US2460900A - Wide-band superheterodyne receiver - Google Patents

Description

Feb 8, @949 w. H. NEWBQLD 2,460,900

WIDE BAND SUPEEHETERODYNE RECEIVER Filed Dec, 31, 1943 sksheets-sheet 1 vi Hw:-

@fa 1 05a Fb E949. W, H, NEWBQLD 2,460,900

WIDE BAND sUPERHE'IERODYNIE:v RECEIVER Filed Dec. 3l, 1945 3 Sheets-Sheet 2 Feb.

Filed Dec, s1, 194:',

8, 1949. w, H, NEWBOLVD 2,460,900

WIDE BAND SUPERHETERODYNE RECEIVER 3 Sheets-Sheet 3 Patented Feb. 8, 1,949

WIDE-BAND SUPERHETERODYNE RECEIVER William H. Newbold, Langhorne', Pa., assignor,y b'y mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application December' 31, 1943, Serial No. 516,479

(Cl. Z50-20) 4 Claims. l

This invention relates to improvements in superheterodyne radio receivers, and more par'i ticularly to superheterodyne receivers which are capable of receiving Wave signals over wide irequency bands.

Wide-band receivers ci the type contemplated by the present invention have been found to be highly advantageous as aircraft beacon receivers, where the beacon receiver, stationed at an airport and associated with a cooperating transmitter, is required, in response to a prearranged code, signal, or interrogation, from an aircraft, to effect, or institute, the transmission of a respondent signal which identifies the airport in question. The aircraft may, of course, be provided with means. for determining. the range and azimuth of the transmitting station, i. e. of the airport.

Ordinarily these aircraft beacon systems operate in the ultra-high frequency regions, and, for various practical reasons which need not concern us here, operateat least insofar as the airborne transmitting equipment is concernedat carrier frequencies which may vary over substantial limits from time to time, and from air'- craft to aircraft. Thus an airborne transmitter employed for beacon purposes, and having a nominal assigned carrier frequency of 1080.5 megacycles, may operate anywhere in the band extending from, say, 1052.5 to 1112.5A megacycles'. Although the included band is 60 megacycles in width, it is important that the beacon receiver at the airport be capable of responding to the aircraits transmission at once and without spe-i cial preadjustment.

Wide-band receivers of the type contemplated by the present invention are also advantageous as monitoring devices for continuously indicating the presence, or absence, of wave signals in a predetermined wave band. In the past it has been customary to monitor very wide frequency bands by manually (or automatically) tuning a conventional receiver back and forth across the band in question, While aurally noting, or automatically recording, the presence of signals as they are encountered during the band-sweeping process. For certain types or" band monitoring service, a receiver embodying the present invention oifers very substantial' advantages.

It is possible, of course, for either ofthe above-A mentioned services, to design aconventionall superheterodyne radioV receiver having very broad radio frequency and intermediate fre--V quency pass-bands so that the entire band of interest may be continuously receivedor monitored without having to resort to av tuning orA band-sweeping process. This has been' tried in" the past, but effortsi-n this direc'tionI have met with no substantial successi, because convention*- alreceivers having very wide pass-bands are 2 notoriously insensitive. t is well known, for example, that the gai-n which can be expected from a single amplier stage is, at least to aiai-r degreeA of approximation, inversely proportional to the width of the ampliers pass-band. Hence such conventional' wide-band receivers require a large number of amplifier stages to provide the desired overall gain', and in consequence they are excessively complicated and costly.

By theA presenty invention there is provided a novel system for passing" a wide band of irequencies (hereinafter termed the reception band) through a high sensitivity narrow-band receiver. This is accomplished through the agency of a special type of superheterodyne circuit having4 twoI or more local oscillators, which operatev at a corresponding number' of different iixed frequencies, and which are switched onV and off in predetermined time sequence to provide, in the narrow-band output circuit of the converter' stage, a series of individual' segments of the desired band (hereinafter termed band segments), which are coextensive as regards their intermediate frequencyband limits, and which, in one' complete cycle of oscillator switching, represent the reception band to be covered. The inventionalso contemplates the simultaneous reception of two suchband segments, this being accomplished by making use lof both the' "pri-f mary and image segments.'

