US2073344A - Superheterodyne receiver - Google Patents

Description

March 9, 1937.` v v.1. K. JOHNSON SUPERHETERODYNE RECEIVER v Filed May 22, 19:54

mi ME (msg/my ou :fn/vizir mmm/Noa Vfl/vizir INVENToR JOHN KELLY JOHNSON M'roRNEYs 'mantos Mor. 9, 1931 'uNlTED STATES f PATENT ol-FlcE Y o amsn Y.

tion of Delaware Application lill Vzia. 1934, sonal No.V reussi 'i4 claim.

mitted on a carrier wave having two sidebands ofv modulation'frequencies, which areofl about 6 kilocycles in width on either side of the carl-ler.`

Under present broadcasting' conditions, the different carrier frequencies are allocated at various positions throughout. the broadcast range, usually 10 kilocycles apart. and in many instances the sideband frequencies of one signal channel either overlap those of an adjacent signal channel or else closely encroach upon them.l In either case, it is difficult, when tuning a radio broadcast receiver to a desired signal in one such channel. to eliminate interference due to signals in 2 the adjacent channels, particularly when such interfering signals are received with a strength comparable to that of the desired signal.

Operation without lil:lterfel'ence in such cases ordinarily requires that the selecting system shall 25 pass a considerably narrowed band of frequencies so that passage of the interfering signals is substantially prevented. Narrowing the selected band in this manner ordinarily impairs the nd'elity of reception of the signals,` since the side,

3o band frequencies corresponding to the higher audio frequencies of modulation are suppressed.

It is well known that in the operation of superheterodyne receivers, there is produced an in-` termediate carrier frequency which is equal to the dierence between the signal carrier frequency andthe local oscillator frequency. This intermediate, ordifference, frequency is ordinarily maintained constant over the tuning range of the signals by operating the signal selecting cir- 40 cults and the oscillator frequency-determining circuit by a uni-control device to maintain the constant fdiil'erence. 'Ihe two sidebands of mod- '50 tioned above is passed by a highly selective ino termediate frequency selective system. u According to oneof the features'of this invention, it is proposed to improve the fidelity of reception and yet maintain the narrowed band 55 selective characteristic of the `selective circuits.

(a. eso-zo) This is accomplished in accordance with this invention by utilizing in a superheterodyne type of receiver a'lccal oscillator system provided with means for 'shifting the omlator frequency relative to the signal frequency.

-A particular feature is the association of a variable' reactive element with the frequencydetermining circuit of theoscillator in such a manner that equal movements of the reactive element produce substantially equal shifts in os- 10 cillator frequency for all frequencies of the tuning range.

When the local oscillator frequency is shifted relative to the signal frequency, as explained above, the intermediate carrier frequency is also 15 altered by alike amount. Since the band selected by the intermediate-'frequency selecting system remains fixed. this .shifting of the intermediate carrier frequency results in further "cutting out" the extreme frequencies of one sideband and of simultaneously admitting more of the extreme frequencies of the other sideband. Finally, when the intermediate carrier frequency is shifted all the way to one extreme or the other of the se'- lected band, only a single sideband is passed. this having a widthA equal to the entire selected band width. 'Ihe direction in which the intermediate carrier frequency should be shifted to produce the greatest freedom from interfering signals is in thedirection away from such interfering sig- 'so nal.

According'to another feature of this invention, there is provided a selective system which is adjustable to pass any band width between a moderately selective width and a more highly selective width. The adjustable feature is provided by varying the coupling between a pair of coupled tuned cil-cuits, and simultaneously varying the resistance introduced into at least one of these tuned` circuits. Since a change of resist- 40 ance in a tuned'circuit also changes the gain, or

:transmission eflciency, .the varying coupling is rig. 2 snows graphically now th loool oscillator Ain,

frequency is shifted according to the invention;

Fig. 3 illustrates graphically the expansion of the band selected by a tuned coupling system according to the invention;

5 Fig.`4 illustrates graphically the combined effect of shifting the local oscillator frequency and of expanding the band selected by a tuned coupling system; and

Fig. 5 shows the impedance variation of one of l the circuits of the tuned coupling system in passing to the expanded condition.

