Continuous-wave Oscillator Research Articles

Frequency Modulation and Control by Electron Beams

General formulas for the effect of electron beams on resonant systems in terms of frequency shift and change in Q are derived from the point of view of lumped circuits and from a general electromagnetic field standpoint. A way of introducing the electrons has been found which materially enhances their effectiveness in producing a shift of frequency in a resonant system. Measurements of the frequency shift produced by such an electron beam in a typical geometry were made which check well with values calculated from the general theory. Possible amplitude and phase distortions are calculated when such a beam is used to frequency-modulate a system, and these are found to be negligibly small, even for very high modulating frequencies. It is shown that this method of frequency control is ideally suited for frequency modulation or automatic frequency stabilization of continuous-wave magnetron oscillators using negative-grid-controlled electron beams for controlling the frequency of oscillations.

A Frequency-Modulated Magnetron for Super-High Frequencies

This paper is based on a wartime requirement for a 25-watt, 4000-megacycle continuous-wave oscillator capable of electronic frequency modulation with a deviation of at least 2.5 megacycles. A satisfactory solution was found in the addition of frequency control to a continuous-wave magnetron by the introduction of electron beams into the magnetron cavities in a manner described by Smith and Shulman. This method is referred to as control. A brief account is given of the method of designing a continuouswave magnetron for the specified power and frequency. The problem of adding spiral-beam frequency control to this magnetron is considered in detail, and a procedure is presented for obtaining the optimum design consistent with negligible amplitude modulation and reasonable cathode-current densities. The unusual features of construction of this magnetron are described, including a method of mechanical tuning and the method of adding grid-controlled beams for frequency modulation. Performance data on the continuous-wave magnetron over a wide range of operating conditions indicate that the required 25 watts output can be attained at 850 volts with 50 per cent efficiency. Experimental results on frequency control demonstrate that the required 2.5-megacycle frequency deviation with no amplitude modulation can be achieved by electron beams introduced into two out of twelve cavities. A deviation of 4 megacycles is attained under conditions allowing some amplitude modulation. Still greater deviation is predicted by using more beams.

A magnetron oscillator with a series field winding

A description is given of an experimental investigation of a continuous-wave magnetron oscillator, the magnetic field for which is provided by an electromagnet energized by the anode current of the valve. Oscillations can be readily obtained, and they persist over a wide range of anode voltages. With a two-segment-anode valve oscillations take place at the fundamental frequency of a lecher-wire circuit connected to the valve. With a four-segment-anode valve, oscillations at the fundamental frequency of such a circuit are observed at low anode voltages, but as the voltage is increased oscillations at 3, 5, 7 ? times the fundamental frequency appear successively. The range of frequency covered by the experiments was 40?750 Mc/s (? 7.5 m to 40 cm). During oscillation the anode current assumes the value necessary to provide the optimum magnetic field, and, within limits, the number of turns on the electromagnet and the magnitude of the filament current do not affect the action of the oscillator. The operational stability is good, and the danger from an excessive anode current is largely removed.

The Inversion Spectrum of Ammonia

Ammonia gas is known to exhibit a strong absorption in the region of 0.8 ${\mathrm{cm}}^{\ensuremath{-}1}$. By sweeping the frequency of a continuous wave oscillator and using a balanced wave guide system with one arm of the wave guide serving as an absorption chamber, thirty lines of the fine structure, as predicted by Sheng, Barker, and Dennison, have been observed in this region. Their intensities and frequencies have been measured for two different temperatures and an empirical expression for the frequencies of the lines in terms of their rotational quantum numbers is given. At pressures of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}1}$ mm Hg and below, the lines are very well resolved. At pressures of ${10}^{\ensuremath{-}2}$ mm Hg and below, a definite hyperfine structure appears. The Stark effect is also observed when a d.c. field is applied to the absorbing gas.

The Inversion Spectrum of Ammonia

Ammonia gas is known to exhibit a strong absorption in the region of 0.8 ${\mathrm{cm}}^{\ensuremath{-}1}$. By sweeping the frequency of a continuous wave oscillator and using a balanced wave guide system with one arm of the wave guide serving as an absorption chamber, thirty lines of the fine structure, as predicted by Sheng, Barker, and Dennison, have been observed in this region. Their intensities and frequencies have been measured for two different temperatures and an empirical expression for the frequencies of the lines in terms of their rotational quantum numbers is given. At pressures of 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}1}$ mm Hg and below, the lines are very well resolved. At pressures of ${10}^{\ensuremath{-}2}$ mm Hg and below, a definite hyperfine structure appears. The Stark effect is also observed when a d.c. field is applied to the absorbing gas.

The action of a split-anode magnetron

An investigation has been made of the generation of continuous-wave oscillations by a split-anode magnetron (excluding the ultra-short waves). It has been found that there are two types of oscillation: the first in which the strongest oscillations over a large range of wavelengths are produced by the same set of values of the voltage and field applied, and the second in which each wavelength has a particular set of values of voltage and field for best results, but these sets are different for every wavelength. A simple relation has been found experimentally between the wavelength, field, and voltage, in this latter case. A theory has been developed (not yet complete mathematically) to show that the first results are due to a pure negative-resistance effect, and the latter to a type of amplified negative resistance which involves a resonance. The theory is confirmed by experiments with a valve having four segments in place of two, and with a valve of larger dimensions.

The action of a split-anode magnetron

An investigation has been made of the generation of continuous-wave oscillations by a split-anode magnetron (excluding the ultra-short waves).It has been found that there are two types of oscillation: the first in which the strongest oscillations over a large range of wavelengths are produced by the same set of values of the voltage and field applied, and the second in which each wavelength has a particular set of values of voltage and field for best results, but these sets are different for every wavelength. A simple relation has been found experimentally between the wavelength, field, and voltage, in this latter case. A theory has been developed (not yet complete mathematically) to show that the first results are due to a pure negative-resistance effect, and the latter to a type of amplified negative resistance which involves a resonance.The theory is confirmed by experiments with a valve having four segments in place of two, and with a valve of larger dimensions.

A Combined Tesla Coil and Vacuum Tube

An oil emersed high frequency resonance coil has been constructed and combined with a vacuum tube in such a way that none of the high voltage parts are exposed to the air. This makes a very compact apparatus, and it is not subject to corona losses. Potentials up to 750,000 volts have been produced on the inner electrode, by exciting the primary circuit with a condenser discharge. An improvement in the design is proposed, which makes use of a system of concentric shields around the inner electrode, for the purpose of distributing the potential drop in a more advantageous manner, and making it possible to go to higher voltages before cold emission occurs. Excitation of the apparatus by means of a continuous wave oscillator, and its use as a high voltage x-ray outfit for practical purposes is discussed.