US2037498A - Variable radio frequency selectivity control - Google Patents

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April 14, 193.6. M. G. CLAY 2,037,498

VARIABLE RADIO FREQUENCY S ELECTIVITTY CONTROL Filed Oct. 20, 1954 INVENTOR v All/8M) a 6M) ATTORNEY Patented Apr. 14, 1936 UNITED STATES PATENT OFFICE VARIABLE RADIO FREQUENCY SELEC- TIVITY CONTROL Murray G. Clay, Hasbrouck Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 20, 1934, Serial No. 749,187

9 Claims. (01. 25 -20) My present invention relates to radio frethat the auxiliary tuning condenser is caused to quency selectivity control devices, and more partune its associated resonant circuit the said freticularly to a variable intermediate frequency quency amount when the receiver has signals of amplifier network selectivity adjustment device. relatively weak amplitude impressed upon it,

One of the main objects of the present invention Still other objects of the invention are to im- 5 is to provide a variable selectivity device for a pr v r ly h imp i y d efficiency of radio frequency signal transmission network, the radio frequency selectivity control devices for network being of type including at 1: 5. 1; two radio receivers, and more especially to provide coupled resonant circuits differing in tuning by selectivity control devices, whether manual or a predetermined frequency, and the selectivity automatic in nature, which are not only reliable 10 device comprising an auxiliary reactance operain Operation, u economically constructed and tively associated with one of the coupled resonant assembled in radio receivers. circuits and an electron discharge device elec- The novel features which I believe to be chartrically connected to the auxiliary condenser in acteristic of my invention are set forth in parsuch a manner that the impedance of the disticularity in the app n d claims, the invention 15 charge device regulates the effect of the auxiliary itself, however, a t both its r niz tion and condenser on the associated resonant circuit, method of operation will best be understood by means being provided for controlling the conrefe ence t the following description taken in 'ductivity ofthe electron discharge device so that connection with the drawing in which I have its impedance is substantially great when transi at d d a m at s v l it 20 mitting signals of relatively'strong amplitude ani a ions whe eby my invention may be carthrough the network, and the impedance of the ried into effect.

electron discharge device being relatively small In the w 1- when transmitting signals of relatively weak am- Fi 1 s h ati y s s a u t mb dy n plitude through the network. the invention, 25

Another important object of the invention is Fig. 2 graphically shows the operation of the to provide a variable selectivity control device e t vity contro device. for an intermediate frequency coupling network, Fig- 3 diag ammatically shows a receivin the couplingnetwork including a pair of coupled cuit embodying an automatic form of the invenresonant circuits differing in tuning by a predetion, 30 termined frequency amount, the selectivity con- Referring now to the accompany drawing,

trol device including an auxiliary condenser in wherein like reference characters in the difierent series with a diode across one of the resonant figures esignate similar circuit elements, Fig. 1 circuits, and the diode being variable in impedshows a schematic representation of a receiver of ance in such a manner that when receiving the superheterodyne type. Between a source of 35 strong signals the auxiliary condenser has no signals l and an amplifier 2 there is disposed a effect on its associated resonant circuit whereas coupling network comprising a pair of resonant when receiving weak signals the auxiliary concircuits A and B. The source of signals I may denser tunes its associated resonant circuit to comprise the usual networks preceding the interthe frequency of the coupled resonant circuit to mediate frequency amplifier and the tube 2 may 40 thereby increase the selectivity of the network. function as anintermediate frequency amplifier Another object of the invention is to provide tube. The tube 2 may be of the duplex diodein a radio receiver equipped with a signal amplipentode type which is known by the designation fier preceded by a network including a pair of 2B7 or 6B7. The pentode section of the tube coupled resonant circuits, the resonant circuits functions as an amplifier section, and the diode 45 difiering in tuning by a predetermined frequency section functions in a manner to be hereinafter amount, an automatic volume control arrangedescribed. ment adapted to produce from signals of varying The plate of the pentode section of tube 2 is amplitude direct current voltage variations varyconnected to a source of positive voltage B ing in the same amplitude sense, one of the resothrough a coupling network M1, the coupling net- 50 nant circuits'having operatively associated therework being tuned to the operating intermediate with an auxiliary tuning condenser and an elecfrequency. The network Ml. may feed a succeedtron discharge device, and the electron discharge ing second detector, and the demodulated outdevice being arranged to be affected by the diput of the latter may then be impressed upon '55 rect current'voltage variations in such a manner a succeeding audio frequency amplifier network,

and finally the audio energy may be reproduced. The diode section of tube 2, to preserve simplicity of disclosure, is shown as including only a single diode anode, and it will be understood that the usual pair of diode anodes are strapped together, and in effect provide a single diode anode.

