Showing papers on "Frequency drift published in 1989"

Off-nominal frequency measurements in electric power systems

Abstract: Some power system protection applications require accurate estimates of voltage magnitude and frequency over a wide measurement range. Most digital techniques for measuring frequency have acceptable accuracy over a small range in the neighborhood of the nominal value. This paper presents a method for extending the measurement range of the Least Error Squares (LES) technique. The paper also examines the effectiveness of the technique and presents some results from simulated studies and laboratory experiments. It is essential to maintain the frequency of a power system close to its nominal value. Usually, frequency deviations in the range of two to three percent only are allowed for short durations of time. In this range, the Least Error Squares algorithm (LES) provides accurate estimates and is, therefore, suitable for implementing in most frequency relays. However, the measurement errors increase as the frequency deviation from the nominal value increases. During start-up and shut-down of a generator, its frequency varies over a wide range. If the generator is connected to a transformer, both the generator and the transformer can experience overexcitation. Excessive volts-perhertz (V/Hz) indicates that overexcitation has occurred. One method of estimating this parameter is to measure the voltage magnitude and frequency and then calculate volts-per-hertz. Voltsper-hertz relays that provide accurate measurements over 10 to 70 Hz range would be desirable for use on steam turbine driven generators. The frequency of the voltage of a thermal generator is low during start-up and shut-down procedures.

Dynamic measuring of frequency and frequency oscillations in multiphase power systems

Abstract: Instantaneous system frequency is defined, and a method of frequency measurement is proposed which determines the angular velocity of the rotating voltage space phasor. Impulses in frequency caused by steps in phase are blanked out, whereas harmonics and noise are suppressed by FIR filters. Using an optimized observer-based filter, frequency modulations are split into and frequency drift and frequency oscillation signals, which can be used in power control and power system stabilization. >

Method and apparatus for controlling the frequency of operation and at least one further variable operating parameter of a radio communications device

Abstract: The operating frequency of a radio transceiver and at least one further variable operating parameter of the radio transceiver are controlled in accordance with pre-stored information. Transmit operating radio frequency information is stored in a memory together with associated optimum modulation deviation level information (e.g., so as to maintain a predetermined modulation level at each operating frequency). Receive operating radio frequency information is similarly stored in the memory along with information indicating whether microprocessor clock oscillator frequency is to be shifted (e.g., so as to avoid spurious interfering harmonics emanating from the clock). When operating in the transmit mode, pre-stored modulation gain level information associated with the selected transmit frequency controls the gain of the modulator circuits to compensate for expected changes in modulator circuit gain with change in transmit radio frequency. In the receive mode, the microprocessor clock oscillator frequency may be shifted in accordance with the selected receive frequency, thereby automatically shifting harmonics of the microprocessor clock oscillator out of the receiver bandpass when necessary to avoid locally-generated signals from interfering with signal reception.

A microcomputer-compensated crystal oscillator using a dual-mode resonator

Abstract: The system described uses microcomputer-controlled compensation wherein the correction is implemented by adding a computed offset frequency to the output of a stable oscillator. Temperature sensing is done by the use of a dual-mode oscillator system using two C-modes of an SC-cut crystal, which are combined to produce a temperature-sensitive difference frequency. System accuracies of +or-20 p.p.b. have been achieved over a temperature range of -55 degrees C to +85 degrees C. >

Automatic frequency control

Abstract: An automatic frequency control tracks a television signal before and after a period of interruption. The automatic frequency control comprises a voltage controlled feedback loop including a voltage controlled local oscillator and a mixer for mixing the output of the oscillator with the TV signal. The output of the mixer is filtered to remove unwanted components of the mixed signal and to equalize sidebands of the television signal. The output of the filter is supplied to a detector which is tuned to a component of the television signal, and produces a voltage corresponding to the frequency of the component. The output of the detector is supplied to the voltage control input of the local oscillator to track the television signal component. Circuitry automatically senses the absence of the TV signal component from the mixer, and automatically applies a reference voltage to the voltage control input of the oscillator when the TV signal component is absent. The detector is operative to detect the television signal component following the application of reference voltage to the oscillator so that when the television signal returns, the detector is capable of tracking it.

