Grid Leak Research Articles

Development of the fluorescent materials test chamber

Alabama A&M university and Nichols Research Corporation have recently completed construction and preliminary testing of the fluorescent materials test chamber (FMTC) with its associated vacuum subsystem at the Alabama A&M University Pelletron accelerator laboratory. The FMTC will provide physicists and material scientists with the capability of exposing a variety of samples in a controlled temperature environment to light ion bombardment. Efforts have also been completed toward the design for a low cost beam pulser subsystem for the FMTC. This equipment will allow the measurement of the fluorescent prompt decay time for a wide variety of phosphors. Current design and equipment capabilities allow for prompt fluorescence decay time measurements down to 1 μs. A vacuum tube based grid leak bias Hartley LC oscillator will be used to generate beam pulses with approximately a 2.2% duty cycle at electric field deflection frequencies of 1 and 100 kHz. This technique provides low-cost post-acceleration beam pulsing when compared to more traditional pulsed ion source arrangements.

Gridded Cross Field Tube

A triode crossed field tube has been operated as a high voltage on-off switch tube. A third, partially transparent (grid) electrode is interposed between the anode and cathode and electrically tied to the cathode by a grid leak resistor. High voltage is first applied to the anode and cathode; the magnetic field is then raised to the conduction level (~ 0.01 Tesla). Ignition does not occur because the magnetic field is too low in the grid-anode gap and the electric field is zero in the cathode-grid gap. Pulsing the grid positive relative to the cathode (~ 1 kV) then results in breakdown of the cathode-grid gap; plasma comunication between the two gaps then fully ignites the tube and closes the main power circuit. Grid ignition has been achieved at 50 kV as well as grid ignition followed by current interruption against 10 kV. These levels were limited by the use of a modified, laboratory tube and not by the physics of the technique.

Criteria for the Amplitude Stability of a Power Oscillator

Criteria for the amplitude stability of a power oscillator are derived in the form of two inequalities. These inequalities involve derivatives which are computed graphically from the constant current design diagram that is customarily used for power oscillators. In the final form of the inequalities, the derivatives used are of the double logarithmic type: that is the fractional increment of one variable divided by the fractional increment of the other. In addition, the criteria involve the ratio of fed-back power to plate power and the ratio of the time constant of the grid leak and condenser combination to the time constant of the tank circuit. There are also involved certain trigonometric functions of the grid current and plate current angles of flow.

Circuit for a Radio-Frequency Spectrometer

A circuit is described for amplitude stabilization of a regenerative detector for use in nuclear magnetic resonance experiments. A remotely located two-stage dc amplifier converts variations in oscillator grid leak voltage into compensating changes in plate supply potential. The stabilizer circuit adds little stray capacity and does not appreciably affect the oscillator design. The level of oscillation is adjustable from zero to approximately 0.3 volt peak rf signal at the detector grid. Once set, it remains essentially constant over the tuning range (4.8 to 6.3 Mc/sec). A noise figure of 2 or better is obtained. Circuit parameters for the regenerative detector and the stabilizer are given.

ON THE OPTIMUM ADJUSTMENT OF GRID LEAK RESISTANCE OF A POWER OSCILLATOR

This paper presents an experimental method to determine the optimum grid leak resistance of a power oscillator. The effect of excitation ratio on the plate efficiency and the proper method of connection of the grid leak resistance are also discussed. The experimental results are checked against the calculated values obtained from graphical method.

Progress Report, Seismological Laboratory, California Institute of Technology, 1949

In addition to those published during 1949 (list at end) the following research activities have been carried forward.Instrumental development under Benioff has included an experimental model seismometer using a capacity transducer. The pendulum is of the torsion type with an inertia reactor of approximately one kg mass. Damping is provided by a coil moving in a magnetic field with leads attached to the upper and lower torsion ribbons which are insulated from each other. In this way damping can be adjusted with an external resistance connected to the coil and any change in period brought about by reaction of the magnetic field on the coil is accounted for. The variable capacity consists of a fixed plate separated about one millimeter from a movable plate attached to a light boom on the cylinder. The mechanical magnification is approximately five. The seismometer capacitor serves to couple the grid and plate circuits of a tuned‐grid tuned‐plate oscillator. For small movements of the pendulum the'amplitude of oscillation varies very nearly linearly with the amplitude of the pendulum displacement. A voltage output of seismic frequency is derived from the oscillator grid leak and is coupled to a one‐stage amplifier by means of a resistance‐capacity network. The output of the amplifier is also coupled through a resistance‐capacity network. The time constant of the coupling circuits are such as to respond to seismic waves up to 60 sec‐period. A short‐period galvanometer (0.1 to 0.3 sec) is used for recording.

