US2204398A - Tuning indicator system for radio receiving apparatus and the like - Google Patents

Description

G. L. BEERS 2,204,398

TUNING INDICATOR SYSTEM FOR RADIO RECEIVING APPARATUS AND THE LIKE June 11, 1940.

2 Sheets-Sheet 1 Filed July 31, 1957 mmmm PM wwbm w m km PM lhwentor ear 6 L. Beers attorney June 11, 1940. G. L. BEERS 2,204,398

TUNING INDICATOR SYSTEM FOR RADIO RECEIVING APPARATUS AND THE LIKE Filed July 31, 1957 2 Sheets-Sheet 2 I Zhwentor George L. Beers 85 I Gttorneg Patented June 11, "1940 UNITED STATES TUNING INDICATOR SYSTEM FOR RADIO RECEIVING APPARATUS THE LIKE George LQBeers, Haddoni'ield, N. J.,,assignor"to Radio Corporation of America, a corporation of Delaware Application July 31, 1937, Serial No. 156, 6'76 v 4 Claims.

The present invention relates to tuning or resonance indicating means for radio receiving apparatus and the like, and more particularly tuning indication means-which is adapted to provide resonance indicationin comparison with a condition of nonresonance, whereby receiving ap paratus provided with such indication means may be tuned more accurately than has heretofore been possible. i

10 As is well known, in tuning radio receiving apparatus, it is desirable to tune xactly to-reschance in order that the fidelity of 'reception'may not be impaired, and, for this purpose, as a tuning aid, accurate visual means sharply responsive to a received signal is highly desirable.

Substantially all radio receiving apparatus is provided with automatic volume control means. As is well understood, automatic volume control devices tend to maintain the signal applied to the second detector and therefore the volume or audio frequency output level substantially constant over an appreciable frequency range at each side of resonance, with the result that it is di-flicult to tune by car a receiver having sucha control. A'

visual indicating device which responds to the signal amplitude at the second detector in such a receiver likewise cannot be used'to give a satmatic volume control that shall'give' an accurateindication of resonance;

It is also a further object of the presentin 5 vention to provide tuning indication means for.

radio receiving systems that shall give an indication of the direction in which the tuning ele-.

- ments thereof are to be adjusted to secure res-' chance with an incoming carrier wave or signal. 40 It may also be considered to be a further object of the invention to provide tuning indication means for radio receiving systems that shall give an indication of one type at resonance, and an indication of another and different type'at each side of resonance. 1 I e In carrying into effect the tuning indication operation of the control'system in accordance with the invention, a phase discriminating net-' work, in connection withtuned coupled high frequencycircuits, is arranged to provide a control potential which is positive when the receiving system is tuned to one side of resonance, and negative when tunedto the other side, the magnitude of the potential increasing sharply as the tuningis varied from 'thej;point' of resonance."

(Cl. 250-20) v V I It is also anobject of the present invention to provide an accurate indication of resonance,

without requiring the use of additionalselecti've circuits even though thereceiving system is arranged to pass a'wide' frequency band.

The invention will, however, be better understood from the following description whenconsidered in connection with the accompanying drawing, and its scope will be pointed out in the, I 10 appended claims.

In the drawings: I p s Figure 1 Ba schematic circuit diagram 'of a tuning indication system embodying the invention; Figure 2 is a modification of thecircuit' of Fig. 1', arranged to provide signal detection and amplification with automatic volume control in addition to tuning indication, and 1 Figure 3 is a modification of the circuit of Fig. 1 arranged'to' utilize radio frequency impulse in' the control of the indicating device. 7

Referring to Fig. 1, the rectangle firepresents any suitable receiving system' having tuning means represented at 6. In the present example, the receiving system may be considered as including a radio frequency amplifier, first detector and oscillator as indicated; The signal channel' of the receiving system following the first detector comprises an intermediate frequency amplifier tube 1 connected with an intermediate frequency input transformer 8 and having an output circuit l3 coupled to the primary winding I0 of a second intermediate frequency coupling transformer 9. The secondary is divided into two equal winding sections II and I2, whereby it is adapted to be connected into a phase discriminating network of the type shown, described and claimed in my copending application, Serial No. 134,105, filed March 31, 1937, for High frequency signal responsive control" system, and' assigned to the same assignee as the present application, for deriving control potentials through a diode rectifier device 23. i I

' As described in that application, in a control system which employs a phase discriminating network 'as 'shown, a change in either the frequency 'of a received signal or a change in the tuning of the receiver is'converted into a difference in phase between two voltages which are then applied to the phase discriminating network; By utilizing the voltageswhich may appear across the primary and secondary of" a coupled tuned circuittransformen'l have' 'provided in one em-v bodiment of my invention, a simplearrangement for converting a change in tuningfrom resonance,-

or a change in the frequency of a received signal, into a change in the phase relation between two alternating current voltages having the same fre-- quency, and means for deriving control potentials from said change in phase relation.