Optical phase control of an optically injection-locked FET microwave oscillator

Abstract: A simple technique is proposed and demonstrated for controlling the phase of an optically-injection-locked 7.2-GHz FET oscillator. The relative phase phi between the oscillator and the locking signal is adjusted by optically tuning the oscillator frequency. Locking characteristics described include locking bandwidth (2.6 MHz), phase tuning range (187 degrees ), phase modulation ( beta =0.69 at 500 kHz), and optical tuning (125 MHz). >

Radio frequency linear accelerator control system

Abstract: A control system for controlling a radio frequency resonant cavity type linear accelerator so as to be power-supplied always at a resonance frequency of a resonat cavity constituting the accelerator. The system consists essentially of a signal pick-up coil inserted in the resonat cavity, a voltage-controlled oscillator assembly, a phase detector for detecting a phase difference between a signal picked up from the cavity by the signal pick-up coil and an output from the voltage-controlled oscillator assembly. An output from the phase detector controls the voltage-controlled oscillator assembly so as to make it oscillate at a frequency equal to a resonance frequency of the resonant cavity.

The devil's staircase: the electrical engineer's fractal

Abstract: A description is given of an experimental technique for rapidly and accurately measuring the devil's staircase, a structure of frequency lockings in a periodically forced nonlinear oscillator. This technique uses a personal computer, a programmable oscillator, and a frequency counter. The authors analyze the method, discuss its limitations, and present experimental results for two simple test circuits. >

High voltage power supply control system

Abstract: The electrostatic potential supply comprises an operating potential source (+Vcc), a transformer (122) having primary (156) and secondary (172) windings, and a resonant frequency, and a high potential rectifier and multiplier (124). The eletrostatic potential supply also includes a phase comparator (110) and a voltage controlled oscillator (114) having a free running frequency of substantially the transformer (122) resonant frequency. The phase comparator (110) generates a control signal related to the frequency difference between the signal from the secondary winding (172) and the signal from the primary circuit (110, 112, 114, 116, 118, 102-1, 2) to synchronize the voltage controlled oscillator (114) output frequency substantially with the frequency of the signal from the secondary winding (172). The voltage controlled oscillator (114) is coupled to the primary winding (156) to maintain the operating frequency of the transformer (122) substantially at its resonant frequency.

Method and apparatus for synchronization of a clock signal generator particularly useful in a digital telecommunications exchange

Abstract: A method and apparatus for synchronization of a clock generator, especially a clock generator of a digital telecommunications exchange. When there is a brief outage of the reference frequency, the voltage controlled oscillator of the phase control circuit whose output frequency determines the frequency of the clock generator continues to operate with the control voltage prevailing until then. Upon resumption of the reference frequency, the phase difference between the reference frequency and the output frequency of the oscillator is measured and corrected with the value of the valid phase difference before the loss of the reference frequency. The corrected value is used as the basis for resumed frequency control.

Spectrum analyzer circuit for pulsed input signals

Abstract: A spectrum analyzer for measuring the frequency spectrum of a pulsed input signal includes a synthesized local oscillator for providing an oscillator signal. The local oscillator is responsive to a digital control input signal to sweep the frequency of the oscillator signal from a start frequency to a stop frequency wherein the values of the start and stop frequencies are determined by the control signal. The oscillator is further responsive to the control signal to stop sweeping the oscillator signal. The oscillator signal is mixed with the pulsed input signal and filtered to determine the peak voltage of a predetermined frequency component of the mixed input signal. A data processor is provided for providing the control signal to the local oscillator. The data processor is adapted to adjust the control signal so that the start frequency of the oscillator is equal to its previous stop frequency minus a predetermined frequency difference. The frequency difference is determined to compensate for the circuit delay of the spectrum analyzer and the sweep start-up transients of the local oscillator. The local oscillator is started at a time prior to the measurement interval so that at the beginning of the measurement interval the frequency of the local oscillator is equal to the stop frequency of the previous measurement interval.

Automatic frequency control in the presence of data

Abstract: OF THE DISCLOSURE There is provided a method of and apparatus for Automatic Frequency Control (AFC) in the presence of data. It comprises removing the effects of data modulated onto the carrier, detecting the frequency difference between the carrier frequency and the phase of the reference oscillator, and adjusting the frequency of the reference oscillator to eliminate the frequency difference. It is further characterized by digitizing the modulated carrier in quadrature, sampling the modulated carrier in quadrature at a multiple of the modulated bit rate, rotating phases toward arctangent (I/Q)=0 to remove the effects of quadrature data (I/Q) modulated onto the carrier, detecting the frequency difference between the carrier frequency and the frequency of a Voltage Controlled type of reference oscillator with a phase trajectory estimator, adjusting the frequency of the reference oscillator by the frequency difference, and establishing and generating the requisite correction voltage for a Voltage Controlled type of reference oscillator (VCO) with the phase trajectory estimator.

