US2312211A - Tuning system - Google Patents

Description

INCREASING,-

Feb. 23, 1943. I R. DE COLA 2,312,211

TUNING SYSTEM Filed Sept. 3, 1940 Patented Feb. 23, 1943 TUNING SYSTEM Rinaldo De Cola, Chicago, mont Radio Corporation,

ration of Illinois 111., assignor to Bel- Chicago, 11]., a corpo- Application September 3, 1940, Serial No. 355,120

1 Claim.

My invention relates in general to a tuning system for a high frequency circuit to maintain a relatively constant wide-band over a relatively large frequency range, and more in particular to such a system for a television essentially constant band widths, a flat response, and high gain over the entiretuning range of the receiver.

Automatic, or push-button tuning as it is variously called, is particularly useful in television because of the relatively few, and sometimes spaced apart channel for broadcasting, spread over a considerable range of frequencies. With the ordinary manual tuning over such a relatively large tuning range, considerable time is required to cover this range, an unskilled operator often has diificulty in obtaining optimum tuning, and in addition, manual tuning necessitates remembering the various frequencies for each station. As is well known, an automatic tuning system substantially eliminates all of these difficulties in selecting the desired station and this has been recognized in television receivers. However, prior to the present invention, television tuning has generally been accomplished by means of individual pretuned circuits for each frequency, with switching mechanism to switch or tune from one pretuned circuit to another. These systems are bulky-and costly, and are inefficient due to losses incidental to the switching mechanism and the associated pretuned circuits. Leads between the switch and the first vacuum tube in the receiver circuit, and in the switch mechanism itself, all combine to add considerable stray capacitance and inductance to the tuning system. This reduces' gain, and results in extremely unequal signal response at various frequencies over the entire tuning range. In fact, because of stray capacitance and inductance resulting from the above-noted conditions in the prior systems, it is diflicult to obtain completely effective tuning under any circumstances, at the higher frequencies.

More particularly, in television preselector or tuning systems it is desirable to build up a maximum circuit impedance at the terminals of the first vacuum tube in the receiver circuit. For example, in a half-wave dipole, having a characteristic resistance of approximately 77 ohms, it is desirable that the matching transformer working into the first vacuum tube amplifier has as high a step-up ratio as possible. This is desirable because the resulting gain from the antenna to the grid of the first tube will be directly determined by the square root of the ratio of the grid circuit impedance to the antenna impedance, and of course, the higher this ratio becom s, the higher the gain becomes. However, the difliculties described with reference to the prior systems definitely diminish this circuit impedance below the receiver to provide desirable maximum, and hence reduce the gain proportionately.

An object of my invention is to provide an improved tuning system for high frequency signal circuits.

A further object is to provide an improved tuning system for television circuits.

It is also an object of my invention to provide efficient continuous tuning in a single circuit to predetermined frequencies over the entire tuning range of a television receiver, in contrast to tuning over such range through a plurality of pretuned circuits which require extensive adjustment in Order to change the pretuned frequency setting.

Another object of my invention is to provide a tuning or preselector system for a television circuit in which stray capacitances and inductances are reduced to a minimum, to in turn provide a maximum circuit impedance and high gain.

Another object is to provide a radio frequency tuning system which maintains a substantially uniform wide band at each tuning frequency over an entire tuning range.

A still further object of my invention is to provide a tuning system for a television circuit effecting a flat response at each signal frequency over the entire tuning range of the circuit, and hence high fidelity.

One of the features of my invention is the provision of a tuning system and circuit for a signal receiving circuit which can be compressed into a relatively small space, with physical components of such type and size that they are readily adapted to various mounting positions so that they may all be placed relatively close to the tube to which they are electrically connected, thus reducing the lead lengths to a minimum and consequently minimizing stray capacitances and inductances in the leads in the system, to thereby provide maximum circuit impedance at the first tube for that particular circuit.

Another feature of my invention is the provision of variable inductances for changing frequency in a television receiver circuit, and coupling means for the inductances which provides a substantially uniform wide band over the tuning range of the receiver.

A still further feature of my invention is the provision of a permeability tuning system for television receivers for improving the sensitivity and the selectivity of the receiver over the entire tuning range thereof.

Other objects, features, and advantages of the invention will be apparent from the following description taken with the drawing in which:

Fig. 1 is a circuit diagram of the tuning portion circuit embodying the in of a signal receiving vention;

Fig. 2 is a graph illustrating certain of the of the circuit of Fig. 1;

operating characteristics tion of my invention Fig. 3 is a fragmentary plan view in actual size of a chassis showing the physical components of the tuning circuit including tube sockets for the.

tenna, and the secondary circuit thereof connected to the first tube in the receiver circuit and being tuned substantially by the capacity of such tube. In addition to the inductive coupling, capacitance in the form of a condenser is inserted between the two circuits to couple the same capacitively, with such capacitive coupling providing a substantially constant band width over the entire tuning range of the frequency. The physical components of the variable inductances for tuning these coupled circuits are such that they may be conveniently grouped around the tube to which the coupling system is connected to thus reduce the physical space required for installation, and reduce the lead lengths between the various components of the system so as to reduce undesirable stray inductance and capacitance to a minimum.

