US2189304A - Station selector - Google Patents

Description

Feb, 6; 19460 A TURNER 2,189,304-

STATION SELECTOR Filed Jan. 14, 1938 Jnnentor i vf dttorneg Patented Feb. 6, 1940 UNITED STATES PATENT OFFICE STATION SELECTOR Alfred H. Turner, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 14, 1938, Serial No. 184,913

10 Claims.

the practice to utilize station selecting means similar to that usually employed in broadcast receivers, namely, a tunable circuit comprising an inductance coil of fixed inductance value and a variable condenser connected across it for tuning to' the desired station. A station selecting circuit of this type has several disadvantages when employed in a receiver designed for the high frequencies employed in television transmission.

For example, in such a circuit, because of minimum circuit capacity,-the inductance is necessarily small in order that the tunable circuit will resonate at the high frequency end of the tuning range. This small inductance, in turn, means very low impedance for two such circuits coupled and loaded to pass the desired 6 megacycle band. The net result is a verylow voltage gain from the antenna to the first detector.

Since plate hiss in the first detector, if it is not exceeded by outside interference, is the limiting disturbance in the video channel of a television receiver, any improvement in gain up to the first detector results in a proportionate improvement in signal-to-hiss ratio.

It is accordingly an object of my invention to provide an improved television receiver.

' of a superheterodyne receiver.

A further object of my invention is to improve the signahto-hiss ratio in a television receiver.

A still further object of my invention is to provide an improved station selector or tuning means for high frequencyvradio receivers such as television receivers. p v

A still further object of my invention is to provide an improved variable inductance tuning device for short wave radio receivers.

In the preferred embodiment of my invention an inductance tuning device is provided with a plurality of tapped inductance coils for the radio frequency selecting circuit and for the oscillator The coils are mounted coaxially on a gang switch of the type having a plurality of wafers carrying contact points, one coil being located adjacent to each wafer whereby very short leads may be run from the coil taps to the switch contact points.

The tuning device preferably comprises two tuned circuits which are so capacitively coupled as to provide a band pass filter. By employing capacity coupling and tapped inductance coils the fi fi ade '60 h rea ass ba w i remains substantially constant over the entire tuning range of the receiver.

The impedance of the receiver input is made to match theimpedance of the antenna transmis- 'sion line over the entire tuning range of the receiver by properly locating the transmission line 7 coupling coil with respect to the tapped coil of the first tuned circuit whereby the inductance of the coupling coil is reduced in value as the receiver is tuned to higher frequencies.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Figure l is a circuit diagram of a portion of a television receiver embodying my invention,

Figure 2 is a plan view of the station selecting unit drawn approximately to full scale, which is utilized'in the receiver shown in Fig. 1,

Figure3 is a view on the line 3-3 in Fig. 2

loo-king in the direction of the arrows,

Figure 4 is a schematic diagram which is referred to in explaining the invention,

Figure 5 is a view illustrating a modification of my invention, and

Figure 6 is a circuit diagram of another embodiment of my invention.

Referring to Fig. l, the receiver comprises a pair of coupled tuned circuits 2 and 3 which function as a bandpass filter for coupling the antenna,

such as a dipole 4, to the first detector tube 6 of the receiver which preferably is of the superhet'erodyne type.

The tuned circuit 2 comprises a tapped inductance coil 1 and a fixed condenser 8 which is effectively in parallel therewith.

The tuned circuit 3 comprises a tapped inductance coil 9 which has effectively in parallel therewith theinput capacity of the first detector tube 6, this capacity being indicated at H.

The circuits 2 and 3 are coupled through a condenser H which, is common to the two tuned circuits, this being substantially the only coupling between the circuits. This series capacitive coupling is preferred, although parallel capacitive coupling'may be used by connecting a very small condenser between the upper ends of coils l and 9 as will be described in connection with Fig. 6.

It will be seen that the tuned primary circuit 2 may be traced from ground through coupling condenser l2; through primary coil 1 and the condenser 8 back to ground. The tuned secondary circuit 3 may be traced from ground through coupling condenser l2, through the secondary coil 9 and the tube input capacity I I back to ground.

tuned approximately to the same frequency and the coupling between circuits is of the correct value to give the two coupled circuits a bandpass characteristic.