Extending the frequency range of a Wien-bridge oscillator using composite operational amplifiers

Abstract: The author describes the use of composite operational amplifiers to extend the useful operating frequency range of an operational amplifier-based Wien-bridge oscillator The effect of the finite gain-bandwidth product (GB) of the operational amplifier (OA) is analyzed This analysis shows that for a certain GB the frequency of oscillation is less than the theoretical value and that the error increases with frequency A composite OA is then used to decrease the errors due to GB This is verified theoretically and experimentally Although the composite OA improves the frequency performance of the oscillator, the amplitude of the output decreases with frequency An automatic-gain control circuit is incorporated to keep an output amplitude constant over the useful frequency range Distortion measurements are presented >

Phase lock clock recovery with aided frequency aquisition

Abstract: In a burst mode fiber-optic transmitter-receiver or bus interface unit (BIU), the clock recovery process by a phase lock loop (PLL), or more generally a servo loop, detects the frequency difference between the PLL oscillator and the transmitter oscillator, and generates a control voltage which is combined with the voltage controlled oscillator (VCO) error voltage to urge the PLL oscillator towards the transmitter's frequency. An activity detector detects the presence of received signal, and "freezes" or holds the control voltage to the PLL so the PLL is free to slew to the frequency and phase of the clock of the received burst. Thus, the PLL is maintained near the expected frequency of the received clock signal for fast acquisition. After the burst, the activity detector once again allows the frequency control signal to assert command of the VCO frequency.

Frequency stability of an optically pumped cesium beam frequency standard

Abstract: Results obtained in an experimental optically pumped cesium beam frequency standard in which a single semiconductor laser is used for the state selection and the atom detection are reported. The separation between the two interaction regions is equal to 21 cm. This gives a 500 Hz linewidth which is observed with a signal to noise ratio equal to 10,000 in a 1 Hz noise bandwidth. A quartz crystal oscillator is frequency controlled by the atomic transition. The measured short term frequency stability is given by σy(τ)=2×10−12τ−1/2 for 1 s<τ≲500 s. Prospects for improvement of this frequency stability are discussed.

System and method of frequency calibration in a linked compression-expansion (lincompex) system

Abstract: A linked compressor-expander (Lincompex) circuit for use in telecommunications utilizes a frequency compensation circuit to frequency shift the total bandwidth of the whole communication channel to eliminate single side band "duck-talk" and data distortion in the transmitting of a complex waveform due to frequency drift or the detuning of the transmitter/receiver system. The demodulator includes an expander for expanding the compressed voice or data signal and a control tone conversion circuit for converting the received control tone into a frequency signal to be used by the expander. The control tone conversion circuit also determines the frequency error of a combination information waveform so that the frequency compensation circuit can carry out the proper frequency shifting process. A fading regulator is also provided to remove any audible level variations not removed by the automatic gain control circuitry in the receiver.

Automatic frequency control circuit for control of reference oscillator in communication apparatus

Abstract: An automatic frequency control device in which a burst frequency is sampled and held on the output side of an IF amplifier, the held frequency is measured and compared with a preset reference frequency data for obtaining a difference therebetween, and a reference oscillator is controlled so as to keep the difference below a predetermined value, by which the stabilization of an IF frequency and a transmitter carrier wave frequency, both of which are controlled by a reference frequency from said reference oscillator, can be accomplished.

Method and device for the demodulation of signals with constant envelope and continuous phase angle modulation by a train of binary symbols tolerating frequency drifts

Abstract: The continuous phase angular demodulation method disclosed digitally processes the signal in baseband after having over-sampled it with reference to the bit period. The processing consists in routinely demodulating sub-sets of differential phases at the bit period T b shifted with respect to one another by fractions T b /q of this period, in correcting them, a priori, by the phase deviations associated with a set of pre-defined d frequency drifts and in computing, for the d.q sets thus obtained, a noise criterion. The set of demodulated bits chosen is the one that reduces this noise criterion to the minimum. The set of bits then enables computation of the phase variation emitted and, using this variation and the measured variation, the real frequency drift. Another demodulation taking this drift into account is done for the q sets of initial differential stages and the set minimizing the noise criterion is then chosen and fixes the synchronization bit by the sampling instants associated with it.

