US2165509A - Oscillation generator - Google Patents

Description

July 11, 1939. D. H. RING ET AL OSCILLATION GENERATOR Filed April 22, 19:58

/N VE N TORS z aHR/NG By WrW//vrR/NGHAM 5% A TTOR/VEY Patented July 11, 1939 UNITED STATES 2,165,509 osoILLA'rioN GENERATOR Douglas H. Ring, Little Silver, and William T. Wintringham, Chatham, N. J., assignors to Bell p Telephone Laboratories, Incorporated, New

York, N. Y., a corporation of New York Application April ,22, 1938, Serial No. 203,558

Claims.

This invention relates to oscillation generators and more particularly to high frequency generators for use in radio receivers for demodulating signal modulated carrier waves.

5 In accordance with the invention, the primary source of the oscillations may be the thermal agitation of free electrons in a resistive impedance or the random fluctuations of the electron current in the space path of a therminoic vaclO uum tube. It is well known that resistive conductors or therminoic electron discharge paths behave as though there were sources of alternating electromotive forces of all frequencies in a continuous range from zero to at least several 5 megacycles per second and of substantially uniform amplitude. We have found that, by 'amplifying the thermal agitation voltages and sharply selecting from the amplified oscillations those lying within a very narrow band of frequencies,

a substantially pure sinusoidal oscillation is obtained having a frequency approximately equal to the mid-frequency of the selected band. The amplitude of the oscillation tends to vary somewhat and in a rather irregular manner, but by the use of an amplitude limiter or other amplitude controlling means, the greater part of the variation may be suppressed. Our experiments have indicated that the amplitude variations tend to become slower as the band width of the selecting device is reduced and that'the range of variation, even without amplitude regulation, is not such as to diminish the utility of the generator for many purposes such as the demodulation of high frequency signal waves.

A satisfactory approximation to a sinusoidal oscillation for the demodulation of speech signals can be obtained by selecting from the source a band of frequencies of width as great as fty cycles per seco'nd located in the superaudible 40 range. Usually a narrower band will be preferred, but if the width is too small diiliculties may arise from the reduction of the total energy of the selected band. Band widths down to at least ve cycles per second are practicable and provide resultant oscillations that are both purer in wave form and less subject to rapid amplitude variations. The location of the selected band in the frequency scale is subject only to the practical restrictions pertaining to the design and construction of narrow band filters. By the use of piezoelectric quartz crystal elements suitable band widths may be obtained at frequencies from thirty kilocycles per second up to several megacycles per second.

Particular features of the invention will appear from the following detailed description and from the accompanying drawing which illustrates one embodiment and its application in radioreception. Of the drawing, Fig. 1 shows the circuit arrangement of an oscillation generator in ac- 5 cordance with the invention and Fig. 2 is a block schematic illustrating the application of the invention in a radio receiver.

Referring to Fig. 1, `resistance R connected be- Y tween the grid and cathode of vacuum tube aml0 plier i0 constitutes a source of thermal agitation electromotive forces from which the ultimate oscillations are derived. Preferably this resistance has a fairly large value, from 20,000 ohms to 100,000 ohms and the grid of tube l0 is l5 negatively biased suiciently to make the fluctuation voltages, Schott effect and the like, in the plate circuit negligibly small.

To the output terminals of the tube is connected an amplifier Il which should have a gain 20 of 50 decibels or more and which may comprise three or more vacuum tube stages. Following the ampliier is a three-section narrow band piezoelectric crystal F. The filter illustrated is of the ladder type but other well-known con- 25 figurations may also be used. The three sections of the lter are similar, each comprising a pair of shunt condensers of capacity 'C2 and a series branch constituted by a piezoelectric' quartz crystal X in Vseries with a condenser of capacity C1. 30.

'Ihe adjacent shunt capacities are provided by single condensers of capacity 2C2`.

