US2323880A - Wave amplitude limiting device - Google Patents

Description

y 1943- c. w. CARNAHAN 2,323,880

WAVE'AMPLITUDE LIMITING DEVICE Filed Dec. 26, 1940 FIG 7 SIGN/4L INPUT- FIG 8 S/GNAL INPUT-- o I l I INVENTOR Y C W. CARA/AHAA/ ATTORNEY Patented July 6,1943

UNITED STATES PATENT OFFICE.

wsvasmn'rzlirz nrmcpavrca Chalon Wesley Carnalian, Oak Park. 111., minor to Zenith Radio Corporation,

corporation of Illinois Application December 26, 1940, Serial No. 371,606

28 Claims.

This invention relates to limiters for signal transmission systems, and more particularly to means for limiting amplitude variations of signals having a characteristic other than the amplitude thereof varied in accordance with transmitted intelligence.

In a radio receiver of the type which responds signal input increases.

Accordingly, the primary object of the present invention is to provide novel means in a radio receiver for limiting the peak voltages of an mcoming signal.

Another object of the invention is to provide a limiter having its output characteristic varied with input in a manner to provide a substantially noise-free receiver when no useful signals are being received, and to provide a transfer of signals having a constant peak signal voltage when receiving useful signals from a desired source, such as a frequency modulated radio transmitter.

Another object of the invention is to provide a multi-stage signal limiting device, in which the signal transfer means is of low impedance at radio signal frequency.

Still another object of the invention is to provide a signal limiting device, in which the portion of the input wave selected by the limiter lies in a region close to the zero axis of the input wave.

The manner of realizing the foregoing and other and more specific objects of the invention will become apparent as the following detailed description proceeds with reference to the accompanying drawing, of which:

Figure 1 is a diagrammatic showing of a radio receiver of the frequency or phase modulated type embodying the invention;

Figures 2 to 6 are curves illustrating successive steps in the operation of the limiter;

Figure 7 illustrates a characteristic curve of a limiter embodying the features of Figure 1 in comparison with a characteristic of known limiters; and

Figure 8 is related to Figure '7 and illustrates the final output of a limiter embodying features of Figure 1 in comparison with known limiters.

Referring to Figure l, a complete receiver is Chicago, 111., a

illustrated having certain parts thereof shown in a conventional manner. An antenna it of any type suitable for collecting signal energy at frequencies ordinarily employed for frequency modulation transmission and reception'is connected to a preamplifier II, which amplifies the incoming waves substantially without change and applies them to a converter l2, which may be of any well-known type employed in a superheterodyne receiver. The signal derived from converter i2 is amplified in the intermediate frequency amplifier It in the usual manner before it is supplied to the limiter of this invention, to be described more in detail later and indicated generally by the reference character ii. The output of the limiter i6 is demodulated in the discriminator H, which may beof any wellknown type capable of changing frequency or phase variations of the signal carrier into audiofrequency amplitude variations. The parts of this discriminator which cooperate with the limlter it will also be explained hereinafter. The audio-frequency output from the discriminator is or may be passed through an intermediate frequency filter i8 to remove undesired frequencies therefrom including those of the order of the intermediate frequency furnished by the converter The output of the discriminator is amplified in the audio-amplifier l9 and translated into sound by the loud speaker 2|. Itwill be understood that any translating device other than a loud speaker may be used at 2i if manifestations other than sound are to be utilized as the output of the receiver, for example, the output of amplifier It may be used to modulate a characteristic of an-' other carrier wave for relay transmission purposes, or for facsimile or television use.

The limiter l6 comprises two electronic devices, such as the vacuum tubes 22 and 23 connected in cascade. In general, the tubes 22 and 23 may 'be of a type suitable for use in radio receivers including tetrode, pentode or multi-electrode types. Preferred types of tubes have a low inter-electrode capacity, a sharp plate current cut-off, andof positive potential 28 such as a battery, rectiand the cathode 30. Where the cathode 33 is grounded, the secondary 23 of this transformer may be connected between the grid 24 and ground, as shown in Figure 1, and produce the same efiect. Withthis connection of the grid to the cathode, no self-bias is produced in the grid-cathode circuit of tube 22, and consequently the plate current cut of! occurs along the line bb' which lies on the negative side of the zero axis X of .the input wave. The working part of the wave at this point thus includes all positive values and a small portion of negative values and, as shown in Figure 2, includes all that portion of the wave above bb. The primary function of the tube 22 is to act as an amplifier which is very easily overloaded, so that the plate current is cut oil on approximately one-half of the signal cycle with strong signals.

Other characteristics of the circuit associated with tubes 22 and 23 will have the effect of limiting the values of the wave that were initially positive and will result in a final working part of the wave that is adjacent to and above the line bb' of Figure 2.

