US2245710A

US2245710A - Frequency changing system

Info

Publication number: US2245710A
Application number: US311901A
Authority: US
Inventors: John A Rado
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1939-12-30
Filing date: 1939-12-30
Publication date: 1941-06-17
Anticipated expiration: 1958-06-17
Also published as: GB546830A

Description

June 17, 1941. J.'A. RADO FREQUENCY CHANGING SYSTEM Filed Dec. s, 1939 INVENTOR JOHN A.RADO

ATTORNEY Patented June 17, 1941 FREQENCY onANGiNo SYSTEM John'A.'jRa'do, Flushing, N. Y., assignor to HazeltineCorporation; a corporation of Delaware Appliance VDecember 30,1929, serial' No'. 311,901

6 Clainis (CL "Z50-0) .cause the reactive current flowing in the controltube anode circuit `incidentally to depart lfrom the desired 90-degree phase with respect to the This invention relates to frequency-changing systems and particularly to Aautomatic.frequency control for suchsystems embodied in modulatedcarrier signal receivers of the superheterodyne type, especially television receivers. s

Superheterodyne receivers provided Witha fre-lv quency-changingsystem for selecting predetermined Inodulated-carrier `signalsand deriving therefrom intermediate-frequency modulated signals are well known. f `Such frequency-chang, ing systems oftenare .provided with automatic frequency control means .including a correction circuit coupled to. the oscillator` circuit of the frequency-changing means kfor varying theleective reactance thereof to maintain .the'frequency of the oscillator circuit atthe proper value necessary for deriving the selected intermediate-frequency signals. One system of controlling-the oscillator frequency involves a control tuloeik the anode circuit of Awhchisin parallel withthe oscillator-tuned circuit. andthe control electrode of which is excited voy a potential which has, by means of a suitable phasing network, been shifted 90 degrees with respect/to the voltage across the tuned circuit of the oscillator sothat the. component of. current which is common to the control-tube anode and lthetuned circuit is 90 degrees out ofphase with the voltageacross. this tuned circuit. The controlJtube/thussimulates a reactance in parallel .with-the oscillator-tuned circuit. The magnitudey of this reactance and hence the oscillatork frequency, is determined by the mutual conductance of thev control tube, which, in turn, is controlled by the bias yvoltage developed and fed to the vcontrol-tube'grid return oy a irequency/.discriminator or detector. In this WayVtheJlOCal oscillator frequency is shifted in a directionV that tends toreduce-the difference between` the intermediate frequency developed and the mean frequency to vwhichI the intermediate-frequency selector circuits are tuned. l

While the operation-ofsuchautomatic frequency conjtrol` systems is-satisfactory''forthe conventional reception of modulatcrd-carrier sigf nais in the broadcast bandv of frequencies; it is often unsatisfactory for the vreception of modulated-carrier Vsignals of ultra-high frequencies, for example, in the frequency range assigned to television, because Vvelectronv transit time Veffects aresuicient to affect Athe operation 'of -both the oscillator and control tube, Which-efiects interfere with the normal `operation :of fthe system when the receiver yis tuned from fone#ultra-high 4 frequency to.another.-Suchtransitfltimerefiects oscillator voltage `as the oscillator-tuned circuit is tunedto diiferent frequencies in the ultra-highfrequency range.

It is an object of the present invention, therefore, to provide animproved frequency-changing system for a modulated-carrier signal receiver of the superheterodyne type which overcomes the above-mentioned diiilculties anddisadvantages of such arrangements of the prior art.

It is a more specific object of the invention to provide an improved automatic frequency control system fortelevision signal receivers of the superheterodyne type.