Fig. 1 illustrates a superheterodyne receiver comprising in tandem in the order named: an antenna I0 and ground Il; a radio-frequency 5 amplifier I2; an oscillator-modulator tube I3; an

intermediate-frequency amplitler Il; and a detector and audio-frequency. amplifier, and loudspeaker I5. The .devices I2, I4, and I6 are indicated only in generalized block form since they constitute no part of this invention.

' The oscillator-modulator tube I3 is of the hexode type and may take the form disclosed in the "copending application of Harold A. Wheeler, Se-

rial No. 654,327, flied January 30, 1933. Patent No.

2,015,321, september 24, 1935. The tube contains a signal input grid I6 coupled'to the radio-frequency amplifier I2 through a radio-frequency transformer l1, the secondary coil of which 'is by a variable condenser IB.

There is associated with tube I3 a local oscillator system I! which includes a coil 20 connected from an inner screen 2i to an inner grid 224 through a condenser 23. A condenser 2l is connected between ground and an intermediate point 2S of the coil. A source of positive operating voltage, designated +B. is connected through a resistor 26 to the point 25 for supplying a proper direct voltage to the screen 2I. A variable condenser 21 is connected between ground and the upper end of coil 20. The condensers 24 and 21 and the portion of coil 20 above point 25 constitute a resonant circuit which determines the tor frequency being variable by means of condenser 21. The oscillator-modulator circuit just described is similar to those which are more fully described in the copending application of Harold 1933, Patent No. 1,958,027, May 8, 1934.

In accordance with a feature of this invention, there is connected a relatively small variable `condenser 28 between ground and a second intermediate point 29 of the coil 20 which is out- 5 side the resonant frequency-determining circuit.

By reason of its connection to ground, condenser 28 is connected across condenser 24 and a portion of coil 2li which is outside the resonant circuit. The function and operation of condenser 2iiewill be more fully explained subsequently.

` Modulation occurs between the received signal frequency impressed on grid I6 and the local oscillator frequency in a manner which is fully described in the above-mentioned application.

Serial No. 654,326, Patent N0. 1,958,027, May 8,

ganged together for simultaneous operation by tunable tosignal channels of the broadcast range frequency of the oscillatorysystem, this oscilla-- A. Wheeler. Serial No. 654,326, filed January 30,

means of a uni-control device indicated generally by U.

A coupling system 3| connected between the anode 30 of tube I3 and the input ofintermediate-frequency amplifier Il is tuned to pass a xed intermediate-frequencyband.

The frequency band selected and passed by the tuned coupling system 3| is shown graphically in Fig. 2. In Fig. 2, the intermediate carrier frequency is represented by the vertical line A which is taken as kilocycles in this case. The coupling system is adjusted to select and pass an intermediate-frequency channel band 8 kilocycles wide, from 171 to-179 kilocycles, these two edges of the selected band being indicated by the broken lines B and C, respectively.

The function of the condenser` 28 associated with the oscillator circuit is to shift the oscillator frequency in either direction by any desired amount up to the width of a sideband, without altering the radio-frequency channel selected. Since the sideband width for the case under consideration is 4 kilocycles, this should be the limit of frequency shift due to condenser 28. If the oscillator frequency is higher than the received signal frequency, asis usually the case, any decrease in the oscillator frequency without an attendant change in the selected radio frequency, will shift the intermediate carrier frequency downward by a like amount, and vice versa. In

-the example under consideration, therefore, the

oscillator frequencywill bef considered to be 175 kilocycles higher than the selected signal frerepresented by line D may be shifted at will to.

any position between the limits of lines B and C.

