Automatic Frequency Control Circuit Research Articles

A low-noise X-band microwave source with digital automatic frequency control for electron paramagnetic resonance spectroscopy

We report a new design of microwave source for X-band electron paramagnetic resonance spectrometer. The microwave source is equipped with a digital automatic frequency control circuit. The parameters of the digital automatic frequency control circuit can be flexibly configured for different experimental conditions, such as the input powers or the quality factors of the resonator. The configurability makes the microwave source universally compatible and greatly extends its application. To demonstrate the ability of adapting to various experimental conditions, the microwave source is tested by varying the input powers and the quality factors of the resonator. A satisfactory phase noise as low as −135 dBc/Hz at 100-kHz offset from the center frequency is achieved, due to the use of a phase-locked dielectric resonator oscillator and a direct digital synthesizer. Continuous-wave electron paramagnetic resonance experiments are conducted to examine the performance of the microwave source. The outstanding performance shows a prospect of wide applications of the microwave source in numerous fields of science.

Operation of synthetic inertia units in electric power systems of various densities

This study is aimed at improving the efficiency of photovoltaic plants operated in the electric networks of various densities by adjusting the synthetic inertia algorithm and automatic frequency control circuits. To this end, the automatic control system of a photovoltaic plant was investigated using hybrid modelling methods in an all-mode online simulation complex of electric power systems. It was shown that the stability of photovoltaic power plants could be improved through the use of synthetic inertia. According to the conducted research, effective operation of this algorithm can be ensured by a correct determination of the bandwidth of automatic frequency control. Operation of this automatic frequency control circuit can lead to the oscillations of various frequencies during the installation of photovoltaic power plants in low-current electrical networks (electrical networks with the short circuit coefficient of less than 10 a.u.) and, subsequently, negatively affect the operability of the synthetic inertia algorithm. In addition, in high-current networks with an increased bandwidth of the automatic frequency control unit, the value of the network frequency reduction decreases (optimal bandwidth of 50 Hz). Conversely, in low-current networks, the automatic frequency control unit, under an increase in the bandwidth, decreases the response rate of the synthetic inertia algorithm, which leads to an increase in the frequency reduction value (optimal bandwidth of 0.3 Hz). Thus, the conducted investigations showed that the automatic frequency control circuit in the control system of a photovoltaic power plant can be used to alter the operation of the synthetic inertia algorithm. However, the nature of this effect depends on the electrical network density and can be both positive and negative. The effect observed in the tested power system was confirmed for a real-dimension power system.

A constant loop bandwidth in delta sigma fractional-N PLL frequency synthesizer with phase noise cancellation

This work presents the design of a new and unique design technique of constant loop bandwidth and phase-noise cancellation in a wideband ΔΣ fractional-N PLL frequency synthesizer. Phase noise performance of the proposed ΔΣ fractional-N PLL frequency synthesizer has been verified using CppSim simulator with the help of transistor level simulation results in Cadence SpecctreRF. Transient response of the proposed ΔΣ fractional-N PLL has been verified in transistor level simulation using Cadence SpectreRF in 0.13 μm standard CMOS process. The proposed phase-noise cancellation and constant loop bandwidth in wideband ΔΣ fractional-N PLL reduces the out of band phase noise by 18 dBc/Hz at 2 MHz offset frequency for a closed loop bandwidth of 1 MHz, when ICP,max is equal to 2.6 mA. PLL locking time has been reduced with phase-noise cancellation and a constant loop bandwidth calibration circuits using the proposed CP unit current cell for the mismatch compensated PFD/DAC in wideband ΔΣ fractional-N PLL frequency synthesizer. Optimum phase noise performance can be achieved with the help of proposed design technique. Proposed ΔΣ fractional-N PLL frequency synthesizer is locked within 14.0 μs with an automatic frequency control circuit of the LC VCO and a constant loop bandwidth calibration circuit through the use of proposed CP unit current cell for the mismatch compensated PFD/DAC for the phase-noise cancellation in worst case condition of KVFC = 10 and KLBC = 150. Our new design technique can be extensively integrated for wideband fractional-N PLL for new type of wireless communication paradigm using the thinnest channel subharmonic transistor and low power devices, and it has the potential to open a new era of fractional-N PLLs for wideband application.

