Variable Frequency Oscillator Research Articles

'Modulation-Rate' Distortion in Frequency Modulators

An analysis of modulation-rate distortion originating in frequency modulators is presented. The analysis is based on the solutions of the differential equations representing two fundamental classes of variable frequency oscillators. It is shown that the VFOs of one class produce ideal frequency modulation, while the VFOs of the other class inherently produce distortion. It is shown that this distortion is a function of the ratio of two rates, the rate of change of energy storage capacity and the rate of change of stored energy.

The Dynamics of High-Field Propagating Domains in Bulk Semiconductors

This paper discusses the dynamics of high-field propagating domains in bulk semiconductors such as gallium arsenide. First, the origin of a high-field domain and its nucleation mechanism are discussed. Next, important properties of a steady-state high-field domain are briefly reviewed. Then, the “unequal” areas rule is derived to explain transient domain behavior. Domain buildup or decay speeds are discussed in detail, and conditions are presented under which two or more domains can exist simultaneously. Finally, the above discussions are applied to explain the high-field domain behavior in pulse circuits, variable frequency oscillators, waveform generators, and domain bypassing schemes. Numerical examples are also given to illustrate how fast these operations can be performed.

A sensitive and rapid bridge for the study of magnetic susceptibilities at frequencies from 200 to 106 Hz

A new piece of equipment is described for measuring χ′/χ0 and χ″/χ0 as a function of field (0 to 4250 Oe), temperature (1.2°K to room temperature) and frequency (200 Hz to 1 MHz). It is about ten times more sensitive than the Hartshorn bridge used in Leiden2) and it is more convenient to operate as the measuring procedure is automatic after initial adjustments have been made. The main component is a bridge circuit of four inductors, built as closely similar to each other as possible. The output from the bridge goes to two phase sensitive detectors which monitor the two outputs, one inphase, one π/2 out of phase, of the bridge. A heterodyne system is used where the input signal to the bridge is obtained by mixing the output from a variable frequency oscillator with that from a 1.5 MHz oscillator and taking the difference frequency, which is phase locked to a master oscillator. The output from the bridge, after preamplification, is mixed with a second output from the variable oscillator and the difference taken again. This gives a 1.5 MHz signal modulated by the magnetic effects in the bridge which is used in the two phase-sensitive detectors. Their output is recorded on an x−y writer. The bridge needs only be balanced to an output of about 50 mV because the sample is moved between two coils and the difference voltage is measured. The use of the same equipment to measure relaxation times longer than 100 ms is also described.

Phase-shifting networks using field effect transistors

The use of the field effect transistor in the below pinch-off mode as a variable resistor permits a wide range of phase shifting networks, filters and variable frequency oscillators to be constructed. The technique is fully compatible with integrated circuit technology and the properties of the circuits so produced are only moderately temperature-dependent.

Proton Magnetometer and Stable Oscillator for Remote Measurement of Strong Magnetic Fields

A simple apparatus for proton-resonance magnetic-field measurement permits remote location of the rf source and indicator. A high stability variable-frequency oscillator is also described.

Absorption Type 1H-{14N} Double Resonance Spectrometer

An absorption (single-coil) type high resolution proton magnetic resonance spectrometer (JNM-3, 40 Mc/s) has been reconstructed into a double resonance spectrometer. The second r-f field at about the 14N resonance frequency is supplied with a variable frequency oscillator using a Clapp circuit. A double resonance probe houses two r-f coils and a glass Dewar tube (20 mm o.d.). The coil for 14N resonance is of Helmholtz type and is wound on the Dewar tube coaxially with the proton resonance coil. Nitrogen gas of the constant temperature is streamed around the sample tube, to avoid the temperature rise in the sample due to the large second r-f field supplied during the operation. The completely decoupled proton spectrum of formamide has been observed, in which the fine structure expected for the 40 Mc/s spectrum has been clearly resolved.

AN APPARATUS FOR DETERMINATION OF BREAKING-STRENGTH OF SMALL RODENTS BONES.