Since the position of the selected intermediatefrequency band remains fixed, such a shift of the intermediate carrier frequency has the eil'ect of cutting olf the outer frequencies of one sideband and of adding on to the outer frequencies of the other sideband by a like amount. For the amount of shift illustrated in Fig. 2, there relmains in the lower sideband only the frequencies between 171 and 172 kilocycles, corresponding to.

audio frequencies from 0 to 1000 cycles. The

lower intermediate-frequency sideband frequencies corresponding toaudio frequencies of 1 to 4 kilocycles have not been selected. The upper this case) are transmitted with better efficiency r than. the frequencies corresponding to higher audio-frequencies. The yreason for this is that frequencies corresponding to low audio frequencies are transmitted by both sidebands. 'I'hisinequality of transmission can be compensated to a considerable extent by'a properly designed equalizing network in the audio-frequency amplifier.

sov

It is noted that the location of condenser 2l is particularly advantageous. By reason of its l connection across at least some of the mi?? f the frequency-determining circuit and across induc- 5 tance which is outside. the frequency-determining circuit, equal motions or variations of the capacity of 2l produce equal frequency shifts at all frequencies of the tuning range. Such would not .be the case if condenser 28 were connected across l0 inductance alone or capacity alone.

`Since the intermediate carrier frequency may be shifted in either direction, the direction chosen for the shift willordinarily be opposite that in which an interfering signal may happen to be lli present. `'Such' a shift will tune out such an interfering signal at the same is being'improvedl There isv arranged according to this invention a .second intermedlate-frequency coupling sys` tem I2, connected between the output of 'intermediate-frequency amplifier i4 and the detector and audio-frequency amplifier, and loudspeaker IB. ('Means is associated with this coupling system to permit the widthof the band transmitted therebyto be varied to change the selectivity and fidelity. 'This means augments, and may be used in coniuiction with the oscillatory frequencyshifting arrangement. The coupling system 32 comprises a primary coil 33 electrornagneticallyv coupled to a secondary coil 3l. The coil 3l is tuned to theintermediate carrier frequency by a condenser 35, and coil 3l is likewise tuned to the intermediate carrier frequency by a condenser 3B. I There is arranged in series between the lower ends (the low potential ends) of coil 3l and condenser an arrangement of a coil l1 and a potentiometer comprising two resistors 38 and 39. A s'witch 4U is provided to connect coil 31 across either resistor 3l or 39 according to whether switch lll makes contact with switch-point li or switch-point l2. The coil 31 is electromagnetically coupled to primary coil 33. The potentiometer contact arm 43 is connectedvto a point ll, and when in the neutral position makes contact with the neutral point between the two resistors. In this position, the low lpotential ends. of coil 3l and condenser 36 are directly connected. The switch lli is correlated to potentiometer arm" 43 so that when the arm I3 is oper'- ating on resistor 38, switch lll makes contact with point Il; but when arm Il is'operating on resistor 3l, switch Il makes contact with point I2. As arm I3 is moved in either direction from the neutral position. along resistor 3l or resistor 39, more voltage is introduced into the secondary circuit from the coupling between coils 31 and ll, since the voltage across coil l1 also appears across the potentiometer resistance. At the same time, more resistance is introduced inthe secondary circuit from the potentiometer. The amount of coupling and resistance increase in the resonant secondary circuit depends imon how far the arm I3 is moved from neutral. It 'is well-known that when the coupling between a pair of coupled tuned 'circuits is small, the resonance characteristic of the system is sharp. But when the coupling is increased above the "optimum value, the system becomes characterised bytwo resonances which become spread apart, thus increasing the selected band width. The term "optimum"` coupling means that degree ofcouplingbetweenthetwottmedeircuits-which produces the maxnnmnampliilcation, at buta Inthismannenthemtheselected'bandis time that the fidelityy acfas the added resistance.

expanded by movement of the arm 4I. The

secondary circuit would be characterized by two pronounced resonant peaks,` when in the. ex-

' pauded condition, if it were not for the resistance which is introduced. Theeilect of simultaneous- 1y introducing resistance and increasing the coupling is to expand the bandwithout producing objectionable resonance peaks in the secondary circuit. ,Y

The double-tuned coupling system 22v is adjusted so that when arm is in the neutral position (no resistance being introduced), the selectivity is quite sharp. A typical selectivity curve for this condition islcurve E of Fig. 3,

'which is a graph of attenuation plotted against frequency. In the example illustrated, the intermediate carrier..frequency is 175 kilocycles.A 'I'he selectivity represented by curve E is such as to pass a band about 4 kilocycles in width; that is. each sideband passed is 2 kilocycles in width, for a carrier frequencyof 175 kilocycles.