At high intermediate frequencies the plate load impedance of the tube 22 consists largely of the tube and distributed capacities to ground existing in this stage. It has been found that for an intermediate frequency in the neighborhood of four megacycles, this plate load impedance is sufllciently high without having to resonate the shunt capacity. For intermediate frequencies higher than four megacycles it may be desirable to raise the plate load impedance at the intermediate frequency by inserting in series with the resistor 35, a parallel resonant circuit tuned to the intermediate frequency, or by inserting between the plate 32 and grounds. circuit including an inductance in series with a blocking condenser.

The plate 32 is connected to a source of positive potential, which may be the source 23, through a resistor 35.' The tube 23 in the second stage of the limiter preferably possess the characteristics of low interelectrode capacity, sharp plate current cut-off and medium mutual conductance pointed out above and its input circuit including the signal grid 3| is coupled to the plate 32 of the tube 22 through a condenser 34 which is parallelled by a resistor 33, which serves to maintain the grid of the tube 23 positive when there is no signal. Positive grid voltage and a plate supply having regulation characteristics such that the terminal voltage falls with the load combine when applied to the second tube to serve the novel and useful function of lowering the sensitivity of this tube at low signal inputs.

The resistor 33 is by-passed at the intermediate frequency by capacity 34 which is substantially larger than the grid to cathode capacity of tube 23. This permits maximum signal voltage to be developed at grid 3| of tube 2'3. It is to be noted that the combination of the resistor 33 and the condenser 34 has a low impedance at the intermediate frequency and a very high resistance to direct current. Consequently practically the full high frequency voltage of the anode 32 is applied to the grid 3| and the average potential of the grid 3i during reception of signal is such that limiting occurs.

The plate 36 of the tube 23 is connected to a source of positive plate potential, which may be the source 28, by way of primary winding 38 of the wave detector or discriminator coupling transformer 33 and a plate resistor 4!. A high 76 frequency path to ground for the discriminator input is provided by the condenser 42. The plate resistor serves the important function of causing the potential at the plate 33 to fall as the plate current increases. This in effect causes the plate supply for the tube 23 to have poor regulation.

The primary of the transformer 38 is tuned by variable capacity 43 and the secondary of the transformer 33 may be tuned by a variable capacity replacing the capacity 44 or may, if desired. be permeability tuned as shown. The ends of the center tapped secondary are connected to the anodes of two diodes 43 and 41, the cathodes of which are connected to a load resistor 43. The latter is also center tapped, and there is a connection between the resistor and secondary taps. It will be understood that the elements of the diodes may be combined in a single envelope as shown, if desired. The final output of the discriminator appears across the load resistor 48 and is transferred to the succeeding parts of the receiver by conductors 48. capacity 50 serves to transfer the potential of the primary 38 to the discriminator load resistor.

In operation of the limiter shown in Figure l, the input wave of varying frequency and usually of varying amplitude appears on the antenna II and is amplified in the usual manner and converted to an intermediate frequency, which is amplified in the amplifier l4. The final output circuit of this amplifier, as stated above, includes the band pass coupling stage 26, the secondary 29 of which is connected to the grid 24 of the tube 22 and effectively to the cathode 30, so that no self bias is produced on this grid.

Since the grid to cathode impedance of the tube 22 goes to very low values during the positive portion of the input wave, the secondary of the coupling transformer 26 is heavily damped as far as average values are concerned. It might appear that the secondary impedance should be lowered to match the low grid impedance. However, the effect of the circuits of the tube 22 on the signal wave after the grid goes positive and the impedance falls is not a matter of any concern, since the signal wave has already passed the values necessary to produce the ultimate desired working portion of the signal. When the negative portions of the signal waves are applied to the tube 22, the damping is absent and there is no loss of gain in the intermediate frequency coupling unit 26.

In practice it is found that the highest limiter sensitivity is obtained by making the transformation ratio of the intermediate frequency transformer 26 as high as possible consistent with the band width required. A secondary impedance of around ten thousand ohms was found to be satisfactory for an intermediate frequency of around four megacycles. The combination of high secondary impedance and low grid impedance on positive half cycles leads to a partial limiting of the signal on the positive swings for strong signals, which helps in the over-all limiting process.

Referring to Figures 2 to 6, which show the changes in the input wave at successive points in the limiter ii, the input wave to the limiter is shown in Figure 2. This figure shows the inclusion of the zero axis of the original wave, but it should be noted that bb' should be as clase to the zero axis as possible. Figure 3 shows the plate current of the tube 22, and Figure 4 shows the potential of the plate 32 of the tube 22. In

The

aaaaeeo I now to be described. Figures 7 and 8 of the Figure 3 the current wave is cut 01! substantially along the line bb' due to plate current cut-oi! in the tube 22 by reason of its grid swinging negative beyond the cut-oi! point. The positive peaks of the input current wave still extend above the original zero axis, but show partial limitation. This latter effect, as explained above, is due to the combination of high secondary impedance and low grid impedance on the positive waves.

