Showing papers on "Frequency drift published in 1994"

Refractive interstellar scintillation in pulsar dynamic spectra

Abstract: The evolution of the dynamic spectra of eight pulsars over 16 months is reported. Observations were made at Jodrell Bank at 408 MHz from 1984 September to 1986 January. The changing form of the spectra is interpreted as refractive modulation of diffractive interstellar scintillation. The correlation function versus time and frequency was computed for each observation and estimates were made of the apparent decorrelation widths in frequency and time, and the frequency drift rate of scintillation features. A model is developed for how these three parameters vary with epoch under changing angles of refraction. The model successfully describes the variations for four of the pulsars, for which the drift rate shows some sign reversals between epochs. Two pulsars kept a constant sign of drift rate, which we interpret as due to persistent refracting structures in their lines of sight, which are consistent with an intervening H ii region. Studies were made of the variability of the diffractive bandwidth, the visibility of the drifting features in the dynamic spectra, the refractive shifts and the speed estimated for the diffraction pattern. These quantities mostly agree with expectations from the model. The pulsar PSRB2016+28, which has an unusually low proper motion, shows clear modulation of the pattern speed by the Earth's annual motion. In the 11 epochs observed for PSRB2016+28, two showed unmistakable periodic modulation in the dynamic spectra. Several other pulsars exhibited occasional periodic patterns with low modulation. The phenomenon varies substantially between the pulsars, being more common for lower strengths of scattering.

Gps synchronized frequency/time source

Abstract: A GPS-based frequency/time source of the present invention provides an accurate, traceable, low-cost reference. In particular, the GPS-based frequency/time source includes a GPS receiver (31), a variable frequency oscillator (35), and a microprocessor (33). The GPS receiver receives and produces as output signals GPS information, whereas the variable frequency oscillator has a frequency control input terminal and produces an output frequency signal, and is coupled to the GPS receiver. The microprocessor is coupled to receive the output signals produced by the GPS receiver and produces an error signal indicative of a difference in frequency between the GPS synchronized frequency and the output frequency signal of the variable frequency oscillator. Circuitry is responsive to the error signal to produce an electronic frequency control signal (37), which is applied to the frequency control input of the variable frequency oscillator so as to cause the difference in frequency to be reduced. In this manner, a low cost oscillator may be employed in such a way as to produce a very accurate output frequency signal.

Dual mode FM and DQPSK modulator

Abstract: A dual-mode radiotelephone terminal includes a π/4-shift DQPSK modulator (11). A dual-mode modulation is achieved by mixing an output of a transmitter oscillator (16) with an output of an offset oscillator (18) to form an injection signal (LO) at a final transmitter frequency, the injection signal being further modulated with a quadrature modulator in a digital mode of operation. In an analog mode of operation the transmitter oscillator or the offset oscillator is frequency modulated and the quadrature modulator is disabled with a bias signal, thereby passing the frequency modulated injection signal without substantial attenuation. The LO signal is regenerated and also phase shifted with a circuit (35) having a frequency multiplier (30) and a frequency divider (34). The circuit outputs two local oscillator (LO) signals (LOA and LOB), each of which directly drives an associated quadrature mixer (36, 38) of the modulator. A control signal is employed for selectively enabling and disabling various of these components as a function of the operating mode, thereby conserving power.

On-chip voltage controlled oscillator

Abstract: An on-chip voltage controlled oscillator for use in an analog phase locked loop receives power from a voltage regulator which greatly reduces the noise seen by the voltage controlled oscillator. The voltage controlled oscillator has a DC bias section which supplies a relatively constant current to the multivibrator to assure a minimum operating frequency. A control signal is used to provide additional current which increases the speed of oscillation. The bias current reduces the transfer characteristics (MHz/volt) of the voltage controlled oscillator making it more immune to noise in the control signal.