The effective inductance'of each of the coils 1 and 9 may be varied by shorting a certain number of turns in each coil. To accomplish this, coil 2 is tapped at a plurality of points and connections made to switch contact points l3 through short leads IS. A switch arm l4 makes a coil shorting connection between the lower end of the coil 1 and selected contact points l3. While it is preferred to short the unused coil turns, this is not necessary in all cases.

Similarly, the coil 9 is tapped at a plurality of points which are connected to switch contact points [6. A switch arm I1 which-is connected to the lower end of the coil 9 shorts a. certain number of turns, the number depending upo the position of the switch arm.

It may be noted at this point that it would be expected that capacity coupling between two inductively tuned circuits would give. a constant percentage pass band width over the tuning range, the actual band width varying over the tuning range. For equal width of television channels it is very desirable that the band width of the selecting circuit be substantially the same at all points in the tuning range. In my selecting circuit this desirable result is obtained since the combination of capacity coupling and tapped inductance coils results in a variation of distributed capacity across the coils as the circuit is tuned which tends to keep the band width constant. This feature will be discussed in .more detail hereinafter.

The antenna 4 is coupled to the coil I through a coupling coil IS, the coils 1 and IBbeing inductively coupled. Preferably, the antenna is connected to coil l8 through a transmission line I9, the mid-point of coil l8 7 being connected to ground.

As will be explained at another point inthis specification, the coupling coil 18 is. so located on the coil l with respect to the turns which are shorted by the switch N that the receiver input impedance (the impedance at the terminals of coil l8) remains substantially constant over the entire tuning range of the receiver. Because of this, it is not necessary to provide any taps and associated switch for the coil Hi to keep the receiver input impedance matched to the transmission line impedance.

For the purpose of making the tuned circuits 2 and 3 symmetrical, a coil 2| is wound adjacent to the coil 9 and its center point is grounded.

' a screen grid 24, a suppressor grid 26 and a plate 21. The control grid 23 may be biasedslightly negative by means of a self-bias resistor 28 which is by-passed by a condenser 29. The usual bypass condenser 3| is provided for the screen grid 24. The bias is applied to the grid 23 through aresistor 20 connected between ground and the tap We of the coil 1. Resistor 20 also provides the necessary loading for circuit 2.

The tunable oscillator 30 of the superheterodyne receiver includes a vacuum tube'32' which may be of the three-element type having an indirectly heated cathode 33, a control grid 34 and a plate 33. This oscillator produces oscillations 10 megato the ends of the arm 6|.

cycles higher than the carrier frequency of the incoming signal.

In the specific example illustrated, the osci1- later 30 has a tunable plate circuit comprising a tapped inductance coil 31 which is coupled to a coil 38 in the grid circuit. In the grid circuit there preferably is the usual grid condenser 39 and grid leak resistor .7 ,A positive operating voltage is supplied to the plate 36 through a filter resistor .2 and the coil 31.

The tuning capacity across the coil 31 is provided by the output capacity of the tube 32 indicated at 43 and by the trimmer condenser 44 in parallel with capacity 43. The capacities 43 and M are connected across the coil 31 through a condenser 46.

The tuning of the oscillator is accomplished (except for fine adjustments by condenser 44) by means of a switch 41 which makes contact with one of the switch contact points 48 to short out sections of the coil 31.

The coupling of the oscillator 30 to the first detector 6 may be through a coupling condenser 49. p

The short-circuiting switches I4, I! and 41 for the tuned circuits 2 and 3 and for the oscillator, respectively, are ganged whereby the receiver may be tunedby a single knob.

- In Fig. 2 there is shown the mechanical structure by means of which I obtain satisfactory operation with the above-described circuit at the high frequencies involved in television transmission and reception. In Figs. 1 and 2 like parts are indicated by the same reference numerals.

