Noise In Tubes Research Articles

Observations on ion oscillations in a cylindrical-beam tetrode under hard vacuum conditions

A study of the physical mechanism of ion oscillations was begun after the discovery of spurious sidebands in the spectrum of a DC-operated low-voltage reflex klystron which was undergoing development at this laboratory. These sidebands were observed to be as strong as 40 db below 1 mw in some tubes and were separated from the carrier by as little as 0.6 mc or as much as 15 mc, depending on which klystron was examined and what voltages were applied to it. It was observed that the frequency separation and magnitude of the sidebands were a function of the exterior cooling conditions, an observation which suggests a relation between the sidebands and the gas pressure within the tube. It has been recognized 1 that ion plasma oscillations can be a source of noise in microwave tubes; consequently it was decided to build a cylindrical tetrode structure whose DC configuration was similar to that of the klystrons and to study the details of the oscillations.

Some Recent Advances in Microwave Tubes

This is a review paper which discusses in simple terms high-power klystrons, double-tuned circuits for reflex klystrons, noise in traveling-wave tubes and its reduction by velocity-jump or space-charge-wave deamplification, periodic magnetic focusing of electron beams, and the backward-wave oscillator.

A Multichannel Noise Spectrum Analyzer for 10–10 000 Cycles

A multichannel noise spectrum analyzer is described which allows the rapid measurement of flicker noise in vacuum tubes. The instrument gives either the equivalent saturated diode current or the equivalent noise resistance of the tube at 10, 30, 100, 300, 1000, 3000, and 10 000 cycles. The seven channels are read simultaneously; a whole spectrum may therefore be measured in 5 minutes. The instrument incorporates a quadratic thermistor-detector circuit with a dc voltmeter of variable time constant and wide-band filter circuits. The latter have a band width of 0.75 times the center frequency, but still give an accurate measurement of the noise at the center frequency. These large band widths reduce the time constant of the detector circuit needed for performing an accurate noise power measurement.

Noise mechanisms in oxide-coated cathodes

Synopsis A critical review is given of a number of mechanisms that have been (or might be) invoked, to explain flicker noise in vacuum tubes. It is shown that only Schottky's emission center theory and a new depletion layer theory can account for the observed noise. The former theory most likely applies to W, Th-W and L-cathodes, whereas the latter theory seems to give the right order of magnitude of noise for oxide-coated cathodes.

Low frequency noise in electron tubes: B. Measurements on triodes under normal operating conditions

Measurements have been made on the noise in electron tubes in the range 1–5000 c/s. For W-cathodes deviations from the white shot-noise spectrum were found only for very low frequencies. W-Th-cathodes showed flicker effect with an intensity ∼ v −1 . With oxide-cathodes two kinds of flickernoise were found: 1) intensity ∼ v −1 , probably due to changes in the interior of the oxide layer; 2) intensity ∼ ( v 2 + β 2 ) −1 , probably due to changes at the surface of the cathode. The current dependence (range 10–2000 μ A) could be derived from the measurements according to the known space charge reduction of the emission fluctuations. Some existing theories on flicker noise are confronted with the results of our measurements.

Low frequency noise in electron tubes: C. Electrometer tubes

Synopsis This paper describes some noise measurements on electrometer tubes of the types 4060 (Philips), BM4A (Ferranti), VX41 (Victoreen) and on a normal pentode of the type AF7 (Philips). A satisfactory agreement exists between the grid current derived from direct measurements and from noise measurements.

Low frequency noise in electron tubes: A. space charge reduction of flick er effect

Synopsis An analysis is given of the space charge reduction of flicker effect in diodes and triodes. This analysis shows that the influence of this effect can be given by an equivalent input intensity (or input noise resistance) which is nearly a constant part of the intensity of the fluctuations in the total emission current if the cathode temperature is kept constant. This paper is presented in three parts, of which this is the first one, which describes the results of some theoretical considerations concerning the behaviour of flicker effect in diodes and triodes under normal operating conditions. The later parts of this paper will show results of low frequency noise measurements.

Noise in Traveling-Wave Tubes

After a discussion of gain, noise factor and attenuation in conventional traveling-wave tubes (twt), a dispersive twt is described and experimental results given. Examination of results and theory lead to the recognition of several types of partition noise as the cause of much of the noise in traveling-wave tubes. These are analyzed and approximate expressions for their magnitude are obtained. Noise due to electron-wave type amplification and thermal noise originating in the resistive part of the helix are also discussed. Experiments are described.