The primary winding is tuned to the intermediate frequency by a shunt adjustable tuning capacitor i5, and the secondary is similarly tuned by a shunt adjustable tuning capacitor Hi. The

tuned primary and secondary windings provide two coupled tuned circuits I! and I8 resonant at the same frequency.

The windings II and I2 are arranged to form a continuous winding inductively coupled to the winding IE! and the high potential or outer terminal ends thereof are connected to the secondarytuned circuit terminals, indicated at H! and 20, and to the anode electrodes 2| and 22, respectively, of a rectifier device which is included within a combined rectifier and amplifier tube indicated at 23.

The rectifier anodes 2| and 22 are associated with a common cathode 24 of the device 23 which cathode is connected through a lead 25 with the tuned primary circuit ll. Continuing through the circuitv H the anode connection includes the positive anode supply lead 14. The secondary winding sections are each connected with the lead l4 and a secondary center tap terminal 26 through two resistor sections 27 and 28. The latter are connected in series between the adjacent terminal ends of the secondary sections H and i2 through leads indicated at 29. The resistor sections are provided with intermediate frequency bypass capacitors 30 and 3|.

The signal or carrier potential applied between the cathode M and the center tap 2% of the output resistors 2'! and 28, is derived from the primary or input circuit i'i through the connection lead 25'between the cathode, Z4 and the high potential terminal 26A. The opposite side of the tuned primary winding is bypassed to the negative B supply terminal 52 and to ground 13 through the bypass capacitor M. Likewise the center tap 26 is bypassed to ground through the lead M and the bypass capacitor 44. Direct current control potentials are derived from the resistor output terminals 32 and 33 through output leads indicated at 34 and 35.

In this circuit, advantage is taken of the fact that at resonance the primary and secondary voltages of the coupled tuned circuit. intermediate frequency transformer are out of phase with each other. By applying the signal or carrier voltage across the primary of the transformer between the cathode 26 and the resistor center tap 26, and the secondary signal or carrier voltage between. the two diode anodes 2i and 22, the desired phase relations for the phase discriminating network are obtained. If the frequency of the signal applied to the transformer is changed either because of a slight change in the tuning of the receiver, or a change in the frequency of the transmitter, the phase relation between the primary and secondary voltages will change. A phase change of approximately will be obtained between the primary and secondary voltages as the frequency of the signal applied to the transformer is varied through the frequency range included within the resonance characteristic of the transformer.

When the signal applied to the intermediate frequency amplifier tube 7 is in resonance with the transformer feeding the double diode rectifier 23, a voltage will be applied between the cathode 24 and the resistor center tap 26 which will be 90 out of phase with the voltage developed across the secondary coil sections H and i2 and applied between the anodes 2| and 22.

If only the voltage developed across the trans former primary is applied to the double diode between the cathode 24 and the resistor center tap 26, the rectified current flowing through the resistor section 27 will be equal and opposite in direction to that flowing through resistor sec tion 28 and the voltage between the terminals 32 and 33 will be Zero. Likewise if only the voltage developed across the secondary of the transformer is applied to the double diode rectifier between the anodes 2i and 22, the rectified current flowing through, the resistor section 2? will be equal and opposite in direction to that flowing through resistor section 28 and the voltage between the terminals 32 and'3t will be zero.

If both the primary and secondary voltages are applied to the double diode and these voltages are 90 out of phase, as will be obtained if the signal applied to the transformer has the frequency to which the transformer is resonant, then the signal voltages applied to the two anodes will still be equal and the rectified currents flowing through the two resistor sections will be equal and opposite and the voltage between terminals 32 and33 will be zero.

If, however, the signal potential applied to the transformer has a frequency which difiers slightlyfrom the resonance frequency of the transformer, the phase relation between the primary and secondary voltages will no longer be 90 and the signal potentials applied to the two anodes will not be equal and a difference in potential will exist between the terminals 32 and 33 because the rectified currents flowing through resistor sections 2! and 28 will be unequal.

