US2280563A - Automatic selectivity control circuit - Google Patents

Description

April 21, 1942. J. wx-:lNBERGER AUTOMATIC SELECTIVITY CONTROL CIRCUITl Filed Nov. 25, 1935 lNvENToR JULIUS WEINBERGER ATTORNEY Patented Apr. 21, 1942 AUTOMATIC SELECTIVITY CONTROL CIRCUIT Julius Weinberger, Bronxville, N. Y., assigner to Radio Corporation of America, a corporation of Delaware Application November 25, 1935, Serial No. 51,363

5 Claims. (Cl. Z50-2G) My present invention relates to automatic control circuits for regulating the gain and selectivity of radio receivers, and more particularly to an automatically operating regulator network for high delity receivers utilizing automatic selectivity control, the regulator network functioning when the receiver is subjected to two powerful signals on adjacent channels.

In the usual form of automatic selectivity control circuit for a high fidelity receiver, a powerful signal .broadens the tuning of the receiver; that. is to say, the action of the selectivity control circuit is to lower the selectivity of the receiver. Accordingly, if a powerful signal is being received, and if another powerful signalI comes in on a channel adjacent to the desired one, the receiver will have insufficient selectivity against the interfering signal by virtue of the fact that the receiver tuning has been broad'- ened by the desired signal. may arise, for example, in receiving broadcasting stations in a given urban locality, wherein the stations are on adjacent channels and are both powerful, and a high fidelity receiver is being employed, which receiver is equipped with the usual form of automatic selectivity control circuit. In the event of the construction of additional high power broadcasting stations, this type of interference necessarily must increase.

Accordingly, it may be stated that it is one of the primary objects of my present invention to provide in a radio receiver of the type utilizing automatic selectivity control, a device which functions automatically to sharpen the tuning of the receiver to a suicient degree to enable it to discriminate against an undesired powerful adjacent channel signal, when the selectivity control is broadening the tuning of the receiver by virtue of a powerful signal on the desired channel.

Another important object of the invention may be stated to reside in the provision of a pair of automatically operating control circuits, both circuits being dependent upon received signal energy; one of the control circuits functioning automatically to regulate the selectivity and signal transmission through the receiver in such a manner that the receiver selectivity and sensitivity is reduced when powerful desired signals are received; and the other signal-actuated control circuit functioning to act upon the selectivity control device in such a manner that the selectivity of the receiver is increased when any undesired adjacent channel signal which is powerful is received.

Such a condition r Still other objects of the invention are to improve generally the eiciency and reliability of `high fidelity radio receivers utilizing automatic selectivity control, and more especially to provide a regulator network for vsuch types of receiver, which network shall not only be dependable in operation, but also economically manufactured and assembled in a radio receiver.

The novel features which I .believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a radio receiver of the superheterodyne type, the receiver being equipped withA an automatic selectivity control network of a type generally disclosed by G. L. Beers in U. S. P. 2,017,523 of October 15, 1935. Those skilled in the art are fully aware of the circuit details of a superheterodyne receiver of the type shown in the drawing. It is believed sufficient to generally point out that such a receiver includes the usual signal collector l which may supply signal energy of a desired carrier frequency to one or more stages of tunable radio frequency amplification. The numeral 2 designates the tuning means of the receiver, and it is to be understood as including the tuning condenser employed in Vthe rst detector and local oscillator networks.

The numeral 3 designates the tunable converter, and it is to be understood that this converter may be of the composite local oscillator-first detector type, or it mayv comprise independent first detector and local oscillator tubes.

In any case the resonant output circuit 4 of the converter network 3 is tuned to the operating intermediate frequency, and the latter may have any value commonly employed in radio reception, say from 175 to li5() k. c. Between the output circuit 4 of the converter network 3 and the demodulator, or second detector, 5, there may be employed one or more stages of intermediate frequency amplification. Two stages 6 and 1 are shown by way of example, and it should be noted that the coupled resonant circuits 4 and 5 provide the coupling network between converter 3 and amplifier 6; coupled resonant circuits 8 and 9 provide the coupling network between ampliers 6 and 1, and coupled resonant circuits Ill and II provide the coupling network between amplifier 1 and second detector 5. Each of circuits 4, 5, 8, 9, I8 and I| is tuned to the operating intermediate frequency.

The output of the second detector is transmitted through any type of audio frequency coupling |2 upon one or more stages of audio frequency amplification followed by a. nal reproducer of any well known type. The coupling between each pair of circuits 4 and 5; 8 and 9; is made relatively loose so that in the absence of any damping on each of these circuits, the resonance curve characteristic of these I. F. networks will be single peaked and relatively sharp, and, therefore, the selectivity of the networks in question will be a maximum.

