Showing papers on "Frequency drift published in 2010"

An On-Chip CMOS Relaxation Oscillator With Voltage Averaging Feedback

Abstract: An on-chip CMOS relaxation oscillator with voltage averaging feedback using a reference proportional to supply voltage is presented. A voltage-averaging feedback (VAF) concept is proposed to overcome conventional relaxation oscillator problems such as sensitivity to comparator delay, aging, and flicker noise of current sources. A test-chip with typical frequency of 14.0 MHz was fabricated in a 0.18 μm standard CMOS process and measured frequency variations of ±0.16 % for supply changes from 1.7 to 1.9 V and ±0.19% for temperature changes from -40 to 125°C. The prototype draws 25 μA from a 1.8 V supply, occupies 0.04 mm2, and achieves 7x reduction in accumulated jitter (at 1500th cycle) as compared to a oscillator without VAF.

Accurate and Less-Disturbing Active Antiislanding Method Based on PLL for Grid-Connected Converters

Abstract: An accurate and less-disturbing active antiislanding method suitable for grid-connected converters using phase-locked loop (PLL) based grid synchronization is proposed in this paper. The orthogonal signal generator required by a single-phase PLL is built using a second-order generalized integrator. The inverter current reference is slightly modified each output cycle by an injected signal, and a feedback signal is extracted from the voltage of the point of common coupling (PCC). When the grid becomes unavailable, the feedback signal moves outside of a preset threshold value. The disturbance caused by this method is small compared to other active antiislanding methods, such as active frequency drift or frequency shift up/down. The method does not affect the zero crossing of the injected current, can be used to estimate the grid impedance, and is highly robust to different grid disturbances and stiffness. The performance of the proposed method has been studied through extensive simulations using MATLAB/Simulink and experiments. Selected results are presented to confirm its validity.

Analysis and Design of an Ultralow-Power CMOS Relaxation Oscillator

Abstract: This paper presents the design of a low-voltage ultralow-power relaxation oscillator without external components. The application field for this oscillator is the clock generation of low-power wake-up functions for battery-operated systems. A detailed analysis of the oscillator, including the temperature performance, is derived and verified with experimental results. The oscillator operates at a typical frequency of 3.3 kHz and consumes 11 nW from a 1-V supply at room temperature, and a temperature drift of less than 500 ppm/°C is achieved over the temperature range of -20°C to 80°C. An efficient design implementation has resulted in a cell area of 0.1 mm2 in a standard 0.35- μm digital CMOS technology.

Regulating power consumption of digital circuitry using a multi-layer ring oscillator

Abstract: A computing device is disclosed comprising digital circuitry fabricated on a multi-layer integrated circuit including a first layer and a second layer, and a multi-layer ring oscillator operable to generate a propagation delay frequency representing a propagation delay of the integrated circuit, wherein the multi-layer ring oscillator comprises a first interconnect fabricated on the first layer and a second interconnect fabricated on the second layer. The propagation delay frequency is compared to a reference frequency to generate a frequency error, and at least one of a supply voltage and a clocking frequency applied to the digital circuitry is adjusted in response to the frequency error.

Discovery Of A 552 Hz Burst Oscillation In The Low-Mass X-Ray Binary Exo 0748–676

Abstract: We report the detection of pulsations at 552 Hz in the rising phase of two type-I (thermonuclear) X-ray bursts observed from the accreting neutron star EXO 0748?676 in 2007 January and December, by the Rossi X-ray Timing Explorer. The fractional amplitude was 15% (rms). The dynamic power density spectrum for each burst revealed an increase in frequency of 1-2 Hz while the oscillation was present. The frequency drift, the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months, confirms this signal as a burst oscillation similar to those found in 13 other sources to date. We thus conclude that the spin frequency in EXO 0748 ? 676 is within a few Hz of 552 Hz, rather than 45 Hz as was suggested from an earlier signal detection by Villarreal & Strohmayer. Consequently, Doppler broadening must significantly affect spectral features arising from the neutron star surface, so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there. The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain.

