Narrow-band Amplifier Research Articles

Application of a variable transformer to the study of low temperature thermal expansion

An air-cored variable transformer can be used in connection with a mutual inductance bridge to obtain fractional Angstrom sensitivity for the study of thermal expansion of solids at liquid helium temperatures. This method has a number of attractive, though not unique, features. First, it provides a linear measurement—permitting the study of anisotropic materials. Second, it is well adapted to an absolute measurement in which only the sample changes temperature—eliminating the need for expansion estimates of other parts. Third, it allows direct mechanical calibration. Fourth, it does not require precise sample dimensions or shapes. Fifth, if cost is a consideration, the only inherently expensive part of the measurement system is a good narrow-band amplifier. This statement, however, neglects the cost of the Dewar system, the liquid helium, and a good thermometer. The major limitations appear to be an applicability solely to low temperature measurements (where the resistance of the variable transformer is low) for the highest sensitivity, and then only to samples of high thermal conductivity where thermal gradients can be minimized. The results obtained for the linear thermal expansions of copper and aluminum are comparable in precision with those obtained using different methods, and the agreement within this precision is gratifying. The anomalous (non- T 3) thermal expansion observed for sapphire is of interest, and it is unfortunate that the more routine heat capacity data do not exist in the same temperature region as our thermal expansion data. Further investigations of both the thermal expansion and heat capacities of dielectric solids in the temperature region below θ D 50 would be profitable, and are being planned.

Some elementary statistical properties of a modulation noise limited detection system

Some of the basic statistical properties of the noise generated by a 1/f power spectral density function, introduced into a narrow band amplifier, are set forth. A noise process of this type is often the limiting factor in the sensitivity of an infrared detection system particularly when the uncooled PbS detector is employed.

Atomic Absorption Spectrophotometer

The construction of a single-beam atomic absorption spectrophotometer is described.A hollowcathode tube is operated by a stabilized d.c.power supply.The emitted radiation, after being chopped at 75 c/s, is passes through the atomic vapour in flame and falls on a monochromator.The output from the photomultiplier is led to a narrow band amplifier with “Twin-T net work.” The amplified signal is rectified and displayed on a recorder.By this procedure, only the signal due to radiation from the source is amplified without the effect of radiation orginated in the flame.

Submillimeter Wave Radiometry

The sensitivities of square-law detectors and superheterodyne receivers presently available at the shorter millimeter wavelengths are reviewed and extrapolated to the submillimeter region. It is concluded that, for applications which permit wide reception bandwidths, significantly more sensitivity will result from use of a direct-detection system consisting of a chopper, square-law detector and narrow-band amplifier than will be attainable with superheterodynes in the foreseeable future. The expected sensitivity of a direct-detection system at wavelengths of the order of 1 mm is compared with that of the most sensitive centimeter radiometers.

A narrow-band peak-clipping speech amplifier

Overcrowded conditions within the radiotelephone portions of the lower-frequency amateur radio bands often make communication with low signal levels impossible because of interfering signals or noise. A design is presented, based on an analysis of the human voice, of a narrow-bandwidth high-intelligence speech amplifier for amateur and possible general use.

Microcalorimetric Technique for the Study of Damping and Hysteresis in Ferromagnetic Films

Experience with experimental techniques for the study of damping and hysteresis in thin films has suggested that many of the difficulties and complications encountered might be avoided by the direct approach of measuring the heat generated by the irreversible processes which are of interest. This suggestion has led to the development of a simple and reliable microcalorimetric apparatus with which accurate loss determinations may be made over a frequency range of about 100 cps to 100 Mc. The loss detector employed is a thin-film thermocouple which developes an emf proportional to the total heat flux emerging from the ferromagnetic sample. The zero drift of the heat-detecting system is eliminated,and the overall noise level greatly reduced by imposing a low-frequency square-wave modulation on the magnetic losses, amplifying the resultant ac component of the thermal emf with a narrow-band amplifier,and rectifying the output by means of a commutator phase-synchronized with the modulator. It appears that losses as small as 106 erg/sec per cc of sample volume are readily detectable. The thermal method thus has a degree of sensitivity which compares favorably with that of other experimental techniques, and has the added advantage of providing a direct measure of energy loss.

