US2167400A - Variable selectivity radio receiver - Google Patents

Description

Patented July 25, 1939 PATENT 1 orr ce 2,167,400 vnnranm .samcrrvrrv aanro aroma John F. Farrington, Flushing, N. Y., assignor to Hazeltine Corporation, a corporation of Del- Application January 21, 1937, Serial No. 121.410

12 Claims. (or. 119-171) This invention relates to modulated-carrier signal receivers, and more particularly to adjustable band-pass selector systems suitable for controlling the selectivity or fidelity of reproduc- 5 tion of such receivers automatically in accordance with the strength of received signals.

In receivers including automatically adjustable band-pass selector systems, it is desirable that the width of the frequency band passed by the selector be increased gradually with increasing signal inputs from a minimum, at signal strengths of the order of the sensitivity limit of the receiver, to a desired maximum, for signals of moderate intensity and that the width of theband l5 thereafter be maintained constant for further increases in signal strength. It is further desirable that the threshold of signal input at which the band width of the selector begins to expand from its minimum value be manually adjustable 20 so that, when the noise level is high, signals of greater strength are required to produce band expansion. Also, it is desirable that means he provided for manually contracting the band width when receiving through strong interfer- 25 ence.

Heretofore there have been proposed numerous band-pass selector systems useful in modulatedcarrier signal receivers and adjustable automatically in accordance with the intensity of received 30 signals to adjust the selectivity of the receiver,

thereby to enable the listener to procure the maximum fidelity of reproduction consistent with conditions of reception obtaining. Among such adjustable band-pass selectors, one type that has been found to be particularly satisfactory, in that it provides symmetrical expansion of the pass band about the mean resonant frequency thereof, is of maximum simplicity, requiring a minimum of additional tubes and circuit elements, and has .10 maximum flexibility of control, is that comprising a pair of circuits resonant at frequencies within the pass band, a directivecoupling means, such as a vacuum tube, coupling the circuits in one direction and a separate coupling means,

5 such as a second vacuum tube, coupling the circuits in the other direction, one or both of the coupling means being adjustable to vary the mutual impedance between the circuits, thereby to vary the selectivity of the receiver.

In band-pass selectors of the type described above, it has been found that a particularly Simple automatic control of the band width may be obtained by directly'controlling the responsiveness of one of the coupling means, as, for exam- 5 pie, the forward coupling tube, in accordance with received signal intensities and indirectly controlling the responsiveness of the other coupling means in accordance with the response of the first coupling means, but in an opposite sense. However, the selectivity characteristics of such 6 band-pass selectors depend upon the joint efiect of both of the coupling means. With the circuit constants and tube characteristics ordinarily encountered in band-pass selectors of conventional receivers, the joint effect of the forward and 10' backward coupling means, controlled as described above, often reaches a maximum value within the range of the automatic controlling eflect utilized as the direct control of the first coupling means so that, for further variations of the direct control beyond that corresponding to the maximum joint eifect,- the selectivity characteristics are varied in a reverse sense; that is, the band passed by the selector is actually contracted, which is I opposite to the result desired.

In other automatically adjustable band-pass selectors, on the other hand, the band width tends to expand indefinitely in accordance-with increasing signal inputs, resulting in excessive band widths which make the receiver vulnerable to interference without corresponding improvement in the fidelity of reproduction once the desired maximum band expansion has been exceeded.

It is an-object of the invention, therefore, to

provide, in a modulated-carrier signal receiver, an improved automatically adjustable band-pass selector system which will overcome the abovementioned disadvantages of arrangements of the prior art and which will be simple in construction and eflicient and flexible in operation and control.

It is a further object of the invention to provide, in a modulated-carrier signal receiver, an automatically adjustable band-pass selector sys- 40 tern by means of which the selectivity of the receiver is varied-in accordance with received signal inputs and in which the band width passed by the selector is limited to a predetermined maximum value for signal inputs of a, given amplitude and is maintained constant for signal inputs of greater amplitude.

More specifically, it is an object of the invention to provide a band-pass selector system including a pair of circuits resonant at frequencies within the band, directive coupling means coupling the circuits in one direction and separate coupling means coupling the circuits in the other direction and in which the joint effect of the two coupling means reaches a maximum value within the range of control of one or both of the coupling means, by means of which variations of the selectivity of the receiver in a reverse sense for variations in the direct control of one of the coupling means beyond that corresponding to the maximum value of their joint efiect are avoided.

