US2166805A

US2166805A - Control circuit

Info

Publication number: US2166805A
Application number: US76789A
Authority: US
Inventors: Robert B Dome
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1936-04-28
Filing date: 1936-04-28
Publication date: 1939-07-18
Anticipated expiration: 1956-07-18

Description

Jul 18, 1939. R. B. DOME 2,166,805

CONTROL CIRCUIT) Filed April 28, 1936 Irwverwtor: Robert B.Dome,

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Patented July 18, 1939 UNITED STATES CONTROL CIRCUIT Robert B. Dome, Bridgeport, Conn, assignor to General Electric Company, a corpcration of New York Application April 28,

6 Claims.

My invention relates to reactance control circuits and more particularly to such circuits as utilized in the control of frequency. Still more particularly my invention relates to signal seeking circuits employed in radio receivers, particularly of the superheterodyne type, and it has for one of its objects to effect certain improvements in the control of the local oscillator utilized in such circuits.

In signal seeking circuits employed in superheterodyne receivers means have been employed to control the frequency of the local oscillator in response to variations of the intermediate frequency of the receiver and in such a way as to compensate for such variations. Thus this compensating means has the effect of maintaining the intermediate frequency constant notwithstanding that the tuning control is moved over a considerable range in proximity to a position where the receiver is tuned to a desired frequency. One means for effecting this control involves the use of an electron discharge device having electrodes connected in shunt with the oscillatory circuit of the local oscillator and varying the impedance between the electrodes in response to variations of the intermediate frequency to control the frequency of the local oscillator. This means presents certain difliculties due to misalignment between the local oscil later with the other tuned circuits of the receiver, this misalignment producing variations in the intermediate frequency as the tuning control is varied from one end of its range to the other. One object of my invention is to provide means for obviating these difficulties.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which the single figure represents an embodiment of my invention.

Referring to the drawing, I have conventionally indicated therein a radio receiver having an antenna l, a tuned input circuit 2, which may include one or more radio frequency amplifiers, a first detector 3, an intermediate frequency amplifier 4, a frequency discriminating circuit 5, a second detector. 6, and an output circuit 1. This receiver also comprises an electron discharge oscillation generator 8, the oscillations of which may be supplied through a switch 9 and 1926, Serial No. 76,789

condenser I!) to the'fii'st detector 3 whereby they are caused to beat with the received oscillations to produce a desired intermediate frequency which in turn is supplied to the amplifier 4. The

electron discharge oscillation generaton 8 in- Isis radio frequency input circuit 2 of the-receiver I whereby the frequency of the oscillator is maintained different by a constant amount-from the frequency to which the radio receiving circuits of the receiver are tuned. In this way a constant intermediate frequency is produced for all received signals.

The first detector 3 of the receiver may of course be of any suitable type but is indicated as one in which the oscillations received from the antenna l are supplied to a grid 20 near the cathode of the discharge device employed whereas oscillations from the local oscillator are supplied to a grid 2! more removed from the cathode and which is shielded by shield grids 22 and 23 both from the anode and from the control grid 20, these shield grids being supplied with suitable positive operating potential from a source 24 and connected to the cathode through a bypass condenser 25.

The signal seeking circuit employed in the receiver illustrated comprises an electron discharge device 25 having its anode and cathode effectively in shunt with the oscillating circuit ll of the electron discharge oscillation generator 8. It will be noted that its cathode is connected through bias resistor 21 and bypass condenser 28 to ground and hence to one side of the oscillating circuit IL Its anode is also connected through resistance 30 to the opposite side of the oscillating circuit. The control grid of the discharge device 26 is connectedto the same side of the oscillating circuit as the anode through blocking condenser 29 and resistance 3|, this control grid being also connected through a conductor 32 to the output of the rectifier 6.

