US2061991A - High-frequency signaling system - Google Patents

Description

Nov. 24,

H. A. WHEELER 1 HIGH FREQUENCY SIGNALING SYSTEM Filed May 22, 1935 DETECTOR FREQUENCY O O O O O 0 O 3 Sheets- Sheet 1 INVENT OR.

HAROLD A. WHEELER I ATTORNEY.

NOV. 24, 1936. WHEELER 2,061,991

HIGH FREQUENCY SIGNALING SYSTEM Filed May 22, 1935 5 Sheets-Sheet 2 *5 x" g VFfEQUE/VC) 5 w E v; d g o 5 2 2 Z 49 g FREQUENCY k I g b E; 4 s Q- a K g I 1%. 6. k FREQUENCY LOW f comps/r552 557mm M/v.

INVENTOR.

AROLD 4. WHEELER FPEQUE/YCV ERROR ATTORNEY.

Nov. 24, 1936. H, A WHEELER 2,061,991

HIGH FREQUENCY S IGNALING I SYSTEM Filed May 22, 1935 3 Sheets-Sheet 3 INVENTOR.

TUNING FEEQUENC) (K/LOCYCLLS) f/flAOLD A. WHEELER 05cm LKIIOB 510202 (K/LOC/(ZE5) ATTORNEY.

Patented Nov. I 24, 1936 HIGH-FREQUENCY SIGNALING SYSTEM Harold A. Wheeler, Great'Neck, Long Island, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application May 22, 1935, Serial No. 22,699 18 Claims. (Cl. 250-20) This invention relates to high-frequency signaling systems and particularly to systems of this character having a plurality of circuits which are simultaneously tunable to different frequen-' cies and over different frequency ranges, as, for instance, tunable selector coupling circuits for antennas and/or radio-frequency amplifiers and tunable oscillator circuits, such as are generally employed in superheterodyne radio receivers. This application is a continuation-in-part of application, Serial No. 691,927, filed October 3, 1933, Patent No. 2,050,679, August 11, 1936, for Radio receiver.

More particularly, theinvention is directed to maintaining constant the frequency difference of circuits as above described by the provision of improved aligning means in connection with one 'or more of the circuits.

Various arrangements have heretofore been devised in attempts to provide correct alignment of circuits such as'those mentioned, or,'in other words, to maintain constant the frequency difference between these circuits over their respective ranges. According to one approved and widely employed practice, there are provided tuning condensers having a. constant ratio of capacitances,

that is, having platesof similar size and shape which are all similarly adjustable at the same rate, and unicontrol means therefor, for tuning, or changing the resonant frequency of, a selector circuit or circuits simultaneously with that of an oscillator circuit, over different frequency ranges and with an approximately constant frequency difference therebetween. With such an arrangement, the respective resonant frequencies of the circuits vary in accordance with characteristic curves of frequency versus condenser adjustment which are spaced in the frequency scale one above the other, the higher one-usually being that of the oscillator circuit. These curves, however, are not exactly parallel to each other, as would, of course, be the case if the frequency difference was exactly constant over the entire ranges. In other words, the frequency difference varies over the ranges of the circuits and the frequency of the circuit having the higher curve is ordinarily too high at the upper end of the range and too low at the lower end thereof.