Power Gain and Stability of Multistage Narrow-Band Amplifiers Employing Nonunilateral Electron Devices

This paper gives an analysis of the transducer power gain and stability of a multistage, narrow-band amplifier employing nonunilateral electron devices. The amplifier is assumed to consist of n identical stages with the input and output terminations. The individual amplifier stage consists of a general active two-port device, such as the transistor, characterized by its four short-circuit admittance parameters, plus a two-terminal interstage network and an ideal coupling transformer. Both the individual amplifier stage and the over-all cascade of n amplifier stages, considered as a composite active two-port, are also characterized by their short-circuit admittance (Y) parameters. Relations between these Y parameters of the individual amplifier stage and the Y parameters of the over-all iterative amplifier have been derived. The transducer gain of the amplifier as a function of the interstage and the terminating network parameters has been studied. The transducer gain is optimized with respect to the external passive terminations and is expressed in terms of a design parameter \gamma , which is directly related to the terminating conductances of the amplifier. It is shown that for an amplifier employing inherently stable active devices, there is a value of \gamma which gives maximum transducer power gain; for an amplifier employing potentially unstable active devices, the optimum transducer power gain of the amplifier will, in general, be a monotonically decreasing function of \gamma . In any case any prescribed value of \gamma determines the maximum gain obtainable from the amplifier. The amplifier's margin toward instability is prescribed through prescribing a number \rho_{l} . For \rho_{l} greater than unity, the amplifier will be stable. Control of \rho_{l} is effected through appropriate choice of the design parameter \gamma . A relation relating \rho_{l} , and \gamma has been derived. Some fundamental considerations in the design of multistage, narrow-band amplifiers employing general active two-port devices are given. Results of experimental two-and three-stage transistor amplifiers are presented which show excellent agreement between the theoretical and the experimental results.

A transistor d.c. amplifier for use in analogue computers

The use of transistors in place of thermionic valves and electromechanical relays in d.c. amplifiers for analogue computers provides a potential means of reducing their size and power consumption and of increasing their reliability. The paper starts by analysing the transistor operational amplifier when used as a summing amplifier and as an integrator. This specifies the characteristics required to give a computing accuracy of better than 0.1% per stage in conventional real-time analogue computers.The amplifier designed to meet these requirements is in two parts. The first amplifier has a transfer impedance of 1000 volts/μA and gives an output swing of ±30 volts when feeding a 10-kilohm load. It is built up of five direct-coupled stages employing, where possible, feedback to minimize the effect of transistor parameter changes. Stabilizing networks are designed for overall resistive or capacitive feedback. The amplifier closed-loop bandwidth is 18 kc/s and the phase shift at 100c/s is less than 0.1° (for a 1-megohm feedback resistance).The second amplifier, which feeds from the virtual-earth point into the first, is a low-drift narrow-bandwidth d.c. chopper-type amplifier with a current gain of 500. In the complete system this reduces the drift of the first amplifier by a factor of 500 and gives a resultant input drift current, over a temperature range of about 5°C about room temperature, of 10−9 amp and over the range 25°–50°C of 10−8amp. With a 1-megohm feedback resistance the former leads to an outputvoltage drift of 1 mV.The overall bandwidth is extended to that of the first amplifier aloneby shunting the second narrow-band amplifier by an RC network. An analysis of the resulting double-loop amplifier enables the principal parameters to be chosen to maintain a stable system with the necessary gain over the range of operating frequencies.Some test results obtained on a printed-circuit version of the amplifier, which is to form the main computing element of a small generalpurpose analogue computer show the amplifier performance to beadequate for the majority of analogue computing applications.