A fast and efficient constant loop bandwidth with proposed PFD and pulse swallow divider circuit in [formula omitted] fractional-N PLL frequency synthesizer

This work presents the design of a ΔΣ fractional-N PLL frequency synthesizer with a new loop bandwidth calibration and automatic frequency control (AFC) circuit, applicable for wide band RF communication system. This new and unique loop bandwidth calibration circuit has been implemented using logarithmic and anti-logarithmic (Base2) architecture. This architecture is an efficient design technique as well as faster operation in CMOS domain. The operating frequency range of the ΔΣ fractional-N PLL frequency synthesizer is from 2.158 to 5.133GHz. The variation of LC VCO gain (KVCO) is obvious due to wide band application and it varies from 30.65MHz/Volt to 368MHz/Volt for the frequency range of 2.158–5.133GHz. Constant loop bandwidth is maintained by controlling the charge pump current. Power consumption of the ΔΣ fractional-N PLL frequency synthesizer is 34mW from a 1.2Volt power supply and the work has been carried out in 0.13μm standard CMOS process. Also, this design includes an automatic frequency control unit for the LC VCO circuit which is coarsely tuned within 1.825μs for KVFC=10 for worst case condition and it completes the loop bandwidth (LBW) calibration within 7.84μs for KLBC=150 for worst case condition. The maximum locking time of the ΔΣ fractional-N PLL frequency synthesizer with loop bandwidth calibration and automatic frequency control circuit is 12.7μs for KVFC=10 and KLBC=150 for worst case condition. The ΔΣ fractional-N PLL is locked much faster than any work reported earlier using the proposed PFD, CP, proposed pulse swallow divider, efficient AFC circuit for LC VCO and a new loop BW calibration technique in transistor level simulation using Cadence SpectreRF. The main advantage of this loop bandwidth calibration technique is that the calibration time can be adjusted according to the PLL output frequency, loop bandwidth calibration accuracy and tuning frequency range of the LC VCO.

High-Precision AFC Circuit Applied to 64QAM Point-to-Multipoint Burst Communications

In this paper, we propose a novel open-loop Automatic Frequency Control (AFC) circuit suitable for 64QAM point-to-multipoint (P-MP) burst communications. The proposed AFC contains two frequency offset detectors. One estimates the phase rotation over long intervals to obtain accurate estimates at the cost of phase ambiguity. The other estimates the phase rotation over short intervals and its output is used to resolve the ambiguity in the following phase ambiguity'compensator. Thus, the proposed AFC circuit calculates the phase rotation over sufficiently long periods to yield accurately estimate the carrier frequency offset while suppressing the phase-unwrapping problem. The proposed AFC approaches the Cramer-Rao bound (CRB) and so achieves very small residual frequency offset. The proposed AFC circuit can be implemented with much smaller circuit scale than the conventional devices. Computer simulations and experiments confirm that its residual frequency error is less than of 10 -5 for the frame format considered; this performance is sufficient for the 64QAM - 40 Mbaud system targeted.

