March 26, 1940.
H. L. MOIR STATIC ELIMINATING METHOD AND APPARATUS Filed May 20, 1938 m kwx HIM" Patented 26, 1940 I non AND APPARATUS I I HarryL.Moir, ChicagoQ Application May 20, 193 8, Serial-No. 209,131
. 2.Claims. (01. 178-44);
This invention relates to" static'eliminating method and apparatus, and-more particularly to improved means for reducing interference during the reception of a note ofafixed audio frequency.
One feature of this' invention is that it substantially completely eliminates from the output of a radio receiver all undesired interference created bystatic andthe like,"during receptionof a radio Wave modulated by a fixed audio frequency wave or note; another feature "of this invention is that it enables the-signal to .noise level ratio to be greatly increased in connection withreception of continuous wave, t'elegraphyan'd radio direction finding signals; other features and advantages of this inventionwill beapparent from the follow ing specification and the? drawing; in which:
Figure 1 is an illustrative diagram, partlyfin circuit form, of one embodiment ofmy invention; and Figure 2 is a schematic diagram of amodification ofmyinventionr w z There are certain forms of communication and radio reception wherein it is only necessary to reproduce in the loud speaker or otherftranslating device an audio waveiof a given note, which note may be of a. fixed and predetermined frequency. This is true, for. example, in continuous wave telegraphy, where theaerial of; the transmitter-is energized with a continuous carrier wave, modu+ lated at intervals-with anaudionote or wave of a; fixed frequency. Ingthe reception :of suchga signal the audio note isseparated from the car rier, as, by a'conventional detector unit, and it is ,this fixed audio Wave WhlChlS amplified in the audio amplifier and delivered tothe output of the receiver. Another important commercial :use of an audiowave:of fixed frequency ,is in'.,the'
sending of signals for-.radio. direction. finding.
The signal transmitted in this work is in many respectssimilar to "continuous wave tele graphy a carrier being modulated at intervals by a fixed audio note, except that the modulation sequence is generally repeated in a fixed pattern.
In the reception of such signals it has been customary to use a radio receiver which is responsive throughout to a band, of frequencies, this being the conventional receiver adapted to also receive broadcast or phone signals Such a receiver has atuned radio frequency or an intermediate frequency portion adapted to pass a band several thousand cycles wide, and the audio frequency portion is generally uniformly respon-f sive, or substantially so, to frequencies from about fifty tofive thousand cycles'per second. Regardless of how sharp the tuning of the radio fre quency portion may be in such a receiver, noise" or undesired static impulses .inevitably pass through the receiver with the signal, and arer'eproduced inithe speaker or earphones; a
The present invention providesmeans-for receiving signals comprising a fixed audio frequency 5 wave in such a manner astoreproduce in the translating device only such audio wave, and substantially none of the undesired linterfering impulses which have passed through the radio frequency portion of. the receiver with the signal. 10
This is accomplished r by using, in the audio; frequency portion of the receiver, a crystal orother vibratory element very sharplytunedto the frequencyof the signal, so that undesired impulses having an effective frequency of only a very few 15 cycles difference from the .desiredvsignal wave are. eliminated from theloutput of the receiver.
In the particular-embodiment of my invention illustrated in Figure 1, only the basic elements of a radiorz-receiverare illustrated, it being under- 20 stood-that my invention is: applicable to receivers of the more complex modern type,;;and;';-in fact,
"toany type." In the illustrativefig'ure an antenna'system lllis adaptedto receiveradio waves,
and particularly those of the type comprising a 25 carrier modulated with a fixedjaudio wave, and
to'deliverthem to a radio frequency amplifier ll of conventional type,; which wouldalso include tuning means. The ..outpu.tl of, the radio frequency-amplifying"portion of the receiver is *.30
transmitted to a detector portion l2, here, shown .as separateffor purposesof; convenience in illustration. The Outputofthe detector portion) [2,
comprising the 'desiredrsignalz vof,:.a.-fixed :audio wave and other undesired. interfering impulses,i235
would then be delivered to the audio frequency amplifying portion of the -receiver,:here shown-as being 'coupled thereto by the transformer 13.