Referring to Fig. 1, the reference character In denotes a half-wave doublet antenna system including a transmission line comprising a twisted pair of appropriate impedance for properly matching the antenna to the input circuit of the receiver. The transmission line of the antenna system is connected to the input at H and i2 and is properly adjusted with reference to the input circuit by means of capacitors l3, I4, I6, and IT. This input circuit including the plurality of capacitors is commonly known as a balanced input system and operates very satisfactorily with a doublet antenna system. However, the operais not limited to this particular input system alone, and other suitable antenna terminations may be employed. Generally, capacitors I3 and I1 are smaller in value than capacitors l4 and I6. Either one of the capacitors l3 and I! can be made adjustable to provide a tuning means for the primary circuit of the preselector system of my invention.

Included in the primary circuit of the tuning, coupling or signal translating system is a variable inductance unit [8 which is preferably of the slidable iron-core ity tuned. Inductively coupled to the variable inductance I8 is a similar variable inductance unit IS in the secondary circuit, and capacitively coupled to inductance I9 is an iron-core variable inductance unit 20. The latter inductance is in the oscillator tube 1. The three permeability tuned variable inductance-units l8, l9 and 20 are adjusted simultaneously by a coupling bar 2|, operatively connected with the slidable iron cores therein as shown in Fig. 3, to tune a receiver circuit over the entire tuning range or frequency interval thereof. The bar 2|, in turn is suitably connected to a suitable push button tuning device (not shown) of either the manually operated type or electrically operated type, with a plurality of selector units in the device acting to selectively move the cores of-the coil units to any one of a plurality of predetermined station settings. The bar 2| may likewise beconnected into a manually operated type and hence permeabilcircuit as is evident from Fig.

ating range for structure for moving the cores over the entire tuning range of the receiver, as might be indicated on an appropriate dial mechanism. With the inductance coil units l8 and I9 permeability tuned, the coupling factor M varies with the frequency to which the circuits therefor are tuned. At the lowest frequency in the operthe circuit, when the cores are completely within the coils for the units, the coupling factor M is a maximum. However, at the highest frequency in the operating range, when the cores are practically out of the field of the coils, the coupling factor M between the two units is considerably reduced. This relationship is shown in an approximate manner in the full line curve in Fig. 2 with zero (0) representing the low frequency end of the range, and infinity representing the high end. The general shape and slope of the curve is determined by the proximity of the coils, with the slope being greatest When the coils are well removed or displaced from one another as can be understood from a consideration of Fig. 3, and the slope is a minimum when the coils are spaced closely together. T These conditions are generally known and are conditions which the present invention overcomes.

This inductive coupling between the units i8 and I9, if used alone would, when properly adjusted for the low frequency stations in the operating range, provide too narrow a band-pass at the higher frequencies. In order to provide an essentially constant band-width over the entire tuning range, a small capacitance 23 is introduced between the coils in the units l8 and i9 and in series connection with each as indicated in Fig. 1. This may be a small fixed condenser which acts upon the band width in a manner complementary to that provided by the mutual inductive coupling between the coils i8 and I9.

With the mutual coupling factor M, and the value of capacitance 23 properly adjusted, essentially constant band-width is provided over the entire tuning range of the system as shown by the straight line dotted curve of Fig. 2. This is in contrast to the variable band-width as illustrated in full line in Fig. 2, and as previously described. Since the eflects of the capacitance 23 (dotted line curve in Fig. 2) and the mutual coupling factor M are additive, the mutual inductive coupling between the coils must be so phased as to have a negative sign.

In arriving at the most desirable values for M and the capacitance 23, the coil units i8 and I! are positioned on a chassis 24 with reference to one another, as shown in Fig. 3, so that the mutual inductive coupling M between them is sufiicient for a desired band-width at the low frequency end of the tuning range. With this coupling satisfied, the capacity.23 is introduced and provided in such a value that the same desired band-width is obtained at the high frequency end of the tuning range. With coupling factor M satisfactory at the low frequency end, and the capacity coupling through the condenser 23 added to the inductive coupling M providing a satisfactory band-width at the high frequency end, it is known that the band width is substantially uniform over the entire tuning range.

In order to have the most eflicient tuning over the entire operating range of the receiver, it is important that the impedance ratio between the antenna terminals ii and i2 and the grid G of the first tube T in the tuning circuit be at a maximum. The tube T which is the first tube in the preselector system, or tuning circuit, will have different characteristics depending upon the particular receiver circuit employed, and whether it be for radio or television. For purposes of illustration for the present invention, the receiver circuit may be considered to be a superhet'erodyne, and the tube T a mixer tube. It may also be variously known as a converter tube. The socket TS for this tube mounted on the chassis 24 is shown in Fig. 3. An oscillator tube T is connected to the inductance coil unit 20 and is considered to be in the tuning system for the receiver. The socket for this tube on the chassis 24 is indicated by the reference character TS' in Fig. 3.

