticity rather than vorticity must be satisfied. The eigenfunctions of the unstable modes have the largest amplitudes nearer to the rotational pole than the equator. We discuss the implications for burst oscillations. Growth of shear instabilities may explain the brightness asymmetry in the tails of X-ray bursts. However, some fine-tuning of the level of differential rotation and a spin frequency near 300 Hz are required in order for the fastest growing mode to have m = 1. If shear instabilities are to operate during a burst, temperature contrasts of 30% across the star must be created during ignition and spreading of the flash. The frequency drift may be related to the nonlinear development of the instability as well as the cooling of the burning layer. The properties and observability of the unstable modes vary strongly with neutron star rotation frequency.

Direct phase digitizing apparatus and method

Abstract: A radio receiver directly digitizes the phase of an intermediate frequency (IF) signal with a desired resolution. The frequency of the reference oscillator in the direct phase digitizer is reduced when compared to the frequency previously required for the same resolution. The reduction in the reference oscillator frequency is accomplished by differentiating between IF zero-crossings that occur during the first half of a reference oscillator cycle and zero-crossings which occur during the second half of the reference oscillator cycle. The apparatus utilizes 2 zero-crossing detectors, the first zero-crossing detector is driven by a positive edge of the reference oscillator signal and the second zero-crossing detector is driven by a negative edge of the reference oscillator signal. Depending upon the alignment of the negative edge zero-crossing indicator and the positive edge zero-crossing indicator, the N-bit phase signal is modified or shifted by one-half a phase sector.

Carrier wave synchronization for multi-level two-dimensional modulation alphabets

Abstract: In a method for realizing carrier wave synchronization in the receptio of a multi-level, two-dimensional modulation signal, the received signal is demodulated by using at least one local oscillator (3) in order to generate the local oscillator frequency In the method, the local oscillator (3) performs a frequency scanning within the frequency range of the reception; during the frequency scanning, a demodulated signal point system is formed of the received signal by using the frequency generated by the local oscillator (3) Furthermore, the demodulated signal point system in the method is examined in order to determine at which point of time the frequency of the local oscillator is synchronized to the carrier wave frequency of the received modulation signal

Frequency synthesizer with rapidly changeable frequency

Abstract: A frequency synthesizer capable of rapidly changing to a different desired output frequency includes a voltage-controlled oscillator and a variable frequency divider arranged in a phase-locked loop. A first frequency-control voltage is initially applied to the voltage-controlled oscillator to cause a rapid change in its output frequency toward a desired value. Then, the divisor of the variable frequency divider is changed to produce a second frequency-control voltage which precisely sets and maintains the oscillator output frequency at the desired value.