It is a further object of the invention to provide, in a modulated-carrier signal receiver, a band-pass selector of the character described in which, for values of the direct controlling action beyond that corresponding tothe maximum joint eflect of the forward and backward coupling means, the.control on the directly controlled coupling means is maintained at a constant value corresponding to such maximum joint effect.

In accordance with one embodiment of the invention, there is provided, in a modulated-carrier signal receiver, a band-pass selector system including a pair of circuits resonant at frequencies within the pass band of the selector, directive coupling means coupling the circuits in one direction, separate coupling means coupling the circuits in the other direction, control means for adjusting at least one of the coupling means to vary the mutual impedance between the circuits, thereby to vary the selectivity of the receiver, and means for limiting the action of the control means in altering the selectivity characteristics of the receiver to a predetermined value, which may be adjustable.

More specifically, the band-pass selector in accordance with the invention comprises a vacuum tube repeater in each of .the coupling circuits, the repeater tubes having a common cathode circuit including a biasing resistor, and means for applying an adjustable bias to the forward coupling tube to vary its space current and thereby indirectly adjust the bias and-transconductance of the backward coupling tube. In such a system, the selectivity of the receiver varies as the product of the transconductances of the forward and backward coupling tubes. The characteristics of the tubes and the constants of their associated circuits are generally such that the space current of the forward coupling tube approaches a small minimum value within the limits of the adjustable bias so that, for higher values of bias voltage, the transconductance of the forward coupling tube continues to decrease while the transconductance of the backward coupling tube does not correspondingly increase. As a result, the product of the transconductances of theaterward and backward coupling tubes goes through v a maximum value. There is, therefore. provided means for limiting the bias applied to the forward coupling tube to a preselected value equal to or less than that required to attain such maximum value of the said product to prevent variation of the selectivity of the receiver in a reverse sense for values of said adjustable bias in excess of the preselected value. There may be provided also manually adjustable means for'increasing the negative bias potential applied to the backward coupling tube to maintain its transconductance at zero and the selector band with at minimum for signal levels and resultant variable control bias potentials greater than the threshold value normally efiective to initiate expansion of the band width of the selector, for receivin through particularly high noise levels.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following specification taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. l is a schematic circuit diagram of a complete superheterodyne receiver employing an automatically adjustable band-pass selector including means for limiting'the maximum band width of the selector in accordance with the invention; Fig. 2 is a circuit diagram, partially schematic, of a complete superheterodyne receiver embodying a particular form of automatically adjustableband-pass selector sys- I tem, shown in detail, together with means for limiting the maximum band width of the selector; Fig. 3 is a simplified circuit diagram of the band-pass selector of Fig. 2 and its control cir cuit to aid in the understanding of the invention; while Fig. 4 is a graph illustratingoperating characteristics of the control systems of Figs. 1

and 2.

Referring now more particularly toFig. 1 of the drawing, there is shown schematically a superheterodyne receivenincluding, an adjustable band-pass selector and means for limiting the maximum band width passed thereby. In general, the receiver includes a tunable radio-ire quency selector and amplifier I having its input circuit connected to an antenna-ground circuit 2, and its output circuit connected to a tunable frequency changer 3, the output circuit of which is, in turn, connected to an intermediate-frequenc-y selector and amplifier d. The selector and amplifier l comprises an adjustable bandpass selector of any suitable type well known in the'art. Connected to the output circuit of the selector and amplifier 4 in cascade, in the order 'one or more of the tubes of the unit 4 by way of connection 9 and a high resistance element H). In shunt with the source of A. V. C. potential in the control connection to the unit 4 is a unilaterally conductive device, such as a diode II in series with a source of adjustable negative potential, suchas a voltage divider l2 connected across a battery l3. The diode H is connected with such polarity that it is non-conductive except when the negative A. V. C. bias exceeds that derived from the voltage divider [2.