The provision in the last-named circuit, as in usual practice, of series and parallel padding or aligning condensers, or their equivalents, andthe adjustment of the main inductance thereof changes the frequency-condenser adjustment curve of this circuit. The resulting curve, while not exactly coinciding with the curve of ,ideal alignment, improves the alignment at 'the lower and upper ends of the frequency range and passes through the ideal curve at three points, to wit: a central point wherein the proper alignment is effected by the adjustment of the main inductance of the circuit; a point near the lower frequency end where alignment is corrected by the provision of the series padding condenser; and a point near the upper frequency end where the alignment is corrected by the provision of the parallel padding condenser. The remaining frequency errors are evidenced by the departure from the-ideal of portions of the curve at the opposite sides of'th'e upper and lower points of alignment mentioned, which portions indicate that the tuning frequency above the points is then too low and below the points is too high. It will thus be seen that, at the intersection points of the actual curve 'with the. ideal curve, the frequency difference is at adesired normal value and that at all other frequencies within the respective frequency ranges of the circuits to be aligned the frequency difference departs from this desired normal value in one direction or the other. The present invention, however, serves to maintain the frequency difference of the circuits more nearly constant over their respective tuning ranges, or, in other words, reduces the remaining alignment errors by increasing the number of frequencies at which the frequency difference is at the desired normal value and decreasing the departure of the frequency difference from this normal value at other frequencies within the ranges. The primary object, therefore, of this invention is to provide, in a system embodying circuits simultaneously tunable to different frequencies and over different frequency ranges, novel and improved means, effectively connected in at least one of the circuits, for maintaining the frequency difference between the circuits substantially constant over their respective tuning ranges.

Further objects of, and advantages achieved by, the invention, including the provision of aligning means for tunable circuits of the character described which are characterized by simplicity of construction and efficiency of operation with improved results, will be apparent from the following descriptions of approved embodiments of the invention.

In accordance with the present invention, there are provided the usual alignment capacitances effectively in series and in parallel, respectively, with the tuning condenser of a particular circuit to be aligned with another circuit or other circuits of the same system, such as above mentioned. The tuning condensers. of the circuits are arranged for unicontrol and have a constant ratio of capacitances, thereby to maintain the frequency difference of the circuits approximately constant. In addition to these aligning capacitances, auxiliary means is connected effectively in series in this circuit for procuring a more nearly constant frequency difference.

The auxiliary means comprises reactance means, whichmay be either a capacitance or an inductance or both, connected in circuit with a resistance. In certain forms of the invention the auxiliary means is connected directly in series in the tunable circuit that is to be aligned, and

' in other forms it constitutes a separate circuit capacitances in the usual manner.

and is reactively coupled to the tunable circuit. The reactanc e of the auxiliary means is variable with frequency, the elements thereof being proportioned to effect increases and decreases in the effective reactance of the tunable circuit at the proper frequencies more nearly to approximate a constant frequency difierence between the several circuits. The resistance varies the effective reactance of the auxiliary circuit gradually with frequency at the rate necessary to change the reactance of the tunable circuit, as required.

A better understanding of the present invention will be had from the following detailed description, taken in connection with the accompanying drawings, and its scope will be apparent from the appended claims.

In the accompanying drawings, Fig. 1 is a circuit diagram, partially schematic, of a complete superheterodyne radio receiver including radiofrequency amplifier circuits and an oscillator circuit with the present invention applied in connection therewith; Fig. 2 is a circuit diagram of another embodiment of the invention applied in connection with a receiver having a tunable antenna coupling circuit and an oscillator circuit;

Figs. 3-7 are curves illustrating the principles of Referring particularly to Fig. 1 of the drawings, the receiver there illustrated includes an antenna Ill connected to ground ll through a primary winding l2 of a radio-frequency transformer, a secondary winding l3 of which is tuned by an adjustable condenser I4 providing a tunable radio-frequency selector circuit IS. The circuit I5 is connected at one point with the grid of an amplifier tube l6, of the conventional pentode type, and at another point, through ground and a cathode biasing circuit, to the cathode of this tube. The output circuit of the tube l6 extends from the anode of the tube through a primary winding I! of a second radio-frequency transformer and through a source of B voltage and ground to the cathode of the tube. The secondary winding l8 of the last-mentioned transformer is tuned by an adjustable condenser l9 providing a second tunable selector circuit 20.