The invention also contemplates the use of a single local oscillator, with means for period-4 ically changing itsl frequency to simu-late the ac-r tion of the plurality of oscillators hereinbefore` referred to'.

It is, accordingly, aprincipal object of the in vention to providev a novel frequency converter system for' r'ia'ssing a wide reception lea-ndv through a high sensitivity narrow-band frequency amplier'.

It is another object' of the invention to provide a' wide-band Su'perhete'rodyne radioreceiver having' novel local'r oscillator' circuits adapted t'odivide a desired recept-ion band into a plurality of band segments. and to pass said band'seg'e ments through a' narrow-band I. E. amplifier.

It isv a further object oi'- the invention to pro-V vide a Vhigh gain superheterodyne receiver ca ing of wide frequencyvv ranges without the use 'of' manually` or mechanically movable parts.

Other objectsv and features of the invent-ien will'y be understood with reference tothe accompariyiig drawings, in which intermediate Fig. 1 is a diagrammatic illustration of a preferred embodiment of the invention;

Fig. 2 is an explanatory diagram to which reference will be made in the description of the subject invention;

Fig. 3 is a diagrammatic illustration of an alternative embodiment of the invention;

Fig. 4 is a diagrammatic illustration of another embodiment ofthe invention; Y 'Y y Fig. 5 is a schematic diagram of a-portion of the system illustrated diagrammatically in Fig. 3;

Fig. 6 is a schematic diagram 'of 'aV portionof the preferred embodiment of Fig. 1;

Fig. '7 is a schematic diagram of a multivibrator of preferred design for generating square waves having rounded corners; and

Fig. 8 shows the type of switching signals which can vbe derived from the multivibrator of Fig. '7. Reference may now be had to Fig. 1 which illustrates, by means of a block diagram, a preferred embodiment of the invention. The superheterodyne receiving system shown comprises an antenna I, a wide-band first detector (frequency converter) 2, a narrow-band intermediate frequency (I. F.) amplier 3, a second detector 4, and a suitable modulation-frequency amplifier 5. The superheterodynes local oscillator stage comprises a plurality of separate oscillators adjusted to operate at predetermined different frequencies. Two such oscillators, designated oscillator No. 1 and oscillator No. 2 are employed in the embodiment of Fig. 1. An oscillator switching device 6, preferably electronic in character, is operatively associated with the two local oscillators for the purpose of switching them alternately on and oil' in opposite time sequence. A switching signal source 1 may be connected to the device 6 to control the switching operation thereof.

The operation ofthe system of Fig. 1 may best be understood by referring to the explanatory diagram of Fig. 2. Let us assume that the receiver illustrated is to be used in the aircraft beacon system hereinbefore referred to, and that it is desired to receive a signal in a reception band extending from, say, 1052.5 to 1112.5 megacycles, the said reception band having an over-all bandwidth of 60 megacycles,` The desired signal may, of itself, occupy a band-width of only a few megacycles, but since it may occur anywhere in the 60 megacycle reception band it is important that the receiver be capable of responding to the desired signal whatever its carrier frequency.

The antenna l and the input circuits to the first detector 2 should be designed to respond to and pass the entire 60 mc. reception band extending from 1052.5 mc. to 1112.5 mc. These, however, are the only wide-band circuits in the entire receiver. By the practice of the present invention the band-width of the receivers I. F. amplier 3 (which, for the purpose of the present discussion may be thought of as including the output circuit of the first detector 2) may be reduced by a factor of where N is the number of local oscillators associated with the first detector 2. In ther system of Fig. l there are two such oscillators, and hence the required band width of the I F. amplifier is reduced by a factor of one-fourth, or, in the exampleunder discussion, to an over-all bandwidth of mc. A suitable I. F. amplifier for the system under discussion will have a pass-band extending from approximately 7.5 to 22.5 mc.

