US2355182A - Noise reduction system - Google Patents

Description

F. SHOUP NOISE REDUCTION SYSTEM Aug, 8, 1944;

Filed June 27, 1941 m M M snmu (In/ware 9 Z6 CHOKE wezasa t 3nventor- 6'2 f Patented Aug. 8, 1944 UNITED NOISE REDUCTIQN $YSTEM Frederic E Sho u p, Haddenfield, N} J., assignorto Radio Corporation of America a. corporation of Delaware Application June 27, 1941, Serial No. 400,003

cclain s. (01. 1250-205).

This invention relates to noise reduction systemsfo'r radio signal amplifiers and the like, which'derive operating current from vibrator power supply units, as-automobile receivers, for example. and has for its primary'obj'ect to provide an improved and'simplified form; and distribution, of circuits elements for materially reducing the noise pickup in such apparatus, without materially increasing the cost of construction. It is a further object of this invention, to provide an improved form of high frequency choke coil and circuit distribution-therefor in connection with automobile radio receiver signal and current supply circuits, which are effective to minimize noise pickup from the vibratory power supply system thereof.

It is a still further object'of this invention, to provide an improved arrangement in connection with the power supply system for aradio receiver of the character refe'rred to, for further effecting 'noise reduction in the receiving circuits'from the vibratory power source. 3

The invention will be further understood from the 7 following description, when considered in connection with the accompanying drawing, and

its scopeis pointed out in the appended claims.- In thedrawing, I I

Figure l is a schematic circuit diagram of the signal and power supply system of a radio receiver embodyingthe'inventio'n; r Y 1 Figures 2 and 3 are external and cross sectional :views, respectively,- and substantially full size, iof a "noise'reduction element employed in' the system of Fig.1;

Figure 4 is an external view, substantially full size, of another noise reduction element also employed in the system of Fig. 1; and

Figure 5 is a schematic circuit diagram of a modification of the circuit of Fig. 1, showing the signal and power supply'circuits of an automobile radio. receiver embodyingthe invention.

-\ Referring to Figures 1 to 4, inclusive, 5 is an antenna connection and 6 is a'battery connectionfor supplying signal and power, respectively, to a shielded radio receiver 1 comprising a signal input circuit 8, a vibratory power supply device 9,

a power transformer I0 and a power switch I I.

. The lead 6 may be considered to represent the ,high side of the battery supply circuit for an automobile or other mobile device and is supplied with energy from a battery indicated at I2, having one terminal connected with the lead 6, while ,the op osite terminalis connected toiground or .chassis, as indicated: at I3. The system of the .present example, also includes the usual circuits for the ignition system indicated at: I 4. The latter imparts to thelead 6 a noise signal which must be eliminated from the receiving circuits. Likewise, the vibrator 9 imparts a noise signal to the distributedwiring of the vehicle and to the receiving circuits, unless precautions are'taken' to prevent such pickup.

As is the usual practice, the receiver system is included ina shield case asindicated, having a metallic chassis member I6 with the vibrator 9 mounted on one side thereof, while the power transformer I0 is mounted on the opposite side as shown. Above the chassis are the input circuit 8, and othercircuits of the receiving system which are not shown for the sake of simplifying the drawing. 0

The signalinput circuit 8 is coupled through a transformeril having a primary winding I8 in the antenna circuit 5 which is extended from the exterioriof the receiver to the interior through a choke coil or inductance indicated at I9 and a supply lead'20 which is bypassed to chassis by suitable. means, such as a capacitor 2|. The cir-' cuit. iscompleted through a lead 22'connected with the chassis or ground, as indicated. The choke coil I9 is close to the casing substantially at the entrance point-of the antenna lead.

On the. power supply side of the receiver, a

.similarchoke coil indicated at 25 is provided directly externally of the shield casing I and connected with an interior supply lead 26 which is also bypassedto chassis or ground. as indicated by the parallel connected capacitors 21 and 28-, the latter .being a'high frequency bypass for the former. The switch I I is included in the lead-26 which provides a connection through. a lead '30 with the power transformer I0. Leads 3I "con- .nect the transformer with the vibrator, asindicated; which serves to interrupt the primary current with a-full waveaction in the present an ample. .As the form of vibrator and transformer do not concern the invention, further description .is believed to'be unnecessary.