The phase of the signal applied to the second limiter grid 3i, from the plate 32, as shown in Figure 4, is reversed in relationship to the phase of the input signal as shown by Figure 2, so that the positive half of the input wave, which was only partly limited by the tube 22, may be completely limited by plate current cut-oil action of the grid of the tube 23. It is to be noted in Figure 4 that the entire signal variation lies above the axis ii representing zero plate voltage of the tube 22.

Figure 5 of the drawing shows the voltage on the second limiter grid 3i and also shows a shift of the zero axis from Figure 4. The coupling between the plate 32 and grid 3| is mainly capacitive since the capacity 34 has a low impedance at frequencies in the neighborhood of the selected intermediate frequency. The line 0'-0' represents zero voltage and is the main axis of Figure 4. The line marked XX is the zero axis of the original input wave. Line H is the cut-off voltage of the grid of the tube 23 and negative values of the applied signal swinging beyond this point do not afiect the plate current of the tube. The upper line bb' shows the maximum values of the positive portions of the signals applied to the grid of the tube 23.

The line H is spaced from the line b-b' by a distance equal to the desired working range of the signal wave which was discussed in connection with Figure 2 of the drawing. It will be noted that at this stage the original zero axis X-X of the wave may lie outside the area between lines H and b-b', as is shown in Figure 5. It is possible to adjust the values of circuit components so that XX may lie either above or below aa' in any relation desired.

Turning to Figure 6, the final output of the limiter is shown as a current wave, the ordinates of the curves representing values of plate current of the tube 23 and the horizontal axis representing time in the usual manner. rent wave is further varied in a novel manner to be later described to produce a quiet receiver.

The physical connection between the Plate 32 of the tube 22 and the grid 3! of tube 23 in known limiters produces symmetrical limiting by including the direct current component in the input to the second limiter. The condenser 34 across the coupling resistor will tend to prevent a symmetrical output, since it allows the grid of the tube 23 to establish some degree of selfbiasing, and with the intermediat frequency normally used of from two to ten megacycles, the coupling between the tubes 22 and 23 is predominately capacitative. At these frequencies the plate load of tube 22 will be largely limited by the shunt capacity to ground associated with the plate of this tube, the grid of the tube 23, and the coupling network. In order toprevent loss of voltage on the grid of the tub 23 th condenser 34 must be several times larger than the input capacity of the tube 23.

The presence of the resistor 33 aids in the attainment of valuable novel results in a manner hiscur-- drawing graphically illustrate the new results obtained in a limiter of this invention when the signal applied toa frequency modulation receiver drops below the level at which satisfactory limiting occurs as, for example, whenthe receiver is not tuned to a transmitter, or 'is tuned to a very distant transmitter.

Noise in the audio-output of receivers with known limiters rises rap dly under conditions just set forth and at zero excessive. This noise e ect on low or zero signal is particularly annoying when manual tuning is employed. The same noise effect isv apt to occur during the warm-up period of the receiver when the appearance of a signal, for which the receiver is set, is preceded by a period when the noise output is very high.

Flares '7 and 8 will be referred to in explaining e action of the limiter and its circuits resulting in noise elimination at low or zero signal strengths.

Noise reduction is produced by the Joint action of the resistor 33 connected between the control grid 3| and a suitable source of positive potential, and the large resistor 4| by-passed by the capacity 42, the resistor 4| being in series with the supply voltage to the plate 36 of the tube 23. With this combination, the plate voltage of the limiter tube 23 will fall to quite a low value, for

ple, several thousand ohms, which effectively shunts the discriminator primary 38. Since the discriminator primary has an impedance in typical receivers, for example, of about five thousand ohms, the low plate resistance effects a considerable reduction in discriminator output.

For the purposes of illustration, I describe below a specific example of a limiter embodying my invention and refer to typical tubes, resistors, condensers, etc., and set forth typical voltages which appear therein during operation when no signal is received and when a signal of satisfactory intensity isreceived. It must be understood that I do not intend to limit my invention to the specific details set forth in this example.