Each of circuits A and B includes a coil and a tuning condenser in shunt therewith in the usual manner. Assuming that the operating intermediate frequency is kc., and that the receiver operates in the broadcast band, circuit A may be tuned to a frequency of 1'75 kc., while circuit B is tuned to a frequency differing from the frequency of circuit A by a predetermined frequency amount not exceeding 5 kc. By way of example let it be assumed that the frequency of circuit B is 1'77 kc. The coupling between circuits A and B is made such'when receiving strong signals from source i that the resultant curve, shown in solid line, in Fig. 2 results. It will be observed that the mean frequency of this re sultant curve is 176 kc. However, this is not sufiicient because of the inherent characteristic of the signal and local oscillator circuits, to cause any undesirable effects on the tuning of the system.

If now signals of relatively weak amplitude are received, such as in distant reception, the resultant resonance curve of the network including circuits A and B can be considerably narrowed by tuning circuit B to the frequency of circuit A. In other words, considering Fig. 2 it is desirable to impart to the coupling network AB the resonance curve characteristic of circuit A, and this is done by tuning circuit B so that its resonance curve corresponds to that of circuit A. It will also be. observed that when this is accomplished the mean carrier frequency of the resultant curve will now be the operating intermediate frequency. Hence, for the reception of signals of relatively strong amplitude, as in local reception, the coupling network AB will have a wide resonance curve characteristic, but when receiving relatively weak signals the resultant characteristic will be essentially that of a single peaked curve.

This shift in resonant frequency of circuit B is accomplished in a simple and effective manner by providing an auxiliary condenser 3 in the resonant circuit B. The condenser 3 is connected between the grid side of circuit B and the anode of the diode section of tube 2. The cathode of tube 2 is grounded through the conventional bias network 4, and it is to be understood that the signal input grid of the pentode section of tube 2 may receive its normal amplifier bias by virtue of the network 4, or, if desired, the bias on the signal grid may be variable and derived from an automatic volume control network, in which case a direct current blocking condenser 5 is connected between the low alternating potential side of circuit B and the cathode side of network 4.

Besides being connected to one side of condenser 3, the diode anode of tube 2 is connected to a source of diode biasing voltage P, which is shown as a supply resistor, through a path which includes a coil 6 and a fixed bias source 1 of positive voltage. It will be seen that the condenser 3 and the impedance of the diode section of tube 2 are connected effectively across the resonance circuit B. A tap 8' is provided on the supply resistor P so that the potential difference between the cathode of tube 2 and the diode anode may be readily adjusted. In Fig. 1 this adjustment is shown as manual. An intermediate point of resistor P is grounded, and thus when the tap 8 is connected to the ground point of resistor P the diode anode of tube 2 is positive with respect to the cathode thereof by the amount of the voltage of source I.

The source 1 may be given a value which depends upon the signal amplitude at which broadening of the over-all characteristic of network AB is desired. As shown in Fig. 1 the tap 8 has been adjusted to render the diode section of tube 2 conductive, and therefore the impedance in series with the auxiliary tuning condenser 3 is very small.

In effect, then, the condenser 3 is connected in shunt with circuit B, and the total capacity of the circuit is increased. This results in a lowering of the frequency of the circuit. The magnitude of the auxiliary condenser 3 has been chosen, so that when the diode in series therewith is conductive, the effect of the auxiliary condenser will be to shift the tuning of circuit B so that its resonant curve corresponds to that of circuit A. As the tap 8 is shifted towards the negative side of the ground point of resistor P there will be impressed on the diode anode of tube 2 an increasingly greater negative voltage. That is to say, the effect of the positive bias source 1 is decreased as the tap 8 is moved to the left of the ground point of resistor P, and when the tap 8 has been moved to a point on the resistor such that the positive bias of source 1 is overcome, then the diode section of tube 2 becomes non-conductive.

When this point has been reached the impedance due to the diode in series with condenser 3 approaches its substantially maximum value, and renders ineffective the auxiliary condenser 3 insofar as the effect of the latter on tuning is concerned. The result of removing the effect of condenser 3 from circuit B is to shift the frequency of circuit B to the 177 kc. value, and the solid line resultant curve is then secured for network AB. The coil 6 is inserted to present a high impedance at the operating intermediate frequency, and this coil may have a natural period, represented by the dotted line condenser across coil 6, or it may be resonated to have such a high impedance by an external condenser.

In Fig. 3 there is shown a conventional type of superheterodyne receiver wherein circuits A and B provide a coupling network between a pair of intermediate frequency amplifiers. In this circuit the bias on the diode anode of tube 2 is varied automatically by connecting coil 6 to the AVG lead 20. The AVC network is of any conventional type, and has been schematically represented as including an AVC rectifier which receives signal energy through a condenser 2| from the input circuit of the second detector. The AVC rectifier has its direct current voltage output impressed upon the signal grid circuits of the radio frequency amplifier, first detector and the two intermediate frequency amplifiers. Since the circuit details of such an AVC network are well known to those skilled in the art, it is believed sufficient for the purposes of this application that such a network is described by Stuart Ballantine in application Serial No. 376,163, filed July 5th, 1929.