In accordance with v.this invention, there is providedin an ultra-high-frequency modulatedcarrier signal receiver aY frequency-changing system for deriving from selected received modulated-carrier signals intermediate-frequency y modulated-carrier, signals,-Which comprises an oscillator circuit, means for selectively proporv tioning` the constants of the oscillator circuit to oscillateY at any ydesired one of a plurality of different frequencies, and a'frequency-correction circuit coupled to .theoscillator circuit including electron-discharge variableimpedance means to ladjust the frequency of the oscillator'circuit to the proper value necessary forv deriving the selected intermediatef'frequency signals.A In a frequency-.changingsystem of this kind, the phase sangle of fthevariable impedance means control- .,'correctionei the oscillator.

ling Y,the oscillator frequencyincidentally tends tovary with respectptothe oscillator circuit voltage with variations in frequency due to variations vinthe effectief Ielectron transit time lof the-impedance; means, thereby to/impair Ythe frequency To counteract any tendency of` the phase angle'of such variable impedanceineans to vary with variations in fre- .quency resulting from the selectionof any de- Ysiredone.: of ilthelplurality of different oscillator Y. portioning the constants'of the compensating circuit lto counteractfany tendencyof the phase angle ofV such variable impedancenieans to vary with variations in frequency. The invention also comprisesunicontrol means :for simultaneously operating the means for selecting Vthe constants ofthe oscillating' and compensating circuits,

-' `whereby any tendency inthe frequency-correo- -tion circuit toward undesired phase anglevariavention, together With other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing the single figure is a circuit diagram, partly schematic, of a complete superheterodyne receiver embodying the Y present invention.

Referring now more particularly to the drawing, there is shown schematically a receiver of the superheterodyne type including an antenna system l, I connected to a radio-frequency amplifier II, to which is connected in cascade, in the order named, a frequency-changing system including a modulator I2 to which is coupled by condenser 56 a local oscillator 24, an intermediate-frequency amplifier I3, and a video-signal channel comprising a video-intermediate-frequency amplier I4, a detector I5, a Video-frequency ampler It, and an image-reproducing device I'I. Also coupled in cascade with the intermediate-frequency amplier I3 is a soundsignal channel comprising, in the order named, a sound-intermedate-frequency amplifier I8, a detector I 9, an audio-frequency amplifier 20, and a sound-reproducing device 2 I. The elements or components III-2|, inclusive, all may be of conventional well-known construction so that a detailed illustration and description thereof is deemed unnecessary herein. A frequency discriminator |9a, of any suitable or conventional type, is coupled to the sound intermediate-frequency amplifier |8 and its bias output applied to the oscillator24 over a conductor |913, as described in detail hereinafter.

Neglecting for the moment the particular operation of the frequency-changing system embodying the present invention, the system above-described includes the features of a conventional superheterodyne television receiver. The operation of such a receiver being Well understood in the art, detailed explanation thereof is deemed unnecessary. In brief, however, the tunable circuits of the radio-frequency amplifier I I and frequency changer lil- 24, are adjusted to select the desired modulated-carrier signals intercepted by the antenna system III, II) and to convert them to intermediate-frequency signals. These signals are selected and amplied by the intermediatefrequency amplifier I3 and translated therefrom to the video intermediate-frequency amplifier I4 and to the sound-intermediate-frequency amplier I8. In the video-signal channel of the receiver the video-intermediate-frequency signals are translated to the detector l5 wherein the video-frequency modulation components are derived. These video-frequency components are amplified in the video-frequency amplifier I6 and reproduced by the image-reproducing device Il in a conventional manner. The sound-frequency modulation components are derived by the detector I9 and amplified by the audio-frequency nals being reproduced by the loudspeaker 2| in a conventional manner.

Referring now more particularly to the parts of the frequency-changing system comprising the present invention, the oscillator system 24 comprises an oscillator tube 23 and a control tube 22, both of the pentode type. The oscillator, per se, is a. conventional Colpitts oscillator, a type at present commonly used in television receivers. The oscillator is provided with means for selectively proportioning the constants of the oscillator circuit to oscillate at any desired one of a plurality of different frequencies comprising a frequency-determining circuit comprising including a selector switch 25 for selectively including one of a plurality of fixed inductances 26, 2l, 28 in the circuit, each being tuned by inherent circuit capacitance indicated conventionally by dotted-line condenser 29. The capacitance 29 comprises the inherent capacitance of the inductance element in circuit, Wiring capacitance, and tube interelectrode capacitances. In order to develop an oscillator circuit voltage, one terminal of such frequency-determining circuit is connected directly to the screen of the oscillator tube 23, acting as an anode, and to the cathode through a coupling condenserv30. The grid of the oscillator tube 23 is connected to the other terminal of the frequencydetermining circuit through a grid leak 3| and condenser 32 and a by-pass condenser 33. The suppressor grid of the tube 23 is grounded, While the cathode is grounded. for direct current through a radio-frequency choke 35. Condenser SII, in conjunction with the inherent cathode-toground capacitance 30a of tube 23, form the capacitance divider required by the Colpitts circuit.