The cooperation between the two limiter tubes 22 and 23 is essentially of two kinds. First, a single tube is capable only of limiting effectively the intensity only of alternate half cycles of a wave, that is, of only one side of a wave. However, in order to have true limiting of the intensity of a wave both sides of the wave must be limited, or the unlimited side has the same kind of variation in intensity as did the original wave. Second, the first limiter is efiective in reducing greatly the effect of undesired noise pulses received with the carrier wave, which pulses, if applied directly to a final limiter, would affect its operation very undesirably. This is Y particularly true, when as with this arrangegnal this noise may be absence of a carrier wave the signal reproducer 2i is silenced, the device 23 acts by reason of its reduced resistance between its anode and cathode in the absence of a carrier wave or in the presence of a carrier wave of reduced intensity t attenuate very greatly voltages such as carrie wave or noise voltages transferred through the device 23 tothe frequency modulation detector including discharge device 46. This attenuation may be viewed as the result of damping, which causes mismatching of the anode resistance of device 23 with the input impedance of the frequency modulation detector, and which causes a great reduction in efficiency of the frequency modulation detector, whereby, for a given frequency shift of the carrier wave, less signal voltage i produced. Since this attenuation is produced whenever a carrier wave of reduced intensity is received or whenever noise voltage in the absence of a carrier wave is received, two actions simultaneously assist each other in reducing the output of the combined limiter. One of these actions is the muting or reducing of the limiter output due to the drop across resistor 4|, and the other is the great attenuation of signals transferred from the device 23 to the frequency modulation detector.

Assume now that a signal of satisfactory level is received. When a carrier wave is applied to the grid of combination limiter and signal responsive control device 23, the grid assumes a negative bias. This negative bias increases with excitation because of the steady component of rectified voltage across resistor 33 arising from rectification of the wave applied to the grid of discharge device 23 through capacitance 34. When the grid becomes negative, the anode current of the limiter 23 becomes relatively low and the voltage drop across resistor 4| is greatly reduced.

It is desirable that, in the absence of a carrier wave on the grid of limiter 23, the grid be kept at a slightly positive potential with respect to the cathode of limiter 23. Under usual conditions of reception, noise voltages are received by the receiver and appear on the grid of limiter 23 in sufilcient intensity to be rectified there and to maintain the grid of limiter 23 at a slightly negative potential with respect to the cathode of limiter 23, unless means he provided to offset this slightly negative potential and maintain the grid positive in the presence of such noise voltages. Without such means for maintaining the grid of limiter 23 positive, the impression of a strong carrier wave on the grid of limiter 23 would cause relatively little change in the anode current of limiter 23, and would reduce the usefulness of the limiter as an amplifier for the control voltage developed on the grid of limiter 23 by rectification of an incoming carrier wave.

By providing the means comprising resistors 33 and 35 for maintaining the grid of limiter 23 in the absence of a carrier wave slightly positive with' respect to the cathode of limiter'23, the space current of limiter 23 is maintained large in the absence of a carrier, even in the presence of noise voltages of substantial magnitude. Consequently, the space current changes over a very large range from this large current to a small current when a carrier wave of satisfactory intensity is rectified upon the grid of limiter 23 and produces a control voltage for amplification through limiter 23. It must be borne in mind that, while the provision of means for maintaining the grid of limiter 23 positive with respect to the cathode of limiter 23 in the absence of a carrier wave is desirable because it enhances theaction of limiter 23 in amplifying the control voltage, limiter 23 nevertheless acts to some extent in amplifying a rectified control voltage on. the grid of limiter 23 even though such a positive voltage is not applied to the grid.

Tubes 22 and 23 in this particular example are type 6SJ7 having five electrodes. The suppressor grids 5| and 52 are at cathode or ground potential. The signal grid of the tube 22, as pointed out above, is connected so as to be substantially at the potential of the cathode. At zero signal on the signal grid 24, the voltage of the screen grid 53 of the tube 22 is about 60 volts positive. The resistors 54 and 33 have a value of fifteen and thirty-three thousand ohms respectively. The voltage of the screen grid 51 of the tube 23 is about 60 volts positive. The resistors 58 and 59 also have values of fifteen and thirty-three thousand ohms, respectively. The plate voltage of the first tube is about volts positive potential, at zero signal on the grid 24. The resistor 35 has a value of fortyseven thousand ohms. The resistor 33 has a value of two hundred and twenty thousand ohms. The plate supply source indicated by the referance character 28 is 300 volts. The plate voltage of the second tube is about twenty volts posiitve with zero signal on the grid 24 of the first tube. The resistor M has a value of 82,000 ohms. The grid of the second tube 23 is about one-half volt positive when no signal is impressed on the grid 24.

When a signal of sufiicient magnitude to cause the tubes 22 and 23 to exercise their limiting function is impressed on th grid 24, the first limiter screen grid 53 assumes a potential of twenty volts. This limiter 22 exercises its limiting function at values of three-tenths to sixtenths of a volt R. M. S. 'on the grid 24. The plate 32 assumes a potential of one hundred seventy five volts positive. The grid of the second tube, with a limiting signal on the grid 24, changes from one-half volt positive to six volts negative. The screen grid 51 of the second limiter goes from sixty volts positive to fifty five volts positive with a limiting signal on the grid 24. The voltage on the plate 36 increase from twenty volts positive to one hundred twenty volts positive, increasing the limiter sensitivity in the manner pointed out above for signals of limiting intensity. The curves of Figures 7 and 8 show this feature of a limiter embodying the present invention. Limiter action is enhanced by the decrease in the screen grid potentials as the limiter tubes exercise their limiting functions.