The general operation of the'receiver justdescribed is well understood in the art and a detailed description thereof is, therefore, unnecessary. 'In brief,-however, signals interceptedby the antenna 2 are selected and amplified in the radio-frequency amplifier I and are transmitted to the tunable frequency changer 3, wherein they are converted into intermediate-frequency signals in a well-known manner. The intermediatefrequency signals are selected and amplified in the intermediate-frequency amplifier 4 and passed to the detector 5, wherein the audio frequencies of modulation are derived. The audiofrequency signal is further amplified in the amplifier 6 and supplied in the usual manner to the loud-speaker l for reproduction. The unidirectional component of the output of the A. V. C.

An automatic amplification con- Luc rectifier and detector 5 isapplied by way of the A. V. C. connection to one or more of the tubes of the radio-frequency amplifier I and the tunable frequency changer 3 through the connection 8, thereby maintaining the amplitude of the signal input to the detector 5 within anarrow range for a widerange of received signal amplitudes.

The circuit constants and tube characteristics of the adjustable selector unit 4 are so chosen that, for signal inputs of less than a predetermined amplitude usually near the sensitivity limit of the receiver, the width of the frequency band passed by the unit 4 is a minimum, ensuring sub stantial freedom from adjacent signal interference and interchannel noise, but at a sacrifice in the fidelity of reproduction. With increasing signal inputs, the signal-derived bias applied to the unit 4 through the connection 9, effects a gradual expansion of the frequency band passed by the unit 4, improving the fidelity of reproduction of the system. This expansion continues until, for signal inputs of moderate amplitude, the signal-derived bias exceeds that derived from the voltage divider I2 and the diode H becomes conductive. Under this condition the signalderived bias is partially dissipated in the resistor I0 and the bias applied to the unit 4 is thereafter maintained approximately constant, limiting the maximum expansion of the frequency band passed by unit 4 to a desired value and thereafter maintaining such band of substantially constant width for further increases in signal input amplitudes. This limiting of .the band expansion is obtained without affecting the relation between the expansion-control bias and theamplitude of the signal input for signal inputs of lower amplitude," that is, without limiting the normal sensitivity of the expansion control. Because of the high resistance of the element 10, there is no appreciable reaction back on the main A. V. C. circuit 8 to the units 1 and 3. By adjusting, the voltage divider I2, the value of the signalinput at which adjustment of the band passed by unit 4 is arrested may be adjusted as desired.

In Fig. 4 are shown curves representing band width-automatic control-bias characteristics of typical automatically adjustable band-pass selectors, Curve A is typical of a selector in which the band width increases continuously with increasing automatic control bias. Curve B is a characteristic of 'a type of automatically adjustable band-pass selector in which the band width increases from a minimum to a maximum with increasing automatic control bias and theredesired maximum band width. For example, if

the desired maximum'band width is represented by the ordinate C of Fig. 4, further expansion or subsequent contraction can be prevented by applying to the limiting diode II from the voltage divider 12 a negative-bias potential equal to the value D of Fig; 4. This prevents operation of the diode ll until the automatic control bias reaches the value D, thus procuring gradual expansion of the band width with increasing signal inputs up to this point and, as explained above, thereafter maintaining the bias to the unit 4 and the band width passed thereby substantially constant for'further increases in signal input amplitude. The maximum band width of the selector is preferably limited to a predetermined value corresponding to the usual modulation band width of received signals, that is, to a band width sufllciently wide to translate all the sideband frequencies of any received signal.

The application of the present invention to a particular form of automatically adjustable band-pass selector embodied in a superheterodyne receiver is shown in detail in Fig. 2, in which conventional elements corresponding to those of the system of Fig. 1 are indicated by like reference numerals. This system is of the same general type, involving the same general principles of operation, as that of Fig. 1, so that adescription-thereof need not be repeated. In Fig. 2 the intermediate-frequency amplifier and adjustable selector 4 and the signal detector and A. V. C. rectifier 5 are shown in detail, the latter comprising a conventional arrangement of a diode I4 having as its load circuit series-connected resistors l5 and I6 by-passed by condensers l1 and I8, respectively, the A. V. C. connection 8 including suitable filters comprising series resistors 23 and shunt condensers 24. The audiofrequency voltages applied to the amplifier 6 are derived from a voltage divider 19 coupled to the load circuit of the detector i4 through a coupling condenser 20.