According to the invention the 60 mc. reception band is divided into four 15-rnc. band segments,

designated A, B, A1 and Bi in Fig. 2. These four relatively narrow band segments are supplied to tor is so related to the band segment A as to convert it to an I. F. band extending from 7.5 to 22.5 mc. This followslfrom the fact that the differences between the oscillator frequency and the upper and lower frequency limits of the band segment A are respectively 7.5 and 22.5 mc. The band segment A may, for purposes of identification, be referred to as a primary band segment, since it corresponds to the band which is normally utilized in conventional superheterodyne receivers, it being customary to operate the local oscillator at a frequency which is higher than the frequency of the desired signal by an amount equal to the receivers intermediate frequency. Oscillator No. 1 is also so related to band segment Ai as to convert this band segment to the same I. F. band. This follows from'the fact that the differences between the oscillator frequency and the lower and upper frequency limits of the band segment Ai are respectively, 7.5 and 22.5 mc. The band segment Ai may, for purposes of identification, be referred to as an image band segment, since it corresponds to the band of similar name which, in a conventional superheterodyne, is not utilized, and which, in fact, has heretofore been eliminated by means of pre-selectors and the like. By the present invention, however, both the primary and the image band segments may be utilized, although both are not necessarily utilized. Y Theremaining band segments, B and Bi, may be converted in like manner through the agency of a second oscillator, oscillator No. 2, adjusted to operate at 1090 mc., a frequency midway between the band segments B and Bi. As before, the band segment B may be regarded as aV primary band segment, and the band segment Bi as an image band segment.

In the normal operation of the system of Fig. 1, the oscillators Nos. 1 and 2 are switched alternately on and off in opposite time sequence, so that while oscillator No. 1 is operative band segments A and Ai are received, and while oscillator No. 2 is operative band segments B and B1 are received, .and thus in a complete oscillator switching cycle the entire reception band is received. The particular oscillator switching rate employed will ordinarily depend upon the type of signal to be received. In one particular use of the'invention where the signals to be received were in thev nature of interrupted continuous waves comprising carrier periods (pulses) of 2 microseconds duration, occurring at the rate of 400 per second, and separated by zero-carrier (no signal) periods of approximately 2500 microseconds, an oscillator switching rate of 300 cycles per second was found to be suitable. Under these conditions the complete oscillator switching cycle has a period of about 3330 microseconds, eachoscillator being individually operative for periods of `about 1665 microseconds. In practice, however, in order to avoid the possibility of both oscillators being operative simultaneously during the change-over period,` itis preferred that each oscillator be on only, say, 49% of the time, or about 1630 microseconds per switching cycle. Should both oscillators operate simultaneously a strong mc. beat signal would be produced in the output of the first detector 2. Since this frequency falls in the middle of the pass-band of the I. F. amplifier 3, such an occurrence should be avoided, preferably in the manner described.

summarizing, it is emphasized that, by means. of the present invention, an unusually wide reception band has been broken up and converted into a plurality of relatively narrow band segments, al1 having identical frequency limits, and thus capable of amplification in a common narrow-band, high-gain, intermediate `frequency amplifier.

Where two local oscillators are employed, and where it is desired to utilize both the primary and image bands, as in the system described with reference to Figs. 1 and 2, the pass-band of the I. F. amplifier will, in the general case, extend between limits having a, 3to1 frequency relation. Thus where a pass-band of 15 mc. is specified, the band-limits satisfying this condition are 7.5 and 22.5 mc. The construction of an I. F. amplifier covering such a frequency range, and having such a high ratio of maximum to minimum frequency limits, should be accorded special care and precision, and it is therefore preferred that the said amplier be constructed in accordance with the methods disclosed in the copending application of C. T. McCoy, Serial No. 473,989, ied January 29, 1943, Patent No. 2,375,309 granted May 8, 1945, employing the band pass transformer disclosed and claimed in the said McCoy application.

Referring generally to the system of Fig. l, it is apparent that the increased R. F. band width (or, conversely, decreasedI. F. bandwidth) is obtained at the expense of duty factor at any specific frequency. When two oscillators are employed this "duty factor'is approximately 0.5, meaning that reception of a given band segment obtains only 50% of the time. mentioned above this is not a disadvantage, and in other cases it may be minimized by proper choice of switching rate.

With proper modification of the I. F. pass-band and oscillator frequencies, the system maybe modified to cover still greater reception-band widths. For instance with three local oscillators, a duty factor of 33%, and an I. F. amplier'having a pass-band extending from to 40 mc., it would be possible to provide a reception band of 120 me. Or if, in the twin-oscillator system rst described, an I. F. pass-band from 10 to 30 mc. were used, a reception band of 80 mc. would be obtained.