The frequency-changing system is provided with a frequency-correction circuit coupled to the oscillator circuit v including electron-discharge variable impedance means, comprising the control tube 22, connected to simulate a variable impedance to adjust the frequency of the oscillator circuit to the proper value for derivingl the selected intermediate-frequency signals. Speciiically, the plate. of the tube 23 .is connected by a blocking condenser 36 to the control grid of the control tube 22the anode circuit of this tube being connected directly across the .frequency-determininghcircuit, as indicated. The plate of the tube 23 is used here merely as a coupling device. The cathode circuit of the control .tube 22 includes undesired but inherent cathode lead inductance 31 and a conventional biasing circuit 33. As willbe explained hereinafter in greater detail, the phase angles of the impedance means comprising tube 22 and of the oscillator vacuum tube 23 incidentally tend to vary with respect to the oscillator circuit voltage Witlr variations infrequency due to variations in the effect of electron transit time of vrthe vacuum tubes, thereby to impair the frequency correction of the oscillator. Io avoid such variations of the phase angles of these tubes with frequency, the frequency-changing system includes an oscillator voltage compensating circuit comprising a plurality of compensating resistors 39, 40 and 4I. This compensating circuit is coupled to the frequency-correction circuit; specifically, space current is supplied from a source, indicated at -I-B, to the anodes and screens of

tubes

22 and 23 through one of the resistors 35,

Y 40 and 4|. The frequency-changing system adamplier 2li, the amplified audio-frequency sig- 75 ditionally includes means for selectively proportioning the constants of the compensating circuit to counteract any tendency of the phase angles of the :impedance means and of the oscillator vacuum tube to vary with frequency resulting'from `ftrol-'bias"voltage changes' the frequency 'of "the the'selectionofany desired one of theplurality of different oscillator frequencies, this means comprising a 'selector switch 42' which is arranged selectively to connect the anode circuits of tubes 22and'23-through one of the resistors 39, 40 and lll to the space current source +B. The plate circuit of the tube 23 includes a resistor 43 which, with the inherent input capacitance ofthe control tube 22 'and output capacitance of tube 23, represented by the dotted-line condenser 44 and hereinafter referred to as input capacitance of tube 22, comprises a phase-shifting impedance for applying tothe control grid of tube 22 a Voltage somewhat less than 90 degrees displaced in phase from that across the frequency-determining circuit of the oscillator. The automatic frequency control bias voltage from the frequency discrim- Y inator |901. is applied by conductor leb-to the control grid of the control tube 22 to determine its transconductance. VThe selector switches 25 and 42 are connected for simultaneous operation by a unicontrol mechanism, conventionally shown by the dashed line 45.

The value of resistor 43 is so chosen in relation to the values of effective tube transadmittances and the input capacitance 44 of tube 22 that the output current of tube 22 is 90 degrees out of phase with the oscillator voltage at a given frequency.l At the oscillator frequencies required in receiving television signals there is an appreciable transadmittance angle between the voltage applied to the control electrode of tube 22 and the current output thereof and'between the voltage developed at the screen of tube 23 and the anode current thereof which is a function of frequency and of the operatingA voltages. The `values of the resistors 39,' 4Q, and 4I are, therefore, chosen to so alter the operating voltages of

tubes

22 and 23 as to maintain the resultant phase angle between the oscillator voltage and reactive current output of tube 22 at 9b degrees for each selected oscillator frequency.

These oscillations are applied to the modulator f l2 through Vthe condenser 45. This causes the modulator l2 to produce the desired intermediate-frequency modulated-carrier signal. Any slight variation of the intermediate frequency supplied to the frequency discriminator lila., due to drifting or Inistuning of the oscillator, causes the discriminator `I 9a to develop a corrective control-bias Voltage which is applied to the control electrode of control tube 22 over conductor |919 to vary the transconductance Yof tube 22. Since the anode circuit of the control tube 22 is in parallel with the frequency-determining circuit of the oscillator tube 23 and its anode current, which is effectively a control current supplied to the frequency-determining circuit, is 90 degrees out of phase with the voltage across the oscillator circuit, the control tube simulates a variable re- Y actance across the frequency-determining circuit.

