US2339857A - Noise limiter - Google Patents

Description

Patented Jan. 25, 1944 NOISE Lili/[ITER Ralph Srllolmes, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 28, 1942, Serial No. 436,559 5 Claims. ('Cl. 178-7.5)

My invention relates to noise hunting circuits for radio receivers and particularly to noise limiters for television receivers that receive a picture signal containing the direct current component thereof.

An object of my invention is to provide an improved noise limiting circuit that will be eiective in operation and simple `in design and adjust? ment.

A further object of my invention is to improve the effectiveness f a noise limiting circuit in a televisionreceiver that receives a signal containing the directf current lcomponent of the picture, while, at the same time, avoiding detector distortion.

In a preferred embodiment of my invention the limiter circuit comprises a diode and an output resistor in series with each other and also in series with a direct current voltage which normally maintains a flow of current -through said diode and resistor. The picture signal, synchronizing pulses, or the like, are impressed upon the limiter circuit whereby they cause a. variation in the diode current and produce a signal voltage across the output resistor. They are impressed upon this circuit with such polarity that an i'ncrease in signal amplitude beyond a certain limit -causes the ow of diode current to stop. Thus,

any signal, such as noise or synchronizing pulses, exceeding this limit will produce no signal across the output resistor exceeding a certain limit.

Further in accordance with my invention the limiter circuit is conductively connected to the detector and means is provided for keeping the limiter current due to the D. C. limiter or gate voltage) from owing through the detector output resistor, thus preventing detector distortion that would result from s uch current flow.

'I'he invention will be better understoodv from the following description taken in connection with the accompanying drawing in which Figure 1 is a circuit diagram of a portion of a. television receiver embodying my invention,

Figure 2 is a graph which is referred to in explaining the operation of the circuit of Fig. 1, and

Figure 3 is a circuit diagram illustrating another embodiment of my invention.

Referring to Fig. 1 the invention is'shown applied to a television receiver of the superheterodye type comprising the usual radio frequency and intermediate frequency amplifier portions (not shown except for the last I.- F. amplifier stage indicated at I), a `second detector 5 comprising diodes 2 and 3 connected to form a full wave rectifier and supplying signal through a. noise suppressor diode 4 and its output resistor 6, a video ampliertube 'I and a cathode ray tube 8.

The part of the receiver which precedes the second detector 5 preferably is designed in accordance with the teachings of Martinelli application Serial No. 287,027, filed July 28, '1939, and assigned to the Radio Corporation of America, now Patent No. 2,299,333, issued Oct. 20, 1942. Specifically, the time constants of the voltage supply filters are made either very short or very long in order to prevent a noise pulse from affecting the receiver for a period longer than that of the noise pulse itself.

The particular coupling between the last I. F. amplifier tube I and the second detector 5, in

the example illustrated, is of the type described in the Grundmann and Allen Patent No. 2,157,170, issued May 9, 1939, and assigned to the Radio Corporation of America. The primary circuit,

which includes a tunable primary coil 9, ,may be v traced from the anode of the amplier tube I through the primary coil 9, a blocking condenser- II, through a portion of the secondary coil I2 to its midpoint and through a large capacitor I3 to ground. The secondary circuit comprises the secondary coil I2 and the capacity of the diodes 2 and 3, this circuit being tunable like the primary circuit by means of an iron core as indicated.

The detector output resistor I6 sconnected between the mid-point of thesecondary coil I2 and, in this example, the cathodes of the diodes 2 and.3.

Instead of applying the picture signal andv synchronizing pulses appearing across the output resistor I6 directly to the video amplier 1, they u are rst passed through the limiter circuit comprising the diode 4 and the outputl resistor 6.

The diode 4 may be of the usual type having' an indirectly heated cathode Il and an anode Iii..l

A flow of current is normally maintained through the limiter diode 4 by applying a certaiildirectcurrent voltage thereacross. This voltage, which is referred to as the gate or limiter voltage,` may be obtained from a voltage divider I5 by connecte Y ing the lower y,entf/of resistor 6 to a point A on resistor I5, the voltage between points O and A being the gate-voltage. The'path for4 the flow of diode current normally maintained by thegate voltage is completed by a resistor 2I connected between cathode ITand a point B on resistor I5. This path may be traced from thecathode I1 of diode 4 through the resistor 2| to the'l oint B on resistor I5, through a section of resis or I5, rand through the resistor 6 to the anode I8 of diode 4.