Wide frequency range CMOS relaxation oscillator with variable hysteresis

Abstract: A voltage controlled oscillator (VCO) which may be adjusted to provide oscillatory signals for a wide range of frequencies includes a relaxation oscillator in which a ramp signal is compared to a reference threshold which exhibits hysteresis. The frequency of the oscillator is changed by varying the hysteresis range of the threshold level and by changing the rate at which the ramp is generated. At higher frequencies, the signal processing delay through the comparator is a factor in determining the frequency of the signal produced by the oscillator. Current sources internal to the oscillator are controlled by a reference potential that is generated from an externally supplied band-gap reference potential. The VCO is used in a phase-locked loop which includes a charge pump circuit that accumulates charge on a capacitor responsive to limited-width pulses applied to a current source which is controlled by the reference potential generated in the VCO.

Response of a Bloch oscillator to a THz-field

Abstract: In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the THz-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.

Communication system having transmitter frequency control

Abstract: A communication system is simpler and conserves power by eliminating the need for a reference frequency oscillator in the transmitter circuit. In a battery operated transceiver of the present invention, the receiver portion includes a tracking oscillator. The tracking oscillator output is used to synchronize received data and as a reference frequency source for the transmitter. In a half duplex communication system a reply signal is transmitted while no command signal is being received. Therefore, the transceiver in such a system includes circuitry for maintaining the reference frequency during transmission. Thus, the frequency accuracy of the transmission is based on the frequency accuracy of the received command signal. The tracking oscillator in one embodiment includes a phase locked loop circuit having a voltage controlled oscillator (VCO), an up-down counter, and a digital to analog converter (DAC) for determining the VCO frequency. A transparent latch in series between the counter and the DAC is used to maintain the VCO frequency as a reference frequency for transmission.

Mixer using fundamental frequency or second or third harmonic frequencies of a local oscillator for maximized resultant frequency mixer product

Abstract: A mixer is provided having a mixing element, an input signal port for incoming signals at a frequency fIN, a local oscillator signal port for signals from a local oscillator having a fundamental frequency fLO, and an output port from which the resultant frequency may be taken. Means are provided to impose a DC component of bias voltage across the mixing element at one of three levels. The mixer is such that it has a pair of conduction threshold voltages which are substantially symmetrical above and below zero volts, beyond which the mixing element will be conductive at least when a signal from the local oscillator is imposed on it. The signal from the local oscillator has substantially sinusoidal voltage waveform, with a peak-to-peak voltage which is greater than the voltage difference between the pair of conduction threshold voltages. When the first zero DC component bias voltage is imposed across the mixer element, the mixing element is conductive in respective symmetrical positive-going and negative-going senses, and the maximized resultant frequency is |fIN ±2fLO |. When the second DC component of bias voltage is imposed across the mixing element, it conducts only once per cycle of local oscillator voltage, so that the maximized resultant frequency is |fIN ±fLO |. When the third DC component of bias voltage is imposed across the mixing element, the value of voltage across the mixing element exceeds the threshold voltage in the same sense as the voltage shift for about 35% to about 55% of the period of the local oscillator voltage cycle making the mixer conductive, but it is also conductive in the opposite sense for at least a portion of the remaining period of the cycle, so that third harmonic mixing occurs, and the maximized resultant frequency is |fIN ±3fLO |.

Development of circuit for measuring both Q variation and resonant frequency shift of quartz crystal microbalance

Abstract: Although frequency shift is often used for QCM (Quartz Crystal Microbalance) measurement, Q of a resonator also varies when viscous loading occurs. However, it is difficult to measure Q variation in real time in comparison with resonant frequency shift. Furthermore, oscillation frequency shift deviates from real resonant one in case of large viscous loading. Here, the authors have developed the circuit based upon motional admittance method for measuring both Q variation and real resonant frequency shift. It was applied to a quartz resonator gas sensor, and its Q variation was measured by this circuit. Moreover, it was compared with a conventional oscillation circuit and the frequency shift of the former was remarkably different from that of the latter in case of large Q variation. When a quartz resonator coated with PEG20M (PolyEthylene Glycol 20M) membrane was exposed to water vapor, its Q decreased from 38700 to 3650 and a frequency shift of the oscillation circuit was only about one sixth of this circuit. >

A phase lock loop with idle mode of operation during vertical blanking

Abstract: A frequency detector of a phase-lock-loop circuit is used for measuring a frequency error between a frequency of an output signal of an oscillator and a frequency of a synchronizing signal. When the frequency error in each of 32 periods of the synchronizing signal exceeds a predetermined magnitude, the phase-lock-loop circuits begins operating in a coarse frequency correction mode. As long as the 32 periods have not lapsed, the phase-lock-loop circuit operates in an idle mode of operation and the oscillator is not corrected. As a result, during vertical retrace, when equalizing pulses occur, the phase-lock-loop circuit is not disturbed by a large frequency error.