This rectifier 6 and the frequency discriminating circuit 5 associated therewith are arranged to supply to the conductor 32 a potential which varies from a predetermined value, as for example ground potential, either positively or negatively, depending upon the variations of the intermediate frequency from a desired fixed value. In this way the impedance between the anode and cathode of the discharge device 26 is varied, this impedance for reasons presently to be explained effecting a desired change in the frequency produced by the oscillator 8. Thus for example if the intermediate frequency increases or decreases for any cause, such for example as movement of the tuning control within certain predetermined limits, the impedance of discharge device 26 is varied in such a way as to produce a change in frequency of the oscillation generator which tends to restore the intermediate frequency to its normal value.

To effect this control the frequency discriminating circuit 5 is employed, this circuit comprising a transformer having a primary winding 4| and a secondary winding 33, both of these windings being tuned to the desired fixed intermediate frequency. The primary winding is connected between ground and the midpoint of the secondary winding 33 through a condenser 34. The opposite terminals of the secondary winding 33 are connected to the respective anodes of diodes 35 and 36, the cathodes of these diodes being connected together for alternating currents through condenser 31 and for direct currents through resistances 38 and 39. The cathode of diode 36 is grounded and the midpoint between resistances 38 and 39 is connected to the midpoint on secondary winding 33 through a choke coil 40.

As thus described the operation of the system is as follows:

If we assume that the tuning control of the radio receiver is adjusted for accurate resonance with the received carrier wave, then the intermediate frequency has the desired value to which the primary and secondary windings M and 33 are each tuned. The voltage across the secondary winding of the transformer, in accordance with well known theory, is displaced in phase from the voltage across the primary by 90. With the connections shown the voltage of the primary is connected in series with one half of the voltage on the secondary through discharge device 35 and condenser 31 and it is connected in series with the other half of the voltage on the secondary through discharge device 36. Because of the quadrature relation between the primary and secondary voltages the voltage on one half of the secondary leads the voltage on the primary by 90 whereas that on the other half of the secondary lags behind the voltage on the primary by 90. Thus the voltage applied to the two diodes 35 and 36 when the intermediate frequency is at its desired value is equal and accordingly equal values of unidirectional currents flow through each of the diodes and hence through resistances 38 and 39. It will be observed that these resistances are poled oppositely, that is, the voltages across the two are opposite in polarity in the circuit between conductor 32 and ground with the result that the conductor 32 is at ground potentialwhen the intermediate frequency is at the desired value.

The quadrature relation between the primary and secondary voltages of the transformer 5 exist, however, only when the oscillations supplied thereto have the desired intermediate frequency.- If this frequency changes in either direction the phase of the secondary voltage varies from its 90 relation with the primary voltage in one direc tion or the other dependent upon whether the frequency increases or decreases. For example, if the frequency increases, the phase shift may be in such a direction that the voltage on the upper half of the secondary winding approaches the aiding relation with the primary voltage whereas that on the lower half of the secondary winding approaches the opposing relation with the primary voltage. Thus the voltage applied to diode 35 increases and that applied to diode 36 decreases with the result that the unidirectional potential on resistance 38 increases whereas that on resistance 39 decreases and the conductor 32 thus becomes positive with respect to ground. On the other hand, if the intermediate frequency decreases, an opposite shift in phase of the secondary voltage occurs with the result that the larger alternating current voltage is supplied to diode 33 and the potential on resistance39 increases whereas that on resistance 38 decreases and the conductor 32 is driven negative with respect to ground.