A digitally compensated 1.5 GHz CMOS/FBAR frequency reference

Abstract: A temperature-compensated 15 GHz film bulk acoustic wave resonator (FBAR)-based frequency reference implemented in a 035 ?m CMOS process is presented The ultra-small form factor (079 mm × 172 mm) and low power dissipation (515 ?A with 2 V supply) of a compensated FBAR oscillator present a promising alternative for the replacement of quartz crystal frequency references The measured post-compensation frequency drift over a 0-100°C temperature range is <±10 ppm The measured oscillator phase noise is -133 dBc/ Hz at 100 kHz offset from the 15 GHz carrier

Model and Observations of Dielectric Charge in Thermally Oxidized Silicon Resonators

Abstract: This paper investigates the effects of dielectric charge on resonant frequency in thermally oxidized silicon resonators hermetically encapsulated using ?epi-seal.? SiO2 coatings are effective for passive temperature compensation of resonators but make the devices more susceptible to charging-related issues. We present a theoretical model for the electromechanical effects of charge trapped in the dielectrics within the transduction gap of a resonator. Observations of resonance frequency against varying resonator bias voltage are fitted to this model in order to obtain estimates for the magnitude of the trapped oxide charge. Statistics collected from wet- and dry-oxidized devices show that lower fixed oxide charge can be expected upon dry oxidation. In addition, observations of time-varying resonator frequency indicate the presence of mobile oxide charge in a series of voltage biasing and temperature experiments.

Ultra-Low Vibration Pulse-Tube Cryocooler Stabilized Cryogenic Sapphire Oscillator With ${\hbox{10}}^{-16}$ Fractional Frequency Stability

Abstract: A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is σy ≈ 1 × 10-15τ-1/2 for integration times 1 <; τ <; 10 s with a minimum σy = 3.9 × 10-16 at τ = 20 s. The long term frequency drift is less than 5×10-14/day. From the measured power spectral density of phase fluctuations, the single-sideband phase noise can be represented by Lφ(f) = 10-14.0/f4+10-11.6/f3+10-10.0/f2+10-10.2/f+ 10-11.0 rad2/Hz for Fourier frequencies 10-3 <; f <; 103 Hz in the single oscillator. As a result, Lφ ≈ -97.5 dBc/Hz at 1-Hz offset from the carrier.

A 50ppm 600MHz frequency reference utilizing the series resonance of an FBAR

Abstract: A 600MHz thin film bulk-acoustic wave resonator (FBAR)-based differential oscillator fabricated in a 0.13µm CMOS process is presented. The oscillator employs a crosscoupled pair with an FBAR resonator tank providing high Q source degeneration to realize frequency oscillation at the series resonance. The measured phase noise is −126 and −150dBc/Hz at 10kHz and 1MHz frequency offsets respectively; the integrated RMS jitter from 10kHz to 20MHz is 50fs. The oscillator achieves a frequency drift of 50ppm over the temperature range from 25 to 110 °C, providing the potential for quartz replacement in some applications. The figure-of-merit (FOM) of the oscillator is 214dB.

Frequency synthesizer and high-speed automatic calibration device therefor

Abstract: A frequency synthesizer and an automatic calibration device are disclosed. An automatic calibration device for a phase-locked loop based frequency synthesizer includes: a frequency-to-digital converter for converting a frequency of a signal outputted from a voltage controlled oscillator into a first digital value; a frequency difference detector for calculating a difference between the first digital value outputted from the frequency-to-digital converter and a second digital value corresponding to a target frequency; an automatic frequency calibration logic for selecting an optimal control code for a capacitor bank such that an output frequency of the voltage controlled oscillator is closer to the target frequency; and a loop bandwidth calibration logic for tuning a charge pump gain such that a loop bandwidth is kept constant in the optimal control code using the frequency-to-digital converter. Thus, the calibration speed can be increased, and the loop bandwidth can be kept constant within the output frequency range.