Plane Waves in an Ionized Gas with Static Electric and Magnetic Fields Present

General equations are derived which specify such waves in a first approximation when their amplitudes are small. The equation of condition (19) (i.e. the equation of dispersion) is deduced and the nature of its roots is discussed. It is shown that under suitable conditions some of the roots lead to wave-groups which grow as they progress and others lead to waves which everywhere grow in time. The theory includes as special cases many of the results previously obtained for radio-propagation, ?plasma? oscillations, and the like. Applications are indicated to electric oscillations and noise in discharge tubes, and to cosmic noise.

Discussion on “Fluctuation noise in vacuum tubes which are not temperature-limited”

Discussion on “Fluctuation noise in vacuum tubes which are not temperature-limited”

Discussion on “Fluctuation noise in vacuum tubes which are not temperature-limited”

Discussion on “Fluctuation noise in vacuum tubes which are not temperature-limited”

Fluctuation noise in vacuum tubes which are not temperature-limited

This paper is concerned chiefly with a discussion of whether or not the anode stream of a thermionic valve produces a component of fluctuation voltage equal to that which would be produced by a metallic conductor of equal resistance at the cathode temperature. In a recent paper, G. L. Pearson has shown that when the anode current is very small, the fluctuation voltage is equal to that in a metallic conductor of equal resistance at a temperature about half that of the cathode.In this paper it is shown that when the anode current is very small a familiar expression for the shot voltage assumes a form suggestive of a thermal-agitation voltage in a resistance at half the cathode temperature. This form is due to the dependence of anode current on cathode temperature when the anode is negative. Thus it would seem that Pearson's work does not necessarily prove that the anode stream is the seat of a thermal-agitation voltage: his results can equally well be interpreted as a shot-voltage effect, and such interpretation gives a direct explanation of why the relevant temperature is only half that of the cathode.

Fluctuation noise in vacuum tubes which are not temperature-limited

This paper is concerned chiefly with a discussion of whether or not the anode stream of a thermionic valve produces a component of fluctuation voltage equal to that which would be produced by a metallic conductor of equal resistance at the cathode temperature. In a recent paper, G. L. Pearson has shown that when the anode current is very small, the fluctuation voltage is equal to that in a metallic conductor of equal resistance at a temperature about half that of the cathode.In this paper it is shown that when the anode current is very small a familiar expression for the shot voltage assumes a form suggestive of a thermal-agitation voltage in a resistance at half the cathode temperature. This form is due to the dependence of anode current on cathode temperature when the anode is negative. Thus it would seem that Pearson's work does not necessarily prove that the anode stream is the seat of a thermal-agitation voltage: his results can equally well be interpreted as a shot-voltage effect, and such interpretation gives a direct explanation of why the relevant temperature is only half that of the cathode.

Fluctuation Noise in Vacuum Tubes*

The fluctuation noises originating in vacuum tubes are treated theoretically under the following headings: (1) thermal agitation in the internal plate resistance of the tube, (2) shot effect and flicker effect from space current in the presence of space charge, (3) shot effect from electrons produced by collission ionization and secondary emission, and (4) space charge fluctuations due to positive ions. It is shown that thermal agitation in the plate circuit is the most important factor and should fix the noise level in low noise vacuum tubes; shot noise and flicker noise are very small in tubes where complete temperature saturation is approached; shot noise from secondary electrons is negligible under ordinary conditions; and noise from space charge fluctuation due to positive ions is usually responsible for the difference between thermal noise in the plate circuit and total tube noise. A method is deduced for the accurate rating of the noise level of tubes in terms of the input resistance which produces the equivalent thermal noise. Quantitative noise measurements by this method are reported on four different types of vacuum tubes which are suitable for use in the initial stage of high gain amplifiers. Under proper operating conditions the noise of these tubes approaches that of thermal agitation in their plate circuits at the higher frequencies and is 0.54 to 2.18 × 10−14 mean square volts per cycle band width in the frequency range from 200 to 15,000 cycles per second. Below 200 cycles per second the noise is somewhat larger. The minimum noise in different types of vacuum tube circuits is discussed. These include input circuits for high gain amplifiers, ionization chamber and linear amplifier for detecting corpuscular or electromagnetic radiation, and photoelectric cell and linear amplifier for measuring light signals. With the aid of these results it is possible to design circuits having the maximum signal-to-noise ratio obtainable with the best vacuum tubes now available.

