US2140391A - Automatic selectivity control - Google Patents

Description

Dec; 13, 1938. H F, M yER 2,140,391

AUTOMATIC S ELECT I VITY CONTROL Filed Oct. 22, 1936 Inventor:

lg Harry}? Mayer;

His ttor'neg. FHEGIUEALCY Patented Dec. 13, 1938 2,140,391

UNITED STATES PATENT OFFICE AUTOMATIC SELECTIVIT'Y CONTROL Harry F. Mayer, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application October 22, 1936, Serial No. 106,984

10 Claims. (01. 250-20) My invention relates to resonant networks and vice; 2' is the radio frequency amplifier stage, 3 particularly to such networks which are employed is the detector and oscillator stage, 4 is the stage, in radio receiving systems. of intermediate frequency amplification, 5 is the It is well known that when relatively weak second detector and automatic volume control 5 signals are being received by'such systems greatstage, 6 is the'audio amplifier and 1- is the louder selectivity may be obtained by limiting the speaker.

frequencies received to anarrow band and that The detector and oscillator stage 3 is shown when the signals are relatively strong the band coupled to the intermediate frequency amplifier 9 of received frequencies may be broadened out of stage l by the coupling transformer l9 whose 10 thereby improving the fidelity of the'reception primary and secondary windings are tuned to 10:

although at some expense of selectivity. With the intermediate frequency by the adjustable systems as heretofore constructed when the sigcapacitors I I and 12 respectively. Bypass canals are weak and when the band of frequencies pacitors l3 and- Mare shown connecting the lower received is narrowed to effect a higher degree of ends of the windings with'ground in the custom- 15 selectivity of a desired carrier there has been an ary --manner. Conductor l5 supplies negative objectionable loss in fidelity due to a partial cutautomatic volume control voltage to the stages ting oif of the side bands of the desired carrier 2, 3, and-4 which voltage may be obtained in the and an objectionable amount of interference with customary manner from the second detector it signals in closely adjacent channels. Oneobforming apart of stage 5.

ject of my invention, therefore, is to provide'an The intermediate frequency amplifier tube 9 20 improved system of this character by which has the'cathode I8 which is self-biased by the rethose objections may be avoided. More specifisistor 18 through which it connects with ground, cally that object is to provide a systemhaving the resistor being bypassed by the capacitor 20. coupled resonant circuits characterized by a Amplifier has the main control grid 2!, which resonance curve which at the higher signal as shown connects with the upper end of the sec- 25 strengths is more rectangular than that previousondaryof the transformer H], the screen grid 22, ly obtained. Another object of my invention is the suppressor grid 23 and the anode 24'. For the provision of an improved system of this charreasons which will be explained later this ampliacter wherein the band of radio frequencies fier is provided also with the auxiliary grid 25.

passed by the system is relatively wide when the The amplifier 9' iscoupled with the detector I6 30- signal is strong but is automatically narrowed by'the' coupling transformer 21 which like the when the signal becomes weak to a band emcoupling transformer It] has its primary and braced by a resonance curve which is substansecondary windings 28 and 29 tuned to the intertially flat topped and rectangular. mediate frequency by the capacitors 36 and 3! In accordance with my invention I have prorespectively. Like transformer 10 also, the lower 35.

vided a resonant network having means autoends of the windings 28 and 29 are connected matically operative in response to a decrease in with ground by the bypass capacitors 32 and 33 signal strength to decrease the effectiveresistrespectively. The above-described apparatus ance of one of the resonant circuits of the netwith theexception of the auxiliary grid 25 in work and simultaneously to decrease the effective amplifier 9, is like that commonly employed in 40 coupling between two coupled resonant circuits superheterodyne radio receiving systems and thereof and vice versa. functions in a well known manner.

My invention will be better understood from 'I'he'coupling transformer 21 is'shown having the following description taken in connection its primary and secondary windings wound in with the accompanying drawing, and its scope opposite directions on the same axis, the wind- 45 will be pointed out in the appended claims. ings being normally somewhat over-coupled Referring to the drawing, Fig. 1 is a circuit adapting the transformer to pass a relatively diagram showing a preferred form of my invenbroad band of: frequencies, whereby the system tion; Figs. 2, 3 and 4 are circuit diagrams showing has high fidelity at the expense of selectivity.

modifications thereof; Fig. 5 is a vector diagram; The resonance curve of the'transformer thus 50 and Fig. 6 shows resonance curves obtainable unovercoupled is represented at 35 in Fig. 6, which der varying circuit conditions. curve is plotted between units of transmission In Fig. l I have shown my invention as formlevel and frequency. With strong signals when ing a part of a superheterodyne' radio receiving selectivityis of minor importance one may theresystem, wherein l is the antenna or pick-up deby obtain maximum fidelity, as the width of the 55 curve is suflicient to include all the side bands of the desired carrier. With weak signals, however, selectivity becomes of the greater importance.