Electronically Tunable Surface-Coil-Type Resonator for L-Band EPR Spectroscopy

The automatic frequency control (AFC) circuit in conventional electron paramagnetic resonance (EPR) spectrometers automatically tunes the microwave source to the resonance frequency of the resonator. The circuit works satisfactorily for samples stable enough that the geometric relations in the resonance structure do not change in a significant way. When EPR signals are measured during in vivo experiments with small rodents, however, the distance between the signal source and the surface-coil detector can change rapidly. When a conventional AFC circuit keeps the oscillator tuned to the resonator under those conditions, the resultant frequency change may exceed ±5 MHz and markedly shift the position of the EPR signal. Such a shift results in unacceptable effects on the spectra, especially when the experimenter is dealing with narrow EPR lines. The animal movement also causes a mismatching of the resonator and the 50-ohm transmission line. Direct results of this mismatching are increased noise; shifts in the position of the baseline; and a high probability of overdriving the signal preamplifier with consequent loss of the EPR signal. We therefore designed, built, and tested a new surface-coil resonator using varactor diodes for tuning the resonance frequency to the fixed frequency oscillator and for capacitive matching of the resonator to the 50-ohm transmission line. The performance of the automatic matching system was tested in vivo by measuring EPR spectra of lithium phthalocyanine implanted in rats. Stability and sensitivity of the spectrometer were evaluated by measuring EPR spectra with and without the use of the automatic matching system. The overall experimental performance of the spectrometer was found to significantly improve during in vivo experiments using the automatic matching system. Excellent matching between the 50-ohm transmission line and the resonator was maintained under all experimental circumstances that were tested. This should allow us now to carry out experiments that previously were not possible.

Modification of a whole-body NMR imager into a radio frequency EPR spectrometer suitable for in vivo measurements

We report the modification of a low-field whole-body NMR imager to allow radio frequency EPR spectroscopy. The instrument is designed to give optimum sensitivity for in vivo detection of free radicals. The RF circuit is able to operate over a wide frequency range (240 - 320 MHz) and is designed to handle input power levels of up to 12.5 W. The EPR resonator is of the loop - gap type suitable for samples up to 400 ml. A remote resonator coupling method has been developed enabling convenient matching adjustment. An automatic frequency control circuit is able to adjust for frequency deviations caused by animal motion. Where possible, existing imager hardware and commercially available instruments have been used. The instrument is controlled from a central computer via an IEEE 488 instrumentation bus. Here we present sensitivity measurements obtained from a variety of large aqueous samples containing nitroxide free radicals. We show that the instrument is suitable for the detection of exogenous free radicals in 200 g rats.

A convenient monitor of EPR automatic frequency control function

This Note points out the utility of an easily saturated EPR signal for mon itoring the performance of an automatic frequency control circuit in an electron paramagnet ic resonance spectrometer. The fundamental principle is that only when the klystron frequency is exactly at the resonant frequency of the cavity will the detected signal be the absorption signal. When the frequency is off resonance, nonlinearities in the AFC system will cause a m ixture of absorption and dispersion (I). Since the dispersion does not saturate as readily as the absorption signal, a small amount of dispersion signal m ixed in with the absorption signal for a easily saturated signal will produce a large change in the recorded EPR spectrum. We demonstrate this with a sample of irradiated quartz. Most of the measurements were conducted with “spectrosil” quartz from Thermal American Fused Quartz, irradiated to 24 Mrad with Co-60 gamma. Similar results were obtained with a sample of irradiated quartz provided by Dr. M ichael Bowman, Argonne National Laboratory. The spectra are predominantly the E’ signal near g = 2. The electron spin relaxation time for the irradiated quartz signal is so long that operation at about 55 dB (below 200 mw ) is required to achieve a nonsaturated signal. Since the signal is weak and since the AFC does not hold well at this low power, all of the spectra were recorded under weakly saturating conditions. Spectra were run on a Varian E-9 with a El02 bridge and on a Bruker ER-200 with a ER044 MRDH bridge. The standard rectangular cavities from each manufacturer were used. Variable-temperature Dewar inserts were installed in each cavity. They increase the effective B, at the sample beyond what would be present in an empty cavity at the same incident power; 100 kHz magnetic field modu lation was used. Frequencies were measured with a Systron Donner 6245B M icrowave Frequency Counter. The sample of irradiated 2 m m o.d. quartz rod, contained in a 4 m m o.d. quartz tube, was placed in the cavity, and the cavity was critically coupled by adjusting the iris to keep the detector current nearly constant over a 0 to 40 dB attenuation range. Spectra (a) (2) and (b) (2) in F ig. 1 were obtained under these conditions. Next, with the AFC off, the klystron frequency was set about +-2 MHz from the cavity resonant

Automatic frequency control for 636 Mbit/s SF-FSK coherent optical field trial

An automatic frequency control circuit for a 636 Mbit/s SF-FSK coherent optical communication system has been implemented. The intermediate frequency is stabilised at 3.2 GHz ± 5 MHz, resulting in a system power penalty due to intermediate frequency drift of less than 0.1 dB at BER = 10−11. The system features a fully automated cold start circuit.