AbstractAnalysis of age changes in physical characteristics of mouse limb bones requires a sensitive, consistent and relatively simple apparatus for determination of transverse breaking strength. The developed instrument is essentially a one meter counter‐balanced 4‐to‐1 lever with a tracked, motor‐driven weight on its upper surface. At the beam's short end a loading tool impinges upward on a bone mounted in an adjacent holder; at the opposite end is a mercury switch connected to a relay halting the weight motor when fracture occurs. Bones, with proximal ends embedded in convex securing blocks of impression medium, may be positioned in a standard relationship to the holder, loading tool edge, and the balanced beam.Repetitive loading of a steel spring indicates instrument error of 1.8%. Preliminary tests with a random series of paired mouse femora show no significant difference between left and right bones. In the long‐term study it will be possible to correlate physical strength with other characteristics of opposite bones.An air‐gap capacitor across loading tool and bone holder and coupled to a variable frequency oscillator permits monitoring of specimen elasticity on a strip‐chart recorder.

Stabilization of a variable quantity with respect to a single valued reference

A feedback system consisting of two interacting loops is studied, which stabilizes an output variable using a single valued reference. The deviation of the output quantity caused by an external disturbance depends on the equality of the two loops, and can be made to approach zero by suitable adjustment of system parameters. Results of the theory were confirmed experimentally using a system of two variable frequency oscillators and a quartz crystal as a reference.

Level Indicator for Permanent Installation in Liquid Helium Storage Dewar

A liquid helium level indicator suitable for a variety of applications is described. It consists essentially of a super-conducting inductance and coaxial condenser, along with a transistorized variable frequency oscillator and an rf voltmeter to detect the resonance. The resonant frequency is very nearly a linear function of the height of liquid helium in the coaxial condenser. The indicator probe may be mounted permanently in a narrow-necked storage or experimental Dewar, since the small-diameter lead-in wire does not interfere with the operation of transferring liquid to or from the vessel.

A transistorized frequency synthesizer

The synthesizer provides 30 000 discrete frequencies between 2–32 Mc/s in steps of 1 kc/s with the stability of the driving frequency standard, using the arithmetic processes of addition, subtraction, multiplication and division, with the most economical use of a set of readily available basic numbers. Use is made of the phase-locked loop principle. A variable frequency oscillator covers the 2–32 Mc/s range and, for each desired frequency, a suitable arrangement of frequency conversion and filtering allows the oscillator to lock on a fixed frequency directly derived from the standard. Spurious signals and noise generated respectively by mixers and harmonic generators, are greatly attenuated, since the phase-locked loop acts as an “active filter”. In particular the “catching range” of an automatic phase control loop is examined and a method is shown whereby this catching range can be greatly increased by very simple and economical means.

Time-frequency scanning in narrow-band speech transmission

Two basic sampling methods for the transmission of intelligible speech with reduced channel capacity have previously been studied. The channel vocoder developed by Dudley is based on frequency band separation or quantization, and Schisser, and Fairbanks, Everitt, and Jaeger have developed a technique of time sampling. Sampling in both time and frequency offers a third major possibility which should be investigated. For this study a narrow-band speech transmission system was constructed which scans the time-frequency plane. The system scans in either a sinusoidal or in a sawtooth manner over a frequency range of 200 to 7000 cps. In order to achieve the scanning, a variable carrier frequency oscillator, having a frequency shift of ± 30 per cent of the carrier frequency and an amplitude modulation of ± 2 db, has been developed. The process of scanning leaves empty spaces in the time-frequency plane, and a four-channel time-delay system which employs the method of dielectric recording has been used to fill the gaps in the time-frequency plane by repeating the signal samples. When signal reiteration is employed with this system, a score of 75 per cent of monosyllabic phonetically balanced word lists was obtained with a scanning filter of 1000-cps bandwidth and a sinusoidal scanning rate of 30 times per second. This intelligibility is appreciably higher than that which can be achieved with a fixed 1000-cycle filter located in the frequency region of maximum intelligibility.