One manner of increasing the selectivity is to decrease the coupling between the primary and secondary windings of the transformer 21. This may be done in various ways either actually or effectively. I prefer the latter and for that purpose have provided a feed-back circuit comprising the phase shifting network 31 and the amplifier 9. The latter network comprises the capacitor 38 which, for example, may have a capacitance of micro-micro-farads, the resistor 39 which, for example, may have a resistance of 2000 ohms, the impedance of the capacitor being high with respect to that of the resistor at the intermediate frequency employed, and the small reactor 40 whose inductance may, for example, be 250 micro-henries. The network 31 is connected as shown between the upper end of the secondary 29 and a suitable source 4! of negative bias voltage which, for example, may be 4 volts the source being shunted by the by-pass capacitor 42. At a point intermediate the capacitor and the resistor the network is connected by means including the feed-back conductor 43 with the auxiliary grid 25 of the amplifier 9.

The operation of the above-described phase shifting network may be better understood by reference to the vector diagram shown by Fig. 5 where E represents the voltage applied by the secondary 29, E39 the voltage across the resistor due to the current advancing effect of the capacitor and E38 the resulting voltage across the capacitor 38 which is at right angles to E39. The voltage E40 across the reactor, which voltage is 180 displaced in phase from E38, is shown as an extension of E33 in the opposite direction. The vectorial sum of E39 and E40 is Eg, the voltageapplied to the grid 25 by the network. With a reactor having the proper impedance at the frequency employed the angle between E and Eg and hence between E and the anode current Ia of the amplifier 9 may be made exactly 90. Moreover, if desired, it may be made either greater or less than 90 by suitably changing the value of the reactor 40. If should be understood that Fig. 5 does not truly represent the phase angles obtained with the impedance values mentioned above for the actual phase displacement obtained with the capacitor and resistor alone is closer to 90 than that shown. Under certain conditions, therefore, I may dispense with the compensating reactor 40 particularly if the losses resulting from a lack of a full 90 angle of displacement are not serious.

When the received signals are weak the effect of the feed-back voltage supplied by the abovedescribed phase shifting network is a maximum since at that time the automatic volume control voltage being small, the gain in the amplifier is large. The form of the resonance curve will then change from that shown at 35 to that shown at 43 which change is the result of the effective decrease in the coupling between the windings of the transformer 21. The feed-back which I have provided is spoken of as effectively decreasing the coupling only because it has the same effect upon the shape of the resonance curve. as Would be produced by an actual decrease in the coupling.

It is to be understood that the scale of ordihates of curve 43 is much smaller than that of the curve 35, the change in scale being made for the purpose of making a comparison of the curvesmore convenient. The curve 43 while showing a high degree of selectivity also shows by its narrow top that there is a material loss in the side bands received of the desired carrier which results in lack of fidelity and because of its widely flaring lower portions shows that the frequencies of adjacent channels will be received to a considerable extent causing interference therewith.

To avoid these objectionable results I apply to the same auxiliary grid 25 of amplifier 9 another voltage whose effect is to decrease the effective resistance of the primary winding of the transformer 21. Forconvenience and simplicity the letter Q is now commonly employed by those skilled in the art to indicate the reciprocal of the power factor of a winding in a resonant circuit.

Since a decrease in the effective resistance of such a winding obviously is accompanied by a decrease in the power factor thereof, to say that the effective resistance of the winding is' decreased is equivalent to saying that the Q of the winding is increased. For convenience this term Q will be used in the description to follow.

The manner which I prefer and have shown by the drawing of decreasing the effective resistance of the winding 28, namely of increasing the Q of the winding, is by the use of the small coil or tertiary winding 44 comprising, for example, a few turns, which is wound in the same direction as and is coaxial and in close inductive relation with the primary winding. The winding 44 is inserted in the conductor 43, the lower end of the winding connecting with the auxiliary grid 25 whereby it applies to that grid a voltage which is 180 displaced from the voltage of the primary 28; in other words when the voltage of the anode 24 is at a positive half cycle the voltage supplied by the winding 44 is at a negative half cycle.