Transistor feedback amplifiers in carrier telephony systems

Because equipment using transistors will be introduced into communication networks in which valve equipment is already in use, questions of compatibility will arise. In some cases the performance of transistor equipment will have to conform closely to that of existing valve equipment, whilst, in others, no such restriction will occur. With present transistors this latter case would lead to the use of low signal levels and low-gain amplifiers. With feedback amplifiers employed in carrier systems the effect is to divide the amplifiers into two classes. The first class must operate with high gains and high output powers, whilst the second class is essentially one of low-power amplifiers. Examples of both classes are described, and various features of their design are discussed. These are a high-power amplifier suitable for use on an open-wire line system, a low-power wide-band amplifier for use on a system for pair cables, and a narrow-band feedback amplifier of the type used in pilot control systems. Future developments are briefly discussed.

Parametric Amplification and Frequency Mixing in Propagating Circuits

When two circuits such as filters or transmission lines are coupled together by means of distributed reactances which can vary sinusoidally in time and space, energy can be converted between waves of different frequencies in a variety of ways. If the waves on the two transmission lines are characterized by frequencies ω1 and ω2 and phase constants β1 and β2, while the coupling reactances vary as ω=ω1−ω2 and β=β1−β2+Δβ, power P1 at ω1 is converted into power P2 at ω2 and vice versa in a manner reminiscent of waves on coupled passive circuits, except that a relationship (P2/ω2)=(P1/ω1) is obeyed. If the group velocities on both transmission lines are in the same direction, the direction of power transfer reverses periodically with distance. If they are not in the same direction, the power transfer increases monotonically with distance for small Δβ but reverses periodically for Δβ larger than a certain limit. When the coupling reactance varies as given by ω=ω1+ω2 and β=β1+β2, parametric amplification is possible in the form of exponentially growing waves at frequencies ω1 and ω2 if the group velocities are in the same direction, and in a form reminiscent of the backward wave amplifier when the group velocities are not in the same direction. In both cases the excess energy is supplied by the variable coupling reactance and can be indefinitely large. Possible applications of new principles of these coupled circuits to broad-band frequency converters, frequency-channel selectors, wide-band amplifiers, tunable narrow-band amplifiers, and oscillators are described. Noise performances of the circuits are also discussed.

The reflex klystron as a negative resistance type amplifier

A reflex klystron has been tested as a narrow-band amplifier at 11,000 mc. The performance is predicted rather accurately from simple theory and steady gain can now be obtained in the 30-db region with a bandwidth of the order of megacycles. A noise figure of 40 db was measured and the saturation power output was the same as the output of an oscillator. A circulator was used in these tests to separate input from output and we consider it important to use this component with any type of negative resistance type amplifier.

Electron Ionization Cross Sections using Chopped Beams

The paper describes the development of a crossed beam method suitable for measuring ionization cross sections of unstable gases such as atomic hydrogen. Results obtained in He and H2 are compared with those of Tate and Smith. The method makes use of a narrow band amplifier associated with a beam chopper and phase sensitive detector, to eliminate the current due to ionization of the background gas. The electric field in the collision region is such that limitation of the beam crossover volume can make the uncertainty in electron energy much smaller than is otherwise possible, without resorting to magnetic collimation or pulsed collection. An analysis of the sources of error due to the collection of spurious currents at signal frequency is given.