Referring now more particularly to the iritermediate-frequency amplifier and selector system 4 of Fig. 2, this system includes a pair of circuits 25 and 26 tuned to the desired intermediate frequency and coupled with somewhat less than optimum coupling, the circuit 25 being connected to theoutput of the frequency changer 3. Included also in the selector system 4 is a pair of circuits 21 and 28, also tuned to the desired intermediate frequency and coupled with somewhat .less than optimum coupling, the circuit 28 being connected to the A. V. C. rectifier and detector l4. Preferably, loading resistors 29 and 30 are connected across the circuits 25 and 28, respectively, to flatten the resonant characteristics of the system when adjusted for selectivity less than minimum, as described hereinafter. The circuits 26 and 21 are coupled by a vacuum tube 3|, illustrated as of the pentode type, provided with cathodebiasing resistors 32 and 33 by-passed by condensers 34 and 35, respectively. Suitable operating potentials are applied to the screen and anode of the tube 31 from the sources indicated as +Sc and +8, respectively.

In order automatically to adjust the width of the frequency band passed by the system 4, the tuned circuits 26 and 21 are also coupled in a reverse direction by a unidirectional coupling means, such as a vacuum tube, which may be of the triode type, as shown, having a relatively sharp cutoff characteristic. The input circuit of the. triode 31 is coupled to the circuit 21 by means of a winding 38 coupled to the inductance element of that circuit and a blocking condenser 39. The output circuit of the tube 31 is coupled to the circuit 26 by means of a winding 40 coupled to the inductance element of that circuit and a bypass condenser Suitable anode potential is applied to the tube 31 from a source indicated as +13 through an isolating resistor 42. In order to neutralize'any capacitive forward coupling between the circuits 26 and 21 through the interelectrode capacitance of the tube 3|, there is proprovides a capacitive feedback of the proper phase relation to neutralize the incidental capacitive forward coupling. With the use of a triode the interelectrode capacitance thereof is sumcient to accomplish this neutralizing function so that a separate physical condenser 33 may be omitted.

It will be noted that the biasing resistors 32 and '33 are included in the cathode circuit of the feed-back tube and the values of these resistors are so selected that the bias voltage developed thereacross, for signal inputs near or below the sensitivity limit of the receiver, biases the tube 3'?! to cutofi. The direct application of the A. V. C. bias to the forward coupling tube 3i effects a variation in the space current of this tube with signal intensity and a corresponding variation in the voltage drop across resistors 32 and 33, which voltage determines the bias on the backward coupling tube 31 and thus its transconductance. That is, the transconductance of the backward coupling tube is controlled indirectly in accordance with received signal intensities. As dis= cussed above, however, in coupling systems of the type described, the selectivity characteristic is dependent upon the product of the transconductances of the tubes 3i and 37, while the characteristics of the tubes 3! and 37, in connection with their associated circuits having electrical constants to satisfy the other requirements of the receiver, are such that the product of their transconductances usually reaches a maximum value for values of A. V. C. bias substantially less than the maximum normally used for controlling the gain of the receiver. That is, when the A. V. C. bias voltage exceeds that value corresponding to the maximum product of the transconductances of the tubes 3i and 37, further increases in A. V. C. bias voltage corresponding to received signal inputs of larger amplitude, result in an increase in the selectivity of the receiver, which is the opposite of .the desired relationship.

In order to prevent this variation of the selectivity of the receiver in a reverse sense, there is provided an auxiliary control circuit connected in parallel with that portion of the'automatic amplification control circuit of the tube 3| including the source of A. V. C. bias potential derived from a load resistor l6 of the detector M, this auxiliary circuit being effective to limit the bias potential applied to the tube 3! to a. predetermined value for all values of A. V. C. bias voltage ex-, ceeding said predetermined value. This auxiliary control circuit comprises a connection M to the junction'of the resistor it and a filter comprising resistor t5 by-pd ed by a. condenser eta, the cathode-anode circuit of a diode limiter tube 436, which may be included within the same envelope as the rectifier N, the lower portion of the voltage divider 47 between the adjustable tap Ma and ground, which voltage divider is energized from a source of constant-bias voltage such as a battery 48, and, through the ground connection, the cathode-biasing resistor 33 to the cathode of the A. V. C. rectifier it.