Were the voltage of the tertiary winding 44 alone to be applied to the auxiliary grid when the gain is high the result of the consequent raising of the Q of the winding 28 would be to give a resonance curve like that shown at 45. Such a curve it will be noticed has comparatively steep sides indicating a small amount of interference with adjacent channels. The curve has, however, a very objectionable dip in its central portion.

I have found that by combining the phase shifting effect of the voltage obtained from the phase shifting network by which the coupling is decreased with the Q raising effect of the voltage obtained from the tertiary winding I am able to obtain a highly selective resonance curve which is sufficiently broad at the top to include those side bands which are necessary for a desired degree of fidelity and whose sides are sufficiently steep to substantially avoid interference with adjacent channels. The curve obtained by this combined effect is that shown at 46, it being understood that this curve as wellas curves 43 and 45 are drawn with their ordinates at different scales from the ordinates of curve 35 thereby reducing the curves to a common height to facilitate their comparison. Since the resulting resonance curve 46 has substantially vertical sides and a substantially fiat top, it may be said to have a substantially rectangular shape.

iary grid 25 accordingly is small. The coupling between the windings of the transformer 21 then increases to normal and the Q of the transformer decreases to normal. The resonance curve consequently returns to the form shown at 35.

The modified form of my invention illustrated by Fig. 2 is similar in construction and in the results obtained to that illustrated by Fig. 1 except that instead of employing a single amplifier to perform both functions of amplifying the intermediate frequency and amplifying the control voltages obtained from the network 31 and the tertiary winding 44, I employ in Fig. 2 separate amplifiers for performing those functions. The amplifier 50 has its cathode connected to ground through a biasing resistor, its control grid 52 connected to the secondary of the coupling transformer Ill, a screen grid 53 and the anode 54 connected to the primary of the coupling transformer 21 in the same manner as the amplifier 9 of Fig. 1. In this case the amplification of the control voltages obtained by the network 31 and the tertiary winding 44 is performed by an additional amplifier represented at 56, which may be a pentode. This amplifier has the cathode 51 which is connected with ground through the self-biasing resistor 58, the control grid 59 which connects with the lower end of the winding 44, the screen grid 60, the suppressor 'grid El and the anode 52 which connects with the upper end of the primary 28 of the transformer 21. The upper end of the winding 44 connects as before with a. point on the phase shifting network 31 between the capacitor and the resistor thereof.

In Fig. 1 it will be remembered that the eifect of the voltages from the network 31 and the winding 44 was controlled by the gain in the amplifier 9. In Fig. 2, however, the efiect of the voltages from the network 3'! and the winding 44 is controlled directly by the negative automatic volume control voltage which is supplied to the network 31 by the conductor 63 connecting through the resistor 64 with the source of automatic volume control voltage. As so constructed and connected this form of my invention operates in a regenerative manner like that represented by Fig. 1. This form may, however, be employed in a degenerative manner if desired. For this purpose the connections of the secondary of transformer 21 should be reversed, the connections of the tertiary winding 44 should be reversed and a source of positive biasing voltage such as an automatic signal control voltage should be applied to the network 31 instead of the negative automatic voltage control voltage.

The modification illustrated by Fig. 3 is similar to that represented by Fig. 2 except that the phase shifting network 31 is connected with the primary of the transformer 21, the tertiary winding 44 is wound in the same direction as and is in close inductive relation with the secondary of the transformer 21 and the amplifier 56 is arranged to amplify the voltage of the secondary winding before supplying it to the tertiary winding and the phase shifting network. The amplifier 56 is represented as a pentode and is similar to the corresponding pentode of Fig. 2 except that its control grid 59 in this case connects with the upper end of the secondary of transformer 21 and its anode 62 connects with the lower end of the tertiary winding 44, the upper end of that winding being connected with the phase shifting network 31. The degree of amplification of the amplifier 56 is controlled by the connection 66 with the source of negative automatic volume control voltage. The network 31 in this case connects between the upper end. of the primary 28 and the source of anode voltage. This form of my invention is regenerative. only and like that shown in Fig. 2 functions to modify the resonance curve by decreasing the coupling and increasing the Q in the same manner as has already been described above in connection with Fig. 1.