If, in the system described with reference to Fig. 2, it is desired to use only the primary band segments A and B, the image band segments A1 and Bi may be excluded by providing antenna and rst detector input circuits with pass bands extending from only 1052.5 to 1082.5 mc. Of

course in this case the band-reduction effect is.

in the preced- 2 of Fig. 3 are themselves continuously operative, l an electronic single-pole double-throw switch 8 being interposed between the local oscillators and the rst detector 2 in amanner such as to con-f In the examples (lllneet first the one and then the other local oscillator to the rst detector 2. As was the case' with the embodiment of Fig. 1, precautions should be taken to ensure that at no time are the local oscillators both coupled simultaneously to the first detector 2. The electronic switch 8 may be controlled, as in Fig. 1, by a switching signal from the source 7. The explanatory. diagram -of Fig. 2 may be applied to the embodiment of Fig. 3 .in the same way that it was applied to Fig. 1, since the two systems are generically identical.l

The embodiment of Fig. 4' is identical in principle to those described above. It differs chiefly in that, instead of a, plurality oflocal oscillators, only a single local oscillator 9 is provided. This single oscillator, however, performs completely the functions of the dual oscillators hereinbefore described. In order to permit such operation there is connected to the oscillators tank circuit (not shown) a frequency control stage l0 adapted to tune the oscillator periodically from one to the other of the predetermined oscillator frequencies.. Operation of the frequency control stage I0 may be effected by the application thereto of a suitable square wave derivedv from the source ll.

All of the components of Fig. 4 are familiar to those skilled in the art, and hence it is deemed unnecessary to illustrate them in detail.

Reference may now be had to Fig. 5 in which there is illustrated schematically an electronic switch and switching signal source suitable for use with the system of Fig. 3. The continuously operating local oscillators, Nos. 1 and 2, are illustrated diagrammatically, since they may be conventional in every respect. Connected between the two local oscillators and the common conductor l2 (which leads to the'rst detector 2 of Fig. 3) is a two-channel electronic switch which operates, in response to a switching signal from the switching signal source, to connect first the oscillator No. 1, and then oscillator No. 2, to the common conductor I2 in predetermined alternating time sequence.

Since electronic switches are well known to those skilled in the art only a brief description of the operation of the device oi Fig. 5 is deemed necessary. The upper channel of the electronic switch comprises a two-stage resistance-coupled amplifier employing the vacuum tubes V1 and V2. This channel is comiected` between the output of oscillator No. 1 and the common output conductor i. The lower channel of the electronicl switch comprises a second two-stage resistancecoupled amplifier employing theY vacuum tubes V3 and V4, This latter channel is connected between ti'ie output ofoscillator No. 2 and the coinmon output conductor i3'. Vacuum'tubes Vi and V3 are each provided with individual load resistors I3 and lll respectively, but the tubes V2 and.

V4 are provided with only a single, or common, load resistor i5. Tubes V1 and V2 are provided with grid input resistors IS and l1 respectively, l which, at their lower extremities, are connected age source 2l may be connected between the.

center-tap of transformer l2 and ground; the

bias Vprovided is preierably'vof ay magnitude such. .that in theabsence oi ai switchingsignal the 'at or below plate-durf voperation in which, during odd alternations (i'fe.

half-cycles) oi the switching signal, oscillator No. 41 is connected by way of tubes V1 and V2 to the output conductor i2, while during even alternations oscillator No. 2 is connected thereto by way of tubes Va and Vt'.

Although triodes have been shown for convenience, practice it will ordinarily be preferred to use pentodes in the circuits just described. Similarly, in order to reduce to a negligible quantity any possible leakage of signal from an undesired oscillator to the output conductor l2, it may be desirable to shield the several stages from each other in accordance with methods of cornrnon practice.

The circuit oi Fig. 5 is particularly adapted for use at medium and medium-high frequencies where conventional vacuum tubes and circuit components are applicable. At higher frequencies, and particularly in the so-called microwave regions, the systemoi Fig. 6 is more applicable. The circuit shown schematically here is representative ci an adaptation of the system of Fig. l for use at ultra-high frequencies.