However, it will be noted thatthe'transformer is included in a metallic shield casing 33and that I0- is mounted belowthe from the input and other circuits of the receiver,

the vibrator 9 is mounted above the 'chassis'for convenience in servicing and isremovable from the apparatus by the usual contact means indi:- 'cated at 34. In engaging the-contactsthevibrator is caused to seat within a metallic ring member or short-circuited conductive turn 35 which overlaps the joint between the vibrator and chassis sufficiently to prevent magnetic leakage from the vibrator circuits to the input circuit 8 and other circuits (not shown) of the receivin system.

It has been found that this ring must be substantially integral with or welded or otherwise s'ecured to the chassis at a plurality ofpoints in order that the noise pickup from the vibrator may.

elfectively be eliminated.

The casing of the vibrator 9 is grounded to the cup or ring 35 at one or more points, as indicated by the connections 36 to provide electrostatic shielding. This may be provided by any suitable means providing connections as shown between the vibrator casing and the ring 35 and at least one such connection should be provided in each case. It will also be noted that the filament connection for the receiving system is taken from the lead 30 through 'a'suitable high frequency choke coil 31. This may be of any suitable construction provided it is relatively small.

It has-:also been found that the choke coils l9 and 25 must be of relatively small diameter with respect to their length, having substantially the shape and size shown in Figs. 2, 3 and 4.

As shown in Fig. 3, the choke coil may comprise a flexible core 40 of insulating material and of substantially minimum diameterfor a given wire size for the coil or winding 4| thereon. The latter comprises a single layer solenoid of insulated conductor with the ends connected to metallic end ferrules 42 which in turn are connected with flexible soldering leads orwire 43 at the ends of the unit. The winding may comprise several hundred turns of No. 40, B; & S. gauge, enameled copper wire, for example, and is covered preferably with a flexible insulating braid, indicated at 44.

In the case of the coil 25, it is similarly constructed of insulated wire 46 and is covered by an outer insulating braid 41. It will be noted that this wire is relatively heavy for carrying the 1 I battery current to the vibrator and other circuits of the receiver. By way of example, No. 18 B. and S. gauge enameled wire had been used and may comprise a plurality of turns, of the order of 20, for example, in present case;

This system and type of choke coil serves to minimize undesired electrical noise impulses caused by the ignition system and vibrator 9' in the supply circuits entering the sensitive receiving circuits in a more effective'manner than any other known arrangement. It has made possible a greater sensitivity with substantially no noise interference in present commercial installations. Since the coils are relatively small, they may readily be inserted in the supply circuits at any desired points. As shown, these coils are preferably external to the receiver and shield housing at the points of entry of the supply leads thereto. In this case, the capacitance of the connectors and cable clamps with the inductances l9 and 25, provide elfective filters without additional capacitors, which resonate in the range of noise-pulses, for example, between 7.5 and 75 m. c. The external capacitance is indicated in dotted lines in Fig. 1 at 48 and 49 and may-be. of the order of 40 mmfd. in some cases. I

Although filter choke coils have been used heretofore for the reduction of undesired signal impulses, it has-been found that coils of unusually small diameter relative to the length, for exam- 1 same relative low cost.

'tice range in length from one inch to two-andone-half inches depending upon the extent or amplitude of the noise impulses to be eliminated. The diameter 'of the internal core here considered, is off the order of one-sixteenth inch when starting the winding process, and depending upon .the nature of the core material, this may be reduced in various degrees when the coil is completed. The inner core may be a braid or cord of any suitable insulating material and the outer cover may be of any suitable insulating or protectingmaterial such as cotton braid, insulated metallic shielding molded rubber, or so-called spaghetti insulation. 1

It has been customary to ground vibrators and casings with a spring-fingered cup. This, however, has proved to be not eifective in preventing electrical disturbances from entering the receiving circuits. The use of a short circuited ring shield about the vibrator base substantially integral with the chassis minimizes noise pickup from the sources referred to, even though slightly spaced about the vibrator base. The cup or ring thus secured to or formed with the chassis, becomes a grounded short-circuited turn or winding for the electromagnetic field of the vibrator where the electrical connections are made below the casing. In certain apparatus, this ring may be formed from the chassis material. In any case there must be no magnetic flux leakage between the ring and the chassis aboutthe power supply connection to the vibrator device.