Padding condensers I51: and 20a are connected in the circuits l5 and 20, respectively, in parallel with the condensers I4 and IQ of these circuits to correct slight differences in minimum circuit The circuit 20 is connected at one point to a control grid of an oscillator-modulator tube 2| and at another point, through ground and a cathode biasing circuit, to the cathode of this tube. The

tance 23 and an adjustable tuning condenser 24,

the latter being mechanically connected for unicontrol with the condensers l4 and I9, as indicated by the broken lines U. The oscillation circuit 22 is included directly in the oscillator grid circuit of the tube 2| land is coupled to the oscillator anode of this tube through a winding 23a, coupled to or forming a continuation of the inductance 23. A blocking condenser 23b is included in the oscillator anode circuit, and a choke coll 230 is included between the oscillator anode and its voltage source +13.

The signal output or anode circuit of the tube 2| is coupled through a double-tuned intermediate-frequency transformer 25 to an intermediatefrequency amplifier which is, in turn, connected in cascade with a detector, an audio-frequency amplifier and a sound reproducer, or loudspeaker LS. The parts following the transformer 25, being of conventional type, are indicated schematically in the drawings. Suitable resistors, by-

passing condensers and similar elements are inauxiliary aligning means are provided. The auxiliary means comprises a circuit ,28which includes an inductance 29, a condenser 30 and -a resistor 3|, all connected in parallel. The inductance 29 is inductively coupled to the inductance 23 thereby to connect the parallel circuit 28 effectively in series in the oscillation circuit.

Neglecting for the present the particular operation and efi'ect ofthe auxiliary aligning means of the present-invention, which will be hereinafter described in detail, the above-described system constitutes in general a conventional type of superheterodyne' radio receiver the operation of which is well known .to those skilled in the art, so that a detailed description thereof is deemed unnecessary. Briefly, however, the radiofrequency signals intercepted by the antennav oscillation circuit 22; and in addition thereto- ID are selected by the tunable selector circuit l5, amplified bythe tube l6, subjected to a second selection in the tunable circuit 20 and converted by the oscillator-modulator tube 2| into an intermediate frequency, which is the difference in frequencies of the oscillation circuit 22 and the selected signal. The intermediate frequency thus produced, modulated by audiofrequency signals, is amplified in the intermediate-frequency amplifier and then rectified or detected to derive the audio-frequency component, which is successively amplified, and reproduced in the loudspeaker. I

The circuit 22, as in usual practice, is so designed as to be tunable at different frequencies from, and over a, different frequency range than that of, the circuits l5 and 20. More particularly, the inductance 23 is smaller than the inductances l3 and I8 (which are substantially equal) while the condensers l4, l9 and 24 have a constant ratio of capacitances, that is, their plates are of similar size and shape, and they are simultaneously adjustable at the same rate-by the unicon-v trol means U. Thus, the range of frequencies of the circuit 22 is higher than the range of the other two circuits and the particular frequencies to which the circuit 22 is tuned are higher than the corresponding frequencies to which the other two circuitsare tuned. The ,range of the circuits l5 and 20 is preferably from 550 to 1500 kilocyclesfthat is, the broadcast band, while the oscillation circuit and the circuits I5 and 20 should be maintained constant over their respec; tive ranges. I.

Refening now more particularly to the operation of the circuit 28, as appliedin connection with the oscillation circuit 22, the manner in which this circuit is aligned in accordance with this invention will be described with reference to the characteristic curves shown in Figs. 3

In Fig. 3 characteristic curves of resonant frequency versus condenser setting for the circuits in question are shown. Condenser settings, from maximum to minimum, are represented by the abscissae, while the ordinates represent frequencies, for example, from 550 to 1612 kilocycles.

The curve 32 indicates the resonant frequencies of the circuits l5 and 20 for various settings of their respective tuning condensers, while the desired or ideal curve of frequency versus condenser setting for the oscillation circuit 22 is indicated by the dotted line 33, which, of course, is parallel to the curve 32, indicating a constant frequency difference over the entire ranges of the circuits.