The operation of the adjustable band-pass selector system 4 may be explained by considering the flow of signal energy around the loop comprising the input circuit 26, the forward coupling tube 3 I, the output circuit 21, and the backward coupling tube 37 having its input circuit coupled to the circuit 21 and its output circuit coupled to the circuit 26. Considering the voltages induced in the circuit 26 from-the circuit 25 as a reference, it will be apparent that alternating voltages appearingacross the resonant circuit 21 are substantially reversed in phase with respect to those across the circuit 26 at frequencies in the vicinity of the resonant frequency of these circuits, at which frequency these circuits are of high impedance and are substantially resistive. The feed-back voltages are reversed a second time in the backward path in the tube 3?,

while a third reversal is secured in the coupling between the circuit 27 and the winding 38, or between the winding 6t and the circuit 26 so that voltages induced across the input circuit 26 through this backward coupling path are, at the frequencies indicated, substantially reversed in phase with respect to the input voltages directly induced across this circuit from the primary circuit 25, and the system is degenerative to the maximum degree at the intermediate-carrier frequency.

At frequencies substantially above the resonant frequency of the circuits 26 and 21, these circuits are capacitively reactive toward voltages thereacross so that the voltages at these frequencies across the circuit 27 lag behind the input voltages by phase angles approaching 90 degrees as a limit. The feedback voltages at these frequencies developed across the circuit 26 are similarly retarded by an additional angle also approaching 90 degrees as a limit, so that the feed-back voltages are nearly in phase with the input voltages at these frequencies, and the coupling system is regenerative.

At frequencies below the resonant frequency 01 the circuits 26 and 21', these circuits are inductively reactive toward voltages thereacross and a similar phase shift occurs but in the opposite Gil pedances of the circuits 26 and 2'! are much less than at resonance, the transmission emciency of the amplifier stage 3| being reduced, and the amplitude of the feed-back voltages being further reduced so that, while the system is regenerative, it is entirely stable in operation. At frequencies intermediate the limiting frequencies justdescribed, the feed-back voltages, have intermediate phase angles with respect to the input voltages, and the feed-back characteristic of the system thus has a gradual transition from degeneration at resonance to regeneration at frequencies substantially displaced from resonance. Hence, the

resultant reduction in amplitude of the input voltages at frequencies near the intermediatecarrierfrequency and increase in amplitude of the voltages at frequencies substantially above and below the intermediate-carrier frequency impart to the system a band-pass frequency characteristic like that of over-optimum coupled double-tuned circuits.

By adjusting the forward and backward coupling reactions between the resonant input and output circuits, the shape and width of the bandpass characteristic may be controlled as desired. Such coupling control is procured, in accordance with the present invention, by applying the am-,

plification control bias derived from the A. V. C. rectifier l4 directly to the control grid of the forward coupling tube 3|, this bias increasing with increaslng amplitudes of signal input to the v receiver and decreasing the transconductance and space current of the tube 3|, thus correspondingly decreasing the bias voltages across the resistors 32 and 33. The latter resistors are included in the cathode circuit of the backward coupling tube 31, so that its grid'becomes less negative with respect to its cathode, thereby increasing its transconductance.

Since, as explained above, the selectivity of the system is determined by the product of the transductance of the forward coupling tube 3i, it is necessary to increase the transconductance of the backward coupling tube 31 to an even greater extent, so that it is necessary that the tube 31 have a steeper grid voltage-transoonductance most easily secured by utilizing as the tube 31 a triode having a sharp cutoff characteristic and operating it in the vicinity of cutoff. While a triode used in such a manner might introduce some distortion, this ismade negligible by coupling the feed-back winding 38 very loosely to the circuit 21 so that only a small signal input voltage is appliedto the grid of the tube 31.

