US1987984A

US1987984A - Adjacent channel selectivity

Info

Publication number: US1987984A
Application number: US573410A
Authority: US
Inventors: William S Barden
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1931-11-06
Filing date: 1931-11-06
Publication date: 1935-01-15
Anticipated expiration: 1952-01-15

Description

Jan. l5, 1935. w, s. BARDEN ADJACENT CHANNEL SELECTIVITY Filed Nov. 6. 1951 MWNUXM QQ@ ATTORNEY Patented Jan. 15, 1935 UNITED STATES ADJACENT CHANNEL SELECTIVITY William S. Barden, Stapleton, N. Y., assignor 'to Radio Corporation of America, a corporation of l Delaware Application November 6, 1931, Serial No. 573,410

11 Claims.

My present invention relates to adjacent channel selectivity, and more particularly to a method of, and means for, securing such selectivity by great frequency discrimination at radio frequency followed by compensation at audio frequency.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the'invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit .organization whereby my invention may be carried into effect.

In the drawing,

Fig. 1 is a graphic representation of a' portion of the broadcast spectrum, p

Fig. 2 diagrammatically represents a superheterodyne receiver including the present invention,

Fig. 3 graphically shows the operation of the radio frequency discrimination means,

Fig. 4 shows the eect of the discrimination means onthe conditions shown in Fig. 1,

Fig. 5 graphically demonstrates the characteristic of the audioampliiier of the receiver shown in Fig. 2. i

Referring now to the drawing wherein like reference characters designate the same elements -in the different figures, there is shown in Fig. 1 a graphic representation of the intensity of desired and undesired signals in the vicinity of a broadcast receiver. Thus,the heavy line A may designate a desired carrier of one unit strength, while the heavy line C, positioned 10 kilocycles away, designates an undesired carrier of the same strength. yEach of these carriers is attended by a single side band component of one unit strength,

vto wit:y B and D, spaced lat frequencies f and fu respectively from the carriers A and C.

These side band components are on such sides of their carriers that side band interference will be a maximum. This should be clear-from Fig. 1, where E, in dotted line, represents thev side band interference positioned 5 kilocycles from either carrier A and B. It will now be seen that the problem of adjacent channel selectivity is entirely divorced from the problem of interference residing on the desired signalling band. Adjacent channel selectivity is fundamentally a matter of frequency discrimination. But, when the interference and signal reside on all of a common channel, no hypothetical scheme can -possibly represent a result which iS not 11i vhai'- (Cl. Z50-F20) mony with thek conventional theory of wave form analysis. i

In Fig. 2 there is shown a receiver arrangement which employs side band cutting frequency discrimination with `audio compensation of a special type. By means of this arrangement it is possible to solve the problem graphically represented in Fig. 1. Furthermore, this solution involves in no sensedoctrines which are in defiance of well-known principles. The frequency discriminator is preferably employed in a radio receiver of the superheterodyne type for the 'reason that Lthe discriminator employed herein lends itself more readily to such a receiver. Asis well known, a superheterodyne receiver, as shown in Fig. 2, comprises the usual grounded antenna A, yG coupled, as at M,`to the input of a tuned radio frequency amplifier, the latter comprising one' or lmore', stages utilizing electron discharge tubes.

'I'he amplified radio frequencies are impressedL upon the inputcircuit of anrst detector tube, the latter also having impressed upon it the output of a local oscillator. v'l'he first detector tube functions as a frequency changing device Vand produces in its output circuit a carrier of an intermediate, or ysuper--audible beat frequency, having superimposed upon it the audio Lfrequency modulations corresponding to the modulations which were originally carried by the radio frequency carrier. While I have not shown any constructional details up to this point, other than (conventional representation, it isA to befunderstood that the usual uni-control tuning arrangements may be employed for selection Yof desired signals.

The modulated intermediate frequency carrier is impressed upon the input circuit of the first tube of the intermediate frequency amplifier. While the latter usually' consists of more than one stage of amplification, each st'ag'e including an electron discharge tube, for the sake of simplicity the first intermediate frequency -amplier stage only is shown. This stage includes the electron discharge tube 1 having 'connected between itscathode and control electrode a socalled crystal bridge. The crystal bridge consists of four arms, three of the arms including capacities C1, Cz andv C3, while the fourth arm includes a piezo-electric crystal 2 mounted between the electrodes 3 and 4. 'Ihe piezo-electric arrangement is conventionally shown, those skilled in the art clearly understanding the manner of mounting the crystal between the electrodes 3 ande. Y

The point 5 of the crystal bridge is-connected L to the high potential side of the capacity 6 and inductance coil 7, the latter two elements comprising a circuit resonant to the intermediate frequency carrier. The point 8 of the crystal bridge is connected to the low potential side of the resonant circuit 6, 7. The remaining two

points

9 and 10 are connected to the grid and cathoderespectively of the tubel.