Five-year frequency stability of a Zeeman stabilized laser

Abstract: A five-year record of the lockpoint frequency of a Zeeman stabilized laser shows an observed drift rate of 0.3 +/- 0.5 MHz/yr following an initial drift of 5.7 +/- 2.2 MHz/yr in the first eighteen months of intermittent operation. A second Zeeman laser drifted at a rate of -0.8 +/- 1.0 MHz/yr over the last 2.5 yr; the frequency drift was -0.2 +/- 0.6 MHz/yr over the last 3.3y r. Empirical temperature correctionsto laser frequency measurements produce a slight variance reduction in the data but no effective bias in the drift estimates.

Frequency modulator utilizing frequency synthesizer

Abstract: A frequency modulator utilizing a frequency synthesizer is disclosed. The modulator is especially useful in a mobile radio communication apparatus. The synthesizer includes a phase locked loop made up of a voltage controlled oscillator (VCO), a programmable frequency divider, a phase comparator, a second frequency divider, a reference oscillator, and a low-pass filter. When a channel control signal is input to the programmable frequency divider, the modulator sensitivity of the voltage controlled oscillator is undesirably deviated according to the channel selection. An increase of the modulator sensitivity due to an increase of the oscillation frequency is compensated for by adding a simple FET to the VCO bias circuit. A control voltage from the phase locked loop is applied to both the voltage controlled oscillator and its bias circuit. The modulator can compensate for changes in sensitivity over a wider frequency bandwidth without expanding the circuit elements of the bias circuit therefore making the modulator especially useful for portable use.

Channel selection and stabilization technique for a waveguide-type 16-channel frequency selection switch for optical FDM distribution systems

Abstract: A channel selection and stabilization technique for a waveguide-type 16-channel frequency selection switch (FS-SW) was examined at the 1.5- mu m wavelength region. The desired channel could be selected from 16 optical channels with a 5-GHz channel separation. The frequency fluctuation width for the observed short period was 65-250 MHz, and the frequency drift during 30 min was 20-100 MHz. An average crosstalk level of less than -20 dB was successfully achieved in the on state of the FS-SW stabilization. >

High resolution, wide band Chirp-Z signal analyzer

Abstract: Wide band and narrow band Chirp-Z channels are interfaced to enhance signalnalysis resolution for the signal's individual component wave forms over a wide signal bandwidth. The frequency characteristic of a selected component wave form is matched to the sweeping local oscillator and filter frequency parameters of the narrow band channel in accordance with the relative position occupied by the frequency characteristic of the selected component wave form in the frequency segregated time domain of said wide band channel. In one preferred embodiment, a mixer and a voltage controlled oscillator are utilized to accomplish the frequency matching and in another embodiment, a band-pass filter precisely defines the bandwidth of the selected component wave form. For still another embodiment, the voltage input to the voltage control oscillator is controlled in accordance with the difference in frequency between the selected component wave form and said narrow band channel.

Temperature compensated crystal resonator found in a dual-mode oscillator

Abstract: A device utilizing a quartz crystal resonator with an orientation substantially equal to 21.93°/34.10°. The crystal resonator is capable of vibrating simultaneously in two thickness modes, namely the B-mode and the C-mode. Because of the nature of the difference between the B- and C-modes, the B-mode may be used as an indication of the resonator temperature in order to compensate the C-mode frequency signal. A digital technique for temperature compensation by using the crystal itself as a sensor and a feedback loop varies the heater on the surface of the crystal. The temperature sensor compensation system contains a quartz resonator with a heater affixed thereon. The resonator is arranged as part of the oscillator to generate both B-mode and C-mode frequency signals. The C-mode signal is used as a time standard or frequency reference. Initially, the frequency of the B-mode is counted. The count is started at the same time the frequency count of the C-mode is initiated. The C-mode is then counted to a predetermined frequency value. When the predetermined count is reached for the C-mode, a signal is issued to stop the count of the B-mode frequency. The eight least significant bits of the resultant B-mode count are passed to a D-to-A converter and then used to control a switching regulator which, in turn, alters the temperature of the heater disposed on the crystal resonator. This change in temperature alters the B-mode frequency and, thus, provides a feedback loop to stabilize the C-mode frequency signal.