In Fig. 6 the square wave generator 25 corresponds to the switching signal source 'l of Fig. 1. Similarly the electronic switch, comprising the tubes V5 and Ve, corresponds to the electronic switch 6 of Fig. 1, while the oscillator tubes, designated Osc No. i and Osc No. 2, correspond tothe similarly designated components of Fig. l. The square wave generatorl is-preierably of the type illustrated in Fig. 7 and will be described hereinafter. The oscillators are conveniently of the Reiiex Klystron type (see Sarbacher and Edson, Hyper and ultra-high frequency engineering, Wile, 1943, pages 602 et seq.) and they are so represented in the drawing.

Signals are derivedrfroin the oscillator tubes by means of coaxial lines 2b and 2'! which are inductively coupled into the cavity resonators 28 and 29 of oscillators No. l and No. 2 respectively. The coaxial lines 2li and 21 may then be connected individually to separate elements of the rst detector of the superheterodyne, or, as illustrated in Fig. 6, they may `be interconnected at the junction 3u, a single coaxial line 3l passing thence to the rst detector. By means of. the electronic switch, so designated in the Vdrawing and hereinafter to be described, only one of the oscillators is operative at any given time, and hence the design ci the coaxial line system is not critical. In general, however, the t-Wo line lengths, 2% and 2l, extending from the cavity resonators to the junction 35, should be such that an unused line presents a high impedance to the oscillator then in operation. In other words, the lines should be of such length that the junction 3i) will look like a high impedance to the oscillating Klystron. In practice, suitable line lengths are most readily found by trial. It has also been found that, in practice, best results are obtained through the use of coaxial lines 2B and k2l which are somewhat lossy. Line losses of 8 6 db'. from 'each cavity're'sonatorto the 'junction 30 were found satisfactory. n

If desired, band-pass filters may be inserted in the lines 26 and 21 for suppressingundesired oscillator harmonics. Amplitude metering means may also be inserted in these lines to assist in balancing the magnitude of the output signal from the two oscillators. Frequency metering means for checking the operating frequency of each oscillator may also be provided.

Reference is now made to the electronic switch, and to the circuit connectionsbetween the said switch and the oscillators. Since it is desirable to operate the cavity resonatorsv 28 and 2K9 at ground potential, it is convenient to connect` the positive high-voltage terminal of the source 32 to ground, the cathodes 33 and 34 andV other low potential Klystron elements 'suchras the griii's4 35 and 36 and repeller plates 3l and 38 being con` nected to points in the system which are at'nega'- tive potentials with respect to ground. Thus the cathodes 33 and 34 are connected through the common cathode resistor 3S, the conductor lill, and the parallel potentiometers 4l and 62, to the negative terminal of the source 32. The repeller plates 3l and 38 are connected by `way of conductors 43 and 56 to'adjustable points on the potentiometers lll and d2. Through adjustment of these devices, the potentials on the repeller plates 3l and 38 may be individually fixed. A condenser 45, connected in shunt with potentiometers 4l and 42, may be required to prevent feedback between the repeller plates. The control grids 35 and 36 of the Klystron oscillators'are connected directly to the cathodes of triodes V5 and Ve respectively. As will be explained hereinafter, it is from the cathode load resistors Q6 and 41, associated with these triodes, that control voltages are derived'which effect operation of the two oscillators in alternating sequence.

The electronic switch comprises a pair of triodes Vs and V6 each provided with individual cathode load resistors it and lil, respectively, and connected through a common conductor 43 to the upper junction of the potentiometers lll and f d2, and thence to the negativeterininal of the source 32. The anodes of the' tubes V5 and Vs are connected to B+, i. e. to ground, through a common current limiting'resistor 4B. Grid leaksv 50 and 5| are connected directly between theV grid and cathode electrodes'of thetriodes Vs and The control grids are also coupled, through condensers 52 and 53, to a source of switching signals 25. The source'25 preferably supplies a pair of relatively reversed' modified square-,wave signals of the type illustrated generally in Fig. 8, the multivibrator of Fig. 7 (to be described hereinafter) being particularly well adaptedto the generation of signals ofV this character.