The selectivity of the I. F. coupling networks is automatically regulated by shunting across each of circuits 4 and 8 a selectivity control tube. Thus, the tube 28 has its plate connected to the high alternating potential side of tuned circuit 4, the cathode of tube 2D being grounded, and the plate of tube 2| is connected to the high alternating potential side of resonant circuit 8. The cathode of tube 2| is also grounded. The plate-cathode impedance of each of tubes 28 and 2| is connected in shunt with circuits 4 and 8 respectively. The tubes 28 and 2 I may be of any well known type; preferably they are of the 46 type. This type of tube is well known in the art, and employs a pair of control grids, one coarse and one fine, the plate current-grid voltage characteristic of such a tube being characterized by zero plate current at zero grid bias, and the plate current increasing with increasing values of positive grid bias.

The grid bias voltage of tubes 20 and 2| is supplied from a rectier network which comprises diode rectiiier 22, the anode of the rectifier being connected to one side of a resistor 23 through the secondary winding 24 of transformer T. The cathode of diode 22 is connected to the opposite side of resistor 23 and a by-passing condenser 25 is connected in shunt across resistor 23'. Signals are impressed upon the diode rectier 22 through a path which includes the signal transmission condenser 25, the signal amplifier 2 and the primary winding 28 of coupling transformer T. It will now be seen that there is impressed upon the diode rectifier 22 I. F. energy from the input circuit of the second detector 5. This I. F. energy is amplified prior to rectification by diode 22, and it is pointed out that the transformer T is broadly tuned to the I. F.

The cathodes of damping control tubes 20 and 2| are connected by direct current connection 38 to an intermediate point on resistor 23 through a tap 3|, the control grids of the damping control tubes being connected, through direct current connection 32, to an intermediate point on resistor 33, the latter resistor being connected to the resistor 23.

It will now be seen that upon impression of I. F. energy on the diode rectifier 22 the control gridsof tubes 20 and 2| will be biased positively with respect to the cathodes thereof, since the cathodes of the damping control tubes are connected to the anode side of resistor 23 with respect to the point on resistor 23 to which the control grids of tubes 2i] and 2| are connected. In the absence of received signal energy, the direct current voltage across resistor 23 will be zero, and, therefore, the potential difference between the control grids and cathodes of tubes 20 and 2| will. be zero. In other words, the effective vresistance shunted across resonant circuits 4 and 8, in that case, by tubes 20 and 2| will be a maximum.

Consequently the effective damping resistance in these tuned circuits 4 and 8 will be a minimum, and, therefore, the circuits will be highly selective.

As the I. F. energy impressed on diode rectifier 22 increases in intensity, the positive bias on the control grids of tubes 20 and 2| increases, with the result that the resistance shunted across circuits 4 and 8 decreases towards a minimum, and consequently the effective damping resistance in series in each of tuned circuits 4 and 8 increases towards a maximum. This results in increased damping of tuned circuits 4 and 8, with consequent lowering of the selectivity of the tuned circuits. Simultaneously the signal transmission eiiciency to the demodulator is decreased, It will, therefore, be seen that the diode rectifier 22 not only functions to automatically regulate the selectivity of the I. F. networks, but at the same time acts to reduce the signal transmission efficiency as the signal amplitude increases, and, therefore, provides an AVC action.

It will now be realized that when the receiving system shown in the drawing is employed in a locality where a strong local station is being received the ASC-AVC network functions automatically to decrease the selectivity of the tuned circuits 4 and 8 and thereby broaden the resonance curve characteristics thereof; this manifests itself in an apparently increased idelity of reception, and reduction in receiver sensitivity. When weak signals, as from distant stations are received, there is an automatic increase in sensitivity and selectivity by virtue of the action described above.

The automatic regulator network for the selectivity control circuit comprises an additional diode rectifier l40, having its cathode connected through lead 4| to the junction of resistors 23 and 33. The resistor 33 is connected in series between the cathode and anode of diode rectifier 40, and the by-pass condenser 42 is connected in shunt across resistor 33. A resonant circuit comprising coil 43 and shunt condenser 44, is connected in series between one side of resistor 33 and the .anode of diode 48. The coil 43 is cou- -pled to coil 45, the latter being shunted by tuning condenser 46, and the high potential side of resonant circuit 45-46 is connected to the plate circuit of the second detector 5 through a direct current blocking condenser 41; the low a1- ternating potential side of the resonant circuit being grounded. Each of circuits 44-43 and 45-46 is tuned to a frequency which is equal to the difference between the desired signal frequency and the frequency of the undesired strong adjacent -channel signal. It will be realized that there is produced in the plate circuit of the second detector 5 energy of such frequency difference, because there is impressed on circuit the desired intermediate frequency energy, and beat energy resulting from heterodyning of the local oscillator with an adjacent channel signal energy. 'I'he 10 k. c. difference energy'is selected out from the second detector plate circuit. This difference energy will not exist until the adjacent channel signals are received. For -broadcast reception this frequency diierence is 10 k. c., and, therefore, each of these last named resonant circuits is tuned to a frequency of l0 k. c.