For reasons discussed below, the limiter circuit put resistor I6. This connectin may be traced from the cathode I1 through a conductor 22 and the resistor |6, through a conductor 20 to a point intermediate points A and B on voltage divider I5, through a section of resistor I5, and through resistor 6 to the diode anode I8.

It will be evident that the picture signal output of the second detector causes variations in the current flowing through the limiter diode 4 whereby there will be variations in the current flow in the limiter output resistor 6. Since the diode 4 has a substantially linear characteristic, the current variations through the output resistor 6 produce voltage variations thereacross which are faithful reproductions of the second detector output.

The input circuit of the video amplier 1 includes a grid leak resistor 3|. 'I'he value of this resistor and the value of the grid condenser 32 preferably are made such that the periodically recurring synchronizing pulses, which drive the control grid of the amplifier tube 1 positive periodically, produce a grid leak biasing'action such that the direct current component of the picture is reinserted as described in Willans Patent 2,252,746, which issued August 19, 1941. The anode of the video amplifier 1 is then conductively connected to the control grid of the cathode ray tube 8. Operating voltage is applied to the anode of the video amplifier tube 1 from the voltage divider I5 through an anode resistor 33. The cathode of the cathode ray tube 8 is conductively connected to a suitable point on the voltage divider I5.

Referring no w to the operation of the noiselimiter circuit, when a signal is being supplied from the second detector 5 and the limiter diode 4 is conducting, there will be a signal voltage appearing across the output resistor 6. However, if the signal impressed upon the cathode I1 of the limiter diode 4 has sulcient amplitude to raise the cathode I1 to the potential of the diode plate I8, or if it has still greater amplitude, whereby the cathode becomes more positive than the plate, the limiter circuit is effectively opencircuited and the signal appearing across the output resistor 6 is held at a xed limiting value until the cathode I1 again becomes negative withl respect to the plate I8 to permit the vflow of current through the limiter diode,

The action of the noise-limiter circuit is illustrated in Fig. 2, where the curve 4| represents the current ow through the limiter diode 4 plotted against voltage impressed across the diode electrode I 1 and I6. The curves :l: and y represent the signal impressed across the limiter circuit 4-6 for the conditions of a white picture and a black picture, respectively. Since the synchronizing pulses are applied with positive polarity to the cathode I1 of the limiter diode 4, any signals such as noise signals of the same polarity as the synchronizing pulses will raise the cathode I1 to the same potential as that of the diode plate I8 when their amplitude reaches the gate voltage. Therefore, as shown by the curves z'and y' in Fig. 2, any noise signal in excess of the gate or limiting voltage will not appear across the output resistor 6 of the limiter circuit. Preferably the gate xaltage is so adjusted that the signal output contains substantially no noise having ian amplitude in excess of the synchronizing pulses. As a result of the conductive or direct current connection between the detector and limiterthisis true for all the picture signal regardis conductively connected across the detector outless of whether it represents av white picture or a black picture.

Since the limiter-ciments 4 and 6 are conductively connected to the detector 5 there normally would be current ow through the detector output resistor I6 caused by the gate voltage. This current flow, unless held to a small value, would introduce substantial detector distortion because it would put a negative bias voltage on the plates of the detector diodes.

I avoid such detector distortion by balancing out such'current 110W in resistor I6. This is .accomplished in Fig. 1 by means of the balanced circuit comprising resistor 2| which, in the example illustrated, has the same resistance as that of the limiter output resistor 6. It is connected to a point on the voltage divider such that in the resistor I6 the iiow of current through the path comprising the resistor 2| and the output resistor I6 of the second detector balances out the ow of limiter diode current through the path comprising the limiter resistor 6 and the detector output resistor I6. In this particular example the points A and B are at the same voltage (but of opposite polarity) with respect to the tap O.

It may be noted that the resistance of resistors I6, 2| and 6 in parallel should be such as to provide the proper load on the detector 5. In the example illustrated, most of this load is provided by the comparatively low impedance resistor I6. However, the resistors 6 and 2| may be given lower resistance values in which case they will provide a substantial' part of the detector load.