Referring to Fig. 2, the several inductance coils are mounted coaxially with a gang switch of well-known construction. The switch comprises a plate which is suitably attached, as by a press fit, to a bushing 52. The bushing 52 is threaded on the .outside and provided with a nut 53 and washer-55 for'mounting the switch on an upwardly projecting flange 56; of a metal base 51.

A hollow tube shaft 58 having a shoulder 59 extends through the bushing 52, this tube shaft having mounted thereon the switch arms I 4, I! and 41. A knob 5|] is attached to the end of shaft 58 for rotating it and the switch arms.

In order to hold the shaft 58. snugly in the bushing 52 and in order to make the switch arms snap into engagement with successive contact points as the knob 50, is rotated, an arm 6| is welded or otherwise attached to the shaft 58 and a ring 62 of spring steel or the like is attached .Two diametrically opposed balls 63 are held between the spring ring 62 and the plate 5| whereby the shoulder 59 is held againstthe bushing 52.

The plate 5i has formed thereon in a circle a plurality of raised portions 54, one for each switch contact point. As the knob 50 is rotated, the calls 83' ride up over two diametrically opposed projections 54 and then snap into the next switch position.

The three switches l3-'l4, l6|1 and 4841 are of similar construction. This construction will be understood from the following description of switch l3|4 with reference to Figs. 2 and 3. Y

The switch contact points l3 are attached to a wafer 65 of insulating material, this wafer being supported by rods 61 and 68 which are bolted to the plate 5|.

' The switch arm I4 is attached to a disk 69 of insulating material such as fibre. The disc 69 is forced onto a flattened section of the shaft 58 whereby. the disc 69 and arm I! are rotated'when shaft 58 is rotated. The switch arm I 4 is electrically connected to or is an integral part of a ring H whereby there is always electrical contact between the arm l4 and a brush 12 riding on the ring.

In a similar way, the switch arms I! and 41 are always in electrical contact with the brushes connected to terminals 13 and 14, respectively.

Each of the coils 1, 8 and 31 preferably is wound on a grooved or threaded fibre tube. Each coil is supported by means of the heavy leads connecting the tapped points on the coil to the switch contact points. For example, the coil 1 is supported by the leads l5.

. The trimmer condenser 44 comprises a fixed plate 44a supported by the fibre tube 31a upon which coil 31 is wound and a rotatable plate 442) attached to a shaft 16. The shaft I6 extends through the hollow shaft 58 and may be rotated bymeans of a knob 11 for adjusting condenser 44.vv The windings I8, 2| and 38 are of fine wire wound between the turns of heavier wire forming the coils 1, 9 and 31, respectively, as shown in Fig. 2. The location of the coil l8 with respect to the coil 1 is important for best results. By properly locating the coil l8, its inductance is made to decrease as the receiver is tuned to higher frequencies (that is, as an increasing number of coil turns are shorted) whereby the receiver input impedance is matched to the transmission line impedance for all station settings. It will be noted that the turns ratio of coils l8 and 1 changes as the receiver station is tuned and that the ob- VlOllS way to keep the receiver and transmission line impedances matched is to switch out part of the coil l8 simultaneously with switching of the coil 1 to keep the said turns ratio constant. This would. complicate the switching, especially since the transmission line should be kept balanced with respect to ground. 1

I have found that by locating the coil l8 (which in the example illustrated is only one and onethird turns) adjacent to the last tap on. coil 1 with part of coil 18 inside the short-circuited turns and part of it outside the short-circuited turns when switch M is on the last tap, the impedance of coil I8 is controlled in the desired manner as the receiver is tuned. By the last tap is meant the last tap at the high frequency end of coil 1, that is, the tap indicated at |5a nearest the condenser 8. It will be notedthat a small number of taps has been indicatedv in Fig. 1 in order to simplify the drawing, tap l5a being the eleventh tap in the actual structure as indicated in Fig. 4.

With this arrangement, as more and more turns of coil 1 are shorted, the shorted turns react on coil l8 to reduce its inductance an increasing amount. It has been found that the preferred relation of coils is to have the mid-tap l8ai of coil l8 located near the last tap |5a of coil I as shown in Fig. 2.