Integrated circuit frequency generator

Abstract: An integrated circuit frequency generator is disclosed In some embodiments, the frequency generator comprises an electronic oscillator configured to generate an oscillator frequency and calibration circuitry configured to periodically calibrate the electronic oscillator with respect to a reference frequency source When a primary power source is unavailable, an output frequency is generated from the oscillator frequency, and the reference frequency source is powered-on only during calibration cycles

1.5-GHz CMOS voltage-controlled oscillator based on thickness-field-excited piezoelectric AlN contour-mode MEMS resonators

Abstract: This paper reports on the first demonstration of a 1.5 GHz CMOS oscillator based on thickness-field-excited (TFE) piezoelectric AlN MEMS contour-mode resonators (CMRs). The measured phase noise is −85 dBc/Hz at 10 kHz offset frequency and −151 dBc/Hz at 1 MHz. This is the highest frequency MEMS oscillator ever reported using a laterally vibrating mechanical resonator. The high frequency operation has been enabled by optimizing the geometrical design and micro-fabrication process of TFE AlN CMRs, so that a low effective motional resistance around 50 Ω is achieved together with a high unloaded quality factor (Q u ) approaching 2500 and simultaneously high k2 t up to 1.96%. A tunable-supply oscillator design is proposed for fine frequency tuning (or trimming) over a narrow bandwidth. The circuit design enables a novel GHz voltage-controlled oscillator (VCO) without the use of any low-Q tunable component. The 1.5 GHz VCO exhibits a 1500 ppm tuning range by a DC voltage change of 2.5 V. This technique can be utilized for fine frequency trimming and temperature compensation applications.

Nd:YAG lasers at 1064 nm with 1-Hz linewidth

Abstract: Two Nd:YAG lasers are tightly frequency-stabilized to separately located, vertically mounted ultrastable cavities, which are connected by single-mode optical fibers employing fiber phase noise cancellation. The optical heterodyne beat between two independent lasers shows that the linewidth of each laser reaches 1 Hz and the frequency drift is less than 0.3 Hz/s.

Method for matching the natural frequencies of the drive and sense oscillators in a vibrating coriolis gyroscope

Abstract: A method of matching, in a Coriolis gyroscope, the natural frequency of a sense oscillator to the natural frequency of a drive oscillator, comprising; generating a plurality of pilot tones of different frequencies and adding each pilot tone to a feedback signal that drives the sense oscillator; detecting a gain and phase difference in each pilot tone after it has passed through the sense oscillator; estimating a transfer function for the feedback signal using the detected gain and phase difference for each pilot tone; determining an estimated transfer function, which is indicative of the natural frequency of the sense oscillator; comparing the estimated natural frequency of the sense oscillator to a known natural frequency of the drive oscillator to obtain a difference; and controlling frequency matching electrodes, using the difference, to effect a change in the natural frequency of the sense oscillator.

A wide-range VCO with optimum temperature adaptive tuning

Abstract: This paper presents an integrated wide-range VCO with a modified tuning scheme to deal with VCO frequency drift over temperature. In this approach, during the coarse-tune operation, VCO tune voltage is a function of temperature such that it resembles the inverse function of VCO fine-tune characteristic. Without degrading VCO performance, the proposed temperature adaptive tuning optimizes the maximum tolerable VCO temperature frequency drift over which PLL remains locked. As a result, VCO gain can be reduced significantly, making VCO less sensitive to PLL tune voltage noise. Integrated in a multi-standard multi-band transceiver with a small VCO gain of 50MHz/V at 3.90GHz, PLL remains locked despite 45MHz frequency drift of VCO over [−30°C, 85°C]. Using an on-chip inductor, VCO covers from 3.15GHz to 4.60GHz, achieving −138.0dBc/Hz phase noise at 3.0MHz at 3.90GHz by drawing just 8.5mA from 1.60V supply in 0.13u CMOS process.

Method and device for transmitting magnetic resonance power

Abstract: PROBLEM TO BE SOLVED: To efficiently achieve non-contact power transmission using a coil by self-resonance. SOLUTION: A power receiving device includes a frequency characteristic detector for detecting frequency characteristics of receiving power from a power-receiving coil, and a transmitter for transmitting power frequency information comprising the maximum power frequency, where the frequency characteristics or the receiving power obtained from the frequency characteristics is maximized, to a power transmission apparatus. The power transmission apparatus includes a transmission frequency varying device for varying a frequency of transmission power, a frequency scanning device for sequentially changing a frequency of transmission power transmitted from a power-transmitting coil, a receiver for receiving the power frequency information transmitted from the transmitter, and a frequency control unit for allowing the frequency of the transmission power by the transmission frequency varying device to coincide with the detected maximum power frequency based on the power frequency information received via the receiver. COPYRIGHT: (C)2011,JPO&INPIT

