US1751271A - Radio transmitting apparatus - Google Patents

Description

March 18, 1930. J. F. FARRINGTQN 1,751,271

RADIO TRANSMITTING APPARATUS Filed April '7, 1925 AMPL/F/ER f/yz N v w Wrenfar:

(/00!) ffar/Myfm q My Patented Mar. 18, 1930 UNITED STATES PATENT OFFICE JOHN F. FARRINGTON, OF FLUSHING, NEW YORK, ASSIGNOR, BY MESNE ASSIGNMENTS, TO WESTERN ELECTRIC COMPANY, INCORPORATED, A CORPORATION OF NEW YORK RADIO TRANSMITTING APPARATUS Application filed April 7, 1925. Serial No. 21,298.

tively wide range. Various conditions such as the load impedance of the aerial circuit and variations in plate potential of the space discharge tubes tend to change the frequency of the generated oscillations by an amount that is undesirable at high frequencies although the percentage variation may be small.

An object of this invention is to provide a radio transmitter capable of operating at short Wave lengths at substantially constant frequency.

Another object is to provide a constant frequency transmitting system in which the frequency of oscillations is unaffected by conditions in the load circuit. A further object is to provide a short wave transmitting apparatus in which the frequency of the radiated wave may be readily controlled.

These objects and others which will be apparent as the nature of the invention is disclosed are attained by employing a source of oscillations operating at a considerably lower frequency than that which it is desired to radiate whereby variations in impedance in the output circuit are prevented from affecting the frequency of the oscillations generated. Oscillations of this lower frequency, which are more readily controlled, are impressed upon a harmonic generator in which the frequency is doubled. Waves from this generator are passed through a second harmonic generator in which the frequency is again doubled to produce the fourth harmonic of the original frequency, which is modulated in the second harmonic generator, am-

'used as a harmonic generator and as a mode" plified if desired, and radiated. I

A feature of this invention is a source of oscillations operating at a frequency substantially lower than that of the radiated wave.

Another feature is a space discharge device ulator for combining the generated oscillations with signal currents.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the claims appended hereto, the invention itself, its objects and advantages, the mode of its operation and the manner of its organization will bewbetter understood by referring to the following description taken in connection with the accompanying drawing forming a part thereof in which Figure 1 is a diagrammatic representation of a radio signaling system embodying this invention; Fig. 2 is a simplified circuit equivalent to the oscillation circuit of Fig. 1; and Fig. 3 is a vector diagram to be used in the explanation of sr In Fig.1 there is shown a space discharge oscillation generator 1 adapted to generate high frequency currents, the frequency of which is determined by tuned circuit 2. Al-

though this oscillation generator is herein shown as a space discharge device operating with a particular circuit, any other suitable source of oscillations of sufliciently constant frequency may be employed if desired. Space current is supplied to space discharge device 1 from direct current source 3 through choke coil 4 which prevents the high frequency oscillations from being im ressed upon the source 3 and short-circuite to-ground. Gondenser 4: serves to by-pass small residual high frequency currents which are not absorbed by the coil 4. Coil 47, coupled to tuned circuit 2, is controlled by key 48 to vary the frequency of the oscillator slightly for code signaling should it be desired to communicate telegraphically.

Energy is transferred to amplifier tube 7 through a coupling circuit herein shown as a potentiometer 6 having a variable tap to enable the high frequency potential applied to tube 7 to be adjusted. Potentiometer 6 is wound non-inductively to avoid standing waves thereon. 1

Grid polarizing potential, the value of which may be varied by potentiometer 34 is supplied from source 31 through filter 32 and choke coil 33. Condenser 35 operates as passes choke coil 33. Stopping condensers 5 serve to prevent direct current from flowing through the potentiometer 6 and to separate the plate current to tube 1 from the grid polarizing current of tube 7.

By operating this amplifier in such a manner that the grid is always negative, .variations in input impedance are decreased. As the tube acts as a unilateral device, it prevents fiuctuations in load impedance from being transmitted in the reverse direction and affecting the impedance of the circuit into which the oscillator works.