It has been found that the crystal bridge in the input circuit of the rst intermediate frequency amplier tube functions as an extraordinarily great frequency discriminator. Briefly, the capacities C1, C2, C3 and the capacity between the electrodes 3 and 4 are so proportioned that no signal potential is .applied to the grid of the tube 1 unless energy of the intermediate frequency is impressed upon the resonant circuit 6, 7. That is to say, the crystal 2-has a natural resonance equal to the intermediate frequency impressed, and the crystal bridge is in balance for all frequencies other `than this particular intermediate frequency. It will therefore be seen that 4signal potential -is applied to the grid of tube 1 only whenthe crystal 2 isforced into oscillation -by the intermediate frequency energy; in other words the crystal bridge is unbalanced to energize the vgrid Aof tube 1 for energy of intermediate frequency only.

Better results are found to be secured by connecting a resistance 11 between the point 9 and the point 10 of the crystal bridge, the grid of -tube 1 -being normally maintained at a negative bias bythe source 12.

In Fig. 3 there isshown graphically the characteristicfof the -input circuit of the intermediate frequency amplier, and particularly the extent to :which vfrequency discrimination is carried by thecrystal bridge arrangement. In thisgure, :plotting "Response against Frequency, it is seen that the bandfof frequencies passed by the frequency discriminatoris `exceedingly narrow, .the dotted line `I representing the Aintermediate .frequency carrier, vwhile the full line demonstrates `the attenuation of frequencies off either side of the .intermediate frequency carrier. It :will be seen that ypraictically complete attenuation -is ysecured 100 cycles off either side of the intermediate frequency carrier I. In other words, by vutilizing the frequency discrimination means shown in Fig. 1, there is transmitted through the intermediate frequency ampliiier a band of frequencies Yof only about 200 cycles vin Width.

`subjecting the action of the crystal admission circuit disposed between the frequency changing device and the intermediate frequency amplier to further analysis, -assume that the resonator to which the components shown inFig. 1 are applied hassuoha low decrement that at cycles on" resonance the attenuation .is to 10% (i. e. 90% attenuation) ,.and lfrom then on the attenuation is .substantially inversely 4as the extent (absolute) oil' resonance, then it `can be-shown that Fig. 4 is a graphic representation of the effect on the componentshown 4in Fig. 1. It will be seen from Fig. 4 that ldue to the attenuation characteristic of the crystal admission circuit the desired carrier Astill possesses one unitstrength, while the undesired carrier C, .10,000 cycles off resonance, has been attenuated to .0005 of a unit. Similarly, the side band component D of the carrier C has been attenuated to asimilar extent. The interference vcomponent E, 5,000 cycles 01T resonance,

.has been attenuated to .001 of a. unit.

However, it Will be observed, additionally, that a side `band component .B, only 100 cycles oi resonance, which side band component is desired, has been attenuated to .05 of a unit.

The output of the intermediate frequency arnplier is then subjected to the action of a second detector, and the detected output impressed upon an audio frequency ampliiier of special design. It has been found that the primary 20 of the audio frequency transformer M1 should have an inductance of approximately 2 henries. The audiofrequency ampliiier has only its rst stage shown in detail, it being understood that as many stages as desired can be employed. Of course, those skilledin the art understand that the designation To utilization means represents additional audio stages designed in a manner similar to the stage shown in detail, and subsequent headphones or loud speaker, or similar translating means.

The rst audioamplier stage, then, comprises an electron discharge tube 23 having arranged in its output circuit a series resonant path which includes a capacity 2l and an inductance 22. The series resonant path is connected between the anode and cathode of the tube 23 and is tuned to a period of 4800 cycles. that if additional stages of audio amplification are employed, each of such stages will have a similar series vresonant path in its output circuit. The characteristic of such an audiofrequency amplier is graphically represented in Fig. 5.

In this iigure, secured by plotting Audiofre quency gain against Frequency, it is shown that those components least attenuated by the crystal admission circuit are ampliiied in the compensated audioamplier to the least extent.

Thus, the carrier component A of Fig. 4 would be amplified to the least extent, the numeral 1 designating the amplication of the component Aas of one unit. On the other hand, there is a straight line increase in amplification of the components up to 4800 cycles oi resonance, the component at the latter point being amplified 50 units more than the component A. It will be observed that the audio amplier, by means of the series resonant paths in the plate circuit of each stage possesses a sharp cut-ofi at 4800 cycles. It necessarily follows that the side band interference component E, which is 5000 cycles o resonance from the desired carrier A, is not amplined because of this sharp cut-off charac- .i

teristic. It will thus be seen that attenuation of desired audio components by the crystal admission circuit is compensated for by the use of an audio frequency ampliiier having a characteristic as shown in Fig. 5, and possessing a sharp cut-off prior to the point in the frequency range where the undesired side band interference is positioned.