A wide-tuning digitally controlled FBAR-based oscillator for frequency synthesis

Abstract: This paper presents a wide-tuning digitally controlled FBAR-based oscillator in a 0.18µm CMOS process. The oscillator is tuned with a digitally-switched capacitor array to achieve a tuning range of >7000ppm, an over eight-fold improvement over previously published low power FBAR-based VCOs. The high Q FBAR allows frequency tuning to be implemented with a switched-capacitor array with relatively large unit capacitors to achieve a sufficiently fine resolution for frequency synthesis. Our oscillator achieves a measured phase noise of −99dBc/Hz and −142dBc/Hz at 10kHz and 1MHz offsets respectively at a carrier frequency of 1.50GHz while consuming less than 4mW.

A 10 GHz frequency-drift temperature compensated LC VCO with fast-settling low-noise voltage regulator in 0.13 µm CMOS

Abstract: This paper presents an LC voltage controlled oscillator (VCO) with an integrated compensation circuit that reduces oscillation frequency drift due to temperature variations. The temperature compensation circuit consists of MOS inversion varactor as a compensation capacitor, and BJTs and resistors for temperature dependent voltage bias generation, and noise contribution suppression of a bias noise. The effectiveness of this technique is demonstrated in a 10 GHz LC VCO with a frequency divider (divide-by-2), output measurement buffer, and integrated fast-settling low-noise voltage regulator. The VCO achieves current consumption of 21.0 mA, tuning range of 10.68 GHz to 12.40 GHz, phase noise of − 109.9 dBc / Hz at 1 MHz offset from 6.12 GHz carrier. The VCO frequency-drift due to temperature is improved by 82 % with the compensation circuit.

Evading surface and detector frequency noise in harmonic oscillator measurements of force gradients.

Abstract: We introduce and demonstrate a method of measuring small force gradients acting on a harmonic oscillator in which the force-gradient signal of interest is used to parametrically up-convert a forced oscillation below resonance into an amplitude signal at the oscillator’s resonance frequency. The approach, which we demonstrate in a mechanically detected electron spin resonance experiment, allows the force-gradient signal to evade detector frequency noise by converting a slowly modulated frequency signal into an amplitude signal.

An in-situ temperature-sensing interface based on a SAR ADC in 45nm LP digital CMOS for the frequency-temperature compensation of crystal oscillators

Abstract: Multi-radio (3G/4G/GPS) mobile communication devices impose stringent requirements (≪1ppm) on a crystal oscillator's (XO) frequency stability over a wide temperature range (≫100°C). Furthermore, these devices often subject their internal XOs to sudden temperature ramps caused by power switching during sporadic user activities. In order for applications such as GPS to maintain satellite acquisition under such conditions, a low XO frequency drift rate with temperature (≪50ppb/°C) is strongly desired. Traditionally, a discrete analog temperature-compensated XO (TCXO) is often used for crystal frequency temperature compensation. Recently, an integrated digital TCXO (DTCXO) [1] that uses an on-chip PTAT sensor and a ΔΣ ADC interface was presented. However, this system's accuracy is limited by the proximity-induced temperature difference between the crystal and on-chip sensor. Moreover, the high converter latency (up to 0.1s) [2] imposed by the low bandwidth ΔΣ ADC degrades the system's frequency drift control capability in response to the temperature ramp. The temperature-sensing interface presented in this paper addresses these issues by using: (1) an in-situ crystal temperature sensor; (2) a digitally calibrated SAR ADC; (3) mixed voltage domain (2.5V/1V) SC front-end sampling to accommodate the ADC under a 1V supply; and (4) chopping and digital demodulation to reduce the 1/f noise.