Assuming that the applied signal has av frequency which is just inside the frequency response characteristic of the transformer, with a conventional intermediate frequency transformer this signal frequency would differ from the resonance frequency of the transformer by from 2 kc. to 5 kc. Under these conditions a phase difference of approximately 180 would exist between the primary and secondary voltage.

Assuming that with this phase difference, the

voltages are such that the voltage applied to the anode 2i by the primary of the transformer is in phase with the voltage applied to this anode by the secondary of the transformer, the voltages applied to this anode will then. add and if they are equal the anode signal potential will then be twice the value obtained when only the secondary voltage is applied to the double diode. This will cause a corresponding increase in the rectified current flowing through resistor 28. On the other hand the signal potentials applied to the anode 22 will be 180 out of phase and if they are equal in magnitude, the sum of the voltages on this anode will be zero. Therefore, no current will flow through resistor 2'! and consequently no drop in potential will occur across this resistor.

Since considerable current flows through resistor 28, a relatively large voltage will be developed across this resistor and this voltage will appear between the terminals 32 and 33; Since the current causing this drop in potential must flow through the circuit comprising the resistor 28 and the anode 2i and cathode 24, the direction of the current will be such that the terminal 32 will be negative with respect to terminal 33.

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and increase to the initial value again. I change in potentialyacrossterminals 32 and 33 "If the frequeny'i'ofthesignall appliedito the transformer feeding the double diode is :changedso that it justfalls' within the otherside of the transformer frequency response characteristic, the phase relation between the primary and secondary voltages will shift by approximately 180. Under this condition, the large signal potential will be applied to the. anode 22 while the signal potential on the anode '2Iwill be substantially zero. Considerable rectified current will now flow through resistor 21 and no current will flow through resistor 28. :This current will flow through resistor 21 in such a direction that the resultant drop in potential across the resistor causes the terminal '32 to become positive with respect to the terminal 33.

Thus, as the. frequency of the signal applied to the transformer isvaried through the frequency range corresponding .to "the transformers frequency response characteristic, the potential across the terminals 32 and 33 will decrease from a given value, pass through zero, change polarity,

'as the frequency of the signal applied to the transformer is varied causes a large changein control-potential to be made available for a small change in frequency.

While the tuning indication may be "derived from any suitable portion of the signal channel of the receiver, the intermediate frequency amplifier is preferable for the-reason that the latter operates at a fixed frequency and includes fixed tuned circuits at the various interstage transformers. d

In an intermediate frequency amplifier,there-' fore, a pair of "coupled tuned circuits already existing in the signal channel of the receiver'and operating at the same frequency may be used to derive two potentials at the same frequency, having a phase relation which varies as the signalapplied thereto tends to; vary on either side of the predetermined intermediate frequency.

Thus, there may be established in an intermediate frequency amplifier, a phase discriminating network which provides a differential direct current voltage which-varies in, magnitude and'polarity as the frequencyof the received signal varies above or below resonance. By connecting a polarized indicating device or other suitable indicating'device responsive to changes in polarity and voltage magnitude to the output leads 34 and 35,1tuning indication may readily be obtained; which is sharply responsive to changes in tuning and which gives a certain indication at resonance and a different type of indication on either side of resonance. By this means, the operator of a receiver may readily determine in which direction changes in tuning are to be'made to correct for off-resonance tuning and to'bring deflector plates ofacathode ray tube, as shown,

the cathode ray beam maybe adjusted toprovide a'figure'or"pattern' omthe screen 4| which will change with ehanges inth'emagnitude and p0- larity' of-the appl-ied c'ontrolvoltages. When resserially between the secondary windings.

anda coupling resistor 55.

.'oriance i'sbbtained,the" figure may be a circle or spot in thecenter of the tube and, for off-resonance condition, may change to an ellipse formed on ione side or the other of the tube, thereby producing one form of indication for resonance and a differing indication for off-resonanceJtunmary or anode circuit is returned to the center tap-of the resistorsx2'l and 28 and the cathode is connected therewith through the primary so that the rectifier circuit is at anode B potential. This .circuitarrangement permits the deflector plates to operate at positive B potential with direct connection with the output resistors 21 and28.