The pass band of the filter is llocated adjacent to the principal resonance frequency of the crystal and its width may be controlled by adjust- 35 ment of the values of capacities C1 and C2. By making the series capacities C1 very small and the shunt capacities C2 large the band width may be made as small as may be desired. Preferably the quartz crystals are in the form of 40 rectangular plates with their major surfaces in the plane ofthe optical and mechanical axes of the crystal as described in U. S. Patent 2,045,991, issued June 30, 1936, to W. P. Mason. With crystal plates of this type the principal reso- 45 nance occurs at a fairly low frequency and is well separated from resonances representing other vibration modes. In an experimental model of the invention the crystal plates had the dimensions 4559 millimeters in the direction of the 50 mechanical axis, 18.24. millimeters in the direction of the optical axis, and one millimeter thickness. The principal resonance frequency was 59.966 kilocycles per second and, with values of capacities C1 and C2 equal to 200 and 1200 micro 55 microfarads respectively, a band Width of about five cycles was obtained. By the use of quartz crystals of the type described, filters with band Widths of the above order and with very sharp cut-offs are readily constructed to operate at frequencies from about 30 kilocycles to 150 kilocycles or greater,

The output terminals of the lter are connected to the input of an amplifier, the rst stage of which, tube I2, is shown in detail and the succeeding stages are indicated by block I3. All of the stages may be similar. Tubes I and I2 are shown as screen-grid tetrodes, but other tubes such as pentodes or triodes may also be used. The gain of this amplifier need not be large and may be proportioned to provide Whatever output energy is desired. The plate current of tube IU and the grid bias of tube I2 are supplied through choke coils I4 and I5 which preferably should be of sufficiently large inductance to avoid resonance with the lter capacities at a frequency close to the filter band.

Control of the output amplitude is provided by an automatic volume control circuit comprising a diode rectifier I6 coupled to the output terminals of amplier I3 and shunted by a resistance Il. The control voltage is developed across resistance I'I and is fed back to the grid` of tube I2 and the grids of amplifier I3 through a path including a resistance-capacity timing lter I8. Battery I9, or other equivalent source, supplies a normal. negative bias to the amplifier grids. If desired, the last stage vacuum tube of amplifier I3 may be operated under a condition of space current saturation to provide amplitude limitation.

Instead of resistance R, vacuum tube il! may be used by itself as a source of voltage fluctuation. For operation in this manner resistance R may be reduced to Zero and a positive bias applied to the amplier grid. Under this condition the fluctuations of the electron stream in the plate circuit are emphasized.

The application of the invention to the r-eception of single side-band suppressed carrier signals is illustrated by the block schematic diagram of Fig. 2. Single side-band Waves are received on antenna 20 and are reduced in frequency in demodulator 2| by beating with Waves from an adjustable frequency heterodyne oscillator 22. The reduced frequency single side-band oscillations are selected by band-pass lter 23 and amplied by amplier 24. From the output of amplier 24 the oscillations pass to a second demodulator 25 in which they are beat with oscillations from the homodyne generator 26 which is of the type shown in Fig. 1. The demodulated signal curr-ents are then passed to signal amplifier 21 and to the signal output circuit.

Generator 26 provides a Wave of fixed frequency which for accurate demodulation of the side-band oscillations impressed on demodulator 25 must be such as to coincide with the position of the absent carrier wave. This may be accomplished by the adjustment of the rst beating oscillator 22. With a band selecting filter of the particular type described in connection with Fig. 1, the frequency of the homodyne oscillations would preferably be between about 30 and 150 kilocycles. By the use of other types of piezoelectric crystal selectors higher frequencies up to several megacycles may be used.

What is claimed is:

1. An oscillation generator comprising a bandpass wave lter having a single transmission band of Width less than fty cycles per second located in a superaudible frequency range, a high gain amplifier connected to the input terminals of said lter, and a source of thermal agitation voltages coupled to the input terminals of said amplifier.

2. An oscillation generator in accordance with claim 1 in which the said source of thermal agitation voltages is constituted by a metallic resistance element.

3. An oscillation generator in accordance with claim 1 in which the said source of thermal agitation voltages is constituted by the electron path of a thermionic vacuum tube.

4. An oscillation generator in accordance with claim 1 in Which the said Wave lter includes piezoelectric quartz crystals as 4elements determiing the frequency and Width of the transmission band.

5. An oscillation generator comprising a bandpass Wave filter having a single transmission band of Width less than fifty cycles per second located in a superaudible frequency range, a high gain amplifier connected to the input terminals of said lter, a Vsource of thermal agitation voltages coupled to the input terminals of said amplifier, and a second amplifier coupled to the output terminals of said filter, said second amplifier including control means responsive to the output voltage thereof for maintaining the amplitude of the output voltage at a substantially constant level.

DOUGLAS H. RING. WILLIAM T. WINTRINGHAM.

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