Mobile Rayleigh Doppler wind lidar based on double-edge technique

Abstract: We describe a mobile molecular Doppler wind lidar (DWL) based on double-edge technique for wind measurement of altitudes ranging from 10 to 40 km. A triple Fabry-Perot etalon is employed as a frequency discriminator to determine the Doppler shift proportional to the wind velocity. The lidar operates at 355 nm with a 45-cm-aperture telescope and a matching azimuth-over-elevation scanner that provides full hemispherical pointing. To guarantee wind accuracy, a single servo loop is used to monitor the outgoing laser frequency to remove inaccuracies due to the frequency drift of the laser or the etalon. The standard deviation of the outgoing laser frequency drift is 6.18 MHz and the corresponding velocity error is 1.11 m/s. The wind profiles measured by the DWL are in good agreement with the results of the wind profile radar (WPR). Evaluation is achieved by comparing at altitudes from 2 to 8 km. The relative error of horizontal wind speed is from 0.8 to 1.8 m/s in the compared ranges. The wind accuracy is less than 6 m/s at 40 km and 3 m/s at 10 km.

A Ku-Band Push-Push Dielectric Resonator Oscillator

Abstract: A Ku-band dielectric resonator oscillator is presented in this paper The push-push configuration is chosen in the oscillator so that an oscillator and a frequency doubler are realized in one circuit To reduce the interference between the two oscillators at fundamental frequency, a wideband Wilkinson combiner is designed to improve the isolation, so that the influence introduced by the inconsistency of active devices can be minimized The experimental results show that the output frequency is 174 GHz with output power about –14 dBm The phase noise of –69 dBc/Hz at 10 kHz and –95 dBc/Hz at 100 kHz is achieved, while the depression of fundamental frequency and the third harmonic is better than –15 dBc and –23 dBc respectively

Temperature compensation for an oscillator crystal

Abstract: An electronic device is equipped with an oscillator interface to be coupled to an oscillator crystal of an oscillator element. The electronic device includes an oscillator circuit which is coupled to the oscillator interface and generates an oscillator signal. The electronic device is further provided with a temperature measurement interface to be coupled to a temperature sensor of the oscillator element so as to receive the temperature signal. For accomplishing temperature compensation, the electronic device is provided with a measurement controller coupled to the measurement interface and configured to measure a first value of the temperature signal at a first point of time and a second value of the temperature signal at a second point of time. A frequency drift estimator is provided so as to estimate a frequency drift of the oscillator signal on the basis of the first value of the temperature signal and a second value of the temperature signal. By means of a compensation logic, a frequency compensation signal for the oscillator circuit is generated on the basis of the estimated frequency drift.

Temperature compensation in a telecommunications device

Abstract: A communications device, such as a GNSS receiver comprises an oscillator, having a temperature-dependent frequency characteristic, for generating signals at a nominal frequency; receiver circuitry, for receiving transmitted wireless signals using the signals generated by the oscillator; at least one temperature sensor, having a known positional relationship to the oscillator; an estimation device, for estimating a frequency of the signals generated by the oscillator, based on a measurement from the temperature sensor, and based on the temperature-dependent frequency characteristic of the oscillator; and at least one heat source. A change in the temperature of the oscillator is predicted, based on a state of the heat source, and further based on a model of the thermal properties of the communications device, and hence a change in the frequency of the signals generated by the oscillator is predicted, based on the temperature-dependent frequency characteristic of the oscillator. The receiver circuitry uses the estimated frequency of the signals generated by the oscillator, and the predicted change in the frequency of the signals generated by the oscillator, in receiving the transmitted wireless signals.

Comparison between Ramp Pulse Modulation (RPM) and constant frequency modulation for the beat frequency oscillation in voltage regulators

Abstract: In the voltage regulator (VR) applications, the load frequency varies in a very wide range from several kHz to several MHz. When the load transient frequency approaches to the switching frequency of VR, there is a low frequency oscillation in the system, which is the beat frequency oscillation between the load transient frequency and the switching frequency of VR. When this low frequency oscillation becomes significant, the system may be hurt. The root cause of this beat frequency oscillation is constant frequency operation of the VR. In order to resolve this problem, Ramp Pulse Modulation (RPM) method is proposed. A Ramp Pulse Modulation control is one of the ripple based control methods, which is generally used in the single phase voltage regulators. In this paper, RPM method is reviewed and it is compared to the constant frequency control method with experiments.