While the characteristic represented by curve E provides high selectivity, the delity is poor. To improve the fidelity then, the potentiometer arm 43 is moved from the neutral position to increase the couplingy andvto introduce resistance the arm 43 is rotated the maximum distance in one direction or the other, the transmission characteristic takes the form of curve F in Fig.

v3. 4 According to curve F, coupling system I2 addition of 'resistance alone, thecharacteristie` i would take a form somewhat like curve G of Fig. 3. Curve G shows greater attenuation than curve E within. the band, this being due to the reduction of transmission eflciency becauseof Curve-F, however, shows less attenuation than curve G, and shows about the same attenuation as curve E. The difference w in attenuation between curves G and F is made up by the additional coupling due to coil 31. Thus the gain due to coupling coil 31 compensates the loss due to the resistance.

It has been found very satisfactory to operate the frequency shifting condenser 28 and the po,- .Y

tentiometer arm 43 (as well as the switch lil) `simultaneously by means of a single mechanical control device indicated generally by the dotted into the tuned circuit, as explained above. When 2 lines and knob S. The device S is so connected v as to holdcondenser 28 and arm i3 both intheneutral position to produce the minimum b and width, which will be that/of curve E in Fig. 3.

YTo broaden the band, knob S is turned in either one direction or the other. Movement in one direction causes arm Il to move along resistor 30 and simultaneously causes condenser 2B tol shift the intermediate carrier frequency in one direc# tion. Movement of the knob in the other direction causes switch III to move to contactY point l2, causes the arm l! to operate on resistor ls, and causes the condenser 2l to shift the intermediate carrier frequency in the other direction.A

The adJustment. 'should preferably cause the `maximum frequency shift to coincide with theV maximum band =width (curve F). Y K

F18. 4 shows the overall 'effect cf operating the knob S to shift. both the oscillator frequency and to expand the band width of system 32. Curve H corresponds .to curve E of Fig.l3 and shows the overall characteristic in the selective condition. which is the condition for tuning. curve I shows the overall selective characteristic when the knob B is turned to fully expand the band width -in the direction of the lower sideband.

6 Curve I is seen to be of substantially the same width as curve F in Fig. 3 except that it is displaced downwardly. Curve J similarly shows the eect of the full expansion in the direction of the upper sideband.

l It should be understood that the arrangement of the invention is readily susceptible to many variations from the specific embodiment described. For example, if it be found that the 9- 'Ihere is associated with the receiver a system of automatic volume control. This system comprises a path 48 between a point l1 of coil and a point II which is connected through resistor 49 to the signal grid circuit of tube I3. There is 25 included in the path 46 automatic volume control apparatus designated generally by the block 6l. This automatic volume control apparatus may be of any conventional form which ordinarily includes a rectifier for developing a uni-directional 3 voltage which biases the grid IB of tube i3 lnversely with any change of the intermediatefrequency voltage at coil 33.

lI"his arrangement, in addition to automatically controlling the volume output, produces the deu sirable effect of maintaining a substantially uniform output with expansion. This efi'ect is due to the manner in which the impedance of Athe primary circuit 33, 35 changes in shifting from the contracted to the expanded -condition of cou- 40 pling system l2.