Fluctuation Noise in Vacuum Tubes

The fluctuation noises originating in vacuum tubes are treated theoretically under the following headings: (1) thermal agitation in the internal plate resistance of the tube, (2) shot effect and flicker effect from space current in the presence of space charge, (3) shot effect from electrons produced by collision ionization and secondary emission, and (4) space charge fluctuations due to positive ions. It is shown that thermal agitation in the plate circuit is the most important factor and should fix the noise level in low noise vacuum tubes; shot noise and flicker noise are very small in tubes where complete temperature saturation is approached; shot noise from secondary electrons is negligible under ordinary conditions; and noise from space charge fluctuation due to positive ions is usually responsible for the difference between thermal noise in the plate circuit and total tube noise. A method is deduced for the accurate rating of the noise level of tubes in terms of the input resistance which produces the equivalent thermal noise. Quantitative noise measurements by this method are reported on four different types of vacuum tubes which are suitable for use in the initial stage of high gain amplifiers. Under proper operating conditions the noise of these tubes approaches that of thermal agitation in their plate circuits at the higher frequencies and is 0.54 to 2.18×10−16 mean square volts per cycle band width in the frequency range from 200 to 15,000 cycles per second. Below 200 cycles per second the noise is somewhat larger. The minimum noise in different types of vacuum tube circuits is discussed. These include input circuits for high gain amplifiers, ionization chamber and linear amplifier for detecting corpuscular or electromagnetic radiation, and photoelectric cell and linear amplifier for measuring light signals. With the aid of these results it is possible to design circuits having the maximum signal-to-noise ratio obtainable with the best vacuum tubes now available.

Fluctuation Noise Due to Collision Ionization in Electronic Amplifier Tubes

Ions formed by collision in electronic amplifier tubes move toward the cathode and control-grid and produce momentary increases in the space-charge limited current (current pulses). These current pulses are random in time and produce fluctuation noise analogous to the shot-effect. An approximate theory is presented, applicable to cathodes of large diameter (ca. 0.1 cm). Both the noise and the increase in the average value of the electron current due to the presence of ionization are calculated in terms of the ``size of the elementary event'' or the integral of the current pulse. It is not feasible to calculate the latter directly but it may be evaluated indirectly by measurements of the increase of average current. The noise calculated from the values so obtained and from the ionization probability data of Smith, Bleakney and Tate is in close agreement with that observed experimentally. The production of noise in tubes with oxide cathodes containing mercury vapor, argon and the gases naturally evolved from the electrodes and tube walls is investigated experimentally as affected by pressure and electron current density. In the case of a tetrode containing mercury vapor or evolved gases the variation with pressure is linear up to 10−3 mm; in the case of argon the noise varies as the 1.1 power of the pressure. In the mercury vapor tetrode a variation as the 3/2 power of the plate current was found, as compared to a 5/3 law, expected theoretically. These measurements were made at 620 kilocycles. The noise per frequency interval was measured as a function of frequency over the range 500–1500 k.c. At pressures of the order of 10−4 mm in mercury vapor a decrease at the higher frequencies was observed; with argon the noise was uniform. With mercury vapor at higher pressures (ca. 4×10−3) the noise spectrum became peaked, the frequencies of the peaks depending upon the electron current flowing (electrode potentials). The hypothesis that these peaks are due to oscillations of positive ions in the potential trough surrounding the cathode is in accord with the principal experimental facts. With further increase in pressure continuous oscillations are produced, sustained apparently by a regenerative action of the space charge upon the oscillating ions. The frequencies of these oscillations are affected only by the electrode potentials and not at all by the external electrical circuit.

A Study of Noise in Vacuum Tubes and Attached Circuits

The noises originating in vacuum tubes and the attached circuits are investigated theoretically and experimentally under three headings: (1) shot effect with space charge, (2) thermal agitation of electricity in conductors, (3) noise from ions and secondary electrons produced within the tube. A theoretical explanation of the shot effect in the presence of space charge is given which agrees with experiment insofar as a direct determination is possible. It is shown that the tubes used should be capable of operating at full temperature saturation of the filament in order to reduce the shot effect. In the computation of the thermal noise originating on the plate side of a vacuum tube, the internal plate resistance of the tube is to be regarded as having the same temperature as the filament. Noise produced by ions within the tube increases as the grid is made more negative. With tubes properly designed to operate at temperature saturation it is possible to reduce the noise on the plate side to such an extent that the high impedance circuits employed on the grid side of the first tube of a high gain receiving system contribute practically all of the noise by virtue of the thermal agitation phenomenon.