In the operation of the system of Fig. 6, a pair of square-wave switching signals are supplied by the generator 25 to the electronic switch in pushpull relation. When the gridof V5 is driven in the positive direction plate current is Vcaused to ilow, and the upper end of cathode load resistor 41 assumes a positive potential with respect'to the lower end thereof. Since this resistor is in the grid-cathode circuit of oscillator No. 1, a positive voltage is applied to the grid of the said oscillator, permitting the previously blocked plate current to ow, and thereby enabling oscillation to commence. Simultaneously with these events the grid of Ve is driven in the negative direction, cutting off the flow of plate current in Vs, and re-Y ducing the voltage existing across the cathode load resistor 46 to substantially zero. The only voltage in the grid-cathode circuit of oscillator No. 2 is, under these conditions, the bias across the common cathode resistor 39 produced by the flow of space current in oscillator No. 1. This bias is preferably of a magnitude such as to bias oscillator No. 2 to, or below, space current cutoff, thus effectively preventing oscillation of the latter oscillator. During the following half-cycle, the events described above are reversed, oscillator No. 2 being operative, and oscillator No. l being inoperative. Thus the electronic switch, including tubes V5 and Vs, comprises means for periodically effecting operation of the twooscillators in alternating sequence. Through the use =of -a switching signal of the character illustrated in Fig. 8, i. e. one having rounded corners and sloped sides, it is insured that only one of the two oscillators shall be operative at any given instant,

there being, in fact, a short interval between the to ma. during oscillation, and employing a GSN'ZGT (double triode) tube for the tubes V5 and V6 in the electronic switch circuit, resistors of the following values Were used:

Resistor 39=1000 ohms Resistor 4 I :10,000 ohms Resistor 42 10,000 ohms Resistor 49=50,000 ohms Resistor 46:6800 ohms Resistor 41:6800 ohms Resistor 50=1 megohm Resistor 5 I =1 megohm The condenser 45 had a capacity of 0.25 microfarad. The source 32 provided a potential of 700 volts. f

Reference may now be had to Fig. 7 in which there is illustrated, schematically, a multivibrator of preferred design for generating square waves with rounded corners, vand suitable particularly for use with systems of the type illustrated in Figs. 1 and 6. This multivibrator circuit diers schematically from conventional systems only in the provision of the grid-to-grid condenser 54 andthe plate-to-plate condenser 55. These condensers sh-ould be of a magnitude just large enough to slightly round the corners and slope the sides of the output signals, as illustrated, for example, in Fig. 8.

The multivibrator of Fig. 7 is fully described and claimed in my copending application, Serial of said frequency converter, the pass band ofY said amplifier being only as wide as said reception band, and automatic means operatively associatedwith said local oscillator means for effecting, sequentially and at a rapid rate of repetition, cyclical selection of local oscillations at said N diierent frequencies, the frequencies of said local oscillations being so related to the frequency limits of said reception band that said reception band is effectively divided into N primary band segments and N image band segments, all of said band segments being converted by said frequency converter into one common intermediate frequency band, said intermediate frequency band being substantially identical to the said pass band of said intermediate frequency amplier.

2. A superheterodyne radio receiver as claimed in claim 1, wherein said local oscillator means comprises N continuously operating individual oscillators, and wherein said automatic selection means comprises a switching system for sequentially connecting said individual oscillators to said switching system for effecting sequential operal tion of said individual oscillators.

4. A superheterodyne radio receiver as claimed in claim 1, wherein said local oscillator means comprises a single oscillator circuit, and whereinY said automatc selection means comprises a frequency control device operative-in response to an electrical control signal-sequentially to change the operating frequency of said oscillator circuit.

No. 516,480, led December 31, 1943, now abandoned, and since it is not a part of the present invention a detailed description of its operation is deemed unnecessary here.

Although the invention has been described with particular reference to various specic embodiments, it is to be understood that the invention is susceptible of considerable modification,

WILLIAM H. NEWBOLD.

REFERENCES CITED The lfollowing references are ofrecord in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,951,524 Nicolson Mar. 20, 1924 1,771,700 Alexanderson July 29, 1930 2,026,759 Turner Jan. 7, 1936 2,028,212 Helsing Jan. 21, 1936 2,029,035 Roberts Jan.28, 1936 2,167,605 Carlson'V July 25, 1939 2,186,455 Goldmark Jan. 9, 1940 2,273,640 Haantjes et al, Feb. 17, 1942

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