Curve 62 of Figure 7 of the drawing shows the direct current plate voltage of the tube 23 as a function of the input voltage to the receiver. Curve 6| represents the direct current plate voltage that would be observed at plate 36 during operation of the limiter if the coupling between tubes 22 and 23 were replaced by a heretofore known type such as resistance capacity coupling in which the resistor 33 or it equivalent is absent. It will be observed that when the signal input is low, the limiter output is relatively large and, as shown by the slope of th curve, increases rapidly. Even with very small increments of signal asaaaao I input, this large initial sensitivity of previously known limiters causes considerable noise to appear in the loud speaker. Curve 62 illustrates the operation of the limiterof this invention and it will be noted that for zero signal input the output of the limiter is small and stays substantially constant as the signal increases at the low levels. At a point where the signal becomes appreciable the slope of the curve has a high value and reache a maximum value at a point substantially where the incoming signal is at a useful level. This maximum sensitivity is maintained with larger signals as shown by the straight portion of the curve. The anode to grid connection 33 of Figure 1 is effective in producing the initial portion of the curve 62, since it holds the plate voltage of the tube 23 at a low value until the incoming signal is of an amplitude sufiiciently great to be effectively limited in the limiter l6 thereby to remove undesired component causing noise. At this level the plate voltage rises rapidly and full output is obtained from the discriminator, or detecting device, almost immediately. The action of connection 33 holds the grid at a slightly positive potential with respect to the cathode and, therefore, decreases the plate potential by maintaining the direct current component of th plate current at a maximum value.

Curve 63 of Figure 8 shows the rectified voltage across one-half of the discriminator output load element 48 wh ch is substantially a direct measure of the limiter output. It will be noted that this curve rises above the curve 64 for a receiver employing a heretofore known type limiter, as referred to above, and this abrupt increase or change of slope coincides in time with the change of slope of the curve 62.

From the foregoing, it will be seen that the limiter of this invention functions to provide noise free reception from signals having a characteristic other than the amplitude modulated in accordance with speech, music or other intelligence. The limiter of thi invention also provides a quiet receiver when it is first placed in operation and when it is tuned to signals of low amplitude.

While the invention has been described and explained in detail in connection with several illustrative embodiments thereof, it is to be understood that the invention may be embodied in other forms, and, therefore, the invention is not limited except as indicated by the terms and scope of the appended claims.

What is claimed is:

1. A voltage limiting device comprising space discharge devices in a plurality of stages, aresistor in the output circuit of one of said space discharge devices and in the input circuit of the immediately succeeding discharge device by means of which a potential is applied to the input circuit of said succeeding discharge device in accordance with signals rec ived by said voltage l miting device, a sig a transfer path around said resistor, means to limit to a predetermined value in the output circuits the peaks corresponding to the negative voltage peaks'impressed upon the input circuits of eachof said space discharge devices, and means for associating said space d scharge devices with each other in such manner that both positive and negative peaks of the voltage wave impressed upon said limiting device are subjected to the limiting action.

2. An electric wave limiting device comprising grid controlled space discharge devices in a plurality of stages, a coupling means in the input circuit of the first said space discharge device '5 for applying; a signal on its grid, a coupling means connected to the output of first said discharge device, a low impedance connection between last said coupling means and the input electrode of second said space discharge device,

the grid of first said space discharge device being unbiased, means for biasing the grid of second said space discharge device so that it becomes positive when the wave applied to the input circuit of said limiting device falls below a predetermined value, means to limit to'a predetermined value in the output circuits the peaks corresponding to the negative voltage peaks impressed upon the input circuits of each of said space discharge devices, and means for associating said space discharge devices with each other in such a manner that both peaks of the voltage wave impressed upon said limiting device are subjected to the limiting action.

3. A radio receiver comprising an amplifier, a.

detector, a wave limiting device interposed-between said amplifier and detector,- said limiting device comprising a plurality of electronic devices associated in cascade, an input connection for said wave limiting device, a cascade connection between two of said electronic devices, said cascade connection comprising a conductive circuit, means cooperating with said conductive circuit to transfer signal frequencies, said conductive circuit causing the output of the second one of said electronic devices to which said signal .plifier and said wave converting device for limiting the positive and negative peaks of a desired signal wave to a substantially uniform value, said limiting device comprising a plurality of electronic devices associated in cascade, an input connection for said wave limiting device, a cascade connection between two successive electronic devices, said cascade connection comprising a conductive circuit and means to transfer signals of a desired frequency, means including said conductive circuit for maintaining the output electrode of said second successive electronic device at a higher potential during operation of the limiting devicethan during inactivity of the tronic devices in cascade, the first of said electronic devices comprising a control electrode, a cathode, a screen electrode, and an output electrode, an input circuit connected between said control electrode and said cathode consisting of a wave coupling device, a source of positive biasing potential for said output electrode, a connection from said output electrode to said source comprising a resistor, a connection from said screen electrode to said source of positive biasing potential, said second electronic device comprising a control electrode, a cathode, a screen electrode, and an output electrode, a conductive con-' pacity shunting said conductive connection to transfer signal energy of a desired frequency between said electrodes, a connection from said output electrode of said second electronic device to said source of biasing potential including a resistance, a connection from said screen electrode of said second electronic device to said source, said connections to said screen electrodes of both said electronic devices serving to reduce the voltage on said screen electrodes during limiting whereby to increase the wave limiting efiect of said wave limiting device.