Evaluation of Time-Frequency Scanning for Narrow-Band Speech Transmission

Two basic sampling methods for the transmission of intelligible speech with reduced channel capacity have previously been studied. The channel vocoder developed by Dudley is based on frequency separation or band analysis, and Schiesser and Fairbanks, Everitt, and Jaeger have developed a basic technique of time sampling. The possibility of sampling in both time and frequency offers a third major possibility which should be investigated. For this study a narrow-band speech transmission system was constructed which scans the time-frequency plane. The system scans in either a sinusoidal or in a saw-tooth manner over a frequency range of 200 to 7000 cps. In order to achieve the scanning, a variable carrier frequency oscillator, having a frequency shift of ±30% of the carrier frequency and an amplitude modulation of ±2 db, has been developed. The process of scanning leaves empty spaces in the time-frequency plane, and a four-channel time-delay system which employs an electrostatic method of recording has been used to fill the gaps in the plane by repeating the signal samples. When signal reiteration is employed with this system, a score of 75% of monosyllabic phonetically balanced word lists was obtained with a scanning filter of 1000 cps band width and a sinusoidal scanning rate of 30 times per sec. This intelligibility is appreciably higher than that which can be achieved with a fixed 1000-cy filter located in the frequency region of maximum intelligibility.

Some Shock Spectra Characteristics and Uses

Abstract A shock spectrum is a plot showing the peak response of a linear variable-frequency oscillator (of single degree of freedom) to a specific shock wave, as a function of the frequency of the oscillator. Such a spectrum may be measured, for example, by multifrequency reed gages and is sometimes used as a basis either for specifying the shock wave or for computing the response of a multi-degree-of-freedom structure to such a shock. In this paper these applications of the shock spectrum are discussed. In particular, it is shown that, if the fundamental frequency (f1, cps) of the structure is sufficiently high, a close approximation of the peak response of a multi-degree-of-freedom system can be obtained by the algebraic sum (not the sum of absolute values) of the peak responses of the individual degrees of freedom. Numerical results for a uniform cantilever beam subjected to a shock load uniformly distributed over its span show that the high-frequency requirement is satisfied if 2f1tm ≥ 1, where tm(sec) is the rise time of the pulse.

Frequency modulation by inductance variation: a magnetically-stable ferrite modulator

The factors affecting the production of frequency modulation are considered when the modulator is either a reactance valve or a variable inductance employing magnetic variation of the core permeability. It is shown that whilst the effectiveness of the reactance valve is reduced at high oscillator frequencies or with large fractional frequency deviations, the inductance modulator is not affected to the same extent. The losses inherent in the magnetic core of the inductance modulator are considerably reduced by using magnetic ferrites; such a ferrite modulator is described. The effects of magnetic hysteresis, in producing oscillator frequency instability, are minimized by applying a specific level of polarization from a permanent magnet. A figure of merit is developed by which the frequency instability can be calculated from a knowledge of the prevailing magnetic conditions. The ferrite frequency modulator is particularly suitable for use with variable frequency oscillators where the equipment is to be portable and free from spurious sub-harmonic outputs. Results are given which serve to indicate the performance of this modulator.

Novel Circuit for a Stable Variable Frequency Oscillator

In the past, quartz crystals have been used extensively to control single frequencies. This paper describes a circuit where a quartz crystal exercises considerable control over a continuously variable frequency spectrum. The circuit is characterized by multiloop feedback, which enables oscillation to be maintained at three frequencies, with the corresponding voltages confined to certain branches of the circuit and coupled to each other through three mixers. The frequencies of oscillation, to be called f1, f2, and f3, are determined by zero phase shift conditions in the feedback loops and by one frequency condition, f1+f2 = f3. The frequency f3 is restricted by a quartz crystal resonator, whereas the frequencies f1 and f2 are tunable in such a manner, that as f1increases, f2 decreases and vice versa. The LC resonators which control f1 and f2 drift in the same direction when the ambient conditions change. Such a drift cannot influence f1 and f2, since the latter can change only when the LC resonators shift in the opposite direction. The presence of a quartz crystal in the feedback loop also improves the phase stability of the circuit. An experimental model has been tested, and shows a thirty to hundredfold reduction of the frequency drift with temperature, excellent short and long term stability and a remarkably small initial frequency drift with switching on.