Corona Measurement and Interpretation

This discussion of the performance of the more common types of corona measurement circuits has emphasized certain points. They may be summarized as follows: 1. The coulomb charge in individual corona discharges and the number and phase of these individual discharges are considered the significant quantities in corona measurement. 2. The voltage step pulse produced by a single discharge varies directly as the pulse coulomb charge and inversely as the test specimen capacitance and the capacitance in parallel with it. This voltage is then divided between the detector input shunt capacitance and the series-coupling capacitance. 3. The observed shape of the pulse following the very rapid step change in voltage is determined by the inductance or resistance shunting the detector input. These produce, in the case of a resistance, a nonoscillatory exponentially decaying voltage, and in the case of an inductor an oscillatory decaying voltage. 4. A wide-band amplifier is needed to indicate a major fraction of the initial pulse height if the time constant of the exponential decaying is short. This is particularly true of the nonoscillatory pulse produced by a resistance input, where conventional narrow-band amplifiers have an output crest voltage which is only a small fraction of the input pulse crest. A much greater sensitivity may be achieved with a fairly narrow bandwidth amplifier tuned to the natural oscillation frequency of the input circuit having an inductance. A high Q in this input circuit is preferred to give maximum sensitivity. 5.

Photoelectric observations of the electron solar corona.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photoelectric observations of the electron solar corona. Wlerick, Gerard ; Axtell, James ; Lee, Robert Abstract The High Altitude Observatory staff has recently installed in Boulder a new instrument designed for the detection without eclipse of the electron solar corona. This component of the corona, often called the K-corona, is strongly polarized, the direction of the polarization being practically radial. It was successfully detected without eclipse in 1930 by B. Lyot at Pic du Midi with a coronagraph and a visual polarimeter able to detect a percentage of polarization as low as 0.1 per cent. Lyot's observations went as far as 0.35 R0 from the limb. The new instrument, called the K-coronameter, has been designed to permit observations at a greater distance from the edge of the sun, of the order of 0.5 to i.o R0. It is a coronagraph supplemented by a sensitive photoelectric polarimeter. The analyzer of polarization consists essentially of an electrically birefringent crystal excited by an a.c. voltage, a polarizer and a Lallemand photomultiplier. The a.c. signal from the multiplier is fed to a narrow band amplifier with synchronous detection. The band width is of the order of a few cycles per second. To reduce as much as possible the instrumental polarization, the coronal observations are always made on the axis. Thus, when scanning around the limb, the axis of the instrument describes a cone. A complete scan, at one given distance from the limb, takes 4 minutes. During the scan, a half-wave plate rotating at the speed of I round/ 8 min. keeps the plane of polarization of the light entering the analyzer in a fixed direction. With a relatively clear sky, Brightness B5 i~-~ B0, it has been possible in Boulder to detect the main features of the corona streamers up to 0.5 R0 and in one case up to o.66 R0. There is generally a good correlation between the brightness of the green coronal emission line measured with a conventional coronograph. It is planned to move the instrument to Climax this summer, at elevation I I 000 feet where better sky conditions prevail, and to make routine observations on clear days during the International Geophysical Year. The principal development work on this instrument was supported by the U. S. National Committee for the I.G.Y. under a grant from the National Science Foundation; a portion was supported by Radio Corporation of America. High Altitude Observatory, Boulder, Colo. Publication: The Astronomical Journal Pub Date: 1957 DOI: 10.1086/107631 Bibcode: 1957AJ.....62R..39W full text sources ADS |

Measured Amplitude Distribution of Noise Voltages

In this paper a brief theory is given about the amplitude and probability density distribution of a narrow band shot noise. It is found that the probability density distribution follows the normal law and the amplitude distribution is the integral of the normal function. However, if the noise voltage is applied across a nonlinear impedance such as a type 1N34 crystal diode, both the amplitude and probability density distributions are changed. The change depends upon the current-voltage characteristic of the nonlinear impedance. An experimental method is described which can be used to measure the amplitude and the probability density distribution. The noise source was a photomultiplier tube followed by a narrow band amplifier centered at 16 kc. The measured results agree with that obtained by calculations.