In the arrangement described above, in which the selectivity is dependent upon the product of the transconductances of the forward and backward coupling tubes and thus is decreased with increasing amplification control bias corresponding to increasing received signal intensities with-- in a given range,'the usual range of amplification control bias is so wide that the product.of the transconductances of the forward and backward coupling tubes may reach a maximum and decrease within this wide range of bias voltages,

as explained above in connection with Fig. 4. In order to prevent such operation, the auxiliary control circuit described above is utilized. To this end, the automatic amplification control circuit is returned to the positive terminal of the resistor 33 in the common cathode circuit of the tubes 3i and 31, the resistor 32 being provided to ensure proper normal grid bias to the tubes 3| and 31. The auxiliary control circuit, including the connection 44 to the automatic amplification control circuit at the junction of resistors l0 and 45 and the diode 46, is completed by an adjustable tap 41a. of the voltage divider 41, one terminal of which is grounded. This auxiliary circuit has been redrawn in simplified form in Fig. 3, corresponding elements being identified by the same refercnce'numerals, and all elements not essential to this feature of the invention being omitted for the sake of clarity.

Neglecting the ground connection. which is immaterial insofar as this portion of the circuit is concerned, it will be seen that, in the auxiliary circuit, the amplification control bias across the V resistor I6 is opposed to the bias across the cathode resistor 33 and that across the. portion of the voltage divider. 41 included in this circuit, through the diode 46. This diode is connected with such polarity that, for small values of amplification control bias across the resistor I6, it is nonconductive and the system operates as described above. As the amplification control bias rises to a predetermined value slightly exceeding the sum of thevoltages across the resistor 33 and the effective portion of the voltage divider 41, the diode 46 becomes conductive and completes a load. circuit for the resistor 16 through resistor 10,;- the voltage drop across resistor 33 also remains constant. In brief, further increases 'in received signal intensity and amplification control bias across resistor l3 eifect no further increase in the bias voltages applied to the grids of the tubes 3i and 31 and the selectivity of the system is maintained constant, avoiding the tendency towards reversal described above. By properly selecting the circuit constants, the operating point at which the diode 46 becomes conductive may be determined to cor-- respond to the maximum product of the transconductances of the tubes 3! and 31, that is, the minimum selectivity of the receiver. By adjusting the tap 410, the value of signal intensity necessary to overcome the initial cutoff bias of the tube 31 may be adjusted to adjust the delay in operation of the automatic selectivity'control. Adjustment of the tap 41a also adjusts the value of the A. V. C. bias applied to the forward coupling tube 3| necessary to procure maximum band width to such an extent as to compensate for the effect of the increased delay bias upon the minimum selectivity.

While the invention is applicable to a wide variety of band-pass selectors having a" wide variety of selectivity characteristics, there follow specifications of a particular band-pass selector having automatically adjustable selectivity characteristics which are particulary satisfactory:

Forward co'uplingtube 3|, type 6K7 pentode. Backward coupling tube 31, type 6C5 triode. Intermediate frequency, 465 kilocycles.

Coupling transformers:

windings 25 and 28, L=1.2 mh. Universal coil of Litz'wire. 1%" wide and ,4." high,

wound on 7 diameter polyiron core. 3

wound on outside of secondary coil and in r.

same direction. (Finish of coil connects to plate of control tube.)

Feed-back winding 38, 4 turns No. 38 wound on outside of primary and in same direction. (Start of coil connects to grid of control tube.)

Resistor Iii, 1 megohm. Resistor ,i5, 50,000 ohms.

Resistor i8, 250,000 ohms.

Resistor 29, 0.125 me'gohm.

Resistor 30, 1.0 megohm. N

No -signal voltage across 33, +6.5 volts. No-signal voltage across 32+33, +9.5 volts. Voltage of tap 41a, 0 to '4 volts.

,.While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those 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 all such changes and modifications as fall within the true spirit and scope of the invention.

" What is claimed is:

1. In a modulated-carrier signal receiver including means for developing a signal-derived control bias, an adjustable bias controlled bandpass selector stage, means for applying said signal-derived bias to said stage to controlthe band 1 width of said selector, and means for limiting the maximum amplitude of said signal-derived bias applied to said stage, thereby to limit the maximum band width of saidselector to a predeter -mined value corresponding to the usual band in one direction, separate coupling means coupling said circuits in the other direction, said directive coupling means being adjustable to vary the mutual impedance between said circuits in the direction of the directive coupling thereby to vary the selectivity of the receiver, means for adjusting said directive coupling means in accordance with the action of said gain control system, and means for limiting the maximum action of said gain control system on said adjusting means thereby to limit the maximum band width of said selector system to a predetermined value corresponding to the usual modulation band width of received signals.