6. In a, radio receiver, an amplifier including a coupling device, a device for converting frequency variations to amplitude variations, a wave limiter interposed between said amplifier and said converting device, said limiter comprising a vacuum tube having a cathode, an input electrode and an output electrode, an input circuit for said tube consisting of connections from said input electrode to said coupling device and from said cathode to said coupling device, means to impress a wave on said coupling device, a second vacuum tube, said second tube having a cathode, an anode and an input electrode, a conductive connection from said output electrode of said first tube to said input electrode of said second tube, low impedance means to supply signal frequencies directly from said first tube output electrode to said second tube input electrode, said output electrode of said first tube and said input electrode of said second tube being connected to the cathode of said second tube, said connection including a source of positive potential and a high resistance, said low impedance means serving to transfer waves having the positive peaks thereof reduced to a substantially uniform value from said output electrode of said first tube to said second tube input electrode, and a connection from the output electrode of said second tube to said variation converting device, whereby to transfer desired waves having their positive and negative peaks reduced to a substantially uniform value and containing substantially only components of frequencyvariations bearing intelligence to be supplied to a translating device.

7. A wave limiting device comprising two electronic devices in cascade, the first of said devices comprising an input electrode, a cathode, a screen electrode and an output electrode, an input circuit connection between said input electrode and said cathode consisting of a coupling device, a source of positive biasing potential for said output electrode, a connection from said screen electrode to said source of positive biasing potential, said second electronic device comprising an input electrode, a cathode, a screen electrode and an output electrode, a conductive connection from said output electrode of said first electronic device to said control electrode of said second electronic device, means comprising a capacity presenting substantially low impedance to signal frequencies shunting said conductive connection to transfer signal variations freely between said electrodes, a connection from said screen electrode of said second electronic device to said source of biasing potential, a connection from said output electrode of said second electronic device to said source of biasing potential including an impedance device, the space discharge path from said output electrode to said cathode of said second electronic device presenting a relatively low impedance to signal frequencies of low amplitude, whereby said second electronic device has a relatively low energy transferring ratio when said signal frequencies are of relatively low amplitude.

8. In a radio receiver, an amplifier, a detector, a wave limiter interposed between said amplifier and detector, said limiter comprising a vacuum tube having a cathode, input electrode and an output electrode, an input circuit for said tube comprising a connection from said input electrode to said cathode so that said input electrode is unbiased relative to said cathode, means to impress a wave on said input electrode, a second tubehaving a cathode, anode and input electrode, a conductive connection from said output electrode of said first tube to said input electrode of said second tube, a low impedance connection for signal transfer from said first tube output electrode to said second tube input electrode, last said connection serving to transfer waves having the positive peaks thereof reduced to a substantially uniform level from said output electrode of said first tube to said second tube input electrode, and a connection from the output electrode of said second tube to said detector to transfer waves having the positive and negative peaks thereof reduced to a substantially uniform level.

9. In a limiter, an electronic device having a control electrode, a cathode and an output electrode, means to impress an alternating current signal on said control electrode, said signal having a characteristic other than the amplitude thereof varied, a connection from said control electrode to said cathode to maintain said control electrode substantially unbiased relative to said cathode, a second electronic device having a control electrode and an output electrode, a connection from said output electrode of said first electronic device to said input electrode of said second electronic device including a low impedance path for signal frequencies, a source of positive biasing potential for said second electronic device, means for providing a voltage drop in the path from said source to said second output electrode, said voltage drop increasing as said signals decrease, to thereby cause said limiter to become less sensitive as the strength of said signals decrease.

