US2100236A - Intercarrier noise suppressor - Google Patents

Description

Nov. 23, 1937. R. D. BROWN. JR

INTERCARRIER NOISE SUPPRESSOR {filed May 8, 1935 a Patented Nov. 23, 1937 UNITED STATES PATENT OFFICE mrnncanama NOISE SUPPRESSOB ReynoldsD. Brown, In, Philadelphia, Pa... as-

signor to l ny, Philadelphia, Pennsylvania" Philadelphia Storage Battery Com- Pa., a corporation of Application May a, 1935, sci-a1 No. 20,411 In Great Britain May 14,1934- 4 Claims. (01. 250-20) type above mentioned embodying the features of This invention relates to radio receivers and has for'its principal object the provision of a novel inter-carrier noise suppressor; The invention is useful particularly'in a system such as 0 for broadening the fidelity curve or response characteristic of the receiver to give a desired,

substantially rectangular response curve, and to eliminate sideband cutting; and also embodying r a tuning indicator circuit connected to the main circuit so as to derive a portion of the incoming energy therefrom. The present invention is very well adaptedfor use in conjunction with such a system and it will, therefore, be described herein 'with reference to this specific adaptation, al- 0 though it will be understood that the invention is not thus limited but is capable of application in any desired and suitable manner.

The main feature of the invention comprises the use of a resistor which has developed thereacross a unidirectional potential which varies in accordance with the various signal strengths received during the tuning of the, receiver. This variable potential is applied to an amplification control element in one of ,the circuits of the 0 audio frequency amplifier of the main circuit, thus placing [a variable bias on the said control element and effecting gain control of the audiov frequency amplifier. There is thus provided an inter-carrier suppressor which allows functioning 3 of the receiver only when it is properly tunedto thecarrier frequency. In the absence of this suppressor, it would be difllcult to properly tune the receiver by ear as the receiver would respond to the same extent even when tuned consider- 40 ably oil the carrier frequency. The resistor above mentioned has developed across it a nega tive biasing potential which is of such value as I to cut off the audio frequency amplifier excepting when the receiver is tuned to the carrier.

# A further feature of the invention resides in its-particular adaptation for use in connection with automatic volume control. By this feature, objectionable characteristics of the conventional will be fully explained hereinafter.

The invention may be fully understod by refaudio volume control action are eliminated, as

this invention.

Referring to the drawing, there is shown an intermediate frequency amplifier IF such as commonly employed in the intermediate frequency stages of the'superheterodyne receiver and while only one such amplifier is shown, it will be understood that the receiving system may comprise a plurality-of intermediate frequency stages, as is well known. The intermediate frequency amplifier shown is coupled to a detector D, which constitutes the usual second detector of the system, by means -of the inductively associated transformer coils P and S. The detector 1) is coupled to .an audio-frequency amplifier AF in conventional manner, it being understood that there may be a plurality of audio-frequency amplifier'stages as is well known. Automaticvol ume control may be embodied in the system in any manner, for example. by deriving a control voltage from the leak resistance of the detector D and applying such voltage through a filter resistor F to the control grid of amplifier IF inconventional manner. I