With only the inductance 23 and condenser 24 included in circuit 22, its frequency condenser setting curve would be as indicated by the curve 34 of Fig. 3, which-intersects the curve 33 only at the point 35, showing that the frequency difference is at the desired normal value and the alignment of the circuits is correct only at this point. The alignment at point 35 is effected by proper proportioning of the inductance 23. At other frequencies the circuits are not aligned; that is, the frequencies of the circuit 22 above this point are too high while below this point they are too low. The series padding condenser 29, however, serves to correct the lower portion of the curve 34, as indicated by the curve 36,

and the parallel trimmer condenser 21 serves to correct theupper portion of the curve 34, as indicated by the curve 37, thus improvingthe alignment at both ends of i the frequency range. This is according to conventional practice and a detailed explanation of the theory thereof is unnec-- essary., It is apparent from the curves 3 6 and 31, however, that frequency errors are still prestuning ranges. Either a single circuit 28 may be employed, as shown in the drawings, which circuit willfunction mainly at one of the ends of the frequency range, or, if desired, two such circuits may be employed, one for each end thereof.

In the embodiment illustrated in Figl the inductance 29 and condenser 3|! are proportioned to tune the circuit 28 to the frequency indicated at the point 89, where the curve 31 intersects the curve 33, or, where the frequency difference is at the desired normal value. As thus proportioned, the circuit 28 has an effect which is equivalent to adding inductance in the oscillator circuit at frequencies below the point 39 and subtracting inductance at frequencies above this point, with the resultant effect of increasing and decreasing, respectively, the'frequencies of the oscillator circuit above and below this point. The resistance 3|, as heretofore mentionecL'broadens the resonance characteristic of the circuit 28 and effects a more gradual variation of the reactance of the oscillator circuit. The size of the resistance is so limited that the added impedance reflected by the auxiliary means into the circuit .22 will not seriously reduce the amplitude of the oscillations of the circuit.

Figs. 4-7represent curves of frequency error or misalignment of the oscillator circuit over its tuning range. In Fig. 4; the curve 4| indicates the best alignment obtainable by employing only the aligning condensers 26 and 21, the portions of the curve 4| above and below the zero axisindicating positive and negative alignment errors respectively. The point 42 corresponds to the point 39 of Fig. 3. The curve 43 indicates the change in oscillator frequencies which is produced by the inclusion of the circuit 28, tuned to the frequency indicated at 39. The curve 44 of Fig. 5is the resultant alignment curve representing the sum of the curves 4| and 43. Having ob tained this curve 44, the series padding condenser 26 is then readjusted to' increase the frequencies at the lower end of the range, and a resultant alignment is thus obtained such as is indicated by thecurve 45 of Fig. 6. The main inductance 23 is then readjusted, that is, reduced the necessary amount, and a resultant aligmnent is thereby obtained such as is indicated by the curve 46 of Fig. 7. It is thus apparent that by adding the auxiliary circuit, proportioned as described, the fre-- quency difierence is made more nearly constant,

and ideal or exact alignment is closely approximated.

In Fig. 2 there is illustrated another form of the invention, applied in connection with a receiver somewhat similar to that shown in Fig. 1, corresponding elements being indicated by the same reference numerals. Here, however, a tunable coupling circuit 50 is substituted for the radio-frequency selector circuits 5 and 2|) of Fig. 1, the antenna circuit |9|| being directly connected by the circuit 50 to the oscillator-modulator tube 2|. The parts of the system following the transformer 25 are omitted from Fig. 2, but they will be understood to be thesame as those indicated in Fig. 1. f, l The coupling circuit'50 is a parallel resonant circuit the arms of which comprise, respectively, a tuning condenser 5| and a coupling, condenser 52 connected in series, and a fixed inductance 53.