It will, therefore, be seen that at the output of the second detector, there is selected any 10 k. c. signal which may be present, and then there is generated a direct current voltage from this k. c. signal. The direct current voltage is developed across resistor 33, and is utilized to oppose the biasing action of the direct current voltage developed across resistor 23. Thus, the effect due to the desired signal is counteracted by that due to the undesired signal to an extent proportional to the ratio of the two signals.

If, now, a strong local signal is being received, the receiver selectivity being broadened, and a powerful adjacent channel signal is collected, this will result in the impression upon the control grids of tubes and 2| of a negative direct current bias. This results in an increase in the shunt resistance across tuned circuits 4 and 8, with a consequent decrease of the effective series damping resistance in the tuned circuits. Hence, the selectivity of the tuned circuits 4 and 8 is automatically increased, and the degree of selectivity increase will be sufficient to discriminate Vagainst the adjacent channel signal. Further, it will be seen that the increase in selectivity of tuned circuits `4 and 8 will be dependent in degree upon the strength of the undesired adjacent channel signals. Of course, the 10 k. c. regulator network will not become operative until the intensity of the adjacent channel signal is sucient to aect the selectivity control tubes 20 and 2|. The taps 3l and 3l' on resistors 23 and 33 respectively can be used to adjust the extent of action of the various control circuits.

It is to be understood that separate diode tubes 22 and 40 need not be employed; for example,

a single tube having the electrodes of the two i diodes housed therein may be employed in .place of the two independent tubes. Such double diode tubes are Well known at the present time. Furthermore, it will be readily appreciated by those skilled in the art that any other type of tubes may be employed in place of those shown vas tubes 20 and 2l. Additionally, it is within the scope of the present invention to combine the electrodes of amplifier 21 and diodes 22 and 4U within a single tube envelope, and any well known type of multiple duty tube,ksuch as a type tube may be used for this purpose.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my f invention is by no means limited to the particular organization 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 a superheterodyne receiver of the type comprising at least one resonant circuit tuned to the operating intermediate frequency, a demodulator, and means responsive to variations in intermediate frequency energy amplitude for automatically adjusting the degree of selectivity of said intermediate frequency resonant circuit, additional means for regulating the action of said selectivity control means, said additional means including a device which is responsive to energy of a frequency equal to the difference between the intermediate frequency and signal energy representative of an undesired adjacent channel frequency.

2. In a superheterodyne receiver comprising cascaded ampliers tuned to the operating intermediate frequency, a second detector for demodulating the amplified output of said amplifiers, means, responsive to variations in amplitude of the intermediate frequency energy impressed upon the second detector, for automatically decreasing the selectivity of the amplifiers when the intermediate frequency energy impressed on the first of said amplifiers increases in amplitude, and a regulator circuit, responsive to energy of a frequency of the order of 10 kilocycles appearing in the output circuit of the second detector, for automatically affecting the action of said selectivity control means and in a sense to increase the selectivity of said amplifiers when said l0 kilocycles energy exists in the second detector output circuit.

3. In a radio receiver including a plurality of cascaded resonant circuits, each tuned to the operating signal frequency, means responsive to an increase in amplitude of the desired signals for automatically reducing the signal transmission efficiency through said cascaded circuits, and additional means, responsive to energy of a frequency equal in magnitude to the difference in frequency between the desired signal frequency and an undesired adjacent channel signal frequency, for automatically increasing the transmissionefiiciency of said desired signals through said cascaded circuits, and said first means additionally being constructed to decrease the selectivity of said cascaded circuits upon an increase in amplitude of the desired signal frequency.

4. In a superheterodyne receiver comprising a plurality of cascaded resonant circuits eachk tuned to the operating intermediate frequency, a second detector, a rectifier coupled to the second detector input, and being adapted to produce a direct current ,voltage which varies directly in magnitude with the amplitude of the intermedi ate frequency energy, at least .one selectivity control tube having its plate circuit impedance connected in shunt across at least one of said intermediate frequency circuits, means for impressing direct current voltage produced by said rectier upon the input electrodes of said control tube in a sense to increase the effective damping of said one resonant circuit as the intermediate frequency energy amplitude increases, a second rectifier, having an input circuit tuned to a frequency of the order of l0 kilocycles reactively coupled to the second. detector output circuit, said second rectifier being constructed to produce a direct current voltage varying in magnitude with the amplitude of said l0 kilocycles energy, and means for impressing the second direct current voltage upon the input electrodes of said control tube in a polarity sense opposed to the polarity of said first direct current voltage.

5. In a modulated-carrier signal receiver, a band-pass selector for selecting a desired signal comprising a carrier frequency and. a band of modulation frequencies, means for deriving from said desired signal and an undesired signal on a carrier frequency near the desired signal carrier frequency a control signal comprising beat notes of said signals, and means responsive to the amplitude of said control signal for adjusting the width of the band of frequencies passed by said selector inversely in accordance with said amplitude of said control signal, said first means cornprising a selector circuit tuned in the region of 10 kilocycles followed by a rectifier.

JULIUS WEINBERGER.

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