If thedevice 23 in which the rectifier elements are'provided isalso a double diode triode tube as indicated, having a control grid 45 and an output anode 46, the device may be arranged to provide automatic volume control' bias by con- 'necting a resistor 41 between the control grid and lead I4 and connecting an automatic volume control lead 49 to the grid end of resistor 41, as

shown. With this arrangement, the primary voltage is rectified by the diode rectifier action of the tube in which the ugridacts as an anode of the rectifier. A suitable intermediate frequency bypass capacitor 50a is provided across the automatic volume control resistor 41. Signal output potentials may-then be derived from -the secondary circuit between the terminal 26 and the terminal 32. Audio frequencyor modulation potentials may: be derived from the resistor 41. The value of the capacitor 5011 should be such that it provides an effective jb-ypass at intermediate frequencies 5 without appreciably attenuating the audio frequency potentials in the desired'range of operation.

A circuit for deriving signal potentials and automatic volume "control potentials is also shown in Fig. 2, to which attention is now directed. In Fig. 2, like parts as in Fig. 1 are designated by the same reference numerals.

In the present example, the doublediode rectifier is provided by a tube 50 which may be of the type known as an RCA 6H6 having two anode electrodes 5| and 52 each associated with a cathode 53. The anodes 5| and 52 are connected with the terminals I9 and 20 ofthe tunedsecondary circuit 18 which comprises the secondary winding sections I l and I2 and the shunt tuning capacitor Hi. The resistor sections 21 and 28 are included Output leads 34 and 35 are connected with output terminals 32and 33 on the resistors and the lat: ten are provided with bypass capacitors 3B and 3|. 4

-'Thecenter tap ZG on the resistor'elements is connected with the cathode 53 through a lead 54 p The output anode circuit 13 of the amplifier I is connected withthe 'primarywinding Ill which is provided witha shunt variable tuning-capacitor 15 to form the tuned primary circuit I1, and the latter is coupled to thecathode 53 through a coupling capacitor '55 in conjunction with the coupling resistor 55 v The operation of the circuit of Fig; 2 is similar to that of Fig. 1 and the tuning indicator device is connected with the output resistor comp-rising the sections 2'! and 28 in a similar manner to the cathode ray tube of Fig. 1.

In the present example, the tuning indicator device is an electrical meter indicated at H having a movable pointer 12 which moves over an indicating scale is on either side of a zero position in which it is shown at 14.

The meter is connected to the leads 34 and 35 to receive the differential potential from the output resistor 21-23 and the indicator moves to the rightor left as the potential increases from zero in either direction depending upon the polarity of the terminals 32 and 33, one with respect to the other.

The derivation of the variable potential is the same as in the circuit of Fig. 1 except that the cathodes 53 are conductively connected back to the center tap 26 through the primary circuit by way of the coupling resistor 55 with which the primary is coupled in shunt relation through the coupling capacitor 56, thereby permitting grounding of the secondary circuit. A choke coil may be substituted for the risistor 55 if desired.

{my suitable polarized indicating device may be used in connection with the leads 3t and 35 to respond to the signal variable potential.

The circuit has the advantage that the modulation or audio frequency component of the received signals may be applied to an output ampliflying circuit through a second detector comprising a diode rectifier 59 connected with onehalf of the secondary winding. In the present example, a cathode 6i! is connected with the terminal 25 of the tuned circuit l8, and the anode electrode 6| is connected through an output resistor 62 for the detector, to a ground lead 63 connected to the terminal 32. The output resistor is provided with a suitable by-pass capacitor 64.

The intermediate frequency carrier wave received in the tuned circuit I3 is applied in part to the rectifier 58 across one-half of the secondary winding l2, and the modulation and direct current component thereof appear across the output resistor 62.

The output resistor 62 may be connected between the control grid 65 and the cathode 66 of an audio frequency amplifier 61 to apply a biasing potential thereto and audio frequency signals simultaneously, whereby the amplifier is diode biased by the signal. The audio frequency output is derivedfrom an output lead 68 in connection with the output anode 69. The cathode 66 is connected with the ground lead 63, as indi. cated. Automatic volume control potentials may be derived from the output resistor 62 through an output lead indicated at H1 and applied to any suitable portion of the receiving system for automatic volume control purposes, in any wellknown manner.