Spontaneous Resistance Fluctuations in Carbon Microphones and Other Granular Resistances

Voltage fluctuations which occur in resistance elements of the granular type when a direct current is flowing have been measured in the granular carbon microphone, commercial grid leaks, and sputtered or evaporated metal films. The results can be experessed by the formula $\overline{V_c^2} = KV^\alpha R^\beta \log (F_2/F_1),$ where $\overline{V_c^2}$ is the mean square fluctuation voltage, V is the d.-c. voltage across the resistance R, α and β are constants having values of about 1.85 and 1.25, respectively, and F 2 and F 1 are the limits of the frequency range over which the fluctuation voltage is measured. The constant K depends, among other things on the temperature, the surrounding medium, and the dimensions and material of the resistance element; for a commonly used carbon transmitter at ordinary operating conditions its value is about 1.3 × 10−11. The spontaneous voltage fluctuations and the signal due to acoustic modulation are affected in almost an equivalent manner by the applied d.-c. voltage which suggests that the two effects arise from the same type of mechanism, namely a fluctuating resistance at the points of contact between granules. Experiment shows that although the acoustic signal produces a resistance modulation which is in phase at all contacts the spontaneous resistance fluctuations are completely random. On the assumption that a region of secondary conduction, wherein the resistance fluctuation lies, surrounds each area of primary conduction as postulated in recent contact theory a value of β consistent with experiment has been deduced. On the further assumptions that thermal energy produces the mechanical fluctuations and that the equipartition law governs the distribution of energy between oscillators the observed frequency distribution follows.

Radio Data

A SECOND edition of “Radio Data Charts” by Dr. R. T. Beatty (Iliffe and Sons, Ltd. 4s. Qd. net) is to be welcomed, since the opportunity has been taken to extend its scope to cover the most recent advances in radio technique. The various abacs provided now cover wave-lengths down to five metres, while provision is made for audio-frequencies over the range 20-10,000 cycles a second. This publication contains a series of thirty abacs with explanatory notes and examples, by means of which many of the quantities required in radio-frequency work can be obtained directly without the necessity for laborious calculation. In addition to the means for obtaining the inductance, capacitance and reactance of coils and condensers at both audio- and radio-frequencies, charts are given for obtaining the efficiency and time constant of a grid leak and condenser combination, for designing coils to have a minimum radio-frequency resistance and for the design of attenuation circuits, to select only a few typical examples. The whole series of charts is printed in a clear and easily accessible form and will be found to be most valuable to engineers, experimenters and students dealing with radio-frequency work.

Improvements to the “scale of two” thyratron counter

An improved circuit is described for a two thyratron counting unit such as is used in the “scale of two” thyratron counter. The advantages which result are that the minimum time of resolution is reduced and a source of occasional failure (“jamming”) of the counter is removed. Any type of impulse may be counted and, with selected thyratrons, impulses separated by only 0.00015 sec. may be reliably counted as separate. This makes possible a counting speed of 8000 random impulses per minute with a probable loss of only 2%.The modification introduced into the circuit used in the original “scale of two” counter is that small condensers (0·0015 μF max) are cross connected between the grids and anodes of the pair of thyratrons, and the extinguishing condenser connected between the anodes is made small (0·02 μF max). The anode resistances are 10,000 ohms and grid leaks 100,000 ohms for the values of capacity mentioned. A valve amplifying stage is introduced, to pass the impulses from the modified circuit of the first pair to the second pair of thyratrons. For greater convenience the mechanical counting meter operated by the third pair is modified to count in multiples of eight so that no multiplication of its reading is necessary.

A Screen-Grid Voltmeter without External Leak

A highly sensitive screen-grid voltmeter using type 24 (or 32) tubes without grid leak is described. Static and transrectification characteristics of single and twin tubes and of tube circuits including a load resistance are shown. The voltmeter is independent of frequency from low audio to ultra-high frequencies. The useful range of the twin (full wave rectifying) circuit is from 0.05 to 20 volts root-mean-square; this can be extended to higher voltages as desired by suitable choice of blocking condensers. The range from 0.1 to 10 volts has a linear calibration curve. Over this range the transrectification factor ∂I t /∂e g = 2.05 × 10-3. The input admittance of the device is very low. Applications of the circuit for use as a peak voltmeter, as a resonance indicator, as a radio detector, and as a high-frequency intensity meter are briefly discussed.

