US2138123A - Static elimination circuit - Google Patents

Description

Nov. 29, 1938. E. M. SARGENT v STATIC ELIMINATION CIRCUIT Filed May r', 1935 2 Sheets-Sheet l HON ' INVENTOR EDWARD M. SARGE/VT. f? I a? j a A rz ws.

Nov. 29, 1938; E. M. SARGENT STATIC ELIMI NATION C IRCUIT Filed May 7, 1955 2 Sheets-Sheet 2 INVENTORY EDWARD M. SARGENT.

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TTO RN EYS Patented Nov: 29, 1938 UNITED STATES PATENT OFFICE Claims.

5 of the effect induced therein by static discharges.

Among the objects of my invention are: To

provide a means and method of preventing impact excitation of amplifying circuits in radio receivers; to provide a means and method of preventing flywheel effect of tuned circuits in conjunction with thermionic tube amplifiers from prolonging and making audible static interference; to provide a means and method of preventing shock excitation of successive amplifying stages in radio reception by an initial static impulse; to provide a means and method for preventing the prolongation of static discharges in radio amplifying circuits; to provide a static blocking circuit; toprovide a means and method for damping static impulses without substantial interference with the signal passing through the same circuit; and to provide a simple and effective method of reducing static interference in radio reception. My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and-utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limitmyself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, 'within the scope of the appended claims.

In the-drawings:

Figure 1 is a diagrammatic circuit reduced to lowest terms of one modification of my invention.

Figure 2 is another circuit diagram reduced to lowest terms showing another modification embodying the ,same invention.

My invention broadly relates to the use of a diode tube, or more particularly, a thermionic tube having an emitter and a collecting anode, this tube being inserted in series with the connection between a signal source and a, tuned radio frequency amplifier stage when the invention is applied to radio receivers. I am fully aware that .diodes have been used in various ways in radio receiving sets prior to the instant invention and that in such prior uses the diodes havebeen used to solve certain problems involved in static elimination. The objective, however, :of substantially all these prior systems is that of limiting staticvoltage to the signal level. In

other words, the prior art uses the limiting effect due to saturation of the diode to accomplish such limitation. Assuming that such prior art succeeded, the best condition which could be obtained would be an equal amplitude for both static and signal. While this condition might conceivably be of value in radio telegraph reception, it is of no aid in radiophone or broadcast reception.

In the latter case, the carrier level is many times the amplitude of the weaker voice modulations of the carrier; hence, even with such prior art limiting systems working at the optimum condition, a ratio of ten to one or more is still possible between the static voltage and the I net signal value. I utilize the diode in conjunction with radio reception in an entirely different manner.

I have found that the so-calledflywheel effect of tuned circuits greatly prolongs and makes audible, static interference. In the analysis of :the static discharges which are commonly received on the antenna of a radio receptor, it is safe to assume that the log decrement of the static discharge will not be less than .8; this being based on an assumption that legal log decrement for spark transmitters is .2 and that such spark signalstune at least four times as sharp as the average static impulses. The number of waves, therefore, in a decaying static wave train necessary to decrease the amplitude to one per cent of the 4.6 1og decrement log decrement If then, we apply this formula to a static discharge of one million volts at a frequency of one thousand kilocycles, we obtain the following elapsed times: i

Number Total time Amplitude voltage cycles elapse d Volts Sec. 1, 000, 000 0 0 Thus, an impact on the antenna of even such an excessive voltage would be reduced to one .microvolt; the sensitivity limit of the most sensitive receiverin use to-day, in slightly more than 1/25,000 of a second and in spite of its tremendous. initial voltage would be inaudible because original is given by the formula the total period of its existence at sufficient energy content to actuate a sensitive receiver would be too short a period for it to affect the human ear.

It is obvious, therefore, that this original decrement is not maintained and that something happens in, the receiver itself to prolong the effect of the impulse. The flywheel effectof tuned circuits is partially responsible and the better the circuits, the more pronounced the effect. I have also found that a second cause is the concentration of electrons between the grid and cathode of the receiving tube held there by the negative grid charge of the tube. A single impulse from the static voltagein the positive direction will free this entire space charge giving a resultant current, for one or two cycles, equal to hundreds or thousands of times the am,- plitude of the current which could be continuously released by an undamped carrier of the same amplitude. Because of flywheel effect, this disproprotionate resultant current, induced in a subsequent tuned circuit of low decrement, would spacecharge is replaced, thus the static actually modulates the carrier. When this happens, there is no wayin which such modulation can be eliminated later in the circuit. Such a condition, however, may be entirely eliminated by the use of a series diode as there is practically no space charge in the diode unless the diode has a negative bias (cold element negative with respect to emitter) and therefore both static and the desired signal can pass through the diode only by drawing electrons directly from the cathode,

a slower process that favours the continuous undamped carrier over that of the shock excitation given to it by the static impulse.

