US2222424A - Noise reduction system - Google Patents

Description

Nov. 19, 1940.

J. D. REID NOISE REDUCTION SYSTEM Filed June 27, 1959 Patented Nov. 19, 1940 PATENT OFFlCE NOISE REDUCTION SYSTEM John D. Reid, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application June 27, 1939, Serial No. 281,333

9 Claims.

This invention relates to noise reduction systems such as are utilized to eliminate the undes red noise produced in radio receiving apparatus by electrical impulses other than those of the i) desired signal, and has for its principal object the provision of an improved system and method of operation whereby such noise is eliminated.

As pointed out in a copending Landon applicat on, Serial No. 210,323, filed May 27, 1938, and

assigned to the same assignee as the present application, much of the noise encountered in radio reception is due to undesired electrical impulses which come into the receiver over the power supply line and produce a noise voltage between the 15 receiver chassis and ground. While this noise voltage may be reduced by the use of a short ground lead, the use of a sufficiently short lead is usually impracticable. In accordance with the aforesaid Landon application, this difficulty is avoided by the provision of an improved noise reduction system including long and short antennae which are interconnected through the primary winding of a transformer provided with a H tuned secondary winding through which signal W impulses are applied to the input circuit of the first stage of the amplifier. These two antennae are spaced from one another by a distance of a few inches to a few feet, and a variable balancing condenser is connected between the long antenna and the chassis.

If there were no capacity coupling between the primary winding and the secondary winding or chassis of this Landon noise reduction system,

the balancing condenser would not be required. Due to the existence of such capacity, however,

the noise voltage on the chasis induces noise voltge in both the antennae. The function of the balacing condenser is to neutralize the noise volta e otherwise produced in the secondary winding and in the amplifier input circuit. If the noise voltages induced in the two antennae were equal in phase and amplitude, no noise current would follow in the primary winding. The long antenna, however, is generally of lower impedance and has less noise voltage induced in it until the condenser is adjusted to produce a balance. If capacity exists between the primary winding and the high potential side of the secondary winding, a change in the balance point is desirable for the w reason that neutralization of this capacity minimizes the effect of frequency change on the balance point.

The present invention is similar to that of the aforesaid copending application in that it involves the use of a counterpoise and balancing condenser, but is distinguished therefrom in that (1) it is adapted to multi-range reception and (2) it avoids the necessity of any switching on the primary side of the input transformer. In this improved system, noise reduction is effected 5 at high frequencies by means of a balanced input system and a twisted pair antenna lead-in, and at low frequencies by means of a counterpoise and balancing condenser.

The invention will be better understood from the following description considered in connection with the accompanying drawing, and its scope is indicated by the appended claims.

Referring to the drawing:

Figure 1 is a wiring diagram of the improved noise reduction as applied to a two-frequency band receiver,

Figure 2 is a similar diagram of a modified two-frequency band receiver wherein the antenna lead-in and counterpoise are in the form of a concentric line, and

Figure 3 illustrates the application of the invention to a receiver of the three-frequency band type.

The system of Fig. 1 includes a dipole antenna [0 which is connected through the twisted pair lead H to the primary winding l2 of a high frequency input transformer l3. The primary winding [2 is provided with an intermediate ter-' minal M which is connected to one terminal of a balancing condenser l5 and is also connected through the primary winding l6 of a low fre-- quency input transformer I1 to a counterpoiseor relatively short antenna l8. The secondary winding [9 of the high frequency input transformer I3 is connected at one end to acoupling condenser 20 and at the other end, through the secondary winding 2| of the low frequency transformer H, to the chassis which is indicated at 22v Connected tothechassis 22 are also one terminal of the balancing condenser and the low voltage side of a transformersecondary tuning capacitor 23. In order to facilitate high frequency reception, aswitch 24 is provided for short-circuiting the low frequency secondary Winding 2|.

Through thecoupling capacitor 20, signal impulses are applied to the input circuit of aradio frequency amplifier 25. The output circuit of theamplifier 25 is coupled to the input circuit of r anamplifier 26 through a coupling capacitor and either a high frequencyband input transformer 21 or a low frequency band transformer 28, depending on the position of aband selecting switch 29. Aswitch 30 is provided for shortcircuiting the secondary of the transformer 28 antenna input network preceding the amplifier As previously indicated, theswitch 24 is closed during high frequency band reception, and noise reduction is effected by means of the dipole,

twisted pair lead-in and balanced primary input connection. During low frequency band reception, theswitch 24 is open, the dipole, the twisted pair lead-in and the low frequency primary [2 function to pick up the signal, and the noise impulses are balanced out by adjusting the capacitor to such a value that the noise voltages at opposite ends of the low frequency primary winding l6 are equal in value and of opposite phase. Under these conditions, no noise current flows in this primary winding and no noise Voltage is induced in the secondary winding 2 I.