In the system shown, following the teachin of the above-mentioned copending application, there is provided a, variable grid biasing resistor R which is interposed in the cathode lead of the intermediate frequency amplifier and there is also provided a series resonant circuit A which includes a variable resistor R1, these two resistors being preferably adapted for joint or simuitaneous manual operation, as indicated by the broken-line representation. The circuit A also includes a condenser C and an inductance coil L which is overcoupled with respect to S. This device functions to increase the fidelity range of the receiver, whilemaintaining the over-all gain uniform, as fully described in the said copending application. Inasmuch as this subject matter does not form a part of cation, there is provided a branch circuit 13 which is preferably connected to the main circuit of the intermediate stage IF and the detector D as illustrated. This circuit preferably takes the form of a sharply tuned circuit which has connected thereto a rectifier E preferably in the form of a grid-leak detector as illustrated. Unless the grid condenser 03 is so large that audio frequency waves will not occur across it as a, result of demodulation ofthe carrier, other means must be li- 1 to prevent such'audlo frequencies from diathe present invention. it is unnecessary to describe it in further detail is detected or rectified by the grid-leak detector E in the usual manner of operation of such a device. The output current of this detector. flowing through the indicator or meter M varies in accordance with the tuning of the receiver and the meter therefore indicates the proper tuning of the system. With the receiver adjusted for high fidelity reception, it is difiicult to properly tune it by ear due to the fact that even though thereceiver is not properly tuned, reception will nevertheless take place but at a loss of quality. However, since the detector E is in a sharply tuned circuit, it will be energized sufiiciently to operate the meter M only when the receiver is properly tuned. Thus the meter indicates at all times the correct tuning of the receiver, even when the correct tuning cannot be determined contenient- 1y by ear.

In accordance with the present invention, there is provided in the output circuit of the detector E and in series with 'the tuning indicator or meter M a resistor Raoneextremity of which is connected to a screen-grid g of, the audio frequency amplifier AF. There is also provided a suitable filter condenser C2, as illustrated. By. virtue of the output current fiow through resistor R2, there is set up across this resistor a unidirectionalpotential or voltage having the polarity indicated. It will be noted that the negative side of the resistor is connected to the screen-grid g, of the audio frequency amplifier.

The cathode'ofone or more of the tubes AF may be biased above ground by a battery V have its cathode and as the amplification is roughly ing the polarity indicated or this voltage may be supplied by a'circuit in the plate supply. It will be noted that the screen-grid-to-cathode circuit of this tube is shunted by the series circuit comprising the tuning meter M and the internal 'plate-to-cathode circuit of the detector tube E and that both of these ,tubes" are energized through the resistor R2. The condensers C1 and C: are preferably provided as filter condensers and serve to reduce the ripple in unidirectional current supplied to the meter M and screen-grid a. It will also be noted that when the grid of the detector tube E has no bias, corresponding to zero carrier signal, the'plate-to-cathode resistance of the tube E will be small. Hence the major portion of the voltage dropin the circuit will be across R2. By virtue of the bias V on the cathode of tube AF, the screen-grid bias of said tube may be very small or even negative with respect to proportional to the screen-grid bias for'low'bias values, the gain in the amplifier. AF under these conditions 'may 'be reduced to zero; in other words, the tube may be cut off. But as the grid of the detector tube is made negative due to an increase in applied signal voltage, its plate current will decrease and hence the voltage drop across the resistor R2 will decrease and the positive screen-grid voltage of the amplifier tube AF will increase. It will be noted that when the detector tube E is unbiased and consequently draws have little current, then the-screen voltage is determined largely by the current which the grid 9 draws .per se and the relatively small plate current for the detector tube E will have little or no effect. Hence the circuit may be designed so that for zero or very small signals on the detector tube, the amplifier is substantially cut off; but for caused to operate as. an inter-carrier suppressor. Since it ischaracteristic of a grid-leak detector that the output or plate current decreases with increase of input voltage, the output current of the detector E will be a minimum when the receiver is tuned to a carrier wave and a maximum when no signal is applied. Therefore, the negative voltage across resistor Re, which is applied to the amplifier AF, will be a minimum when the receiver is properly tuned to a carrier wave. With proper design, the amplifier AF may be caused .to transmit signals only when the signal input to detector E is above a predetermined minimum value; or, in other words, when the receiver is properly tuned to a carrier wave. A decrease in detector input signal below this minimum value due to a change in the tuning of the receiver or-for any other reason, will cause the plate-to-cathode current of detector E to become sufilciently large to causecut-oif or blocking of the amplifier AF so that the amplifier will not function when the receiver is detuned. Thus, the

device may be made to function effectively as an inter-carrier suppressor.