10. In a limiter, an electronic device having an input electrode, a cathode and an output electrode, means to impress on said input electrode an alternating current signal having a characteristic other than the amplitude thereof varied, a path from said input electrode to said cathode to maintain said input electrode substantially unbiased relative to said cathode, a second electronic device having a control electrode and an output electrode, a connection from said output electrode of said first electronic device to said input electrode of said second electronic device including a low impedance path for signal frequencies, a source of positive biasing potential for the output electrode of last said electronic device, means for providing a voltage drop in the path from said source to last said second output electrode, said voltage drop increasing as said signals decrease, to thereby cause said second electronic device to become less sensitive as the strength of said alternating current signals decrease, a wave detecting device comprising an input impedance included in the output circuit of said second electronic device, said second electronic device having a low interelectrode impedance in the absence of signals whereby to substantially short circuit said wave detecting device and prevent transfer of undesired frequencies to and through said wave detector. 11. In a limiter, an electronic device having a cathode, anode, signal electrode, and a screen electrode, a source of potential for said anode,

means'connecting said anode to said source, said source serving to bias said screen electrode, means for supplying to said signal electrode a signal which swings the voltage of the signal electrode beyond the point of plate current cutoff, and

means to decrease the biasing potential on said screen electrode as an increased signal potential is applied to said signal electrode to thereby as- 'sist in causing a decrease in amplitude of signal cause the potential on said screen electrodes to decrease as the signal increases, to thereby assist in causing alternate signal half-cycles to be limited in intensity in one of said devices, means to reverse the polarity of signal potentials impressed on the input electrode of said second electronic device, said reduced screen electrode potential assisting said second electronic device to limit the other alternate signal half-cycles.

13. In a carrier wave receiver, two electronic devices in cascade, the first of said electronic devices having an input circuit and an output circuit, said second electronic device comprising a control electrode, a cathode, a screen electrode and an output electrode, a conductive connection from said output circuit of said first electronic device to said control electrode of said second electronic device, means comprising a capacity shunting said conductive connection to transfer signal energy of a desired frequency between said electrodes, a connection from said output electrode of said second electronic device to a source of biasing potential, including a'resistance, a connection from said screen electrode of said second electronic device to a source of biasing potential, said connection to said screen electrode of said second electronic device serving to reduce the voltage from said screen electrode during limiting whereby. to increase the wave limiting effect of said second electronic device.

14. In a carrier wave receiver, two electronic devices in cascade, the first of said devices having an input circuit and an output circuit, said second electronic device comprising an input electrode, a cathode, a screen electrode and an output electrode, a conductive connection from the output circuit of said first electronic device to said control electrode of said second electronic device, means comprising a capacity presenting substantially low impedance to signal frequencies shunting said conductive connection to transfer signal variations freely between said output circuit and said control electrode, a, connection from said screen electrode of said second electronic device to a source of biasing potential, and a connection from said output electrode of said second electronic device to a source of biasing potential including an impedance device, the space discharge path from said output electrode to said cathode of said second electronic device presenting a relatively low impedance to signal frequenciesof low amplitude whereby said second electronic device has a relatively low energy transferring ratio when said signal'frequencies are of relatively low amplitude.

15. In a limiter, an electronic device having a control electrode, means to impress signals on said control electrode,an output electrode for said electronic device, a source of positive biasing potential for said output electrode, means gm providing a voltage drop in the path from said source to-said output electrode, said voltage 4 drop increasing as said signals decrease, to thereby cause said electronic device to become less sensitive as the strength of said signals decreases, a wave detecting device comprising an input impedance included in the output circuit of said'electronic device, said electronic device having a low inter-electrodeimpedance during absence of signals whereby substantially to short circuit said wave detecting device and prevent transfer of undesired voltages to and throug said wave detector.

16. In a signal limiter, a plurality of electronic devices, each having a cathode, anode, a signal electrode and a screen electrode, means to ap-.

ply signals to said signal electrode of one of said electronic devices, means to apply a positive potential to said anodes, means to apply a potential to said screen electrodes from said last named means, means to' cause the potential on said screen electrodes to decrease in response to increase of the signal potentials for a given polarity of the half waves, to thereby assist in causing said signals to be limited on a given polarity of the signal waves, and means to reverse the polarity of signals impressed on the input electrode of a successive electronic device, said reduced screen electrode potential assisting said successive electronic device to limit portions of the signals having a different polarity.

17. In a limiter, in, combination, an electronic device having an anode, means for supplying a signal to said electronic device, means for pre-- venting supply of one set of peaks of said signal to said anode, another electronic device having a control grid and an anode, a capacitance and a resistance in parallel connected to first said anode, said control grid being connected exclusively to said capacitance and resistance, and means for preventing supply of the other set of peaks of said signal to last said anode.

18. In a limiter, in combination, an electronic device having an anode, means for supplying a signal to said electronic device, means for preventing supply of one set of peaks of said signal to said anode, another electronic device having a control grid and an anode, a capacitance and a resistance in parallel connected to first said anode, said control grid being connected exclusively to said capacitance and resistance,

' means for preventing supply of the other set of peaks of said signal to last said anode, and a source of high positive voltage connected to first said anode and through said resistance means to said control grid.