The modification illustrated in part by Fig. 4 is similar to that represented by Fig. 2 except that I employ a. single amplifier, shown. as the pentode 68, insteadof the two amplifiers: 50 and 56 and I connect the tertiary winding 44 with the control grid 69 of that amplifier through the secondary of the transformer Ill. The automatic volume control voltage is thus applied'to the control grid 69 through the phase shifting network 31 and the tertiary winding 44. It will, of course, be understood that those'portions of the circuit from the antenna to and including the transformer ID and those portions including the trans;- former 21' to the loudspeaker, while not shown in this figure, are the same as shown in Fig. 2. In this form of my invention the phase shifted voltage from the network 31 and the voltage from the tertiary winding 44 both are applied to the control grid 69 in addition to the signal voltage. With an increase in the signal strength, therefore, the coupling of the transformer 21 is decreased and the Q is increased whereby the form of the resonance curve is modified in the same manner as has already been described in connection with the other forms of my invention.

I have chosen the particular embodiments described above as illustrative of .my inventionv and it will be apparent that various other modifications may be made without departing from the spirit and scope of my invention which modifications I aim to cover by the appended claims.

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

1. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, means for obtaining from one of said circuits a voltage substantially in quadrature with the voltage thereof, means for obtaining from another of said circuits a voltage substantially in phase opposition with the voltage in said other circuit and means for applying said voltages to said other circuit in accordance with the strength of the received signal.

2. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, means for obtaining from one of said circuits a voltage substantially in quadrature with the voltage thereof, means for obtaining from another of said circuits a voltage substantially in phase opposition with the voltage of said other circuit and amplifying means for applying said voltages to said other circuit with greater amplification when the received signals are weak than when they are strong.

3. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, means responsive to the current in one of said circuits for decreasing the effective resistance of said circuit and means responsive to the voltage of another of said circuits for supplying to said one circuit a voltage whose phase is substantially in quadrature with that of said other circuit.

4. In combination, a plurality of coupled circuits tuned to the same frequency and arranged ductively related to one of said circuits for decreasing the eifective resistance thereof and means for supplying to said one circuit a voltage Whose phase is substantially in quadrature with that of another of said circuits.

5. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, means including a reactive circuit connected to one of said coupled circuits for supplying to the other of said coupled circuits a voltage whose phase is substantially in quadrature with that of said one coupled circuit, and means for simultaneously supplying to said other coupled circuit a voltage in accordance with the current therein.

6. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, a grid controlled electron discharge device having its anode circuit connected with one of said coupled circuits, means for supplying to said grid a voltage which is substantially 180 displaced from the voltage of said one circuit and means for supplying also to said grid a voltage which is substantially 90 displaced from that of another of said circuits,

7. In combination, a plurality of coupled circuits tuned to the same frequency and arranged to transmit a signal, a grid controlled electron discharge device having its anode circuit connected with one of said coupled circuits, means connected with another of said circuits for supplying to said grid a voltage substantially in quadrature with that of said other circuit and a coil in series therewith in inductive relation with said one circuit. 7

8. In a radio receiving system havingmeans for producing a desired frequency, a pair of coupled oscillatory circuits tuned to said frequency, a grid controlled electron discharge device having its anode circuit connected with one of said circuits, a winding inductively associated with said one circuit, apparatus including a reactance connected with the other of said circuits for obtaining a voltage substantially in quadrature with the voltage thereof and means connecting said grid, winding and apparatus in a series circuit.

9. In a radio receiving system of the superheterodyne type, an electron discharge amplifier having a main control grid and an auxiliary control grid, a coupling transformer having its primary and secondary windings tuned to the intermediate frequency of said system and having its primary in the anode circuit of said amplifier, and a feed back circuit including means for supplying a voltage to said auxiliary grid to change the Q of said primary and means for simultaneously supplying a voltage to said auxiliary grid to change the effective coupling between said windings, the effect of said voltages being controlled by the amplification of said amplifier.

10. In a radio receiving system of the superheterodyne type, an electron discharge amplifier having a main control grid and an auxiliary control grid, a coupling transformer having its primary and secondary windings tuned to the intermediate frequency of said system and having its primary in the anode circuit of said amplifier, a tertiary winding adjacent said primary wind- 7 ing to produce a voltage for raising the Q of said primary winding, a circuit including a capacitor and a resistor connected in series across said secondary winding to produce a voltage for decreasing the coupling between said primary and secondary windings, means connecting said tertiary winding and said resistor in series to said auxiliary grid and a source of supply of automatic volume control voltage connected with said main control grid.

HARRY F. MAYER.

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