,The antenna circuit is connected directly across the coupling condenser 52 and output terminals of the circuit are connected across the tuning con denser 5|, and connect the circuit 50 to the signal input circuit of the tube 2|. If desired, padding or aligning condensers 54 and 55 may be connected in parallel with the tuning condenser 5i and the coupling condenser 52, respectively. A resistor 58 serves to connect the lower terminal of the inductance 53 with ground to provide a direct current path for the signal control grid of tube 2|, thus to determine its potential and to permit a variable bias to be applied for automatic volume control or other purposes. This particular antenna coupling circuit permits more accurate alignment with relatively higher gain than is ordinarily possible with such circuits. As the coupling circuit per se forms no part of this invention, a detailed description of its characteristics and operation will not be here necessary, such circuit being disclosed and broadly claimed in the copending application of Daniel E. Harriett, Serial No. 18,362, filed April 26, 1935, for improvements in high-frequency coupling circuits.

The oscillator-modulator system of this embodiment and the succeeding parts of the system, as well as the general operation of the receiver, are the same as in the embodiment shown in Fig. 1, and further details in these respects are, therefore, unnecessary. The oscillation circuit 22' includes an inductance 23, a tuning condenser 24', and aligning condensers 26' and 21 similar to the corresponding elements of the circuit 22. An auxiliary alignment circuit 28' is provided in this embodiment, which includes an inductance 29', a condenser 30', and a resistor 3!, all connected in parallel. In this embodiment, the circuit 28' is directly connected in series in the oscillation circuit 22'. The operation and effects. of .the auxiliary alignment circuit 28" are substantially the same as those of the auxiliary alignment circuit 28 of Fig. 1.

While my invention is in no wise limited to any specific circuit design or constants, eminently satisfactory results were obtained in a superhetercdyne receiver embodying the present invention and using substantially the circuit arrangement illustrated in Fig. 2. In this receiver the circuits 22', 28', and 50 had the following constants:

Circuit 2 2'--1 01 -1 960 kilocycles Inductance 23' microhenries 163 Condenser 241---;micro-microfarads- 15420 Condenser 21' micro-microfarads 10 Condenser 26 ..micro-microfarads1 500 Circuit 28' I Condenser 30' micro-microfarads 850 Resistance 3| ohms 250 Inductance 29' microhenries 8 Circuit 50-550-1500 kilocycles Condenser micro-microfarads 15 420 Condenser 52 micro-microfarads 15420 Inductance 53 microhenries 390 Condenser 55 micro-microfarads Resistance 58 0hms 500,000 Condenser 54 micro-microfarads 210 In Fig. 8 there is illustrated another embodiment of the invention, applied in connection with an oscillator-modulator system, comprising separate modulator and oscillator tubes 60 and GI, respectively. This embodiment is shown in Fig. 1, and described with reference to Figs. 1 and 5 01' the drawings of the above mentioned application, Serial No. 691,927, Patent No. 2,050,579. The present application thus constitutes a continuationin-part of said copending application. While only the oscillator-modulator system is shown in Fig. 8, it is, excepting for the application of the present invention in connection therewith, of a conventional design and its operation and manner of connection in a receiver provided with unicontrol tuning means for the several tunable circuits thereof, will be readily understood by those skilled in the art, rendering a detailed description in these respects unnecessary in this application.

The system shown in Fig. 8 includes 'an oscillation circuit 61 comprising a fixed inductance ii", an adjustable tuning condenser 62, which is mechanically connected for unicontrol with the tuning condensers of other tunable circuits of the receiver (not shown), and a series-aligning condenser liw. The equivalent of an additional parallel-aligning condenser, as indicated at 63, may be provided by properly proportioning the condenser 62 to increase its capacity the necessary amount. The oscillation circuit 61 is included in the oscillator grid circuit of the tube 6| and is coupled to the anode of this tube through a winding 54, coupled to, or forming a continuation of, the inductance l9l. A blocking condenser Gla is included in the oscillator anode circuit and an isolating resistance 64b is included between the oscillator anode and its voltage source +13. The inductance i9! is also coupled to a winding 65 serially included in the cath0de-toground connection of the modulator tube 60, whereby the oscillations are fed from the circuit 61 to the tube 60.