19. In a limiter, in combination, an electronic device having an anode, means for supplying a signal to said electronic device, means for preventing supply of one set of peaks of said signal to said anode, another electronic device having a control grid and an anode, a capacitance and a resistance in parallel connected to first said anode, said control grid being connected exclusively to said capacitance and resistance,

means for preventing supply of the other set of peaks of said signal to last said anode, a source of positive voltage, a resistor, and means connecting said source or positive voltage through said resistor to first said anode and to said control grid through said resistor and first said resistance.

20. A wave amplitude limiting device for a si nal transmission system comprising two electronic devices, means associated with each device for preventing one set of peaks of a supplied signal from reaching the anode of said device, said first electronic device being effective to reduce the effect of undesired waves of high amplitude upon the second electronic device and the second electronic device being a screen grid tube, and means associated therewith for decreasing the plate voltage of said screen grid tube in response to a decrease in the intensity of said supplied signal below a predetermined intensity whereby both the plate resistance and noise output of said tube are radically decreased.

21. A wave amplitude limiting device for a signal transmission system comprising two electronic devices, means associated with each device for preventing one set of peaks of a supplied signal from reaching the anode of said device, said first electronic device being efiective to reduce the efiect of undesired waves of high amplitude upon the second electronic device and the second electronic device being a screen grid tube, a high resistance connected to the anode of said screen grid tube, a source of high posi-.

tive voltage connected to said high resistance, and means associated therewith for decreasing the plate voltage of said screen grid tube in response to a decrease in the intensity of said supplied Signal below a predetermined intensity whereby both the plate resistance and noise output of said tube are radically decreased.

22. A wave amplitude limiting device for a signal transmission system comprising two elec-.

tronic devices, means associated with each device for preventing one set of peaks of a supplied signal from reaching the anode oi. said device, said first electronic device being effective to reduce the effect of undesired waves of high amplitude upon the second electronic device and the second electronic device being a screen grid tube, a high resistance connected to the anode of said screen grid tube, a source of high positive voltage connected to said high resistance, a resistance connected between the anode of the first electronic device and the control grid of said screen grid tube, a source of high voltage connected to the anode of first said electronic device and to said resistance, said resistances acting in response to signal intensities below a predetermined intensity to cause the control grid to become positive and the plate resistance and the noise output of said screen grid tube to be radically decreased.

23. A wave amplitude limiting device for a signal transmission system comprising an electronic device, a screen grid tube, means associated with said device and with said tube for preventing one set of peaks of a supplied signal from reaching the anode thereof, a resistance and capac tance arranged in parallel, connected to the control grid of the screen grid tube and connected to the anode of said electronic device and to a source of high positive voltage, and means including said source of high positive voltage for rendering said control grid positive when the signal supplied thereto through said capacitance is inadequate, said resistance being such! that the grid becomes negative in response -to reception of a signal'whose intensity is above a predetermined intensity.

24.1 A wave amplitude limiting device for a signal transmission system comprising a source 01' a signal modulated wave, means for limiting the intensity or a wave from said source below a predetermined intensity, for rectifying said wave to produce a uni-directional potential, and for producing a change in a characteristic of said discharge path in response to said uni-dlrectional potential, said limiting, rectifying and producing means comprising a single electron discharge path having a control electrode, means for utilizing said wave of limited intensity from said discharge path, and means responsive to a change in said characteristic of said path for increasing the attenuation between said path and said utilization means upon a decrease in the intensity of said carrier wave from said source, said last means comprising means for impressing a positive average bias potential upon said control electrode.

25. A wave amplitude limiting device for a signal transmission system comprising a source of a carrier wave, means including two cascade connected electron discharge devices for limiting successively the intensity of alternate half cycles of a carrier wave from said source below a predetermined intensity, the first of said devices acting to reduce the intensity of peaks of a wave high with respect to said predetermined intensity impinging upon said second device, said second device having a control electrode, means for changing an average bias potential upon said control electrode in accordance with the intensity of a carrier wave from said source whereby the output of the second device is made smaller than said predetermined intensity, and means for impressing a positive average bias potential on said control electrode to increase said output to said predetermined intensity.

26. A wave amplitude limiting device for a signal transmission system comprising an electron discharge device having an anode, a cathode, and a control electrode, means for impressing between said control electrode and cathode alternating potentials to be limited in amplitude, means for rectifying said potentials to produce an average negative bias potential on said control electrode, a utilization circuit for said alternating potentials connected between said anode and cathode, means for biasing said control electrode to a positive potential with respect to said cathode in the absence of said alternating potentials whereby in the absence of said alternating potentials on said control electrode the average discharge current through said device increases, andmeans responsive to such increase of average discharge current through said device in the absence of said alternating potentials for substantially decreasing the amplification of said discharge device whereby large alternating potentials between said control electrode and cathode of said device are amplified with greater amplification through said device and transmitted to said utilization circuit in proportionately reater intensity than small alternating potentials, said large potentials being sufllcient to cause limiting action in said device.-

C. WESLEY CARNAHAN.

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