A Simple High Resistance

HIGH resistances to carry small currents, as in grid leaks and electrostatic work, can be very simply made by smoking a rod of silky quartz. A piece of this rod in its ordinary condition is wiped clean and fitted with end metal connexions of closely fitting metal tubes or of copper wire wound round and twisted tight; connecting wires are soldered on or left over from the winding; the dimensions of the quartz rod may be chosen as desired, for 1010 ohms 6 cm. length has been used, for 1014 ohms 10 cm.; the diameter has been 4 mm. The metal-quartz junction is then well covered with india-ink and dried by warming, the quartz surface being kept free from handling. The whole is now smoked over a small smoky gas flame given by burning, at the end of a glass tube, coal gas that has been led through petrol; a good dense deposit is made round the india-ink junctions, and over the rest of the quartz a fairly even deposit not too dense is formed. After the rod is allowed to cool, the resistance is measured and can then be very easily and quickly adjusted in value by evenly wiping off some of the deposit with clean dry cotton wool or by adding a little more soot. The resistance may then be sealed into a clean quartz tube or left bare or coated with paraffin wax, of good white quality, by simple dipping.

A resistor for high-voltage measurements

A cheap and compact 100-megohm resistor for the measurement of direct or alternating voltages of power frequency may be constructed from thin-film resistance units similar to those commonly used as “grid leaks”. The construction and performance of such a resistor are described. Errors due to corona or leakage along supports appear to be absent. There is a change of resistance with loading, due to the self-heating of the resistor. The magnitude of this change is approximately proportional to the square of the applied voltage and amounts to −2.6 per cent at 100 kV. By taking this change into account, voltages up to 100 kV may be measured correctly to 1 or 2 parts in 1000.

An Analysis of Power Detection

The reasons for overloading of detectors as output devices is discussed. It is shown that in grid circuit rectification replacing the grid leak with a high impedance choke extends the overload point considerably. Using a 247 type pentode 800 milliwatts are obtained at the output circuit with seven per cent maximum distortion.

An Oscillator Having a Linear Operating Characteristic

A review of conventional linear equilibrium conditions is given. It is shown that these conditions are not usually at all applicable to practical oscillators because the operating region is not to be so simply described. The relation of nonlinear effects tofrequency modulation is pointed out. A modified type of oscillator which conforms to the elementary linear conditions is described and its properties are discussed. The results are applied to a conventional grid leak and condenser oscillator to explain its modulating characteristics. The simplicity of the new circuit permits a discussion of amplitude inertia effects.

Some notes on grid circuit and diode rectification

The equivalent input resistance of a grid leak and condenser in parallel and the combination in series with a diode or the grid-cathode circuit of a triode are calculated for various combinations by means of the static I g -E g , characteristics and an extension of the work of Colebrook and Peterson and Llewellyn. The equivalent internal resistance of the diode is also calculated and an expression for the equivalent generator is also derived. The relations between X, the grid condenser modulation frequency reactance, R, the grid leak, and m, the percentage modulation, to give good quality are considered theoretically and experimentally. The part played by the values of X, R, and m in the quality of the audio-frequency output was first considered by Terman and Morgan. Limiting values of X and R for the cases of the triode and diode are also estimated.

Variable High Resistance Grid Leaks

THE difficulty of obtaining very high resistances for certain forms of experiment can be overcome by the use of suitable photoelectric cells, as has been suggested by Messrs Adam Hilger, Ltd. Not only have these the necessary resistance, but also they can be made to act with controlled variability as variable grid leaks. This result we have obtained by mounting the photoelectric cell and grid connexions of a capless valve on amber and vitreosil, the valve itself being suspended in a ring of vitreosil. The cathode of the photoelectric cell is attached to the grid, and the anode to whichever side of the filament is found most suitable. The cell is completely darkened except at one point where it is exposed to a small lamp lit from batteries controlled by a continuously variable rheostat. The lamp connexions, batteries, and rheostat are contained in a screened tube and box, with the screen earthed to avoid variations due to the adjustment and handling of the rheostat.

Measurement of Resistance and Impedances at High Frequencies

It is well known that the resistance of a conductor increases with frequency. At very high frequencies the usual method of measurement fail, especially if the unknown resistance is more than about one hundred ohms. In the following it is shown that the absolute value of the unknown impedance, when put across the end of a transmission line, is a simple function of the ratio of the currents, measured at the beginning and the end of a transmission line. This method has been tested out at a wavelength of 21.8 meters, measuring the resistance of a number of grid leaks and of a decade box. The a-c resistance was considerably higher than the labeled d-c value. At the same time, the shunted capacity across the resistors has been measured and valutes have been obtained, which agree with expectations. In general, this method lends itself to measurement of impedances of any kind; but it only gives the absolute value of the unknown impedance. However, by means of a known capacity or resistance, connected in series or in parallel with the impedance to be determined, the phase of the latter and, therefore, its real and imaginary components are found. At frequencies corresponding to wavelengths longer than 100 meters the line becomes rather long. In this case another method, as described in the second part of this paper, can be applied.