The auxiliary aligning means (supplementing the series and parallel aligning capacitances) in this embodiment of the invention constitutes resistors 68 and 69 which may be connected across the windings 55 and 64, respectively, fairly closely coupled to the oscillation circuit 61. These coils thus serve their first-mentioned respective purposes, of coupling the circuit 61 to the tubes 60 and BI, and also constitute reactances connected in parallel with resistances to provide auxiliary aligning means in accordance with this invention.

Each of the auxiliary aligning means functions in substantially the same manner as the auxiliary aligning means shown in Figs. 1 and 2, as hereinabove described; that is, they effectively change the inductance of the tuned oscillator circuit at the frequencies where the circuit is misaligned, so as to correct the alignment errors. The resulting curve, illustrating frequency error as a function of oscillator tuning frequency, is shown by curve l3l of Fig. 10, the alignment curve obtaining when the auxiliary means is omitted being shown at I 30. It will be seen from a comparison of the two curves that the addition of the auxiliary impedance 5 means substantially lessens the alignment error at all frequencies within the band considered. The auxiliary alignment means of this embodiment is most effective at the high-frequency end It will be understood that the circuit 61 and the their respective inducta'nces.

In Fig. 9 there is illustrated another embodiment of the invention, as embodied in an oscillation circuit 10 which includes an inductance ll, a tuning condenser 12, and series and parallel aligning condensers l3 and I4, respectively, in the While only the circuit 10 and its immediate connections are shown, it will be unusual manner.

derstood that this circuit may be substituted for either of the circuits 22 or 22, respectively, of the systems shown in Figs. 1 and 2, it being only necessary to connect the terminals '|5,-16, shown in Fig. 9, to the respective terminals indicated at 15 -16 in Fig. 1 or Fig. 2. The condenser 12 will be mechanically connected to the other tuning condenser or condensers of the system, as in the embodiments of Figs. 1 and 2. In this embodiment, the auxiliary aligning means of this invention is provided by the condenser 13 and the resistor 11 connected across this condenser and thus providing the parallel circuit 18, which circuit is connected directly in series in the oscillation circuit Ill. The condenser 13, by proper adjustment of its capacity, serves as both the usual series aligning condenser and the aligning condenser of the auxiliary means. The operation of this embodiment of the invention is substantially the same as that of the other embodiments, the auxiliary aligning circuit serving effectively to change the reactance of the circuit 70 tomodify the characteristic of the circuit over those portions of the range where the alignment errors occur, and thus to maintain the frequency diiference more nearly constant.

While various forms of the invention have been described above in connection with oscillation circuits, it will, of course, be understood that the alignment means may be equally well applied to a signal selector circuit or to any one or more tunable circuits which it is desired to align with another or other circuits, in systems such as have been described, in order to maintain the frequency difierence between the circuits more nearly constant.

Furthermore, while there has been described what are considered at present to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of this invention. I

What is claimed is: c

1. In a high-frequency signaling system, a pair of circuits tunable to different frequencies and over different frequency ranges, means for simultaneously tuning said circuits over said frequency ranges, reactance means included in one of said circuits for maintaining the frequency difference of said circuits approximately constant. over their respective frequency ranges, and reactance and resistance means connected in parallel and effectively connected in circuit with .said lastnamed tunable circuit and proportioned substantially to maintain said frequency difference more nearly constant.

