US2377475A - Tuning arrangement in radio receivers - Google Patents

Description

June 5, 1945. M E 2,377,475

TUNING ARRANGEMENT IN RADIO RECEIVER Filed May 28, 1945 In venlor .DM, n M BRoSE Attorney Patented June 5, 1945 UNITED STATES PATENT OFFICE- TUNING ARRANGEMENT 'IN RADIO RECEIVERS Dermot Min Ambrose, London,: England, assignor to Standard Telephone and. Cables Limited, London, England, a British company Application May 28, 1943, Serial No. 488,837

In Great Britain June.5, 1942- 5 Claims.

present invention relates to improvements ticularly suitable' for a variety of special applications in electric circuits;

Various different materials are available for the resistance element of athermistor, these various materials having'different properties-in other respects; as one-- example, a resistance material having a high negative temperature coefiicient of resistance comprises'a mixture of manganese oxide and nickel oxide, with or without the addition of certain other metallic oxides, themixture-being suitably heat treated.

Thermistors have been employed in two diiierent'forms: (a) known as a Directly Heated Therm'is'tor' and comprising a resistance element of the-thermally sensitive resistance material pro vided with'suitable lead-out conductors or terminals; and" (b) known as an Indirectly Heated Therm-istor'comprising the element (a) provided in addition with a heating-coil electrically insulated from'the element. A directly heated thermistor is primarily intended to be controlled by the current which flows throgh varies-the temperature andalso the resistance accordingly. Such a thermistor will also be affected by the temperature of .its surroundings and may'therefore be used for thermostatic control and like' purposes with or without direct heating by the current flowing, through it. An indirectly heatedthermistor is chiefly designed to'be heated by-acontrolling current which flows through the heating coil and which will usually;but not necessarily, be'difl'erent'from'the current which flows th'roughthe resistance element, but this type of thermistor. may also be subjected to either or both of the'types'of control applicable to a directly heatedthermistor'.

More-detailed information on the propertiesof' thermistors will be found in an article by G; L.

Pearson in the Bell Laboratories Record, Dec." l940,'-page"106.

In radio receiving sets, it is now common practice 'to supply some means of indicating when the set is accurately tuned to a desired transmitting it and which station. For this I purpose cathode ray 1 tube indicators of varioustypeshave been: used, and

alsogas-discharge tubes and incandescent electric lamps have-been employed where a cheaper arrangementis desired; By'a'ssociating an indi rectly heated'thermistor with the circuit, the sensitivity of these cheaper arrangements may be increased,

According to the invention; therefore, there. is provided a tuning indicating arrangement fora radio receiver provided with automatic volume control, comprising an indirectly heated ther mistor having its heating coil connected in series with the plate circuit of a thermionic valve in the receiver; the plate current of which valve is varied by the operation of the volume control, the resistance elementof the thermistor being connected in a local "circuit with a tuning indicator and adapted to control the indicator in accord ance with the changes in the tuning adjustmentof the receiver.

The invention will be more clearly understood from the following detailed description with reference'to the accompanying'drawing in which:

Fig; 1 shows a schematic circuit diagram of one embodiment of the invention; and

Fign'zshows a modification of part of Fig. 1.

Referring to Fig. l, the valve V is shown as'an intermediate frequency" amplifying valve in a radio receiver, for example. Incoming signals are applied to the control grid circuit from terminals 1 and'2' and after amplification are deliveredto the next'stage (not shown) through the output mtransformer OT and terminals 3 and 4. The

transformer has its primary winding connected in series with the plate circuit of the valve and the primary and secondary windings are tuned by'mean's of the condensers Cs and Cs respectively.

The cathodeof' the valve is suitably, biased by 7 means ofthe series resistanceRz with the usual by- 1 passcondenser Ct and a negative voltage derived froman automatic volume control arrangement (not shown) of any welhknown type is applied through terminal '5 to an auxiliary grid of the valve' V." A blocking condenser C1 completes the circuit for alternating currents.

So far the circuit which" has been described'isi provided insa number conventional; and might be of other well known ways.

In accordancewith the invention; the positive terminal of the high tension supply for the plate of the valve V is connected from terminaLG through'the heating coil r of an indirectly heated. thermistor'T whose-resistanceelement R is connected in parallel with an incandescent electric lamp L which is lighted from a suitable constant voltage source S connected to the terminals I and 8, in series with a constant resistance R1. The source S may supply either alternating or direct current, and might be the same as one of the power supplies for the valve V or other valves of the receiving set.

It will be first assumed that the thermistor has a negative temperature coefficient of resist-* ance, so that it has a low resistance when the current through the heating coil r is large.

When the receiver is not tuned to a station,

the incoming signals at terminals I and 2 will be of very low level, and the automatic volumefcontrol negative voltage at terminal 5 will be small.

The plate current of the valve V will accordingly be large, the thermistor resistance R will be low, and the lamp L will accordingly be heavily shunted and will glow only faintly, if at all;

When the receiver is brought-into tune thesig'? nal level increases, and the negative automatic volume control voltage also increases and'reduces the amplification of the valve Vand also the plate current. The thermistor .cools and its: resistance R increases; sothat the'lamp glows'more bright ly. Accurate tuningwill be: shown by a maxi'-' mum brilliancy of the lamp, since the plate cur:

rent will increase again as soon as the tunin pointispassed. I I

If the lamp L is connected in series instead of in parallel with the resistance R of-the thermistor T, it; will-be,- evident that the indication of accurate tuning will be given by minimumbrilliancyr.

or the lamp instead of maximum brillianclhand this arrangement may ,be preferred.