Measurement of Time Varying Optical Absorption

Time sampling techniques have been applied to the study of optical absorption transients. The method is described in terms of its use for the study of the decay of the concentration of metastable atoms following a pulsed discharge. Light from a capillary source is passed through the discharge tube. The transmitted intensity of one of the spectral lines which is absorbed by the metastable atoms is measured using an interference filter and a gated photomultiplier. The photomultiplier gate occurs at twice the discharge frequency so that alternate pulses of the photomultiplier output are reduced by absorption. The component of the photomultiplier output at the discharge frequency is proportional to the absorption and can be measured with a narrow band amplifier and synchronous detector. Since the time resolution of the system is determined by the width of the photomultiplier gate, the response of the synchronous detector can be made slow enough to average the absorption signal over many decay periods. Fluctuations in the number of electrons leaving the cathode of the photomultiplier limit the useful sensitivity of the present system to approximately two parts in 104. This represents a hundredfold improvement in sensitivity over that available with previous techniques. The results of some studies of helium metastables are presented in order to illustrate the detail which can be obtained with this technique.

The Cascade Backward-Wave Amplifier: A High-Gain Voltage-Tuned Filter for Microwaves

The characteristics of a backward-wave circuit as a beam modulator and as a beam demodulator are investigated theoretically and experimentally. A basic modulator-demodulator configuration, consisting of two periodic circuits (e.g., helices) separated by an arbitrary transducer section, constitutes the basis for a new class of backward-wave tubes. These "cascade backward-wave amplifiers" behave as narrow-band amplifiers whose pass bands can be tuned electronically over a wide range of frequencies. They overcome most of the inherent disadvantages of the simple single-circuit backward-wave amplifier and feature high gain well removed from the oscillation region, high off-signal rejection, internal circuit terminations which smooth out variations in operating characteristics with frequency, and provision for adjusting bandwidth electronically. The characteristics of an experimental model at S-band are presented and compared with theory. Analysis indicates that a minimum noise figure in the same range as that of a conventional traveling-wave tube, i.e., about 6 db, should be attainable.

Apparatus for Nuclear Magnetic Resonance

Apparatus and procedures are described for the high-resolution measurement of resonance shifts and for the observation of broad line shapes. Included are a discussion of magnet design and a summary of empirical results obtained in the construction and field homogenization of two large permanent magnets. Narrow, complex resonance lines, with components separated by as little as a milligauss, are resolved by a combination of homogeneous applied magnetic field, small samples, and slow-sweep field modulation. Broad absorption line shapes are plotted at fixed frequency by a system incorporating a regenerative oscillator, a narrow band amplifier, a recording potentiometer, and an electronic control for varying the applied magnetic field linearly in time. A simple cryostat provides stable temperatures from 85° to 500°K. In the case of samples with short spin-lattice relaxation times, such as the metals, improved signal-to-noise ratios are obtained by adjusting the oscillator to super-regenerative operation and observing the frequency modulation associated with the dispersive component of the resonance.

Audio Modulation Substitution System for Microwave Attenuation Measurements

This paper describes an Audio Modulation Substitution System for high accuracy microwave attenuation measurements. In order to utilize the full advantages of a high-gain, narrow-band audio amplifier, problems of noise, hum, and instability had to be overcome. The techniques used to overcome these difficulties are discussed. The system is used over the frequency range of 240-40,000 mc and can measure up to 40 db.

The measurement of stratospheric density distribution with the searchlight technique

Abstract : An investigation of light scattering from a beam projected into the atmosphere over New Mexico was made by means of the searchlight technique. Modulation of the searchlight beam was necessary in order to differentiate the scattered light from the light of the night sky. Therefore, a modulating technique using a rotating beam is first developed and evaluated, as is the shutter method of modulation used for acquisition of final upper-altitude data. A 60-in. parabolic mirror and photomultiplier tube mounted at its focus comprised the sensing device. A narrow-band tunable amplifier then selected the desired signal component from the photomultiplier output. Absolute values of atmospheric densities were obtained by assuming Rayleigh scattering and matching the measured response at 15 km with the densities obtained from radiosonde measurements at the same height. Eight vertical density distributions to 61.8 km so determined were in good agreement with the Rand distribution for a model atmosphere. A seasonal trend for densities at high altitudes was evident. (Author)