It will be noted that the relation of coil 2| to coil 9 is the same as the relation of coil l8 to coil 1 whereby the tuned circuits 2 and 3 are balanced or symmetrical.

In the oscillator, the grid coil 38, which has two turns, should be so located with respect to the coil 31 that its inductance is substantially reduced by the shorted turns of coil 31 as the receiver is tuned to the higher frequencies. This action in the oscillator circuit is necessary for best operation since, at the highest frequency setting, the grid coil 38 has more effective turns than the plate coil 31 and the grid circuit will resonate to control the frequency unless the grid circuit inductance is substantially reduced.

As shown in Fig. 2, the coil 38 is located substantially within the short-circuited turns of coil 31 when the receiver is tuned to the highest frequency. To accentuate the reduction in the inductance of coil 38 by the short-circuited turns, the two turns of coil 38 are spaced apart by one unused turn space.

As previously mentioned, my station selector is so designed that the pass band stays substantially constant over the entire tuning range of the receiver. By utilizing the combination of capacity coupling and tapped inductance coils, I cause the percentage coupling to be reduced as the receiver is tuned to the higher frequencies whereby the width of the pass band of the two coupled circuits remains substantially constant. As a matter of fact, in some applications the band Width will actually decrease as the receiver is tuned to higher frequencies.

The reasonthe width of the pass band remains as constant as is desired may be explained as follows: The percentage coupling between circuits 2 and 3 (where the tuning capacities are equal) is the ratio of the tuning capacity of a coil to the capacity of coupling condenser I2. Considering coil 1, the tuning capacity is the capacity of condenser 8 plus the distributed capacity of the effective portion of the coil. As switch arm 14 is moved from a lower tap towards the last tap l5a to tune the receiver to higher frequencies, the distributed capacity across the unshorted section of coil 1 decreases. Thus, the tuning capacity decreases and the percentage coupling decreases, whereby the usual large increase in pass band width is avoided.

The resistor 20. is so connected that it also aids in keeping the pass band width approximately constant, although the distributed capac ity is the main factor in obtaining this result. It will be noted that resistor 20 is connected between ground and a point on the coil '1 intermedate its ends whereby it is in shunt to some of the inductance of the tuned primary circuit 2 when the lower part of the coil is not shorted but not in shunt to the tuning inductance when the switch arm 14 is on the last tap l5a, i. e., when the selector is tuned to the highest frequency. In this connection, it should be kept in mind that the coupling condenser I! has a large capacity compared with the capacity of tuning condenser 8.

In some designs, it may be preferred to connect resistor 28 between ground and a point on the secondary coil 9. Whether the resistor is connected across the primary or the secondary, it may be preferred to connect it between ground and a coil tap other than the one indicated on the drawing.

In order to make my station selector operate at the greatest efficiency for the very high. frequencies involved in television transmission without resorting to coils of variable Winding pitch and /or of variable coil diameter, it is so designed that the station settings are scrambled on the tuning dial. That is, if the receiver is tuned to 44 megacycles at one switch position, when the tuning knob is rotated to the next switch position the receiver is tuned, not to the next 6 megacycle channel of 50 megacycles, but to 10 megacycles, for instance. At the next switch position the receiver is tuned to 66 megacycles, etc.

The reason for this will be apparent from an inspection of Fig. 4, this being a schematic diagr met the coil land contact points 13 looking from the back side of the coil l towards the tuning knob. The figures adjacent to the contact points it show the frequency in megacycle's to which'the receiver is tuned. As shown in Figs. 2 and 4, the leads 1% are brought from the taps on the coil l to the nearest switch contact point to keep the leads short. The tap for the 50- megacycle band is three switch contact points away from the tap for 44 megacycles. The contact points on each-side of the one opposite the i l megacycle tap are opposite the taps for '78 megacycles and 102 megacycles. Thus, the teleision bands are not tuned in regular sequence but this fact is of'small importance compared with the advantage of avoiding any long leads between the switch contact points and the taps on the coils.

It may be noted that in the specific example described, the receiver is adjusted to be tuned for reception of bothvideo side bands and both audio side bands, the megacycle markings in Fig. 4 representing the carrier wave irequenciesoi the incoming audio or sound signals accompanying the picture.