' Also, it should be understood that resistors 6 and 2| may have unlike resistance values so long as they are connected to the proper points on voltage divider I5. The main requirement for balancing out the undesired D. C. iiow in resistor I6 is that, with no signal input, the upper ends ofv resistors 6 and 2| be at substantially the same potential.

In Fig, 3, the current flow of the limiter diode 5I through the output resistor 52 of the second detector 53 is prevented by means of a voltage source such as a battery 54 which is connected in the detector circuit to oppose the ow of current from the D. C. limiter voltage source 56.

By means of arrangements, such as shown in Figs. 1 and 3, it is possible to avoid detector distortion while utilizing the advantages of applying the D. C. component to the limiter circuit.

In Fig. 1, the values of certain resistors have been indicated in ohms merely by way of eX- ample.

I claim as my invention: l

1.A A television receiver for the reception of a carrier wave negatively modulated by a composite signal consisting of picture signals'and synchronizing pulses and also modulated by the direct current component of the transmitted picture, said receiver comprising a diode detector for demodulating said received signal to produce picturel signals and synchronizing pulses, ,said dethrough the diode so long as the signal appearing cordance with the changes in background of the` transmitted picture. a receiver comprising a diode detector for .demodulating said transmitted picture signals, said detector having an output resistor, a utilization circuit, and a -signal amplitude limiter through which the output of said detector is fed to said utilizationl circuit, said limiter being direct current connected to said detector and comprising a diode. and an output resistor connected in series; means for inserting a direct current voltage in series with said seriescolnbination of diode and output resistor and with the proper polarity to maintain, a ilow of current through said diode until the signal to 'be limited exceeds a predetermined amplitude,

means including said direct current connection for applying said signal across the series combination of diode, output resistor and D. C. voltage with such polarity as to oppose current ilow through said diode, and means for substantially balancing out any direct current ilow through they detector output resistor ,due to said direct current voltage.

3. A television receiver for the reception of a carrier wave negatively modulated by a composite signal consisting of picture signals and synchronizing pulses and also modulated by thev direct current component of the transmitted picture, said receiver comprising a diode detector for demodulating said received signal to produce picture signals and synchronizing pulses, said detector including an output resistor, and an amplitude limiter circuit comprising a diode. an output resistor and means for providing a gate or limiter voltage in series with said diode and said output resistor, said series combination of diode, output resistor and direct current means being direct current connected across the output resistor i said detector, said diode being connected in such adirection in the circuit that the synchronizing pulses appearing across the detector output resistor and applied to the diode through said direct-current connection oppose 'current ow through the diode, said gate voltage having the proper polarity to maintain a now of current across the detector output resistor having the polarity of the synchronizing pulses does not exceed a predetermined polarity, and means for substantially balancing out any direct currnt flow through the detector output resistor due to said gate voltage.

4. In a television system oi the type in which the direct current component of the picture is transmitted whereby the carrier varies in accordance with the changes in background of the transmitted picture, a receiver comprising a diode detector for demodulating said transmitted picture signals, said detector including an output resistor, a utilization circuit, and a signal amplitude limiter through which the output of said detector is fed to said utilization circuit. said limiter being direct current connected to said detector and comprising a diode and an output resistor connected in series, means for inserting a direct current voltage in series with said series combination of diode and 'output resistor and with the proper polarity to maintain a ilow of current through said diode untilthe signal to be limited exceeds a predetermined amplitude. means including said direct current connection for applying said signal across the series combination of diode, output resistor and D. C. voltage with such polarity as to oppose current ilow" through said diode, and means for substantially balancing out any direct current flow through the detector output resistor due to said D. C.

voltage.

5. A television receiver comprising a diode detector or demodulating transmitted picture signais, a voltage supply, said detector having an output resistor, 'a utilization circuit, and a signal amplitude limiter through which the output of 'said detector is fed' to said utilization circuit.

said limiter comprising a diode and an output resistor connected in series with each other and with one end of the series combination D. C.

lconnected to one end of the output resistor and with the otherend of the series-combination connected to a point on said voltage supply to maintain a flow of current through said diode until the signal t'o be limited exceeds a predetermined amplitude, a D. C. connection from the other end of the detector output resistor to Va lpoint on said voltage supply, and a resistor connected between said oneend of the detector output resistor and a point on said voltage supply such that there is substantially no current flow output resistor due to said RALPH s'. nomma. 7

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