3. In a; modulated-carrier signal receiver including an automatic gain control system, a

band-pass selector system including a pair of circuits resonant at frequencies within said band,

directive coupling means coupling said circuits in one direction, separate coupling means coupling said circuits in the other direction, said directive coupling means being adjustable to vary the mutual impedance between said circuits in the direction of the directive coupling thereby to vary the selectivity of the receiver, means for adjusting said directive coupling means in accordance with the action of said gain control system, means for limiting the maximum action of said gain control, and means, efiective when the action of said gain control system on said adjusting means exceeds a predetermined value, for effecting a fixed adjustment of said directive coupling thereby to limit the maximum band width of said selector system to a. predetermined value correspondingv to the usual modulation band width of received signals.

4. In a modulated-carrier signal receiver ineluding a source of automatic amplification control bias, a band-pass selector system including a pair -of circuits resonant at the mean frequency of said band, directive coupling means coupling said circuits in one direction, separate coupling means coupling said circuits in the other direction, said directive coupling means being adjustable to vary the mutual impedance between said circuits in the direction of the directive coupling thereby to vary the selectivity of the receiver, controlmeans including a control circuit for utilizing said control bias for adjusting said directive coupling means thereby to vary the selectivity of the receiver in accordancetherewith, and means for limiting the bias utilized by said control circuit to a substantially fixed predetermined value when said control bias exceeds said value thereby to limit the maximum band width of said'selector system to a predetermined value corresponding to the usual modulation band width of received signals.

5. In a modulated-carrier signal receiver including a source of automatic amplification control bias, a band-pass selector system including a pair of circuits resonant at the mean frequency of said band, directive coupling means coupling said circuits in one direction, separate coupling means coupling said circuits in the other direction, said directive. coupling means being adjustable to vary the mutual impedance between said circuits in the direction of the directive coupling thereby to vary the selectivity of the receiver, control means including a control circult for utilizing said control bias for adjusting said directive coupling means thereby to vary the selectivity of-the receiver in accordance'therewith, and an auxiliary circuit including an auxiliary source of biasing potential and a. unidirectionally conductive device coupled with said control circuit, said auxiliary source and said control-bias source being efiectively connected in series through said device, with opposite polarity, whereby the bias utilized by said control circuit is limited to that of said auxiliary source when tion, said directive coupling means being ad-' justable to vary the mutual impedance between said circuits in the direction of the directive coupling thereby to vary the selectivity of the receiver, control means including a control circuit for utilizing said control bias for adjusting said directive coupling means thereby-to vary the selectivity of the receiver in accordance therewith, and an auxiliary circuit connected in parallel with 'a portion of said control circuit including said source of control bias, said auxiliary circuit including an auxiliary source of bias potential variable inversely in accordance with said control bias potential, a second auxiliary source of constant-bias potential, and a diode rectifier, "said auxiliary sources being serially-connected to oppose said control source through said rectifier, r whereby the bias utilized by said control circuit is limited to the sum of the potentials of said auxiliary sources when the potential of said control-bias source exceeds the sum of the potentials of said auxiliary sources and the maximum band width of said selector is limited to a predetermined value corresponding to the usual modulation band width of received signals.

'7. In a modulated-carrier signal receiver, a band-pass selector system including a pair of circuits resonant at frequencies within said band, a pair of directive coupling means individually coupling said circuit in forward and backward directions, said coupling means being adjustableto provide a mutual impedance between said circuits which is adjustable to vary the selectivity of the receiver, means for adjusting one of said coupling means, means responsive to adjustment of said one of said coupling means for adjusting the other of said coupling means in an opposite sense, whereby the selectivity of the receiver is dependent on the joint eflect of both of said adjusting means, the response of said last-named means being such that said joint effect reaches a maximum within the limits of adjustment of said one of said coupling means, and means for limiting the adjustment'of said one of said coupling means to a value corresponding to the said maximum joint effect to prevent variation of the selectivity of thereceiver in a reverse sense.