In the embodiment of my invention illustrated in Fig. 3, the radio frequency potentials developed across the primary and secondary of transformer 9 are applied directly .to the deflecting plates 35 and d!) of the cathode ray tube 3'! and the rectifier tubes used with the arrangements shown in Figs. 1 and 2 are eliminated. By applying the voltage across the primary of transformer 9 to the vertical deflecting plates and the voltage across the secondary to the horizontal defleeting plates a circle will be formed on the screen of the cathode ray tube when the applied signal has the frequency to which the trans former'is resonant and provided that the voltages applied to the two sets of deflecting plates are substantially equal. If the frequency of the applied signal is varied on either side of the frequency to which the transformer is tuned the figure on the screen of the cathode ray tube will change from a circle to an ellipse the inclination of which will indicate whether the frequency of the applied signal is above or below the frequency to which the transformer is tuned. In this embodiment resistors 21 and 28 and by-pass condensers'3ll and 3| can be eliminated so that a single coil can be substituted for windings II and I2.

While the tuning indication system has been shown and described in connection with an intermediate frequency amplifier, it may be applied to other portions of the radio receiving system wherein two circuits are provided which operate at the same signal frequency and which provide signal voltages which vary in phase in response to off-resonance tuning. However, for reasons which have hereinbefore been pointed out, the tuning indication system is at present preferred in connection with the intermediate frequency amplifier, since this connection permits existing apparatus to be utilized to advantage.

I claim as my invention:

1. In a radio signal receiving system, the combination of means for tuning said system to resonance with a received signal, means providing two substantially equal signal potentials in response to a received signal, said potentials having the same frequency and a predetermined phase relation to each other when said receiving system is adjusted to resonance with said received signal, means for converting a variation in frequency of said received signal into a variation in phase between said potentials, said first and second-named means including the primary and secondary of a coupled tuned circuit intermediate frequency transformer through which said received signal is conveyed, means for converting said variation in phase into a controlling potential for said system, said controlling potential being variable in response to said phase variation in magnitude and direction of variation from a zero value, indicating means responsive to changes in the magnitude and direction of varir ation of said potential, whereby resonance and off-resonance conditions of tuning of said system are provided thereby, a signal rectifier connected with said first-named means for rectifying at least a portion of one of said signal potentials, and means for deriving the modulation component of said signal and automatic volume control potentials from said last-named rectifying means.

2. In a superheterodyne receiver, the combination with tuning means therefor, of an intermediate frequency amplifier, a coupling transformer for said amplifier having a tuned primary circuit and a tuned secondary circuit resonant at the same intermediate frequency, said secondary winding having two tuning inductance sections, a resistor element having a center tap connected between said sections, a rectifier device having a cathode and two anode electrodes associated therewith, said anode electrodes being connected with the terminals of said secondary circuit and said cathode being connected through the primary circuit with said center tap on the resistor element, means for deriving difierential direct current potential from said resistor element in response to received signals, tuning indication means responsive to the magnitude and polarity of said potential connected with said resistor element,'and mans-for deriving audio frequency signal and automatic volume controlpotentials from at least one portion of said secondary circuit.

3. In a radio receiving system, the combination with means fortuning said system to resonance with a signal, of a signal amplifierprovided with two tuned coupled signal conveying circuits, said circuits being resonant at substantially the same signal frequency, said tuned circuits including the primary and secondary windings of a tuned transformer device, a rectifier having a cathode connected through the primary winding with acenter tap on the secondary winding and having a pair of rectifier anode electrodes connected with the terminals of the secondary windings, an output resistor for said rectifier, a cathode ray indicating device having a pair of beam deflecting electrodes connected with said output resistor, an

audio frequency detector connected with one of said windings, and means for deriving audio fre-f quencyand automatic volume control potentials from said last named detector.

4. In a superheterodyne receiving system, the combination with an intermediate frequency amplifier, of an in'terstage coupling transformer therefor having a tuned primary and a tuned secondary winding responsive to the same intermediate' frequency, apair of resistors interposed in the secondary providing a center tap therefor, a rectifier having a cathode connected through said primary winding with said center tap and having a pair of anodes connectedwith the terminals of the secondary, a cathode ray indicating tube having at least one pair of beam deflecting electrodes connected with one of said windings to receive a differential controlling potential "therefrom responsive to a variation in frequency of a signal,

a rectifier device connected with a portion of the secondary winding and having an output resistor, and an audio frequency amplifier connected with said resistorto receive output signals therefrom GEORGE L. BEERS.

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