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.

Computer controlled in situ radiolysis electron spin resonance spectrometer incorporating magnetic field-microwave frequency locking

Abstract: A lab‐built personal computer‐based electron spin resonance (ESR) spectrometer is described which incorporates a wide range magnetic field/microwave frequency lock as part of its magnetic field control subsystem. Instrument operation is accomplished by keyboard commands, with important experimental variables logged automatically for reference during subsequent data analysis. The spectrometer features both narrowband field modulation and direct‐detection time‐resolved ESR modes. The data acquisition system and field/frequency lock operate such that spectrum recordings consisting of multiple magnetic field sweeps are undistorted by long‐term klystron frequency drift. The spectrometer features low noise microwave preamplification, balanced mixer detection, automatic reference arm phase control, and a fast automatic frequency control system requiring no klystron frequency modulation. Natural abundance 13C and 33S studies of the terephthalic acid radical trianion and sulfite radical anion are presented as proo...

Method and apparatus for maintaining linearity and frequency accuracy of an FM chirp waveform

Abstract: A homodyne radar apparatus and method for performing radar cross-section surement by maintaining the linearity and compensating for frequency drift of an FM chirp waveform produced by a YIG tuned oscillator. A digital computer uses a predetermined data look-up table of each YIG oscillator frequency characteristic in conjunction with a digital to analog converter to provide a compensating sweep signal to linearize the frequency output of the oscillator. The computer uses an analog to digital converter in conjunction with a reference circuit output to compare the actual YIG signal nulls resulting from the reference circuit to the nulls in a signal produced by a lab standard oscillator in the reference circuit to adjust start sample delay of the ADC and to correct any frequency drift evidenced by the difference in the null of the signal.

Method for starting a radio transmitter, and a radio transmitter using a start-up estimated control voltage needed for locking onto selected output frequency

Abstract: When starting a radio transmitter, a transmission frequency is generated by a phase/frequency-locked loop which includes a phase/frequency comparator, a loop filter and a voltage-controlled oscillator. The start-up is performed by switching on an operating voltage to the voltage-controlled oscillator. To minimize the frequency error of the transmitter at start-up, measuring data about relationship between the control voltage of the oscillator and the output frequency of the oscillator at a specified calibration temperature are pre-stored in connection with the oscillator, and immediately before switching on the operating voltage, the control voltage needed for locking onto the selected output frequency at the prevailing temperature is estimated and the estimated control voltage is set as the control voltage of the oscillator.

Carrier derived frequency stabilizer for a radio communication transmitter

Abstract: A method and apparatus in a radio communication transmitter (600) stabilizes a carrier frequency locked to a reference oscillator (102). The reference oscillator (102) is controlled by a control signal and generates a first signal at a reference frequency. The first signal is coupled to a frequency multiplier (212) for multiplying the reference frequency by a predetermined factor to generate a second signal at the carrier frequency. Cycles of the carrier frequency are counted (508, 514) during a predetermined interval to determine (516, 518) a measured cycle count. The measured cycle count is compared (520) with an expected cycle count corresponding to a predetermined carrier frequency to determine a drift error. From the drift error a control signal adjustment required to correct the drift error is computed (522). The control signal of the reference oscillator (102) is adjusted (524) by the control signal adjustment to correct the drift error.