As aresult', a change of transconductance of the tube 22 produced bythe automatic frequency con-v 's'cillatorto maintain the intermediate frequency at itsA desiredv mean value. Howeverfas :stated above, in changing from one oscillator frequency to 'another'by means of switch25, there is an incidental tendency accompanying these shifts in frequency for the current in the correction circuit to vary in phase with respect to the oscillator circuit voltage.

The factors determining the magnitude of the phase angle of the current output of tube 22 with respect to oscillator voltage are:

1. The phase angle at the oscillator'frequency of the impedance of resistor 43 and input capacitance 44 in parallel.

2. The phase angle of theV cathode-to-ground impedance of control tube 22.

3. The transadmittance angles from control electrode to anode of control tube 22 and from screen to anode of oscillator tube 23.

The above three quantities are aifected in magnitude by variation of oscillator frequency; in addition, the magnitude of the two transadmittance angles may be varied over a considerable range by varying the unidirectional potentials applied to anode and screen.

In accordance with this invention, the uni* directional voltages applied to the anodes and screens of the

tubes

23 and 22 are controlled by means of the selector switch 42 to compensate for the tendency of the other factors to change the phase angle of the control-tube current with variation of the selected oscillator frequency.

A specific example will illustrate the operation to be expected in the system of this invention. Let it be assumed, for example, that selector switch 25 connects the tuning inductance 2li into the frequency-determining circuit of the oscillatorl andthat the voltage fromsource +B is applied to the anodes and screens of

tubes

22 and 23 through resistor 39 by switch 42 and that oscillation commences at the desired frequency. Neglecting for the moment electron 'transit time effects in the

vacuum tubes

22 and 23 and all circuit impedances except capacitance 44, a current inphase with the oscillator voltage ows in capacitance 44 and a voltage appears at the control electrode of tube 22 which lags the oscillator voltage by degrees. The resulting anode current of tube `22 is at the proper QO-degree phase angle with respect to `the oscillator Voltage. However, if electron transit time in tube 23 be considered, it causes the plate current of tube 23 to lag somewhat behind'the oscillator voltage and likewise, for the same reason, the anode current in control tube-22 lags behind its control electrode voltage. liurtlfiermore, V Vcathode-to-ground yimpedance ofstube22,particulariy lead inductance 3l, increasesY this-lag. The necessary correction for this set ofconditions can be made by canceiling some of the effect of the reactance of capacitance '44 by-means of resistor'43 in the anode circuit of tube 23.

Butnow should switch Y25 be operated to select another tuning inductance, for example, inductance 21, to develop another oscillator frequency which is higher than the oscillator frequency produced by the tuning inductance 26, the electron transit time, while of the same absolute Value, is now a larger fraction of the period of the generated' wave so that thephase shift of the output current of oscillator tube' 23 and of the anode current of the control tube 22 with respect to the oscillator voltage is increased. At 4the same time, resistor -43Yis now of -tooihigh a value to vcancel the required portion of the phase shift due to capacitance 44, which is lower in reactance at the higher frequency. However, in accordance with the present invention, both these variations in phase angle are compensated for by the simultaneous selection by means of simultaneous voperation of the switch 42 With the switch 25, Ofanother operating voltage of suiciently higher value to shorten the electron transit times in both vacuum tubes and therebyto reduce the transadrnittance angles therein, thus restoring the proper 90-degree phasebetween the anode current of tube 22 and the oscillator voltage.