Determination of the Parameters of Cavities Terminating Transmission Lines

Past methods available for measuring parameters of cavities terminating transmission lines are often considered too involved for production line testing. One common method requires many individual standing wave ratio measurements at frequencies slightly lower and higher than the resonant frequency of the cavity. These measurements take not only an excessive length of time but also require an accurately controlled variable frequency oscillator for precision measurements. A second method in which an oscillographic display of reflected power is used, while much faster, requires more complicated equipment and calls for use of unconventional measurement procedures. The method presented herein was devised to meet time and accuracy requirements on the production line. Its speedy results can be obtained from equipment and calculations with which production line personnel are familiar. Use is made of the same common sweep frequency method utilized in testing transmission and absorption cavities. This paper presents a derivation of the theory of the method, a description of the apparatus used, and a comparison of the results obtained with this and the point by point method.

The analysis of binary gas mixtures by a sonic method

The operation of the instrument described is based on the variation of the velocity of sound in a mixture of two gases, with the proportions of the mixture. The velocity of sound is determined at a fixed temperature by measuring the resonance frequency of a double Helmhoitz resonator containing the mixture. The source of the sound is a diaphragm driven electromagnetically from a variable-frequency oscillator; a similar transducer followed by an amplifier and rectifier form the resonance detector.The method of continuous recording of the mixture proportions, is also described. In this, the gas mixture flows through the resonator, and a simple servo-mechanism is used to maintain the condition of resonance and to operate the recorder.

An ultrasonic method of gauging

A method is described of measuring wall thickness accurately although only one surface may be accessible. A variable frequency oscillator is employed which sets up standing waves between the transducer and the reflecting surface. At the resonant frequency and its harmonics the oscillator anode current increases and measurement of the frequency interval between these peaks enables the thickness to be calculated. The paper describes:—(1) A portable instrument in which the frequency, modulation is by a mechanically-rotated capacitor, and audible or meter indication is given; (2) A larger instrument employing electrically variable inductor tuning and direct reading c.r.t. presentation. Consideration is given to the design of transducers, choice of couplants, applications and limitations, and to accuracy and range.

An Electronic Tachometer

Details are given of an instrument capable of measuring speeds of revolution of a shaft to an accuracy of ± 0.05%. The rotation of the shaft is made to generate an alternating voltage which, after amplification, is connected to the Y-plates of a cathode-ray tube. The X-plates are connected to the output of a variable-frequency oscillator calibrated to ± 0.05% so that the trace appearing on the tube is a Lissajous figure. If the oscillator frequency is adjusted to give a stationary, easily identifiable Lissajous figure, then the unknown frequency is immediately known and the shaft speed given directly. Two methods by which the alternating voltage may be generated are described. Photographs show typical traces and the wave form of the generator output. The instrument can cover any speed range.

Variable-frequency crystal oscillators

The frequency of a crystal oscillator can be varied by means of a variable capacitor, but the range obtainable is extremely small. Methods of increasing the range by associating inductance with the crystal are described. It is estimated that the range of a 3-Mc/s oscillator, using an AT-cut plate, could be increased from 0.8kc/s (267 parts in 106) with variable capacitance alone, to 4kc/s (1 330 parts in 106) or more with added inductance, while preserving far better frequency stability than that of a tuned-circuit oscillator. Experiments have confirmed these results, a range of 7.7 kc/s (2560 parts in 106) being obtained with one 3-Mc/s plate. Spurious responses limit the range with some plates, but changes of rather more than 1000 parts in 106, with good frequency stability, are not difficult to achieve.Many arrangements of inductance and capacitance, capable of giving the same range, are possible. Circuits are described whereby the frequency can be made a linear function of a controlling reactance or capacitance, and the possibility of linear variation by means of a reactance valve is briefly considered. Various fields of application for an oscillator of the kind described are indicated.