Television tuner oscillator with three point tracking

Abstract: The present invention concerns apparatus for changing the effective tuning inductance with a change of frequency of a multi-band oscillator, said frequency being controlled in response to the magnitude of a tuning voltage for tuned signal amplifiers so that three point tuning can be achieved in the lowest frequency band between the signal amplifiers and the oscillator. In accord with the present invention, a reactance network is connected in series with the tuning inductor of the lowest band for tailoring the tracking of the oscillator frequency with change of tuning voltage.

Novel FSK heterodyne single-filter detection system using no IF frequency lock

Abstract: A frequency-shift keyed (FSK) modulation/single-filter detection system which does not require an intermediate-frequency (IF) locking loop is reported. This system utilizes a laser frequency-locked to an atomic line (krypton 2p/sub 8/-3d/sub 1/" transition at 1.5224 mu m) as both transmitter and local oscillator. As a result, communication can be initiated without any manual adjustment to match the wavelengths of transmitter and local oscillator lasers. No significant degradation in the receiver sensitivity was observed due to the slight frequency dither used to lock the laser frequency to an atomic transition line. >

Method and circuitry for automatic control of the frequency for a radio telephone

Abstract: A method and circuit for automatically controlling the frequency of a radio telephone. According to this method, the frequency of the signal detected at the receiver of a radio telephone is mixed with the output frequency of a phase-locked loop in a first mixer. The output frequency of the first mixer is mixed in a second mixer with the frequency derived from a local oscillator. The frequency of the phase-locked loop is controlled on the basis of the frequency deviation of the output frequency of the second mixer. This frequency deviation can be determined by counting the pulses generated by the second mixer during a predetermined time interval, the time interval being equal to the amount of time which elapses when a specified number of pulses generated by the local oscillator is counted. The reference frequency of the phase-locked loop is derived from the frequency of the local oscillator.

Precise time and frequency control using the Navstar GPS and a disciplined rubidium oscillator

Abstract: Shows in the first phase of testing that a rubidium oscillator's frequency can be set and maintained to within a few parts 10/sup -13/ through the oscillator's C-field. It is also demonstrated that a 1-PPS epoch can be steered into synchronism with a reference pulse by offsetting the rubidium's frequency a known amount for a predetermined amount of time to integrate out the error. This method of epoch steering is nondisruptive. It was shown in the second phase that the rubidium oscillator can be reconciled to GPS by averaging daily measurements and adjusting the C-field. Several times during this phase the 1-second epoch was adjusted while tracking, and no corruption of tracking was observed. Many people working in the precise time arena have concluded that environmental mitigation is the key to long-term stability for an atomic frequency standard whether cesium, rubidium, or whatever. The dominant sources of drift are temperature and magnetic field influences, and cesium standards are an order of magnitude better than rubidium on both counts. A major focus of the third phase is therefore on thermal mitigation in addition to developing the drift-averaging software.< >

Oscillator frequency locking and prepositioning circuit

Abstract: A frequency locking device, with pre-positioning in which an oscillator is controlled by the output of a frequency discriminator. Shifts in the frequency discriminator are removed by performing calibrations on this discriminator through two signals with known frequencies Fo and F1 produced by a generator. The signal resulting from this calibration is used to prepare a reference voltage for a digital/analog converter which also receives a digital oscillator control signal and which then produces a voltage representing the control voltage of the oscillator.

Frequency-locked loop circuit providing large pull-in range

Abstract: Presents a frequency-locking circuit that provides a larger pull-in bandwidth than that of a conventional automatic frequency control (AFC) circuit. In addition, the proposed circuit has a stable mode of operation over the full frequency response of the frequency discriminator, and locks the oscillator at exactly the centre frequency of this discriminator. The frequency offset and the unstable mode of operation presented by the conventional circuit outside its pull-in range can thus be eliminated.

Low-frequency, low-distortion digital quadrature oscillator

Abstract: Digital recursive and non-recursive filtering techniques have both been exploited to build a low-frequency (0.5-50 Hz), low-distortion digital quadrature oscillator. This generator shows an amplitude constant to within 0.3% over the frequency range, and the quadrature phase error is within +or-0.05 degrees .