Discharge device 26 responds to these variations in potential of the conductor 32 to produce corresponding variations in frequency of oscillations generated by the device 8. The anode and'cathode of this discharge device may be caused to act effectively either as an inductance or as a capacitance in shunt with the oscillatory circuit ll. With the circuit arrangement shown it acts effectively as an inductance. It will be observed that the oscillations of the circuit II are applied through resistances 36 and 3| and blocking condenser 29 to the control grid 52 of the discharge device 26 and through resistance 30 to the anode. The voltage on the grid 42, since the impedance between the grid 42 and the cathode is essentially capacitive, lags behind the voltage of the oscillating circuit by 90 and since the current in the anode is in phase with the grid voltage it also lags behind the voltage of the oscillating circuit by substantially 90. In other words the anode to cathode impedance of the discharge device is effectively inductive. The value of this effective inductance may be varied by varying the unidirectional potential applied to the control grid 42 thereby varying the intensity of the current flowing in the anode circuit. Thus when the control grid becomes positive the effective inductance is reduced and when it becomes negative the effective inductance is increased. This effective inductance may be caused to vary the natural period of the oscillator circuit I l to effect the desired frequency changes,

Certain diihculties, however, may be encountered in the use of such a system owing to the fact that there is a certain predetermined impedance arising from a capacitance between the anode and. cathode of the discharge device 26 which is effectively in shunt with the oscillating circuit II. This capacity is sufiicient to throw the oscillating circuit II out of alignmentwith the tuned circuits of the amplifier 2. That is, for example, let us suppose that the conductor 32 were permanently grounded. The anode capacitance of the discharge device 26 is such that if the, tuning control be varied throughout the range to which the receiver is adapted to respond, the intermediate frequency may vary from one end of the range to the other notwithstanding that the components of the circuit H are chosen for a constant intermediate frequency at all positions of the tuning control with the discharge dev ce .26 removed from the circuit. To avoid this misalignment produced by the discharge device 26 and in accordance with my invention the resistance 35 is included in the circuit. This resistance may be chosen of such a value that the capacitance between the anode and cathode of the discharge device or other capacitance in the circuit of the discharge device is prevented from affecting the frequency of the oscillating circult H. At the same time this resistance may be small enough so that it does not materially reduce the control of the frequency produced by the discharge device 26 in response to variations in potential on conductor 32. This will be understood, for example, if the discharge device 26 be considered as a capacitance connected across the oscillating circuit through resistance 30. If the resistance 30 were zero the capacitance would be connected directly in the oscillating circuit, whereas if the resistance were infinite the capacity would be disconnected from the oscillating circuit. It has been found, however, that by the use of an intermediate value of resistance 30 the effect of this capacity can be reduced to an extent such that it is unobjectionable in the operation of the receiver. I employ a resistance of about five thousand ohms for this purpose. This value of resistance is very small as compared with the resistance between the anode and cathode of the discharge device 26, and accordingly variations in the latter resistance produced by variations in the grid potential are so large relative to the value of resistance 30 that the latter element does not greatly reduce the control effected by the discharge device 26.

In its practical application the resistance 30 may be connected in close proximity to the oscillating circuit II thereby to minimize capacitance produced by the conductor adjoining it with the oscillating circuit.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since this embodiment is presented by way of illustrating the principles involved. Many modifications may be made both in the arrangement shown and in the instrumentalities employed and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination, in a radio receiver having an adjustable element adapted to tube an input circuit of said receiver over a range of frequencies, an electron discharge oscillation generator having an adjustable element adapted to tune said oscillator over a corresponding range of frequencies, means to produce a beat frequency between received oscillations and oscillations produced by said oscillation generator, means including a mechanical connection of said adjustable elements for maintaining said oscillator frequency different by a predetermined constant amount from the frequency to which said input circuit is tuned, means responsive to variations in said beat frequency to control the frequency of said oscillation generator, said last named means comprising an electron discharge device having electrodes connected to said oscillation generator, said electrodes having a predetermined impedance therebetween which cause an undesired variation of the difference between the frequency of said oscillator and the frequency to which said input circuit is tuned as said receiver is tuned over said range of frequency, and means for greatly reducing the effect of said predetermined impedance on the alignment between said oscillator and said input circuit tuning while permitting said last named means effectively to control the frequency of said oscillation generator.