Method and apparatus for supplying synchronization signals serving as clock signals with defined phase relationships

Abstract: The present invention relates to a method and apparatus for recovering the time base of signals which change at periodic intervals The apparatus comprises gated voltage controlled oscillators (GVOC) that are alternated or exchanged, to reduce phase and frequency deviations in the recovered time base signal, such as the deviations induced by inherent GVCO differences Each GVCO is stabilized by a respective phase locked loop The respective GVCOs are gated only in response to a chosen polarity transition in the input signal, to make the circuit more tolerant of waveform distortions More than two GVCOs may be used to provide improved frequency drift resistance The circuit uses resynchronization control signals, such as the time slot signal in synchronous switching systems, to indicate resynchronization or reassignment the GVCOs in gaps in the data transmission Automatic reassignment is insured when there are periods of non-transitioning data that last longer then the stability of the GVCOs to prevent frequency drift in the recovered clock

Radio receiver with a high speed and accurate search operation and method thereof

Abstract: A controller shifts the frequency of the local oscillator with a frequency shift circuit so that as many channels to be scanned as possible will be included within specified bandwidths, detects a desired received signal by scanning each of the specified bandwidths with the shifted frequency, scans each channel within the bandwidth where the desired received signal was detected, and changes the frequency of the local oscillator or shifts the frequency of the local oscillator with the frequency shift circuit so that the desired channel frequency identified above will be at the center of said bandwidth.

Automatic determination of the presence of a microwave signal and whether the signal is CW or pulsed

Abstract: A method and apparatus for detecting a high frequency signal by down converting the high frequency signal to an intermediate frequency signal by mixing the high frequency signal with the harmonics of a minimal number of local oscillator frequency signals chosen so that at least one harmonic of at least one of the local oscillator frequency signals will convert any high frequency signal in a given frequency range to an intermediate signal in another given frequency range, and applying the result to detectors sensitive to signals in the intermediate frequency range.

Frequency synchronizing method for a reference oscillator

Abstract: A method and apparatus of frequency synchronization is described for a reference oscillator. Using a reference oscillator set to a first frequency, the process begins by scanning for a radio frequency carrier modulated with a predetermined pattern. The radio frequency carrier is located such that a received pattern comprising a frequency shifted version of the predetermined pattern is identified and time synchronization is established. The received pattern, the predetermined pattern, and stepped alterations to the first frequency are combined to provide a set of results that represent pattern correlation versus reference oscillator frequency. The first frequency is offset using the set of results, such that frequency synchronization is improved.

Method for setting the local oscillator of an optical superheterodyne receiver

Abstract: Novel structure and methods for tuning the local oscillator of an optical superheterodyne receiver are disclosed. In a first method, the value of the local oscillator frequency at which the maximum of the chronological average of the output signal from the demodulator occurs is used to set the local oscillator frequency. An alternate method first and second discriminators are used to receive the intermediate frequency signal. The separate discriminator characteristics of the first and second frequency discriminators are used in setting the local oscillator frequency.

Digital frequency multiplier utilizing digital controlled oscillator

Abstract: A frequency multiplier includes a ring oscillator having a number of logic gates arranged in a plurality of rings. Control inputs enable the selection of individual gates so as to connect them into the ring or, conversely, remove them from the ring. As additional gates are removed, the combined delay imposed by the gates remaining in the ring is reduced and the frequency of the oscillator increases. A variable delay element, preferably a group of tri-state inverters connected in parallel, is connected between two of the gates. The oscillator is fine tuned by controlling the delay inserted by the variable delay element. The frequency multiplier also includes a frequency comparator. A reference frequency is passed through a divide-by-K unit and the output of the ring oscillator is passed through a divide-by-N unit, N being greater than K. The frequency multiplier is coarse-tuned by progressively removing additional gates from the ring oscillator, and then fine-tuned by increasing the delay imposed by the variable delay element. At the conclusion of coarse and fine tuning, the frequency multiplier is locked at a frequency which closely approximates a reference frequency multiplied by N/K. An accuracy of 1% or less may be achieved. When the frequency multiplier ceases to be hooked on a frequency, it enters an idle state in which it consumes no power.