. By properly 'designing the resistor R2 in rela-. tion to the amplifier AF, the device may thus be It will be noted that the present invention has I important advantages in connection with automatic volume control. The automatic volume con- Y trol action is controlled byenergy derived from energy derived from portions or-circuits of the system whichare sharply resonant to the frequency of the received signals and, therefore, the suppressor cuts off the audio amplifier before the said noisy regions'can .be reached and while a large amount of automatic volume control action is stillpresent, thereby eliminating the undesirable noisy operation. The suppressor action is at all times sharper than the automatic volume control action 'under any condition of adjustment of the fidelity of response. The relatively broad frequency response characteristics of the automatic volume control will maintain the voltage supplied to the branch circuit B substantially constant over the small range of frequencies to which the response of the inter-carrier noise sup pressor is limited by the sharply tuned input circuit of detector E. As the voltage supplied to it is not reduced by the automatic volume control, the inter-canier noise suppressor is not critical with respect to voltage.

Only such portion of the receiving system as is necessary to illustrate a simple, embodiment of the invention has been illustrated, it being unnecessary to illustrate or describe the conventional receiver in further detail. It will be understood Edi . a oaaae L that various modifications, includingthose above a branch circuit comprising at least one sharply tuned circuit energized by said intermediate frequency amplifier, rectifying means energized by said intermediate frequency amplifier for demodulating said modulated signal, an'audio frequency amplifier comprising at least one space discharge device having an anode, a cathode and two control elements, means for supplying said demodulated signal to said cathode and one of said control elements, a second rectifying means connected to said branch circuit and having an output circuit wherein there is obtained a signal dependent upon said carrier signal amplitude, and means including filter means for supplying said last-named signal to said cathode and said other control element so as to reduce the amplification of said audio frequency amplifier when said carrier signal amplitude is less than a predetermined value;

' 2. In a superheterodyne radio receiver, an intermediate frequency amplifier for amplifying modulated intermediate frequency carrier signals, a branch circuit comprising at least one sharply tuned circuitenergized by said intermediate frequency amplifier, rectifying means energized by said intermediate frequency amplifier for demodulating said modulated signal, an audio frequency amplifier comprising atleast' one space discharge device having an'anode, a cathode and two control elements, means for supplying said demodulated signal to said cathode and one of said control elements, a second rectifying means comprising a grid-leak detector having an input circuit, a cathode and an anode, means for supplying said detector input circuit with signals from said branch circuit, a resistance connected to said detector anode and to a voltage source, a connection between said detect-or cathode and said voltage source, a condenser connected to said detector anode and to said voltage source, a' conductive connection between said detector anode and the other of said control elements of said discharge device, and a conductive connection between said cathode of said discharge device and said cathode of said grid-leak detector.-

3. In a superheterodyne radio receiver, an intermediate frequency amplifier having a substantially rectangular frequency response characteristic for amplifying modulated intermediate frequency carrier signals, automatic volume control means controlled by said intermediate frequency amplifier, a branch circuit comprising at least one sharply tuned circuit energized by said intermediate frequency amplifier, rectifying means energized by said intermediate frequency amplifier for demodulating said modulated signal, an audio frequency amplifier comprising at least one space discharge device having an anode, a; cathode and two control elements, means for supplying said demodulated signal to said cathode and one of said control ele ments, a second rectifying means connected to said branch circuit and having an output circuit wherein there is obtained a signal dependent upon said carrier signal amplitude, and .means' including filter means for supplying said last named signal to said cathode and said other control element so as to reduce the amplification of said audio frequency amplifier when said carrier signal amplitude is less than a predetermined .value.

4. In a superheterodyne radio receiver, an

device having an anode, a cathode and two con-' trol elements, means for supplying said demodulated signal to said cathode and one of said control elements, a second rectifying means connected to said branch circuit and having an output circuit wherein there is obtained a signal dependent upon said carrier signal amplitude, a tuning indicator in said output circuit, and means including filter means for supplying said last named signal to said cathode and said other control element so as to reduce the amplification of said audio frequency amplifier when said carrier signal amplitude is less than a predetermined value.

REYNOIDS 1). BROWN, in.

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