2. The combination, in a radio receiver adapted to receive oscillations varying over a range of frequencies, a source of oscillations having a frequency determining oscillatory circuit, means to produce a beat frequency between said received oscillations and oscillations produced by said source, means to vary the frequency of said source to maintain said beat frequency at a desired constant value for reception of any frequency in said range, means responsive to variations in said beat frequency from a normal frequency to control the frequency of said source, said means comprising an electron discharge device having an anode and cathode with a normal impedance therebetween connected across said oscillatory circuit through a resistance, and means to vary the impedance between said anode and cathode from said normal value to vary said frequency, said resistance having a value large in comparison to the inherent anode to cathode impedance and small in comparison to the controlled anode to cathode impedance.

3. In combination, a reactance, an electron discharge device having electrodes connected at difierent points on said reactance and having a predetermined impedance between said electrodes, a resistance connected in circuit between one of said electrodes and said reactance and having a value large in comparison to said predetermined impedance between said electrodes and small in comparison to the controllable impedance between said electrodes whereby said predetermined impedance between said electrodes has substantially no effect upon the reactance between said points, and means to vary the effective reactance between said points by controlling the impedance between said electrodes.

4. In combination, two circuit points between which alternating electromotive force exists, means to vary the effective reactance between said points, said means comprising an electron discharge device having a pair of electrodes connected respectively to said points, said electrodes having capacitance therebetween, a resistance connected in circuit between one of said points and the respective electrode, said resistance being sufficiently high substantially to reduce the effect of said capacitance upon the effective reactance between said points yet sufficiently low as to have negligible effect upon the simulated reactance between said two circuit points by said electron discharge device, means to phase the current flowing between said electrodes relative to said electromotive force to simulate a reactance connected between said points, and means to vary the magnitude of said current.

5. The combination, in a superheterodyne type of radio receiver having aligned input and oscillator circuits tunable over a range of frequencies, of means for producing from locally generated oscillations and received signal oscillations beat frequency oscillations having a desired constant value as said receiver is tuned over said range of frequencies, means responsive to variations in the frequency of said beat frequency oscillations from said desired constant value for controlling the frequency of said locally generated oscillations, said last named means including an electron discharge device having electrodes with a predetermined impedance therebetween con- 'nected to points on said oscillator circuit, said predetermined impedance causing an undesirable variation of alignment between the tuning of said oscillator and said input circuit as said receiver is tuned over said range of frequencies, and means including an impedance connected in series relation with said discharge device electrodes across said points on said oscillator circuit for greatly reducing the effect of said predetermined impedance on the alignment between said oscillator and said input circuit while permitting said last named means eifectively to control the frequency of said oscillation generator.

6. The combination, in a superheterodyne type of radio receiver having aligned input and oscillator circuits tunable over a range of frequencies, of means for producing from locally generated oscillations and received signal oscillations beat frequency oscillations having a desired constant value as said receiver is tuned over said range of frequencies, means responsive to variations in the frequency of said beat frequency oscillations from said desired constant value for controlling the frequency of said locally generated oscillations, said last named means including an electron discharge device having electrodes with a predetermined impedance therebetween connected to points on said oscillator circuit, said predetermined impedance causing an undesirable variation of alignment between the tuning of said oscillator and said input circuit as said receiver is tuned over said range of frequencies, and means including a resistance connected in series relation with said discharge device electrodes across said points on said oscillator circuit and having a proportioned value of resistance greatly to reduce the effect of said predetermined impedance on the alignment between said oscillator and said input circuit while permitting said last named means effectively to control the frequency of said oscillation generator.

ROBERT B. DOME.

. CERTIFICATE OF CORRECTION. Patent No; 2,166,805. y 8, 959'- ROBERT B. DOME.

It is hereby certified' that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 5, first column, line 55, claim 1, for the word "tube=" read tune; andthat the said Letters Patent should be read with this correction therein that the same \may conform to the record of the case in the Patent Office.

Signed. and sealed this 29th day of August, .A. D. 1959.

Leslie Frazer, (Seal) Acting Connnissioner of Patents.