My invention, therefore, comprises broadly the use of diode'circuits in conjunction with regular low decrement circuits to damp out flywheel may be balanced to maintain the overall capacity at a minimum. Such capacity would tend to bypass the diode action of the tube and allow a strong static Voltage to-reach a point in the circuit where it would become audible. I also utilize a form of limitation entirely different from that described in the prior art. I do not try to limit the static to the signal level but to limit static voltage to a maximum which will not allow it to swing the grids of the various space amplifier tubes positive or nearly'positive. For example, if the amplifier tube has a grid bias of three 7 volts'negative, a static impulse could swing it to two volts or even one volt negative without doing a great deal of harm. If this happened to the input tube on areceiver having a one microvolt sensitivity and a one microvolt signal, there would be a ratio of two million to one between the harmless static voltage and the signal voltage before the grid would go sufficiently positive to release space charge energy. 5 Specifically, I use a system employing a pair of diodes in series with the signal current so inserted that their capacity neutralizes each other. I may also use a single series diode with either no bias or a positive bias and a neutralizing '10 capacity, or again a single diode having special electrodes reducing the capacity to a minimum. I also prefer to utilize a sharply tuned circuit in conjunction with the diode in order that the damping factor as related to the signal passing 15 through the diode may be corrected in order to give the necessary selectivity. The diode, being resistive is a strong damping factor in a tuned circuit.

' Referring directly to the embodiment in Figure 20 1 an antenna circuit I is coupled to a sharply 'tuned'intermediate or link circuit 2 which in turn couples to a tuned circuit comprising a inductance 4 and a variable capacity 5. This tuned circuit feeds through a connecting wire 6 to ;25 diode l which may be in the form of a diode or a standard three-electrode tube or triode having the grid 9 and plate ID coupled together to form a diode. The cathode may conveniently be a unipotential surface I l heated by the usual type of {330 heater l2. H r Y The cathode II connects to a primary coil l3 of a multi-winding transformer having a secondary M tuned by a secondary conde'nser I5. Thecurrent through primary winding I3 passes toa35 ground through a resistor I6 shunted by a capacity I'l. Connected to wire 6 ,which leads to the anode of the diode is a small neutralizing condenser I9 in series with a second primary coil 20 which is wound in the opposite direction to 4 that of primary I3, the inductance 20 thence being connected tothe input circuit between the inductance 4 and blocking condenser 2|. Bias is applied by bias assembly 22 through resistor 23. The output of the multi-winding transformeraa comprising secondary l4 and secondary tuning condenser I5 is applied to the, grid 24 of. a radio frequency pentode25, provided with the usual biasing assembly 26,. the output of the pentode passing into a tuned output circuit 21. Other 5 usual amplifying tube stages are deemed full equivalents. Coupled to the output circuit 21 is a sharply tuned diode input circuit 29 feeding a second diode 30 hooked up 'in the identical manner with diode 1, the output of diode 30 J5 appearing across terminal 3| and beinglapplied preferably to another pentode stage to form a cascade amplifier. Y

In operation, it will be seen that the input voltage is applied through diode 1 to the pentodecs 25, the output of pentode 25 being applied to other amplifying tubes through diode 30. Referring directly to diode 1 it will be seen that inasmuch as primary 20 is wound in the opposite direction to primary l3, that if neutralizing 5.1

condenser 19 is so adjusted as to equal the capacpotentials, they are of extremely short duration. When a high voltage pulse of' static type is impressed upon the grid of the conventional vacuum tube, such as pentode .25, for example, it can release a disproportionately large amount of energy into the output circuit of the amplifying tube, during a positive half cycle, because of the high space charge collected behind the grid.

The large instantaneous pulse thus released causes impact excitation of the tuned circuits in the output of the amplifying tube, which in turn impresses high voltage pulses of longer duration upon the next amplifying tube. Where the first tube is a diode, however, .as in my present invention, there is no such reservoir of energy waiting to be tapped. The diode acts as a strong damping factor and the short period, high voltage energy of the static exerts itself in giving some slight additional acceleration toelectrons passing between cathode and anode, but usually not sufficient to cause impact excitation of the output circuit. There is of course, some small kick applied to the output circuits but since a diode is preferably used between each amplification stage, the suppressing action prevents build-up of the interference to the point where it can unduly prolong its existence.

The intermediate sharply tuned circuits 2 and 29 are used to provide the necessary selectivity since the damping of the signal introduced by the diode would otherwise broaden the tuning too greatly. It is of course to be understood that diode may be fed by a beat frequency oscillator as well as with the input signal and the transformer l3--i42ll may be tuned to an intermediate frequency instead of to the radio frequency. The result, however, is exactly the same. t will also be understood that each of the circuits herein described should be properly shielded as is well known in the art, the indication of shielding being omitted here for purposes of simplicity.