Impedance characteristics of the primary circuit II, 85 are Aillustrated in Fig. 5 in which the impedance isfplotted against the intermediate frequency. Curve K illustrates the relatively sharp impedance curve obtained when the band passed by the coupling system is contracted by placing the arm 4I inthe neutral position. When the band passed byl the coupling system is expanded. as by moving the arm in either direc- 50 tion. and the coupling between the primary and secondary circuits of the coupling system thereby increased, the impedance at the primary circuit becomes characterized by the double resonance effect due to over-optimum coupling. The

56 impedance of `the primary circuit under the ex- `duced into the primary circuit when the couplingis increased. 66 Since the voltage applied to the automatic volume control apparatus is taken from the primary circuit, the automatic volume control effect is caused to relax when the band widthselected by coupling system '-32 is expanded. The reason 10 for this is that the intermediate carrier frequency lies within the dip' in curve L. In consequence the volume output rises somewhat when the band width is expanded; or in other words.

the sensitivity would be somewhat less in any u except the expanded condition.

` This counteracts the tendency for the volume to drop due to the moving of the carrier to one side of the sloping resonance characteristic and thus decreasing in amplitude as the band is expanded. This also compensates for the'net drop in output which results from the cutting off of a large portion of one of the sidebands..

If the receiver is tuned in the expanded con.- dition. this primary impedance characteristic produces a noise suppression and sharp tuning eect. y0n either side of the exact resonance vrfrequency the voltage actuating the automatic volume control is abnormally high, thus decreasing the receiver gain, while at exact resonance the automatic volume control voltage is abnormally low. 'I 'his condition produces a rapid and easily distinguished output volume rise` when tuning through resonance, which indicates the exact point at which the receiver is correctly tuned.

I claim:

1. In a 'superheterodyne receiver comprising means forf'receiving modulated carrier signals, means including means for generating oscillations for deriving from said received signals modulated intermediate carrier-frequency signals and a selecting circuit for selecting a band of frequencies including said intermediate carrier-frequency, means for varying thelei'iective width of an intermediate frequency modulation sideband passed` by said selecting circuit comprising means for shifting said oscillator frequency relative to the frequency of said signals.

2. In a superheterodyne receiver comprising means' for receiving modulated radio-frequency signals, means including means for generating oscillations for deriving from said received signals intermediate carrier-frequency signals, said latter means including. a resonant frequencydetermining vcircuit having inductance shunted by capacity and a selecting circuit for selecting a band of frequencies including the intermediate carrier-frequency, means for varying the effective width of an intermediate frequency modulation sideband passed by said selecting circuit comprising a condenser connected across at least part loi' said capacity and coupled to said inductance,

by a` substantially constant intermediate frequency is produced at the output of said modulator and a selecting circuit for selecting a band of frequencies including said intermediate frequency, means for varying the effective width of an intermediate frequency modulation sideband passed by said selecting circuit comprising a variable reactive element associated with said frequency-determining circuit and operable independently of said adiustable -means for shifting said oscillator frequency relative to the frequency of said signals, whereby the intermediate carrier-frequency may be shifted within said band.

4. In a superheterodyne receiver, a tunable radio frequency signal receiving circuit, means including meansfor generating oscillations `for deriving from received signals intermediate carrier frequency signals, saidlatter means including a tunable frequency determining circuit and a selecting circuit for selecting a band of frequencies including the intermediate carrier frequency, means for varying the effective width of an intermediate frequency modulation sideband passed by said 'selecting circuit comprising an.

adjustable'reactance element coupled to' one of said tunable circuits and so related thereto that equal adjustments of said reactance element prov l0 duce substantially equal adjustments in said inthe effective width of an intermediate frequency modulation sideband passed by1 said selecting circuit comprising an adjustable reactance element included in said frequency determining circuit for' shifting said oscillator frequency in either direction relative to said signal frequency, said reactance element being so connected to said frequency determining circuit that equal adjustments thereof produce substantially equal frequency shifts at all frequencies in thetuning range. 6. In a superheterodyne receiver comprising a signal-selecting circuit tunable over a range in frequency, an oscillator-modulator tube upon which the selected signals are impressed, said tube having associated therewith an inductance and a capacity included in a tunable local oscillator circuit associated with said tube and a selecting circuit coupled to the output of said tube responsives to the intermediate-frequency band of frequencies, means for varying the efl fective width of an intermediate frequency modulation sideband passed by said selecting circuit comprising a variable condenser operable independently of the tuning of said oscillator circuit andconnected across part of said inductance and at least part of said capacity for shifting the oscillator frequency relative to said signal frequency, equal movements of said variable condenser producing substantially equal frequency shifts at all frequencies of said range.