8. In a modulated-carrier signal receiver, a

band-pass selector system including a pair of circuits resonant at frequencies within said band, I a, first directive coupling means coupling said circuits in a forward direction, a second directive coupling means coupling said circuits in a backward coupling means in an oppodte sense, where by the selectivity of the receiver isdependent upon the product of the adjustments of said coupling means, the response, of said backward "coupling adjusting means being such that said product reaches a maximum within the limits of adjustment of said forward coupling means, and means maintaining the adjustment of said .for-

ward coupling means constant at a value corresponding to the maximum value of said product for all adjustments of said forward cmipling adljusting' neans corresponding to values of said product beyond said maximum, to prevent variation in 'the selectivity of the receiver in a reverse sense.

9. In a modulated-carrier signal receiver, a

'band-pass selector system including 'a pair of circuits resonant at the mean frequency of said coupling said circuits in the forward direction, means including a second vacuum tube repeater coupling said circuits in a backward direction,

said repeater tubes having a common cathode 4) circuit including a biasing resistor, means for applying an adjustable bias to said forwardcoua pling tube to vary its transconductance and fthereby indirectly adjusting the bias and trans-- conductance of said backward coupling tube, whereby the selectivity of the receiver varies as the product of said transconductances, the characteristics of said tubes being so related to the constants of their cathode-biasin circuit that said product reaches a maximum within the lim- 50 its of said adjustable bias, and means for limiting the bias applied to said forward coupling.

tube to a preselected value to prevent variation of the selectivity of the receiver for values of said adjustable bias in excess of said preselected 55 value. j

10. In a modulated-carrier signal receiver, a

band-pass selector system including a pair of circuits resonant at frequencies'within said band,

means including a vacuum tube repeater coum pling said circuits in the forward direction,

means including a second vacuum tube repeater coupling said circuits in a backward direction,

band, means including a vacuum tube repeater:

said product reaches a maximum within the limits of said adjustable bias, and means for limiting the bias applied to said forward coupling tube to ,a. preselected valtre corresponding to the maximum value of said product to prevent 5 variation of the selectivity of'the receiver in a reverse sense for values of said adjustable bias in excess of said preselected value.

11. In a modulated-carrier signal receiver. .a band-pass selector system including a pair of circuits resonant at frequencies within said band. means including a vacuum tube repeater coupling said circuits in the forward direction, means including a second vacuum tube repeater coupling-said circuits in a backward direction 15 whereby the selectivity of the receiver varies as the product of the transconductances of said tubes, said repeater tubes havinga common cathode circuit including a biasing resistor, a source of. adjustable bias voltage, means for applying 20 anadjustable bias derived from said source to said forward coupling tube to vary itsspace current and thereby indirectly adjusting the bias and transconductance of said backward coupling ,tube, the characteristics oi'said tubes being so 25 related to the constants of their cathode-biasing circuit that said product reaches a maximum.

within the limits of said adjustable bias, and

a pair of circuits resonant at frequencies within said band, means including a vacuum tube repeater coupling said circuits in the forward direction, means including a second vacuum tube repeater coupling said circuits in a backward direction whereby the-selectivity of the receiver varies as the product of the transconductances of said tubes, said repeater tubes having a common cathode circuit including a biasing resistor, means including a control circuit for applying a control-bias derived from said resistor to said forward coupling tube to vary its transconductance inversely in accordance with received signal inputs and thereby indirectly to.vary the transconductance of said backward tube in an opposite sense, the transconductance characteristics of said tubes being so related to the con- ,stants of their cathode-biasing circuit that said product reaches a maximum for an amplification control bias of a'given value, and an auxiliary.

circuit connected in parallel with a portion of said control circuit including said source of control bias, said auxiliary circuit including an auxiliary source of bias potential variable inversely in accordance with said control-bias potential, a second auxiliary source of constant- 5 bias potential and a diode rectifier, said auxiliary sources being serially-connected to oppose said control source through said diode rectifier,

whereby the bias applied to said forward coupling tube by said control circuit is limitedto the sum of the biases of said auxiliary sources whenv the potential of said control-bias source exceeds the sum of the potentials of said auxiliary sources, thereby to prevent variation in the selectivity of the receiver when said control bias exceeds said given value.

' JOHN F. FARRINGTON.

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