As shown in 5, the t immer condenser 44 .iay'be omitted and replaced by a magnetic core iii for tuning the oscillator coil 31. Rotation of the hoop l'i (Fig.2) causes the core M to move in or out to give a fine tuning of the oscillator. core iii may be of magnetite manufactured in accordance with the teachings of application Serial No. 56,993, filed December 31, 1935, in the name of R. L. Harvey and assigned to the Radio Corporation of America.

may be noted that my selector may be made tunable over the entire frequency range of from 44: megacycles to 108 megacycles for the reception of audio signals by making the width of the radio frequency pass band at each tap or switch pc tion equal to the frequency between tap positions, and by makingthe oscillator tuning range on each tap cover this pass band. The oscillator tuning range may easily be designed to cover the entire pass band as the trimmer condenser 44 rotated or, in the case of Fig. 5, as the magnetite plug "cl is' moved with respect to the oscil- 5'0' later coil.

It will be understood that my variable band pass filter may be utilized for coupling two amplifier tubes or for coupling an amplifier tube and a detector as well as for coupling an antenna to the input tube of the receiver. 7

denser i2 (Fig. 1). t are indicated by the same reference numerals.

In Fig. 1, the values of certain elements have been indicated in micromicrofarads and in ohms' by ways of example. The values used for dif-'- ferent units obviously depend upon the type tubes employed, size of coils, etc.

shown in Fig. 6, the circuits 2 and 3 may be coupled by a condenser 90, which provides parallel capacitive coupling, instead of by the con- In Figs. 1 and 6 likeparts .lected one of a plurality of transmitting stations, said device comprising a tuned primary circuit and aturid secondary circuit,'said circuits being coupled by means of a coupling condenser common to said circuits; eachof said circuits including a tapped inductance coil, said circuits being so tuned and coupled-as to form a band pass filter having a pass range which is narrow enough to provide selectivity between adjacent frequency bands utilized by said transmitting stations, and means means associated with the taps on said coils for varying their inductance simultaneously in such manner that said pass range may be shifted to receive signals from a selected one of said stations, said inductance coils having distributed capacity of such value thatsaid pass range remains approximately constant as said selector device is tuned over "its tuning range.

2. A station selector device for high frequency receivers designed to receive signals from a selected one of' a plurality of' transmitting stations, said device comprising a tuned primary circuit and a tuned secondary circuit, said circuits being coupled'by means of a coupling condenser common to'said circuits, each of said circuits includ- 3 ing a tapped inductance coil, said circuits being so tuned and coupled as to form a'band pass filter having a pass range which is narrow enough to provide selectivity between adjacent frequency bands utilized by said transmitting stations, means including switching means associated with the taps on said coils for varying their inductancesimultaneously in such manner that said pass range may be shifted to receive signals from a selected one of said stations, and a resistor connected effectively across the tapped section of one of said coils whereby the damping effect of said resistor decreases as said selector device is tuned me higher frequency.

3. A station selectingd'evice for high frequency receivers designed to receivesignals from a selected one of a pluralityof transmitting stations, said device comprising a primary circuit including'an inductance coil, a tuning capacity and a coupling condenser all connected in series, a

secondarycircuit including an inductance coil,

a tuning capacity and said coupling condenser all'connected in series, said circuits being so tuned and coupledas to form a band pass filter having a pass band narrow enough to provide selectivitybetween adjacent frequency bands utilized by said transmitting sections, each of said coils having 'a plurality of tapped points and switching means for connecting to said points, and means for simultaneously varying the inductance of said coils to shift said pass band by actuating said switching means.

{L A station selector device for high frequency receivers for the reception of signals from a selected one of a plurality of transmitting stations, said device comprising a tuned primary circuit including an inductance coil and a tuned secondary circuitincludi'ng an inductance 0011, said circuitsbein'g coupled by means'of a coupling condenser common .to said circuits,'said circuits being so tuned and coupled as to form a-band pass filter having a pass range which is narrow enough to give selectivity between adjacent frequency bands utilized by said stations,. and switching means for simultaneously shorting a portion of each of said coils and thereby varying the inductance ofIsaid circuits in such manner that said pass range'is shifted to receive signals from a selected one of said stations, said inductance coils having distributed capacity of such value that said pass range. remains approximately C011:

including switching stant as said selector device is tuned over its tuning range.