Piezoelectric-controlled tuning capacitor for surface coils.

The design of a piezoelectric-controlled trimmer capacitor and its use as a tuning element of an MRI surface coil at 63.8 MHz are described. It is found that image quality does not depend strongly on match of the coil to the transmission line but that tuning can be a significant factor. The work is intended to lead eventually to an automatic frequency control circuit in which the capacitor serves as the active element in holding the resonant frequency of the nuclei and the resonant frequency of the coil in coincidence.

EPR automatic frequency control circuit with field effect transistor (FET) microwave amplification

An electron-paramagnetic-resonance (EPR) reference-arm microwave bridge is described that incorporates a FET microwave amplifier and a balanced quadrature mixer. An automatic frequency control (AFC) circuit is described that employs 70-kHz frequency modulation of the oscillator. AFC feedback is based on formation of the sum of squares of the I and Q outputs of the mixer using analog devices. The resulting AFC circuit is independent of the phase angle α of the reference-arm microwave voltage with respect to the signal-arm microwave voltage at the mixer. An L-band bridge is described that incorporates the circuit. Good AFC lock was obtained from 1 μW to 100 mW of incident power independent of α using a loop-gap resonator of Q=500.

Hold-In Characteristics of an Extended Range Gunn Oscillator System

This paper describes a new Gunn oscillator system having an additional arrangement for controlling the instantaneous frequency of the oscillator through an automatic frequency control circuit. By utilizing this new technique, based upon the principle of self tracking, the locking bandwidth of an injection-locked Gunn oscillator can be increased to a huge extent without affecting its stability. Experimental observations are found to be in good agreement with the conclusions of the analytical approach.

FREQUENCY FLUCTUATIONS IN A GAS LASER WITH NONLINEAR ABSORPTION

Technical fluctuations of the frequency of a gas laser with nonlinear absorption are analyzed theoretically. The spectrum of frequency fluctuations is calculated for a laser with automatic frequency stabilization and an external automatic frequency control (AFC). The frequency instability σ(τ) is calculated as a function of the averaging time τ, the time constant of the automatic frequency control circuit τ0, and of the nonlinear frequency pulling factor S. The limiting values of the short-term and long-term stability are deduced allowing for the intrinsic noise of the AFC circuit. A frequency-stabilization system with two nonlinearly absorbing cells (tandem system) is discussed. It is shown that this system can raise considerably the short-term and long-term stabilities by automatic frequency stabilization.

Frequency modulated NMR spectrometer for measuring of internal magnetic fields

A Robinson-type oscillator with an automatic frequency control circuit suitable for the measurement of internal magnetic fields, is described. An AFC circuit ensures that the tuning of the tank circuit is such that the first harmonic of the modulation frequency is kept at a minimum in the detector output. Cascade amplifier states permit operation at higher frequencies with a better signal-to-noise ratio and larger sweep amplitudes than with other types of amplifiers. The use of the equipment is demonstrated by measuring the 57Fe NMR signal in iron and iron-cobalt alloys.

Design and Analysis of the Pull-in Range of Horizontal AFC in Television Systems

Previous analyses of the automatic frequency control (AFC) pull-in phenomenon in television horizontal deflection systmes have not yielded results that would permit the quantitative determination of the pull-in range during circuit design. This paper describes phase-plane analysis of this phenomenon that clarifies the relationship between the pull-in range and the various AFC circuit parameters.