Some Properties of Grid Leak Power Detection

It is shown theoretically that grid leak power detection can be obtained without undue distortion provided the grid condenser reactance X to the modulation frequency, and the grid leak resistance R are related to the degree of modulation m in such a way that X/ R . m/ V1 -iM2. The load limit of the grid leak power detector is the point where plate rectification becomes appreciable, i. e., when the straight line part of the plate characteristic is exceeded. Theory is developed which indicates that the maximum carrier voltage a grid leak power detector will handle is slightly less than one-half the input voltage that the tube will take as a properly adjusted amplifier operating at the same plate voltage; and that the maximum undistorted audio-frequency output voltage of the detector is in the neighborhood of one-third the output voltage developed by the corresponding amplifier. The equivalent input resistance of a grid leak power detector is proved to be greater than one-half the grid leak resistance. Experimental results are given which verify the theoretically determined point at which distortion becomes excessive. These results indicate that the grid leak power detector is more sensitive than the C bias power detector, and will ordinarily give at least as much undistorted output when operated at the same plate voltage.

AN AMPLIFIER FOR MEASURING SMALL CURRENTS

An amplifier is described which uses a four electrode tube of the ordinary type, and gives maximum current sensitivities of the order of 10−18 amperes. A novel feature consists in using as grid leak the positive ion emission of the filament.

The valve-maintained quartz oscillator

A quartz resonator connected between the grid of a valve and either of the other electrodes gives rise to oscillations in the plate circuit when the constants of that circuit satisfy certain conditions. The frequency of the oscillations depends mainly upon the natural frequency of the quartz, and is, therefore, fairly constant when the temperature is constant. In addition to temperature effects, which are not considered in the present paper, variations of frequency can be produced by (i) variations of the air-gaps of the resonator, i.e. the air-gaps between the quartz and the two electrodes of the resonator; (ii) variations of the constants of the plate circuit; and (iii) variations of the inter-electrode capacities and conductances. The object of the investigation is the study of these variations, both theoretically and experimentally.In the theoretical treatment, the quartz is replaced by a circuit made up of an inductance, a resistance and a capacity in series, the whole being in parallel with a second capacity. The validity of such a substitution has been studied experimentally by Dye.The combination of quartz resonator, valve and plate circuit is represented by a network of admittances, from which three circuit equations are obtained; to these is added the fundamental equation of the valve. These four equations yield a single linear differential equation of the fifth order, with constant coefficients. In the case under investigation, it is only required to find the frequency, and the amplitude of oscillations after the latter have reached a constant amplitude. This consideration affords a means of reducing the differential equation to one of the same type, but of the second order. From the latter equation, of which the general solution is known, formulae are developed for the changes of frequency resulting from variations in the resistance and capacity of the plate circuit, in the resistance of the grid leak, in the inter-electrode capacities, and in the air-gap. These formulae provide useful information as to the influence of each variable on the frequency, and can with advantage be applied to the choice of the best circuits and the best valve for a quartz oscillator.As the oscillations build up after switching on, the average plate conductance of the valve alters gradually. When the amplitude of oscillation has reached its final value, the value of the average plate conductance is such as to annul the damping term, i.e. the coefficient of dy/dt in the differential equation. From this condition the average plate conductance can be determined for any values of the circuit variables. The amplitude of oscillations in the plate circuit is a function of this average plate conductance.Formulae are developed which give the conditions for the maintenance of oscillations, and an investigation is made of the variations of amplitude with variations in (a) capacity in the plate circuit, (b) resistance in the plate circuit, and c) grid conductance.Points of importance in the choice of circuits and valves are deduced from the formulae and the theoretical curvesTwo types of quartz oscillator are investigated, namely, the cases in which the quartz resonator is connected between i) the grid and the plate of the valve, and (ii) the grid and the filament of the valve.A fairly complete experimental investigation is described. The admittances of the valve and of the circuits were measured, and the equivalent electrical network of the quartz was determined. The behaviour of the quartz oscillator was then investigated by altering in turn each variable in the circuits. The theoretical and experimental results are compared and discussed.The paper does not deal with circuits especially devised for obtaining large outputs from quartz crystals. Its object the study of the influence of the circuit variables on the frequency and on the power generated as regards the simple Pierce circuits.