5. A station selector device for high frequency receivers, said device comprising a gang switch of the type having a rotatable switch shaft, switch arms mounted on said shaft and rotatable therewith, said switch arms being spaced longitudinally alon said shaft, a plurality of disc or plate elements each having a plurality of switch contact points mounted thereon and insulated from each other for cooperating with said switch arms, each of said disc elements being mounted adjacent to one of said switch arms, terminals for said contact points mounted on each of said disc elements and arranged substantially in a circle, a plurality of inductance coils supported in coaxial relation to said gang switch, each of said coils being positioned adjacent to the terminals on one of said disc elements, each coil having tapped points thereon connected to adjacent terminals, and capacity means connected across said coils whereby a plurality of tunable circuits are formed which may be tuned to different frequencies by rotating said switch shaft.

6. A station selector device for high frequency receivers, said device comprising a gang switch of the type having a rotatable switch shaft, switch arms mounted on said shaft and rotatable therewith, said switch arms being spaced longitudinally along said shaft, a plurality of disc or plate elements each having a plurality of switch contact points mounted thereon and insulated from each other for cooperating with said switch arms, each of said disc elements being mounted adjacent to one of said switch arms, terminals for said contact points mounted on each of said disc elements and arranged substantially in a circle of a certain diameter, and a plurality of inductance coils supported in coaxial relation to said gang switch, each of said coils having'an external diameter less than said certain diameter and being positioned adjacent to the terminals on one of said disc elements, each coil having tapped points thereon, each of which is connected to the nearest one of said terminals by a short lead.

7. The invention according to claim 6 characterized in that said leads are sufficiently rigid to act as supports for said coils.

8. A station selector for a high frequency superheterodyne receiver, said selector comprising a gang switch having a rotatable switch shaft and having at least three switches spaced longitudinally along said axis, each of said switches comprising a switch arm atached to said shaft and a.

plurality of contact points mounted in a circle substantially in the plane of said arm and in cooperating relation therewith, at least three inductance coils, each of said coils being mounted adjacent to one of said switches and in coaxial relation thereto, a tuned primary circuit including one of said coils, a tuned secondary circuit including another of said coils, a coupling condenser common to said primary and secondary circuits, said circuits being so coupled and tuned as to form a band pass filter having a certain pass range, said one coil and said other coil having tapped points each connected to the nearest switch contact point by a short lead whereby said pass range may be shifted by rotating said switch shaft, and oscillator having a tuned circuit including a third one of said coils, said third coil having tapped points each connected to the nearest contact point of the adjacent switch whereby the oscillator frequency is changed simultaneously with the change in said pass range as said shaft is rotated.

9. The invention according to claim 8 characterized in that said oscillator comprises a vacuum tube having a plate circuit and a grid circuit, said oscillator tuned circuit being included in said plate circuit, a grid coil in said grid circuit, said grid coil being inductively coupled to said oscillator tuned circuit and being so located that it is located within the short-circuited turns of said third coil when said selector is tuned to the highest frequency in its tuning range.

10. A station selector device for high frequency receivers for receiving signals from a selected one of a plurality of transmitting stations, said device comprising a tuned primary circuit and a tuned secondary circuit, said circuits being coupled by means of a condenser common thereto, each of said circuits including a tapped inducttance coil, said circuits being so tuned and coupled as to form a band pass filter having a pass range which is narrow enough to provide selectivity between adjacent frequency bands utilized by said stations, switching means including switch contact points connected to said tapped points for shorting out portions of said coils whereby said pass range may be changed, a coupling coil for coupling a transmission line to'said primary coil, said coupling coil being located partly within and partly outside the short circuited turns of said primary coil when said device is tuned to the highest frequency in its tuning range.

ALFRED H. TURNER.

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