It will be evident that a thermistor with a positive temperature coefficient of resistance may also be used. This will give minimum brightness when the circuit is properly tuned in the arrange-' ment of Fig. 1, and maximum brightness in the series with the resistance R.

Whichever of the four possiblejarrangements is.

employed, it will, of course, be'necessary to choose the voltage of the sources, the resistances 'of the lamp L, and R1, and the range of resistance change of the thermistor T, so that the-largest variationof the brightness of the lamp is robe tained corresponding to the-known variationsfof" the plate current of the valve V caused-by action of the automatic volume control; Ihis will not present any difficulty when the conditions are given, and the range of choice of' the thermistor resistance is very wide.

The thermistor T may also be arranged to control a neon tube indicatorin the manner shown in. Fig. 2. This figure shows the modification of that part of Fig. 1 which is below and to-the right-hand side of terminal A; everything above and to the left-hand side of A is the same.

N is a neon tube indicator or otherlike gas discharge device having a cathode H or relatively large area, a small control electrode, and a alternative arrangement when the lamp L isyin terminal to the positive high tension terminal 6 through a resistance R3. This other terminal of the resistance R is also connected through a resistance R4 to the control electrode I! of the neon tube.

Assuming first of all that the thermistor has a negative temperature coefiicient of resistance, the resistance R will be a maximum when the receiver is in tune, the plate current of the valve V-in Fig.1 being a minimum under this condition. This will raise the control electrode l2 of the neon tube N (Fig. 2) to a maximum positive potential, and will in turn cause the glow in the tube N to be of maximum brightness and to cover a maximum area on the cathode II. When the receiver is untuned, the control electrode I2 will be at a low potential and there will be little or no glow.

A thermistor with a positive temperature 00- efficient of resistance will produce a substantially similar'effect if R and R3 are interchanged in Fig. 2. In this case the resistance R will be a minimum when the receiver is in tune so that it will cause the control electrode l2 to assume a maximum positive potential as before.

It will generally be preferable to operate the neon-tube from'the same high tension supply as the valve-V as indicated in Figs. 1;and'2;;but'if preferred, a separate supply could be used, .which would be connected to terminal ill, the connece I tion Bbetween terminals 6 and ill being removed.

It is of course not essential to apply the invention to an intermediate frequency valve. 1 [Any other valve similarly controlled so that its plate current is varied in accordance with level'could be used instead; 1

By suitable choice of the resistance 'r of the heating coil of the thermistor T in Figs. 1 and 2, the sensitivity of the indicators can be made the signal greaterthan if the thermistor were not used. 1 I

What isclaimed is: i i

1. In a radio receiver having an amplifying valve including a plate circuitthe-plate current of which valve is variedbythe level of signals entering said vane; a tuning indicating devicecom, prising. a thermistor having a thermally sensitive resistance element and a heating coil;electrically insulated .from'. said element, a series connection from s'aidcoil to said plate circuit a. local circuit connected to said resistanc element and a tuning indicator in said local circuit connected to SaidIesistanceeIernent, wherebysaid resistance element is adapted to control said indicator in accordance with the changes in the tuning adjustment of the radio receiver, and whereby no transfer of electricalenergy between said plate circuit and said indicator takes place.

v2. In av radio. receiver having an amplifying valve'including a plate circuit the, plate current of which valve is varied by,,the level of signals reaching said valve, a tuning indicating device comprising a thermistor having a thermally sensitive, resistance element and cheating coil electrically' insulated from said element, a series conmotion from said coil to saidplate circuit, a local circuit. connected to said resistance element, an

incandescent lampin said local circuit connected to said resistance element and a source of electromotiveforcefor supplying current to said lamp, whereby. {said resistance element controls said incandescent. lamp in accordance with the changes in the tuning adjustment of the radio receiver, and whereby no transfer of electrical energyfbetween said plate le? tim iece. a H

circuit and said indi 3. In a radio receiver having an amplifying valve including a plate circuit the plate current of which valve is varied by signals entering said valve, a, tuning indicating device comprising a thermistor having a thermally sensitive resistance.

element and a heating coil electrically insulated from said element, a series connection from said coil to said plate circuit, a local circuit, an incandescent lamp in said circuit connected in parallel with said resistance element, a source of electromotive force for supplying current to said lamp and a, constant resistance connected in series with said source of electromotive force, whereby no transfer of electrical energy between said plate circuit and said indicator takes place.

4. In a radio receiver having an amplifying valve including a plate circuit the plate current of which valve is varied by signals entering said valve, a, tuning indicating device comprising a thermistor having a thermally sensitive resistance element and a heating coil electrically insulated from said element, a series connection from said coil to said plate circuit, a local circuit connected to said resistance element, a gas-discharge tube in said local circuit connected to said resistance element and a source of electromotive force for supplying current to said gas-discharge tube, whereby no transfer of electrical energy between said plate circuit and said indicator takes place.

5. In a radio receiver having an amplifying valve including a plate circuit the plate current of which valve is varied by signals reaching said valve, a tuning indicating device comprising a thermistor having a thermally sensitive resistance element and a heating coil electrically insulated from said element, a series connection from said coil to said plate circuit, a local circuit, a gas-discharge tube in said local circuit, said gas-discharge tube comprising a cathode, an anode and a control electrode, a source of constant voltage connected to supply said tube, a constant resistance across said source and to which said resistance element is connected in series, a connection from said control element to the junction point of said resistance element and said constant resistance, and a second constant resistance in series with said last named connection, whereby no transfer of electrical energy between said plate circuit and said indicator takes place.

DERMOT MIN AMBROSE.

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