As a further precaution to maintain con stant frequency the various units employed in the oscillator circuit are constructed to have constant values and the entire assembly is enclosed in a metallic shield and suspended by springs to prevent mechanical vi brations from disturbing the constants of the elements. 2

Energy is transferred from amplifier tube 7 to the space discharge tube 8 operating as a harmonic generator by means of a coupling circuit consisting of inductances 9 and 10 and condenser 11. Grid polarizing potential is supplied from source 31 through potentiometer 36 and filter 37 to harmonic generator 8.

The positive cycles of the oscillations supplied to tube 8 from amplifier 7 are of sufficient amplitude to counteract the grid biasing potential and to make the grid strongly positive thereby producing a space current in tube 8 rich in harmonic components. Although the tube 8 and the other tubes are herein represented as having their filament current supplied from source 12, any suitable type of tubes and any desired source of filament current may be employed. Space current is supplied from source 13 through choke coil 14 which prevents oscillations of high frequency being impressed upon the space current source and being short-circuited to ground. Condenser 38 by-passes high frequency currents and prevents them passing through the direct current sources.

A tuned coupling circuit composed of condenser 15 and inductance 16 selects the second harmonic of the originally applied frequency. The coupling circuit 15, 16 is of relatively low impedance to the fundamental and the undesired harmonics. The low impedance of the coupling circuit to the fundamental is of importance. The reason for this is that the operating frequency is so high that the tube 8 has a backwardly directed transducing effect due to the tube capacity and other causes. The low impedance for the fundamental frequency approximates a short circuit so that changes in impedance of any part of the circuit to the right of coupling circuit 15, 16 are caused to have lessened effects upon devices to the left of circuit 15, 16 and particularly upon the oscillator 1.

The selected harmonic v is transferred by means of the coupling circuit 15, 16 to a second harmonic generator 17 operating in a manner similar to harmonic generator 8. I

Space current for tube 17 is supplied from source 13 through radio frequency choke coil 18 and audio-frequency choke coil 23. Grid polarizing potential is supplied from source 31 through potentiometer 39, filter 40 and choke coil 41. Condensers 42 operate similarly to condensers 38. Direct current is prevented from flowing through the coupling circuit by stopping condensers 43. v

The second harmonic of the frequency impressed upon tube 17, corresponding to the fourth harmonic of the original frequency applied from source 1, is selected by the tuned circuit consisting of condenser 19 and inductance 20. This fourth harmonic is modulated by modulating current from microphone 21, amplified by space discharge amplifier 22 operating in the usual manner. rent is supplied to the amplifying tube 22 from source 13 through audio-frequency choke coil 23. Grid polarizing potential is taken from source 3]. through potentiometer 44 and filter 45. Condenser 46 by-passes the audio frequency to the filament of tube 22. Choke coil 24 prevents high frequency currents present in tube 17 from being impressed upon tube 22.

The principle of the constant current method of modulation described in U. S. patent to Heising lfo. 1,442,147, issued January '16, 1923, is employed, choke coil 23 in the anode supply lead of tubes 17 and 22-being of such value as to hold the total space current to the two tubes constant as set forth in the above patent. By modulating the fourth harmonic in the plate circuit of harmonic generator 17 the necessity for a separate modulating tube is avoided. Condenser 25 serves as a by-pass for high frequency oscillations and prevents currents of the modulating frequency being short-circuited to ground. Condenser 49 prevents direct current and components of speech frequency from flowing in coupling coil 20.

Modulated fourth harmoniccurrents selected by the tuned circuit 1920 are impressed upon the amplifier 26 which consists of a suflicient number of stages of vacuum tube amplifiers to raise the high frequency wave energy to the value desired for transmission. From this amplifier, energy is transferred to the antenna circuit 27 in the usual manner.