The fiXed-inductance elements 26, 21, and 28 are selectively proportioned to cause the oscillator circuit to oscillate at any desired one of three differ-ent frequencies. The fixed compensating resistors 39, 40, and 4| likewise are selectively proportioned so that the voltage applied by the compensating circuit to control the electron transit times of the

vacuum tubes

22 and 23 counteracts any tendency of the control current supplied to the oscillator-tuned circuit by the vacuum tube 22 in the correction circuit to vary in phase with respect to the oscillator circuit voltage by virtue of variations in frequency resulting from the selection of any desired one of the three different oscillator frequencies. Thus, the unicontrol means 45 operates the switches 25 and 42 simultaneously to select the constants of the oscillating and compensating circuits s that any tendency toward variation in phase of the. control current resulting from the selection of a desired oscillator frequency is minimized.

The following values have been found satisfactory in the design of a frequency-changing system according to the present invention:

Video-intermediate frequency megacycles 12. 75 Sound-intermediate frequency do 8. 25 Radio-frequency band range (l) do 44-50 Radio-frequency band range (2) do 50-56 Radio-frequency band range (3) do 66-72 Oscillator frequency (1) do 5S Oscillator frequency (2) do 64 Oscillator frequency (3) do 80 Inductance 26` microhenries 0.20 Inductance 27 do 0. l5 Inductance 28 do 0. 10 Inductance 37 do 2.00 Resistor 39 ohms 5, 000 Resistor 40 do 2, 500 Resistor 41 do 1, 000 Resistor 43 do 15G-200 Capacitance 29 micro-microfarads 30 Capacitance 44 d0 20 Control tube 22 6AC'7 Oscillator tube 23 GAC? Voltage source +B volts 250-300 A. F. C. Control-bias voltage range volts 2-6 Phase shift due to electron transit time at U megacycles degrees 35-40 While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious tothose skilled in the art that various changes and modifications may be made therein Without departing from the invention, and it is, therefore, aimed in the appended claims to cover al1 such changes and modifications as fall within the true spirit and scope of the invention. Y

What is claimed is:

carrier signal receiver, a frequency-changing system for deriving from selected received modu- 1. In an ultra-high-frequency modulatedlated-carrier signals intermediate-frequency modulated-carrier, signals comprising, an oscillator circuit, means .for selectively proportioning the constants of said oscillator circuit to oscillate at any desired one of a plurality of different frequencies, a frequencyecorrection circuit coupled to said'oscillator circuit including electron-discharge variable impedance means to adjust the frequency of said oscillator circuit to the proper value for deriving the selected intermediate-frequency signals, the phase angle of. said impedance incidentally tending to vary with respect to the oscillator circuit voltage ,with variations in frequency due to variations in the effect of electron transit time of said impedance means thereby to impair the frequency correction of said oscillator, an oscillator voltage compensating circuit coupled to said correction circuit, and means for selectively proportioning the constants of said compensating circuit to counteract any tendency of the phase angle of said impedance to vary with frequency resulting from the selection of any desired one of said plurality of different frequencies.

2. In an ultra-high-frequency modulated-carrier signal receiver, a frequency-changing system for deriving from selected received modulated carrier signals intermediate frequency modulated-carrier signals comprising, an oscillator circuit, means for selectively proportioning the constants of said oscillator circuit to Oscillate at any desired one of a plurality of different frequencies, a frequency-correction circuit coupled to said oscillator circuit including electron-discharge variable impedance meansto adjust the frequency of said oscillator circuit to the proper value for deriving the selected intermediate-frequency signals, the phase angle of lsaid impedance incidentally tending to Vary with frequency due to variations in the effect of electron transit time of said impedance means thereby to impair the frequency correction of said oscillator, a compensating circuit coupled to said correction circuit, means for selectively proportioning the constants of said compensating circuit to counteract any tendency of the phase angle of said impedance to vary With frequency resulting from the selection of any desired one of said plurality of different frequencies, and unicontrol means for simultaneously operating the means for selectively proportioning` the constants of said oscillating ,and compensating circuits, whereby said tendency of the phase angle of said impedance to vary with frequency resulting from the selection of the desired oscillator frequency is minimized.