2. In a high-frequency signaling system, a pair of tunable circuits each including an inductance and an adjustable condenser, said condensers being connected for unicontrol adjustment and having aconstant ratio of 'capacitances whereby the resonant frequencies of said circuits may. be varied over different ranges of frequencies with afrequency difference therebetween, reactance' means included in one of said circuits for maintaining the frequency difference of saidcircuits approximately constant overtheir respective frequency ranges, and reactance and resistance means connected in parallel and effectively connected in circuit with said last-named tunable circuit and proportioned substantially to main- Q tain said frequency difference more nearly con: "stant. 1

3. In a high-frequency signaling system, a pair of circuits tunable to different frequencies and over different frequency ranges, means for simultaneously tuning said circuits over said frequency ranges, means included infoneof said circuits for maintaining the frequency difference of said circuits-approximately constant over their re circuit, and auxiliary impedance means effectively connected in circuit with said last-named tunable circuit and proportioned substantially to maintain said frequency differencemore nearly constant. r

v 4. In a high-frequency signaling system, a pair of tunable circuits each including an inductance and an adjustable condenser, said condensers being connected for unicontrol adjustment and having a constant ratio of capacitances whereby the resonant frequencies of said circuits may be varied over different ranges of frequencies with afrequency difference therebetween, means included in one of said circuits for maintaining said frequency difference approximately constant over their respective frequency ranges" compris ing condensers included, respectively, in parallel and series with said tuning condenser of said lastnamed tunable circuit, and auxiliary means effectively connected in circuit in said last-named tunable circuit and proportioned substantially to maintain said frequency difference more nearly constant over a portion of said frequency ranges including a reactive element and a resistance connected in parallel therewith."

5. In a high-frequency signaling system, a pair .maintain said frequency difference more nearly constant.

6. In a high-frequency signaling system, a pair of circuits tunable to different frequencies and over'different frequency ranges, means for simultaneously tuning said circuits over said frequency ranges, reactance means included in one of said circuits for maintaining the frequency difference of said circuits approximately constant over their respective frequency ranges, and an inductance and a resistance connected in parallel and effectively connected in circuit with said last-named tunable circuit and proportioned substantially to maintain said constant.

7. In a high-frequency signaling system, a pair of circuits tunable to different frequencies and over different frequency ranges, means for simultaneously tuning saidcircuits-over said frequency ranges, reactance means, included in one of said circuits for maintaining the frequency difference of said circuits approximately constant over their respective frequency ranges, and auxiliary means including an inductance, a condenser and a refrequency difference more nearly sistance connected in parallel, said auxiliary imately constant over their respective frequency ranges, and auxiliary means including an inductance, a condenser and a resistance, connected in parallel, the inductance of said auxiliary means being coupled to the inductance .of the last-named tunable circuit and the elements of said auxiliary means being proportioned substantially to maintain said frequency difference more nearly constant.

9. In a superheterodyne radio receiver, a signalselector circuit and an oscillation circuit tunable V to different frequencies and over different frequency ranges, means for simultaneously tuning said circuits over said frequency ranges, reactance means included in one of said circuits for main taining the frequencydifference of said circuits approximately constant over their respective frequency ranges, and resistance and reactance means connected in parallel and effectively connected in circuit with said last-named tunable circuit and proportioned substantially to maintain said frequency difference more nearly constant.

10. In a superheterodyne radio receiver, a signal-selector circuit and an oscillation circuit, tunable to different frequencies and over different frequency ranges, means for simultaneously tuning said circuits over said frequency ranges, reactance means included in said oscillation circuit for maintaining the frequency difference of said circuits approximately constant over their respective frequency ranges, and resistance and reactance means connected in parallel and effectively, connected 'in circuit -with said oscillation circuit and proportioned substantially to maintain said frequency difference more nearly constant. 11. In a superheterodyne radio receiver, a signal-selector circuit and an oscillation circuit, each including an inductance and an adjustable condenser, said condensers being connected for unicontrol and having a constant ratio of capacitances whereby the frequencies of said circuits may be varied over different ranges of frequencies with a frequency difference therebetween, means included in one of said circuits for maintaining said frequency difference of said circuits approxi-' mately constant over their respective frequency ranges, comprising condensers included, respectively, in parallel and series with said tunin contuning reactance means in said last-named circuit for maintaining said denser of saidlast-named tunable circuit, and auxiliary means effectively connected in circuit in said last-named tunable circuit and proportioned substantially to maintain said frequency difference more nearly constant over a portion of said frequency ranges, said auxiliary means including a reactive element and a resistance connected in parallel therewith. 12. Ina superheterodyne radio receiver, a signal-selector circuit and an oscillation circuit each including an inductance andan adjustable condenser, said condensers being connected for unicontrbl and having a constant ratio of capacitances whereby the frequencies of said circuits may be varied over different ranges of frequencies with a frequency difference therebetween, means included in one of said'circuits for maintaining said frequency difference of said circuits approximately constant over their respective frequency ranges,