As previously mentioned, in the prior preselector or tuning systems, a bulky switching mechanism for pretuned circuits prevented a compact flexible assembly for the physical elements of the tuning system. Consequently, the leads to' the tube or tubes in the circuit and to the various elements therein, as well as the leads within the switching mechanism itself, were all generally so long as to introduce stray capacitance and inductance, and reduce the impedance of the circuit, and consequently the gain thereof.

As is apparent from the bottom 'planrview of the chassis 24, in actual size, the inductance coil units I8, I9, and 2| are grouped closely around the base of the tube sockets TS, andT'S'. The coil units as well as the other physical components, which will be described in detail, adapt themselves to a mounting that makes possible short wire leads between the sockets and these various elements. As a result the undesirable stray inductance and capacitance is reduced to a minimum and the impedance to the grid of the first tube is a maximum thereby providing the maximum gain in the tuning circuit.

A typical example of an installation of my invention includes condensers I3 and I I of 42.4 micromicro-farads each, and condensers I4 and I6 of IOO-micromicro-farads each. The inductances I8, I9 and 20, are all .21 microhenry each, and the condenser 23 is 2.7 micromicro-farads. The e'ifective capacitance of tube T, represented by the character CT is 15 micromicro-farads. The spacing between inductance units I8 and I9, determining the mutual inductance M in this example, is approximately /16 inch, as can be determined from the full scale drawing of Fig. 3.

As to the remaining elements illustrated in Fig. 3, these include a small adjustable capacitor 25 which allows the secondary circuit including the coil unit I9 to be properly tuned. A condenser, or capacitor 26 permits the adjustment of the oscillator frequency of the tube T. A fixed condenser 21 is incidental to the oscillator circuit itself. Reference characters 28 and 29 indicate the grid condenser, and grid leak respectively for the tube T. A radio frequency choke 3I is connected to the plate of the tube T to prevent short circuiting of the capacitor 21 at radio. frequency. The condenser 32 acts to impress some of the oscillator radio frequency voltage upon the control grid of the tube T in order to provide proper mixing of the signal voltage with the oscillator voltage to produce the intermediate frequency when the superheterodyne receiver is used. The condenser 33, and resistor 34, provide filtering for the application of AVG voltage to the tube T. The resistor 36 acts to stabilize the input impedance of the tube T. A condenser 31, and resistance 38 provide the necessary filtering and resistance for the application of bias voltage to the tube T.

The condenser symbol indicated by the reference character CT is symbolical of the capacitance of the vacuum tube T itself, together with the incidental capacitance of the tube socket and associated elements on the socket itself. With the undesirable capacitance reduced in the timing circuit as the result of the shortened leads and more efficient transmission of energy, as previously explained, the impedance in the grid circuit is increased to a maximum. Where the prior art systems, as previously described, required loading in the tuning system in order to provide the proper matching therein, in the present system the first tube T provides all of such necessary loading to bring the impedance of the circuit up to maximum. Loading as used herein means theadding of resistance to the circuit in order to match the impedances of the two coupled circuits including the coil unit I8, and the coil unit I9, respectively. In this permeability tuned circuit, therefore, with all of the advantages previously described, tube T furnishes all of the required loading and the gain is a maximum. The capacity of this tube is an irreducible minimum and may vary as the commercial practice improves the construction thereof. As a result, a fiat response curve with high gain is provided from the two coupled circuits. Of course, in the prior switching mechanism for tuning a receiver circuit, resistance can be added to obtain the necessary loading so that a flat response curve results, but the gain is so materially reduced that the overall efficiency of the prior system is much lower.

From the preceding description, and from the drawing, it is apparent, therefore, that my invention provides a highly efficient wide-band coupling and tuning system in compact flexible form. With the two mutually coupled variable inductance circuits, and complementary capacitance coupling as well, high gain operation and uniform band-width is obtained over the entire tuning range of a receiver. The iron core variable inductances lend themselves readily to connection with push button, or automatic tuning devices which can be set very readily to any desired frequency, or coupled to manually operated tuning structure so that tuning of any type can be readily accomplished over ing range of the receiver.

Although my invention has been illustrated and described in its preferred embodiment, it is understood that the invention is not limited thereby, but is limited only by the scope of the appended claim.

I claim:

A wide band coupling circuit for coupling a source to a vacuum tube load and provide a substantially constant band width over an entire tuning range of a receiver including in combination a transformer having a pair of inductively and capacitively coupled inductance tuned inductance units and a vacuum tube unit electrically connected to the secondary of said transformer at the high potential end, said tube unit including a socket and a tube, conductor means for electrically connecting all of said units, with said inductance units arranged relative to one another and relative to the socket so that the conductor means are short to reduce stray reactances, and with a maximum impedance being provided at the input electrode of said tube.

RINALDO DE COLA.

the entire tun-

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