Referring now to Fig. 5, the chassis of a radio receiver is indicated at 50 and is connected to ground as indicated at 5|. This serves as a shield between radio receiving circuits. above and power supply circuits below, the latter comprising a vibrator device 52, the power transformer 53 and A battery supply .lead 54 connected to the transformer and to a filament circuit 55 through an external inputchoke coil 56 and suitable internal choke coils indicated at 51 and 58, the latter being an iron core choke coil, provided with a bypass capacitor 59 to ground. i

The distributed capacity of the supply lead external to the receiver is indicated at 60. With the chokecoil 56 this provides an effective external filter for removing from the receiving circuits, radiation back from the A" battery lead 54 to the vehicle or other power supply means with which the system is used. Internal bypass capacitors are indicated at 62 and 63. providing low and high frequency .bypass means for the Ab y s pply. A Above the chassis, the antenna lead, indicated at 65, isconnected through a coupling capacitor 66 with an input primary Winding 61 coupled to .a, secondary winding -68 connected through a choke coil 69 and a bypass capacitor 10 with ground; The choke coil.69 may be located internally of the receiver shield casing indicated by the dotted outline l2, withoutinterfering with the operation of the noise reduction system. This is for the reason that. thechoke coil is of such relatively small diameter that it sets up a relatively small field for coupling with other elements of the receiver, such as the input circuit indicated at 13 for the first amplifier tube 14 and the output circuits l and 16.

The distributed capacity of the antenna input lead is indicated at 18. With the choke coil 69, this provides a filter for the undesired ignition and other noise impulses received on the antenna lead. The-operation of the circuit is otherwise the same as that described in connection with Fig. 1, and the choke coils 69 and 56 correspond respectively with the construction shown in Figs. 2-3 and 4. 1

While the invention has been described in connection with automobile radio receiving systems, it is not limited thereto, but may be provided in connection with any receiving system haying noise impulse sources associated therewith such as vibratory power supply and ignition circuits.

I claim as my invention:

1. The combination with shielded signal amplityin apparatus, of external signal and power supply leads extending into said apparatus, flexible inductive filter elements adjacent to ,the points of entry connected serially in said firstand second named leads, providing with the distributed capacity thereof with respect to said shield and said power supply and leads high frequency noise filter for said leads, said inductive elements each comprising a single layer conductive winding having an internal diameter of the order of one-sixteenth of an inch, and having a length at least of the order of one inch.

2. In a radio receiving system, the combination with a signal receiver having a shield fcasing, a chassis and signal and power supply leads for said receiver, of an unshielded elongated small diameter flexible choke coil in each of said leads external of and adjacent to said casing providing filtering elements for said leads to prevent the entry of high frequency disturbances into said receiver, the internal diameter of said choke coils being of the order of a small fractional part of an inch and substantially a minimum consistent with effecting a single layer winding with a relatively small wire gauge, and having a length to diameter ratio greater than 10:1, and capacity means for causing said choke coils to resonate in a relatively high frequenc disturbance range.

3. The combination with signal amplifying apparatus, of a supply lead'therefor, a high frequency noise filter in said lead adjacent to said apparatus comprising the distributed capacity of said lead and an unshielded elongated single layer flexible inductance winding comprising a plurality of conductive turns of relatively small gauge wire having an internal diameter of the order of one-sixteenth of an inch and a length of the order of at least one inch.

4. A choke coil element for a high frequency noise filter system, comprising an unshielded elongated flexible inductance winding of relatively small gauge wire having an internal diameter of the order of a small fractional part of an inch, and a length to diameter ratio in excess of 10:1.

5. A choke coil element for a high frequency noise filter system, comprising an unshielded elongated flexible inductance winding of relatively small gauge wire having an internal diameter of the order of a small fractional part of an inch, and a length to diameter ratio in excess of 10:1, and means providing spaced terminal ends for said winding and aprotective covering for said winding interposedbetween said terminal ends.

6. A choke coil element for a high frequency noise filter system, comprising an unshielded elongated inductance winding of relatively small gauge wire having an internal diameter of the order of a small fractional part of an inch, and a length to diameter ratio in excess of 10:1, means providing a-fiexible core element for said Winding, a pair of spaced terminal ends for said choke coil element to which said Winding is connected, and means providing a protective covering for said winding between said terminal ends.

FREDERIC F. SHOUP.

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