said means comprising condensers included, respectively, in parallel and series with said tuning condenser of said last-named circuit, and auxiliary means effectively connected in series in said 'l'ast-nam'ed tunable circuitand proportioned to maintain said frequency diflerence' more nearly constant, said auxiliary means including an inductance, a condenser and a resistance connected in parallel, said auxiliary inductance and condenser being tuned to a one of said ranges.

- 13. In a superheterodyne receiver with unicontrol tuning, the arrangement comprising a selector to be tuned to any signal frequency in a given tuning range,. an oscillator circuit to be simultaneously tuned to a different frequency,

frequency at one end of several reactance'elements connected in the oscillator circuit and adjusted to maintain the frequency difference approximately uniform, and

parallel connected resistance and reactance in the oscillator circuit, adjusted to substantially improve the uniformity of the frequency diflerence.

14. In a superheterodyne receiver with unicontrol tuning, the arrangement comprising a selector to be tuned to any signal frequency ins. given tuning range, an oscillator circuitto be simultaneously tuned to a different frequency, several reactance elements connected inthe oscillator circuit and adjusted to maintain the frequency difference approximately uniform, and parallel connected resistance and inductance coupled to one of said reactance elements, adjusted to substantially imat the higher frequencies in the oscillator tuning range.

15. In a high-frequency signaling system, a pair of circuits, adjustable reactance means for simultaneously tuning said circuits to different resonant frequencies and over different frequency ranges whereby a frequency difference exists between the resonant frequencies of said circuits, means connected in one of said circuits for maintaining said frequency difference approximately constant as the tuning of said circuits isgvaried to vary said resonant frequencies over their respective frequency ranges, and additional/means comprising an impedance network including an impedance element of a and effectively connected frequency difference more nearly constant.-

16. In a high-frequency signaling system, a pair of circuits tunable to different frequencies and over difl'erent frequency ranges, means for type different from said prove the uniformity of the frequency difference simultaneously tuning said .circuits over said 7 5 aocnoei ranges, reactance means included in one of said circuits for maintaining the frequency diiference of said circuits approximately constant over their respective tuning ranges and additional means comprising a third circuit resonant at a frequency at one end of the tuning range of said one of said tunable circuits and effectively connected in circuit therewith to maintain said frequency difference more nearly constant.

17. In a high-frequency signaling system, a pair of circuits tunable to difierent frequencies and over different-frequency ranges, means for simultaneously tuning said circuits over said ranges, reactance means included in one of said circuits for maintaining the frequency difference of said circuits at a normal value only at predetermined frequencies within said ranges, and additional means comprising a third circuit resonant at one of said predetermined frequencies and effectively connected in one of said pair of circuits for increasing the number of frequencies at which said frequency difference is at said normal value and for decreasing the departure of said frequency difference from saidnormal value at frequencies above and below the resonant frequency of said third circuit.

18. In a high-frequency signaling system, a pair of tunable circuits each including an inductance and an adjustable condenser, said condensers being connected for unicontrol adjustment to tune said circuits over different frequency ranges with a frequency difference therebetween, reactance means included in one of said circuits for maintaining said frequency difference at a normal value only at predetermined frequencies within said ranges, and additional means comprising a third circuit resonant at one of said predetermined frequencies and includthird circuit.

HAROLD A. WHEELER.

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