However, the utility and desirability of the present invention becomes more evident when a so-called dynamic method of determining adjacent channel selectivity is employed. When two signals exist 10 kilocycles apart, cross heterodyne among their side bands, and even between their carriers, as well, brings an entirely different aspect to the problem. The present low-decrement audio compensa-tion system can now be compared from the standpoint of dynamic selectivity with the dynamic selectivity of a standard superheterodyne receiver. It can be theoretically demonstrated by the dynamic method, that the heterodyne of an undesirable component 5000 cycles oiT resonance from its carrier with a desirable component ranging from zero to 5000 cycles off resonance from its carrier f It will be understood is Athe characteristic: of .most` intense.: heterodyne; and plainly :thisresponse Iwill varywith'the numa ber of; cycles off resonanceof the'vdesirable component, the` cross yheterodyne being va maximum where thestrengthof thedesirable componentl off-resonance is` greatest, 1 f

' rItcanfber shown, Afor example,"` that withlthe desirable-1 component 4700 cycles off resonance. fromits carrier, theratio of desired yto possible undesired iis 60:1. If the modulation frequency onthe undesired signal .carrier never exceeded 47.00 cycles, the presentfsystem would be sixty times as Ameritable 'as a conventional superheterodyne receiver which is rated'to be flat 60.4700 cycles onl its fidelitycharacteristic.

If theundesired signal be modulated intermittently and generally feebly,V 'at .about 4700 cycles, the ratio ,rapidlyibecomes less favorable, and at 5000 cycles, the side .band interference might break through occasionally` so as to be noticeable, .The chances are remote that-it would ever be `noticed unlessit is being looked for, because during program modulation, in general, only a small percentage of the energy is concenv., trated near 5000-cycles, andtheentire desired program exists to `easily overwhelm the undesired effect.' Thus, to befextreme, and accomplish the ideal theoretically the undesired signal could be modulated to 5000 cycles, progressively monotone without lowering the-'60:1gratio, theaudiocompensation'being cutoff at audiofrequency below 4700 .cyclespthus,maintaining substantially ideal legitimate fidelity. l

. However, it isimportant topoint outthat the regionof possibly greatside band interference is so confined that only a very slight sacrifice in fidelity need be' ma'detc eliminate it, and that when not taken care of (i. e. no cut-off in cornpensator) the situation is not bad, for reasons already stated. It should be clear that when dealing with interference outside of the signalling band, the theory of proportionate loss of fidelity does not hold. Again, it is well to note that in practice the selectivity merit of the present system is four times as great as has been pointed out heretofore.

It has been assumed throughout the present analysis that all side band potentials are concentrated on one side of their carriers. Actually, of course, it would be half on each side of the carrier, and, due to the product principle, (since both desired and undesired components on adjacent sides are halved), the audio developed is one-fourth of what has been shown, thus increasing the ratio by four.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, in a superheterodyne receiver, a frequency changing device, means for amplifying the changed frequency, a frequency discriminator disposed between the frequency changing device and the amplifier, said discriminator having a frequency response characteristic which decreases rapidly for frequencies off resonance, a second detector, and an audio amplifier having a characteristic inverse to that of the frequency discriminator, the amplifier being provided with means having a sharp cut-off prior to the point vinjth'e frequencyk rangewhere .undesired side'band interference is positioned. 2.1In a superheterodyne receiver, meansfor collecting. signal energy, means for generating local energy, means for combining thesignal and local energies, a'crystal admission circuit adapted to ytransmit a band'of frequencies including the desired carrier and its band components, and the circuit transmitting substantially the greater portion of the desired band components greatly attenuated with respect to the carrier, [and audio compensation means for amplifying the transmitted components in a manner inverse to their attenuation by `the-crystal 'admission circuit, the amplifier being provided with means having a sharp cut-off prior to the point in the frequency range where undesired side band interference is positioned. l3. In a radio receiver, Aa frequency discrimination circuit of low-decrement adapted to transmit a band of frequencies including the desired carrier and substantially the greater portion of the desired'components greatly attenuated with respect to the carrier, and an audio-amplifier including compensation means whereby the ampliner has an ,amplification characteristic'inverse to the discrimination characteristic of the said discrimination circuit, the amplifler compensation `means additionally imparting a sharp cut-off prior to the point in the-frequency range where undesired side band interference is positioned.