'1. In a superheterodyn'e receiver comprising a* signal-selecting system, means including means for generating oscillations for deriving from said selected signals an intermediate carrier-frequency and associated sidebands, a selecting cir-'f cuit for selecting a band of frequencies including said intermediate carrier-frequency, said osdilation-generating means comprising a resonant owillation frequency-determining circuit including an inductance and a capacity connectedacross two points of said inductance, means for varying the effective width of an intermediate '65 frequency modulation sideband passed` by said selecting circuit comprising an auxiliary variable condenser connected between a third point of saidY inductance and a`. point of said capacity.

8.\In a superheterodyne receiver comprising a .70 signal-selecting'system, means including-means for generating oscillations-for deriving from said signals an intermediate carrier-frequency and associated sidebands and a selecting circuit for selecting a band of frequencies in- 75 cluding saidintermediate carrier-frequency, said oscillation-generating means comprising Vs. resonant oscillation frequency-determining circuit including part of an inductance and a variable capacity effectively connected across said part. means for varying the effective width of an intermediate frequency modulation sideband passed by said selecting circuit comprising an auxiliary adjustable condenser connected across at least a portion of said'capacity and at least a portion of said inductance which is outside lsaid part, whereby equal movements of said auxiliary condenser produce substantially equal frequency shifts at any frequency within said range, and thereby produce equal shifts of said intermediate carrier-frequency.

9. In a l superheterodyne receiver comprising means for receiving radio-frequencysignals,means including means for generating oscillations for deriving from said received signals intermediate carrier-frequency signals, means for varyingthe effective width of an intermediate frequency modulation sideband passed by the receiver comprising a selecting circuit for selecting a band of frequencies including said intermediate carrier-frequency, means for shifting said oscillator frequency relative to the frequency of said signals whereby the intermediate carrier frequency may be shifted, and means for adjusting the width of the band selected by said selecting circuit While said intermediate carrier-frequency is being shifted.I

10. Apparatus according to claim 9 in which a single mechanical control device serves to shift the frequency of said oscillations and to increaseA the band width selected Iby said selector.

11. In a superheterodyne receiver comprising means for receiving modulated carrier-frequency signals, means including means for generating oscillations for deriving from said received signals intermediate carrier-frequency signals, said latter means including a resonant frequencydetermining vcircuit having inductance shunted by capacity, a selecting circuit for selecting a. band of `frequencies including the intermediate carrierfrequency means for varying the effective width of an intermediate frequency modulation sideband passed by said selecting circuit comprising a variable condenser connected across at least part of said capacity and aportion of said inductance, said variable condenserbeing normally ina neutral position but being movable in either direction from said neutral position, a selecting circuit for selecting a band of frequencies including the intermediate carrier-frequency, said selecting circuit being provided with movable means for varying the selected band width, said band width being least when said movable means is in a neutral position, and a mechanical control device i'or simultaneously moving said variable condenser and said movable means away from their neutral positions to improve the fidelity of reception.

12. In a superheterodynev receiver comprising a tunable input circuit for carrier signals. means inycluding oscillation generating means for deriving fromsaidsignalsmodulatedlntermediatefrequency signals, said latter-means including a tunable frequency determining circuit, and means for simultaneously tuning said input circuit and said frequency determining circuit and maintaining a substantially constant frequency diil'erence therebetween, means for adjusting said frequency difference by a constant value throughout the respective tuning ranges of said circuits comprising an additional adjustable tuning reactance elementV coupled to one of said circuits.