CMOS wireless temperature sensor with integrated radiating element

Abstract: A novel fully integrated wireless temperature sensor realised in 0.35 μm CMOS technology, operating at 2.4 GHz, is presented. The realised chip emits an electromagnetic field at regular time instants, set by an on-board digital control circuit. The high frequency oscillator is a simple three-stag ring oscillator. The signal is transmitted by a small on-chip loop antenna structure. This ring oscillator with the on-chip antenna represent a building block of a wireless temperature sensor, which transforms the silicon substrate temperature variation into a frequency modulation. To easily recover the frequency- vs .-temperature information, the electronic design of the ring oscillator has been carefully tuned to get a linear dependence between them. However, the frequency oscillation of a ring oscillator presents an undesired dependence on the bias voltage. To solve this problem a technique was implemented based on the monitoring of two different signals, allowing the extraction of reliable information on temperature, regardless of the bias voltage variation. This concept is implemented using two three-stag ring oscillators with slightly different frequency- vs .-temperature characteristics, both close however to 2.4 GHz, switched alternatively on for a few milliseconds each. The experimental measurements show that the oscillators provide an electromagnetic field with a power level of −45 dBm at a distance of 1 m. The peak and average power consumptions are 3.3 mW and 500 μW, respectively.

Improved active frequency drift anti-islanding method with lower total harmonic distortion

Abstract: As more distributed generators join the utility grid, the concern of unintentional islanding increases. This concern is due to the safety hazards this phenomena imposes on the personnel and equipment. Passive anti-islanding methods monitor grid parameters to detect islanding, whereas active methods inject perturbation into current waveform to drive theses parameters out of limit. The performance of active methods, such as conventional active frequency drift method (AFD), is limited by the amount of total harmonic distortion (THD) they inject into the grid. In this paper a novel anti-islanding method is presented, which generates 30% less THD than the AFD, which results in faster island detection and better non-detection zone. The performance of the proposed method is derived analytically, simulated using MATLAB and verified experimentally using a prototype setup. A single phase grid-tied photovoltaic distributed generation system is used for the simulation and experimental setup, and considered as potential application.

Method for realizing network access current of unity power factor of grid-connected inverter and internal anti-islanding

Abstract: The invention discloses a method for realizing a network access current of a unity power factor of a grid-connected inverter and internal anti-islanding, which belongs to control technology of inverters. The method comprises the following steps: phase difference detection, frequency droop calculation, reference current generation, grid voltage feed-forward and a proportional integral controller. A frequency droop algorithm comprises the following steps: performing real-time calculation on a phase difference between the network access current and a grid voltage; multiplying the phase difference by a droop coefficient to obtain a frequency variable quantity; and subtracting the frequency variable quantity from a previous frequency output quantity of the last frequency droop algorithm so as to obtain an output quantity of a new frequency droop algorithm. The method realizes phase control of the network access current of the grid-connected inverter, realizes the network access current of the unity power factor, and internally realizes an anti-islanding measure of frequency drift positive feedback of the grid-connected inverter at the same time. Therefore, the method is of a simple and effective realizing mode.

A 5 GHz quadrature relaxation oscillator with mixing for improved testability or compact front-end implementation

Abstract: We demonstrate by measurements on a test circuit that a 5 GHz relaxation oscillator with accurate quadrature outputs and low phase-noise can be obtained, and that these favorable properties can be preserved while the mixing function is performed by this oscillator. This is useful either to measure the quadrature error at a low frequency, or to implement a low-intermediate frequency (IF) or zero-IF (homodyne) radio frequency front-end. Copyright © 2008 John Wiley & Sons, Ltd.

Phase locked oscillator and radar unit having the same

Abstract: An error detecting unit of a phase-locked oscillator evaluates difference between a reference phase error signal output from a phase detector and a phase error signal actually output from the phase detector when a reference frequency modulation signal is output from a voltage-controlled oscillator and further detects a frequency error of the frequency modulation signal from the voltage-controlled oscillator based on a rate of change of the difference. A correction unit of the phase-locked oscillator calculates an average value of the frequency error in a predetermined section of the frequency modulation signal and corrects center frequency of the frequency modulation signal by correcting the average value to be zero, and changes the rate of change of control voltage per control step based on comparison between at least two frequency errors in one cycle of the frequency modulation signal. Thus frequency shift of the frequency modulation signal is corrected.