4. In a radio receiver, a crystal admission circuit of low-decrement adapted to transmita band of frequencies including the desired carrier and substantially the greater portion of the desired components greatly attenuated with respect to the carrier, and an audio-amplifier including compensation'means arranged to impart an amplification characteristic inverse to the characteristic of the said admission circuit, said amplifier having a sharp cut-off at 4800 cycles.

5. In a radio receiver, a crystal bridge circuit of low-decrement adapted to transmit a band of frequencies including the desired carrier and substantially the greater portion of the desired components greatly attenuated with respect to the carrier, and an audio-amplifier including compensation means imparting to the amplifier an amplification characteristic inverse to the characteristic of the said bridge circuit, the amplifier additionally having a sharp cut-off prior to the point in the frequency range where undesired side band interference is positioned.

6. A receiver for wave signals comprising a resonant circuit for wanted incoming signal energy, such as is constituted by a modulated carrier wave, a local generator of oscillations to combine with the incoming energy to'produce energy of an intermediate frequency, resonant means including a piezo-electric device for transmitting the intermediate frequency energy therethrough, whereby the amplitude of wave-form energy corresponding to different modulation frequencies is varied substantially in inverse proportion to the frequency value, a detector and an audio frequency amplifier, means for varying the amplitude of wave-form energy corresponding to the different modulation frequencies substantially in proportion to the frequency values, the amplifier including in its output means for imparting a sharp cut-off to the amplifier at 4800 cycles.

7. A receiver for wave signals comprising a resonant circuit for wanted incoming signal energy, such as is constituted by a modulated carrier wave, a local generator of roscillations to c0111- bine .with :the :incoming AenergyA toA produce energy of an intermediate frequency, resonant means `ncludingv 'a `piezo-electric device for 'transmitting the vintermediate frequency energytherethrough', whereby the'ampltude of wave-form energy corresponding to ldifferent modulation frequencies is relatively'altered substantially throughoutthe erltire rangeof such frequencies, a detector to which the energy transmittedfthrouglr sadpiezo-electric deviceis supplied', and an audio ampliiier'associatedwith said detector including means for altering the amplitude of the wave-form energy corresponding to diierent modulation frequencies in the oppositesense, said last named means being adapted to impart to said amplifier sharp cut-off effects at substantially 4800 cycles.

8. In a superheterodyne receiver provided with a rst detector circuit adapted to produce in its output a band of frequencies including a desired intermediate frequency carrier'and its modulation side band frequencies, an intermediate frequency amplifier including at least one tube having means inits input to normally maintain a negative bias on the tube grid, a bridge circuit coupled between the amplifier tube input and the detector output, said bridge including as an arm thereof a crystal resonant to said desired carrier, a second detector, an audio frequency amplifier including at least one tube, and a tuned path shuntedv across said tube `tuned to a period of substantially 4800 cycles.

9. A- selecting system for radio receivers and the like adapted to receive incoming signal energy, such as is constituted by a modulated carrier wave, comprising in combination, a signal interceptor and signal -conveyer arrangement provided'with output terminals,a resonator capacity bridge circuit provided ywith input terminals and output terminalszoneiarnrzot said bridgezinnluding aime, chani'cal resonatordevice;V said i input," terminals being connectedfto thezfirstnamedoutput terminals; .whereby energy'finterceptedzrby the inter.- ceptorris iconveyedthrough the conveyer torthe resonant circuit, a detector and amplifier arrangement connected` toftheoutput terminals ofthe resonant circuit,said resonant circuit having such sharpfrequency characteristics due to the resona-V tor device thataportion of the desired. signal energy'fedtheretorfrom the interceptor conveyer arrangement'is discriminated!against to such an extent asto cause substantial. distortion yoi' the signals, means connected. to the amplifier arrangement forgiving tosaidampiier predetermined distortion'al characteristics `oisuch vcharacter that. the portion: of the desired` signal. energy discriminated againstdue to the'resonant circuit is de-distorted tothe extent that theportion Aof the desired signal discriminated againstis brought backso as to bear'substantially'its original, relationship. with respect to the remaining portion of thesignal energy.

10; 'Ihe system described in. claimt 9 wherein the resonant circuit comprises a, piezo-electric crystal 'bridge and wherein the input'terminals are formed bytwo oppositepoints of the bridge and the outputterminals are formed by'two other opposite points of the bridge.

11. The system describedin claim 9 wherein the'means connected to they amplinerfarrangement comprises a circuit vshuntedacross the output of the amplifier which circuit is tuned'to a frequency which' closely approximates. the frequency oflthe portion ofthe signal energy discriminated against.

WILLIAMT S. BARDEN.