13. In a superheterodyne receiver comprising a tunable input circuit for carrier signals, means including oscillationI generating means for deriving from said signals modulated intermediate frequency signals, said latter means including a tunable frequency determining circuit, and means for simultaneously tuning said input circuit and said frequency determining circuit and maintaining a substantially constant frequency difference therebetween, means for adjusting said frequency difference by a constant value throughout the respective tuning ranges of said circuits comprising a fixed reactance element external to one of said tunable circuits but coupled thereto and an additional adjustable tuning reactance element connected across said fixed reactance element and at least a portion of its associated tunable circuit.

14. In a superheterodyne receivercomprising a tunable input circuit for carrier signals, means including oscillation generating means for deriving from said signals modulated intermediate frequency signals, said latter means including a tunable frequency determining circuit, and means for simultaneously tuning saidinput circuit and said frequency determiningfcircuit and main'- taining a substantially constant frequency dif' ierence therebetween, means for adjusting said 'frequency dierence by a constant value throughout the respective tuning ranges of said Vcircuits comprising a fixed inductance element external to said frequency determining circuit but coupled thereto, and an additional adjustabletuning condenser connected across said inductance and a capacitive portion of said frequency determining circuit. JOHN KELLY JOHNSON.

Patent No. 2,073,344.

`first column, line 54, strike out the word "the"; page 5, first column,

' IJOHN KELLY JOHNSON.

l March 9', 1937.

lIt is hereby certified that error appears in the printed specification` of the above numbered patent requiring correction as follows: Page 2,

line 2?, for "conjuiction" read conjunction; page 5, first column, line 57, claim 7. strike out the comma aft er' '-'sidebands"; second column, line 44, claim'll, insert a' comma after "frequency" and lines 51-53, strike out the words and comma "a selecting circuit for selecting a. band of frequenr ies including the intermediate carrier-frequency., and that the said LettersPatent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

` signed and sealed 'this 1st day of June, A. D. 1937'.

`(Seal) fI-ienry` Van Arsdale Acting Commissioner of Patents.-y

13. In a superheterodyne receiver comprising a tunable input circuit for carrier signals, means including oscillationI generating means for deriving from said signals modulated intermediate frequency signals, said latter means including a tunable frequency determining circuit, and means for simultaneously tuning said input circuit and said frequency determining circuit and maintaining a substantially constant frequency difference therebetween, means for adjusting said frequency difference by a constant value throughout the respective tuning ranges of said circuits comprising a fixed reactance element external to one of said tunable circuits but coupled thereto and an additional adjustable tuning reactance element connected across said fixed reactance element and at least a portion of its associated tunable circuit.

14. In a superheterodyne receivercomprising a tunable input circuit for carrier signals, means including oscillation generating means for deriving from said signals modulated intermediate frequency signals, said latter means including a tunable frequency determining circuit, and means for simultaneously tuning saidinput circuit and said frequency determiningfcircuit and main'- taining a substantially constant frequency dif' ierence therebetween, means for adjusting said 'frequency dierence by a constant value throughout the respective tuning ranges of said Vcircuits comprising a fixed inductance element external to said frequency determining circuit but coupled thereto, and an additional adjustabletuning condenser connected across said inductance and a capacitive portion of said frequency determining circuit. JOHN KELLY JOHNSON.

Patent No. 2,073,344.

`first column, line 54, strike out the word "the"; page 5, first column,

' IJOHN KELLY JOHNSON.

l March 9', 1937.

lIt is hereby certified that error appears in the printed specification` of the above numbered patent requiring correction as follows: Page 2,

line 2?, for "conjuiction" read conjunction; page 5, first column, line 57, claim 7. strike out the comma aft er' '-'sidebands"; second column, line 44, claim'll, insert a' comma after "frequency" and lines 51-53, strike out the words and comma "a selecting circuit for selecting a. band of frequenr ies including the intermediate carrier-frequency., and that the said LettersPatent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

` signed and sealed 'this 1st day of June, A. D. 1937'.

`(Seal) fI-ienry` Van Arsdale Acting Commissioner of Patents.-y

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