"The output of the transformer l3 ishere-shown,
as developed across'the grid 14 and cathode I5 40 of a tube It, which tube may be self-biased by the resistor IT. The output of the tube l6, or first audio stage, is delivered by the plate l8 to q the primary'of the transformer IS. The secondary of this transformer developes a voltage differential between the grid 20' and cathode 2| of the second amplifying tube 22; but the voltage differential developed between these two-elements controlled by filter means 23 comprising an 5 element adapted to vibrate mechanically at a definite frequency.
This vibratory element is here illustrated as a piezoelectric crystal 24, preferably of Rochelle 'sa1t,'in'e1ectrical contact with two elements, such 55 as metal foil, and 26, through which the circuit is completed.
In the position shown in Figure 1, the vibratory element acts as a filter, passing to the grid of the tube 22 only those voltages which have a frequency substantially conforming to the frequency of mechanical resonance of the vibratory element. This element must, of course, be so designed as to have a .very sharp. frequency characteristic curve. Quartz crystals used in radio frequency portions of receivers generally pass a band several thousand cycles wide; the vibratory element 23, however, should be so sharply peaked that if its period of mechanical resonance is 145'0'eycles'p'er second,
the energy passed thereby should be in the neighborhood of five times or more down at five cycles off of this frequency, on either side. The frequency of mechanical resonance of the" crystal is not here spoken of as the naturalf, frequency of vibration, since a crystal allowed to vibrate without restraint generally passes too'wide a band for the purposes of this invention; and. the natural frequency of a -crystal of convenient size may not be within the audio band- It is necessary, in orderto have a crystal peaked very sharply at a frequency within the audio range, to mechanicallyor otherwise constrain it to damp out the vibration, this damping resulting in vibration substantially only at the desired'fixed frequency. This damping may be conveniently accomplished, for example; bymass loadingof the crystal.
' The output of the tube 22.is here shown as delivered from the plate 21 thereof .to a coupling transformer 28 which effects delivery of the output to a translating device 29, .here shown as a loud speaker. As a result of the filtering'action of 'the vibratory element. 23 substantially only the desired signal wave is reproduced in the loud speaker. If all of the undesired impulses were of wave form of a definite frequency there would be no audible interference; but because of the steep 'wave'fronts' of some of the static impulses shock excitation may still result in a few undesired impulses in the speaker. The proportion of these undesired impulses, however, is. only a very few percent of those which would otherwise be present in the speaker; and the signal. to-noise ratio is greatly raised, frequently ten to twenty times over the-ratio existing thout the use of :this invention.
v In the modification of my invention illustrated in Figure 2 an antenna system 31 delivers signals to a radio frequency amplifier 32, a detector unit 33, and an audio amplifier 34, these various portions of the receiver being arranged in cascade as shown. .In this modification of my invention, however, two vibratory elements, here shown diagrammatically as 35 and 36, are used, here being placed immediately prior to the translating device or speaker 31, onebeing placed in each of the leads thereto. While these vibratory elements are here shown for illustrative purposes as outside of the audio amplifying portion of the receiver it should be understood that as a practical matter they would be incorporated therein. The use of two vibratory elements, one in the return circuit for the other, has been found to result inimproved operation, even fewer undesired impulses gettingthrough to the translating device.
Because of the fact that the vibratory elements are generally damped or physically constrained in order to sharpen their frequency-response characteristic curve, it is preferable to place at least one or more stages of audio amplification ahead of;the1vibratory element. In Fig. 1, for example, the vibratory element 23 is shown in use after the first stage of audio amplification; and inFigure 2 the vibratory elements 35 and 36 are shown in use following the last, stage of a several-stage,
audio amplifier. An arrangement of the type illustrated inFigure 1, wherein the vibratory element is followed by at least one stage of audio amplification, is preferable if a large output is desired, since such a large output might overload the elements when used in the position shown in Figure 2.-
' i While I have described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the. construction and arrangement may 'bemade without departing from the spirit and scope of the invention as disclosed-inthc ap- I pended claims in which it is my'intention to claim all novelty inherent in my invention as broadly as permissible in view of the prior art. 1
Iclaim: 1 I T 1. Apparatus of the character described for reducing undesired interfering impulses duringreception of a radio carrier .wave modulated'by audiofreq'uency' components including a wave of fixed frequency, including: a receiver com- HARRY L. MOIR.