Resistance 28 and capacity 29 are inserted in the grid. lead of vacuum tube 1 in order to Space cur-' driving voltage simplified dia ram, Fig. 2, and vector diagram, Fig. 3. n Fig. 2, the coil brepresents that part of the coil of the tuned circuit 2 of Fig. 1 between the plate and the filament; the coil a represents that part of the 'coil 'of circult 2 between the grid and the filament;

the condenser 0 represents the condenser ofthe tuned circuit 2; the condenser a represents the stopping condenser 5 and is to be regarded as of negligible impedance at'the -operating frequency; the condenser f represents the condenser 29 of Fig. 1, and the resistance g represents theresistance 28. Condenser 29 and resistance 28 constitute a phase shifting network whose operation is explained as follows: As a preliminary requirement, the oscillator must be adjusted so that the high frequency external impedance between the anode and cathode at the operating frequency is a pure resistance. This insures that the space current is in phase with the grid voltage. By reference to Fig. 3, assume a space current i flowing through the coil 6. This produces in the oscillation circuit beat a driving voltage 6 which lags 90 behind the current i The capacity a and inductance a constitute a capacity reactance at the operating frequency. In order that the reactance between the points m and n shall be zero, it is necessary that the operating frequency be slightly higher than the frequency at which the circuits a, b, 0 and 1' in series would have series resonance. The current in,circuits a, b, c, and r is designated 1' 1* representing the inherent resistance of the coil a and any resistance introduced thereinto. Current i will lag slightly behind series driving voltage e Current 5 flowing through a, 1" will pro-- duce in mesh III (a, f, g, r) a driving voltage a. which lags behind 11 by an angle slightly greater than 90, that is, angle6 Since the voltage applied to the grid of the tube is derived from resistance 9 and this must be in phase with 2,, current i flowing in mesh III must lead the driving voltage 6 by the angle 0 This requires that mesh III have the proper capacity reactance to produce the desired angle of lead. The condenser 'f, in combination with resistance g, is assigned a value suitable for this purpose. Resistance g can be assigned any value within reasonable limits but is assumed as of high resistance as compared with the reactance of coil (1 for the purposes of the present explanation. The voltage drop across resistance 9, due to current '6 is the voltage e impressed upon the grid of the tube, which in turn produces the e in phase with the anode current, or, in other words, in phase opposition with the anode voltage. By this means it has been found possible tokeep constant the frequency of the oscillations produced by the oscillation generator tube 1 when the plate voltage is varied over a range of from 150 to 7 50 volts, under which conditions, without the use of the phase shifting network,

the oscillations would vary in frequency over a range of several thousand cycles at an operating frequency of around one million cycles.

In order'that tube 8 shall generate a strong series of harmonics, it is necessary that the grid become strongly positive and that a large negative biasing potential be employed.- If connected directly to the oscillation generator, tube 8 thus operated would constitute a variable impedance which would alter cyclically the constants of the frequency determining circuit 2 and cause a frequency change at each alternation. To avoid this and also to obtain an oscillating current of sufficiently large value to counteract the negative bias and make the grid'positive, the amplifier tube 7 is connected between the harmonic generator 8 and oscillation generator 1. The grid of tube 7 is maintained sufficiently negative by the source of grid biasreached- Summarizing the preceding statements, the precautions taken to render the frequency of the radiated oscillations constant are the following:

(1) Use of the phase shifting network 28, 29 to produce unity power factor in the anode circuit of-the oscillation generator.

(2) Radiation of a'frequency which is a multiple of the base frequency, whereby,

' (a) reaction of the load circuits on the oscillator is reduced; and

(b) the oscillator is permitted .to oscillate at a lower frequency than that radiated.

(3) .The use of tube 7 operating at 1owpower and with highly negative grid potential which prevents, to a substantial extent,

reaction from the elements to the right of the tube 7 upon the elements to the left ofthe tube 7 and which prevents the input or grid filament impedance from varying as much as it would if the grid ever became positive.

- (4) The low impedance of the circuit 15, 16 to the fundamental frequency.

(5) The use of the potentiometer 6.

(6) Selection of extemely constant elements for the circuit of the oscillation generator.

(7 Careful shielding of the oscillation generator and its associated elements.

(8) Suspension of the oscillation generator from springs whereby external mechanical vibrations are prevented from being ilnpressed upon any of its elements.

(9) Modulation of the oscillations at a point electrically remote from the oscillator thereby preventing any possible frequency variation due to modulation. (In a suitable circuit the use of an oscillator having a phase

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