The operation of the arrangement shown in Fig. 3 is similar to that explained in connection with Figs. 1 and 2. As the received signals increase in amplitude, the output of the AVG rectifier will increase thereby increasing the negative bias on the grids of the controlled tubes. Si-

multaneously the positive voltage from the fixed bias source I will be overcome and at a predetermined value of signal amplitude the diode anode of tube 2 will become negative with respect to the cathode. When this point is reached the impedance in diode with condenser 3 will become a maximum and the circuits A and B will have the solid line resultant curve shown in Fig. 2. When the received signal amplitude decreases below the predetermined amplitude level, the output of the AVG rectifier will be less, and the diode anode of tube 2 will become positive with respect to the cathode of the tube, and the condenser 3 will then become effective to shift the tuning of circuit B to correspond with the frequency of circuit A.

It is to be understood that the present invention is not restricted to manual and automatic control of selectivity, but a combined manual and automatic selectivity control network may be utilized. It is believed that the features of Figs. 1 and 2 can be readily combined by anyone skilled in the art from the disclosure given above. Furthermore, it is to be understood that the invention is not restricted to the control of selectivity in an intermediate frequency amplifier network, but may be readily applied to vary the tuning of a couple of tuned circuits of a band pass network. Additionally, the selectivity control arrangement may be varied so that the solid line curve of Fig. 2 is obtained when the diode controlling auxiliary condenser 3 becomes conductive. In this case the increased selectivity characteristic is obtained when the diode controlling the eifect of condenser 3 becomes non-conductive. In this latter arrangement, when applied to automatic control, the lead 20 could be connected to a point of positive potential in the AVG rectifier.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a source of signals, a load network, a pair of resonant circuits connecting the source to the load, means for tuning said circuits to different frequencies, the difference in the tuning of said circuits being equal to a predetermined frequency amount, an auxiliary tuning condenser connected in one of said resonant circuits, a space discharge device having its impedance connected to said auxiliary condenser to control the effect of said condenser on its associated resonant circuit, and means for adjusting the magnitude of the impedance of said space discharge device to vary the selectivity of the network including the coupled resonant circuits.

2. A coupling network adapted to transmit signal energy, said network comprising a pair of coupled resonant circuits, one of the resonant circuits being tuned to a frequency differing from the frequency of the other circuit by a predetermined value, a selectivity control path, including a tuning reactance in series with a space discharge device, operatively associated with one of said coupled circuits, and means for regulating the impedance of said discharge device in response to variations in received signal amplitude.

3. In an intermediate frequency amplifier network of a superheterodyne receiver, a pair of cou pled tuned circuits, one of the circuits being tuned to the intermediate frequency, and the other circuit being tuned to a frequency slightly above the intermediate frequency, a selectivity control device operatively associated with said other circuit and comprising a condenser in series with a diode, and means for varying the conductivity of the diode to regulate the tuning effect upon said other circuit, of said condenser in series with the diode.

4. In a radio receiver provided with an amplifier of broadcast signals, a pair of cascaded, coupled tuned circuits connected to the input electrodes of the amplifier, said circuits being tuned to frequencies differing by a predetermined small frequency amount, a selectivity control device operatively associated with the second of said tuned circuits, said selectivity control device comprising an auxiliary tuning condenser and a space discharge device whose impedance is in series with said auxiliary condenser across the said second tuned circuit, and means for varying the conductivity of the space discharge device to regulate the effect of the auxiliary condenser upon the tuning of the said second circuit.

5. In a system as defined in claim 4, said conductivity varying means including a bias control device which produces a direct current voltage proportional to the amplitude of received signals.

6. In a system as defined in claim 4, said conductivity varying means being responsive to variations in received signal amplitude, and additionally regulating the amplification of said amplifier.

7. In a system as defined in claim 4, said space discharge device comprising a diode whose electrodes are included in the same tube envelope as the electrodes of said amplifier.

8. In combination with a source of signals, a load network, a pair of resonant circuits connecting the source to the load, means for tuning said circuits to different frequencies, the difference in the tuning of said circuits being equal to a predetermined frequency amount, an auxiliary tuning condenser connected in one of said resonant circuits, 2. space discharge device having its impedance connected to said auxiliary condenser to control the effect of said condenser on its associated resonant circuit, and means for adjusting the magnitude of the impedance of said space discharge device to vary the selectivity of the network including the coupled resonant circuits, and a reactive impedance in circuit with said space discharge device, said impedance being tuned to the frequency of one of said resonant circuits.

9. In an-intermediate frequency amplifier network of a superheterodyne receiver, a pair of coupled tuned circuits, one of the circuits being tuned to the intermediate frequency, and the other circuit being tuned to a frequency slightly above the intermediate frequency, a selectivity control device operatively associated with said other circuit and comprising a condenser in series with a diode, and means for varying the conductivity of the diode to regulate the tuning effect upon said other circuit, of said condenser in series with the diode, and a coil in circuit with said diode and condenser, said coil being tuned to said intermediate frequency.

MURRAY G. CLAY.

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