The modification of Fig. 2 is similar to that of Fig. 1, with the exception that the twisted pair lead-in is replaced by a concentric line 3l32. It is important to keep the capacity low and symmetrical from either side of the line to the grid end of thesecondary winding 19, and to keep the capacity of the low frequency transformer low so that the balance point will remain substantially the same over the low frequency band. Thebalancing capacitor I5 may be preadjusted or used for exact balancing which varies as the frequency of the tuning is varied. vAs

disclosed by the aforesaid Landon application, a resistor in series with the capacitor I5 is often effective in securing a better balance, and a ground connection including a capacitor may be fe'O utilized instead of the counterpoise.

Fig. 3 illustrates a three-band receiving system which functions to receive a medium frequency band intermediate the bands received by the systems of Figs 1 and 2. To this end, there is 45 provided anadditional input transformer 33 and a frequency band selection switch 34 which is operated to its upper closed position for selecting the frequency band passed by the transformer I3, is operated to its central closed position for sew lecting the frequency band passed by the transthe secondary winding of the transformer I1.

This system, like those previously described, has the advantage that it avoids switching in the primary circuits, thus enabling the primary to ground and the primary to secondary capacicf, tances to be kept low. If an antenna of dimen- 70 tion over-the conventional receiver grounded to sions suitable for the long wave band is used, it

is resonant in the medium band and a balance cannot be obtained on the medium band, a1 though there is an improvement in noise reducthe chassis. In most cases, further improvement in noisereduction can be secured by the used of a variable resistor connected in series or parallel with the balancing resistor, as pointed out in the aforesaid Landon application.

I claim as my invention:

1. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of high and low frequency band input transformers each provided with primary and secondary windings, antenna means connected to the opposite end terminals of said high frequency band primary winding, and counterpoise means connected through said low frequency band primary winding to an intermediate terminal of said high frequency band primary winding.

2. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of high and low frequency band input transformers each provided with primary and secondary windings, antenna means connected to the opposite end terminals of said high frequency band primary winding, counterpoise means connected through said low frequency band primary winding to an intermediate terminal of said high frequency band primary winding, and capacitor means connected between said chassis and said intermediate terminal for excluding said noise voltage from said low frequency band primary winding.

3. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a plurality of different frequency band transformers each provided with primary and secondary windings, antenna means connected to the opposite end terminals of one of said primary windings, counterpoise means connected to an intermediate terminal of said primary winding through another of said primary windings, and a balancing capacitor connected between said chassis and said intermediate terminal.

4. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a plurality of different frequency band transformers each provided with primary and secondary windings, antenna means connected to the opposite end terminals of one of said primary windings, counterpoise means connectedto an intermediate terminal of said primary winding through another of said primary windings, a balancing capacitor connected between said chassis and said intermediate terminal, and means for connecting said secondary windings in an output network having its low voltage side connected to said chassis.

5. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a plurality of different frequency 'band transformers each provided with primary and second ary windigs, antenna means connected to the opposite end terminals of one of said primary windings, counterpoise means connected to an intermediate terminal of said primary winding through another of said primary windings, a balancing capacitor connected between said chassis and said intermediate terminal, means for connecting said secondary windings in an output network provided with a high voltage terminal and with a low voltage terminal connected to said chassis, and means for connecting the high voltage side of either of said secondary windings to said high voltage terminal.

6. In a radio apparatus including a chassis Vided with primary and secondary windings, antenna means connected to the opposite end terminals of the highest frequency primary windings, counterpoise means connected to an intermediate terminal of said primary winding through the lower frequency primaries of said primary windings, a balancing capacitor connected between said chassis and said intermediate terminal, means for connecting said secondary windings in an output network provided with a high voltage terminal and with a low voltage terminal connected to said chassis, and means for connecting the high voltage side of either of said secondary windings to said high voltage terminal.

'7. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of a plurality of different frequency band transformers each provided with primary and secondary windings, antenna means connected to the opposite end terminals of one of said primary windings, counterpoise means connected to an intermediate terminal of said primary winding through another of said primary windings, a balancing capacitor connected between said chassis and said intermediate terminal, a shunt condenser connected across the lowest frequency primary, means for connecting said secondary windings in an output network provided with a high voltage terminal and with a low voltage terminal connected to said chassis, and means for connecting the high voltage side of either of said second windings to said high voltage terminal.

8. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of high and low frequency band signal input transformers each provided with primary and secondary windings, said primary winding having two end terminals and an intermediate terminal, means providing a dipole antenna connection for the end terminals of said high frequency band primary winding, and means providing a second antenna connection serially through said low frequency band primary winding to said intermediate terminal of said high frequency band primary winding, and means providing a balancing capacity between said chassis and said intermediate terminal.

9. In a radio apparatus including a chassis which tends to be maintained at a noise voltage with respect to ground, the combination of high and low frequency band input transformers each provided with primary and secondary windings, said primary winding having two end terminals and an intermediate terminal, a transmission line, antenna means connected to the opposite end terminals of said high frequency band primary winding through said line, and counterpoise means comprising a conductor connected through said low frequency band primary winding to said intermediate terminal of said high frequency band primary winding and associated with said transmission line.

JOHN D. REID.

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