3. In an ultra-high-frequency modulated-carrier signal receiver, a frequency-changing system for deriving from selected received modulated-carrier signals intermediate-frequency modulated-carrier signals comprising an oscillator circuit, means for selectively proportioning the constants of said oscillator circuit to oscillate at any desired one of a plurality of different frequencies, a frequency-correction circuit including a vacuum tube connected to simulate a variable impedance coupled to said oscillator circuit to adjust the frequency of said oscillator circuit to the proper value for deriving the selected intermediate-frequency signals, the phase angle of said impedance incidentally tending to vary with respect to the oscillator circuit voltage with variations in frequency due to electron transit time effects in said vacuum tube, a compensating circuit coupled to said correction circuit, and means for selectively proportioning the constants of said compensating circuit to counteract any tendency of the phase angle of said impedance to vary with frequency resulting from the selection of any desired one of said plurality of different frequencies.

4. In an ultra-high-frequency modulated-carrier signal receiver, a frequency-changing system for deriving from selected received modulated-carried signals intermediate-frequency modulated-carrier signals comprising, an oscillator circuit including a vacuum tube, means for selectively proportioning the constants of said oscillator circuit to oscillate at any desired one of a plurality of different frequencies, a frequencycorrection circuit including a vacuum tube connected to simulate a variable impedance. coupled tc said oscillator circuit to adjust the frequency of said oscillator circuit to the proper value necessary for deriving the selected intermediatefrequency signals, the phase angles of trans-admittance of said vacuum tubes incidentally tending to vary with variations in frequency due to electron transit time effects in said vacuum tubes, a compensating circuit coupled to said correction circuit, and means for selectively proportioning the constants of said compensating circuit to counteract any tendency of said phase angles of transadmittance to vary with variations in frequency resulting from the selection of any desired one of said plurality of different frequencies.

5. In an ultra-high-frequency modulated-carrier signal receiver, a frequency-changing system for deriving from selected received modulated-carrier signals intermediate-frequency modulated-carrier signals comprising, an oscillator circuit, means for selectively proportioning the constants of said oscillator circuit to oscillate at any desired one of a plurality of different frequencies, a frequency-correction circuit including a vacuum tube coupled to said oscillator circuit for supplying control current thereto to adjust the frequency of said oscillator circuit to the proper value f or deriving the selected intermediate-frequency signals, said control current incidentally tending to vary in phase with respect to the oscillator circuit voltage with variations in frequency due to electron transit time eiects in said vacuum tube, and a compensating cir-v cuit coupled to said vacuum tube for controlling its electron transit time effects and having constants selectively proportioned to counteract any tendency of said control current to vary in phase with frequency resulting from the selection of any desired one of said plurality of different frequencies.

6. In an ultra-high-frequency modulated-carrier signal receiver, a frequency-changing system for deriving from selected received modulated-carrier signals intermediate-frequency modulated-carrier signals comprising, an oscillator circuit including a vacuum tube having an anode circuit for developing an oscillator circuit voltage, means for selectively proportioning the constants of said oscillator circuit to oscillate at any one of a plurality of different frequencies, a frequency-correction circuit coupled to said oscillator circuit including a vacuum tube having an anode circuit which carries a current that is desirably degrees out of phase with respect to said oscillator circuit voltage whereby said lastnamed anode circuit simulates a variable reactance to adjust the frequency of said oscillator circuit to the proper value for deriving the selected intermediate-frequency signals, the current in the anode circuit of said frequency-correction tube incidentally tending to vary from said desired 90-degree phase relation With respect to the oscillator circuit voltage with variations in frequency due to electron transit time effects in both of said tubes, a conpensating circuit coupled to said anode circuits, and means for selectively proportioning the constants of said compensating circuits to counteract any tendency of said anode circuit currents to vary in phase with frequency resulting from the selection of any desired one of said plurality of different frequencies.

JOHN A. RADO.

CERTIFICATE 0F CORRECTIQN. Y Patent No. 2,215,710. 'June 17, 19111.

JOHN A. RADO.

It is hereby certified that error appears in the printed specification of the abovel numbered patent Ilequiring correction as follows: Page 2, firstv column, line 5'?, for "'l22l1." read '12,2l1; and second column, line` lll, lstrike out the word comprising page l1, first column,- line 62', in the table, for "2- 6" read -2 -6 and that the said Letters Patent should be readwith this correction therein that the: same may conform to the record of the case n the Patent Office.

Signed and sealed this 50th day of September, A. D. 191.11.

Henry Van Arsdale,

(Seal) Acting Commissioner .of Patents.