Receiver with function to adjust local frequency following reception frequency

Abstract: A receiver which has a linear approximation expression showing a relationship between the frequency deviations and corresponding changes to the voltage which is to be supplied to the voltage controlled reference oscillator to adjust the deviation following a reception frequency. The frequency deviation of the voltage controlled temperature compensated crystal oscillator is calculated based on the count values of the counters which count the frequency of the second local oscillator and the second intermediate frequency. The control voltage supplied to the voltage controlled temperature compensated oscillator is changed based on the linear approximation expression. Further, the approximation expression is represented as a series of powers of 2 based on a sensitivity coefficient of the voltage controlled reference oscillator. The approximation expression is changed by renewing the sensitivity coefficient of the voltage controlled reference oscillator. Moreover, in addition to the control of the voltage controlled reference oscillator, the second local oscillator is controlled in the similar way of the control of the voltage controlled temperature compensated oscillator to obtain more accurate intermediate frequency.

Can a charge‐exchange induced density rise at the heliopause explain the frequency drift of the 3 kHz Voyager signal?

Abstract: A physical model for the recently advanced interpretation of the 3 kHz data [Gurnett, Kurth et al., 1993] is proposed. Basing on the results of the flow dynamical model of the heliosheath [Czechowski and Grzedzielski, 1993, 1994] we conclude that the post-shock solar wind (SW) plasma, cooled down by charge-exchange with neutral hydrogen of LISM origin, should form a high density layer on the inner side of the surface of the heliopause. The model allows us to estimate the plasma density profile and the size of the layer. These we use as input for MHD calculation of (1) a transient shock moving through this layer towards the heliopause and (2) of resulting 2ωp (frequency) drift of VLF emissions generated at the shock. For the heliopause distance L= 150-180 A.U. our model reproduces both the time duration of the emissions and their typical frequency drifts.

Dichroic design for the orbiting VLBI earth station antenna

Abstract: The design and performance of a single screen frequency selective surface (FSS) with gridded square loop patch element are described for diplexing the Xand Ku-band signals in an orbiting very long baseline interferometer (OVLBI) earth station reflector antenna system. The FSS grids were designed using an analysis code incorporating the accurate integral equation technique. The effects of the dielectric substrate and superstrate are accurately evaluated by this technique. Excellent agreement is obtained between the predicted and measured results. This validates the FSS design using the gridded square loop element. If the grid is sandwiched between two 0.889 mm thick Teflon slabs, the resonant frequency drift is noticeably reduced as the incident angle steered from normal to 40 degrees . >

Fast and stable frequency switching employing a delayed self-duplex (DSD) light source

Abstract: A novel frequency-agile light source, the delayed self-duplex (DSD) light source, is constructed using a tunable DBR laser and an optical switch. The DSD light source overcomes the tuning speed limitation of the tunable laser, and also offers suppressed frequency drift due to the thermal effect by employing periodic laser tuning. For 160-GHz-spaced frequency switching, the 4-ns switching time and 2.2-GHz thermal frequency drift inherent to the DBR laser are reduced to 0.4 ns and 0.3 GHz, respectively. >

Automatic frequency control which monitors the frequency of a voltage controlled oscillator in a PLL demodulator

Abstract: An automatic frequency control circuit measures the deviation in frequency of a selected signal from a reference frequency and when the deviation exceeds a selected threshold which is variable, steps the frequency of the selected signal back to the proper frequency. The selected signal is typically an intermediate frequency (IF) signal in a radio receiver. For detection of frequency deviation, an exclusive OR (EX-OR) circuit receives a reference signal having a frequency proportional to a desired IF carrier frequency and an input signal having a frequency proportional to an actual IF frequency. A divider coupled to a voltage controlled oscillator (VCO) used in demodulation of the IF signal provides the input signal. An output signal of the EX-OR circuit includes a component with a frequency proportional to the difference between the actual and desired IF frequencies. A low pass filter isolates that component, and a charge pump generates a control voltage proportional to the frequency of that component. A comparison of the control voltage and the threshold determines whether the actual IF frequency should be changed.

AFC circuit having local oscillator phase locked loop

Abstract: An automatic frequency control circuit which can set a local oscillator synthesizer to a small frequency division number, without requesting the exact required frequency. The automatic frequency control circuit includes a voltage source for producing a desired voltage to offset the oscillation frequency of a voltage controlled oscillator, and an adder for adding the output from the voltage source to a frequency control signal of the voltage controlled oscillator. The automatic frequency control circuit is applicable to the receiver for a satellite telecommunications apparatus.