I may also desire to utilize a double diode system which need not be neutralized as the capacity of the two diodes can be used to produce an inherent neutralization; and in addition, I am able to obtain a limitation of static voltage to a maximum that will not allow it to swing the grid of the following tricde, tetrode, or other amplifying tube, positive or approaching a positive potential.

Referring directly to Figure 2, the antenna system I is coupled to the sharply tuned intermediate circuit 2, and thence to the input circuit 45 as before. Connection 3, however, in this case passes to the anodes 32 and 33 of two diodes 34 and 35. Cathode 36 of diode 34 passes through first primary l2 and a resistor 4i shunted by a variable capacitance 3D to ground. Cathode 31 of diode 35 passes through the second primary and thence is connected by means of wire 33 to the input circuit between a series capacity 44 and a bias resistor 45 supplied by a variable bias battery 42, the positive end of which is grounded. The side of capacity id opposite the connection point of wire 33 is also grounded. The secondary M of the multi-winding transformer l2, I4, 20 is connected directly to pentode as is shown in Fig. 1. However, I do not wish to limit myself to this structure alone for it is very apparent that the entire circuit shown in Fig. 2 could be built as an attachment which might be placed ahead of any ordinary receiver with output of transformer l4 being fed directly to the antenna and ground connections of the receiver. Of course, with this arrangement the pentode 25 and tuning condenser !5 would preferably be omitted. It is also possible to place another pair of diodes connected as shown in Fig. 2 between other amplifying tubes in a cascade 1In the double diode static blocking circuit,

' the cathode 36 returns to ground through variable condenser 40 and resistor ll to duplicate as far as possible in diode 34 the DC. characteristic of diode 35 and to have the same capacitative .efiect as the variable condenser E9 of Fig. 1. Cathode 3'! returns to. the input circuit, without bias. In operation, the bias assembly 4245 places a negative bias on diode 33 which renders At the point that diode 34 pacity of the two diodes, being the same, acts together to neutralize the capacity current and only diode current from unbiased diode 35 flows under non-interference conditions.

I thus obtain the shock reduction by the use of the diodes with neutralization of capacity current; and in addition, provide a limitation of the static voltage to a maximum which will not allow it to swing the grid of the following amplifier tube positive. It can be seen, of course, that in a circuit such as I have described, that there is some loss of energy due to the use of the diodes. I find, however, that this energy loss may be easily made up for by the use of an additional amplifying stage. The use of such an additional stage is not at all objectionable because of the extreme quietness of the circuit.

While I believe that the theory of operation above described is the correct one, I do not wish to be bound thereby, but I have found in actual practice there are two factors that indicate the correctness of the theory as outlined, one, indicating directly, the other indirectly. The indirect evidence is that where a signal is so disrupted by static as to be unintelligible a modulation meter in the output circuit will faithfully follow the signal and respond to the static little if at all, thus, indicating that the static impulses, although large in initial amplitude, have a relatively short period of duration. The second factor directly observed is that the circuit as described has been built and is extraordinarily quiet under very bad static conditions.

I claim:

1. In a radio frequency amplifying stage having a space charge amplifying tube therein, a tuned circuit excited by radio frequency currents, and a damping link between said circuit and said tube comprising a pair of diodes connected in parallel, one of said diodes being negatively biased, means for equalizing the capacitative reactance of said parallel diode paths, and. means for feeding the output of both diodes to said tube in opposite phase relation.

2. In a radio frequency amplifying stage having a space charge amplifying tube therein, a tuned circuit excited by radio frequency currents, and a damping link between said circuit and said tube comprising a pair of diodes connected in parallel, one of said diodes being negatively biased to reduce space current to zero, and. means for feeding the output of both diodes to said tube in opposite phase relation, both of said diodes having substantially the same internal capacity, and means for equalizing the capacitative reactance of said parallel diode circuits.

3. In a radio frequency amplifying stage havmg a space charge amplifying tube therein, a tuned circuit excited by radio frequency currents,

and a damping link between said circuit and d ing a space charge amplifying tube therein, a

tuned circuit excited by radio frequency currents,

and a damping link between said circuit and said tube comprising a pair of diodes connected in parallel, one of said diodes having a zero bias and the other diode having a negative bias, means for equalizing the capacitative reactance of said parallel diode circuit paths, means for feeding I the output of both diodes to said tube in opposite phase relation, and selective means for correcting the damping of the signal impulses only.

5. In a radio frequency amplifyingsstage'having a space charge amplifying tube therein, a. tuned circuit excited by radio frequency currents, and a damping link between said circuit and said tube comprising a pair of diodes connected in parallel, means for equalizing the capacitative" reactance of said parallel diode paths, means for regulating the anode voltage on one diode to a value preventing, current flow over a predetermined voltage range under dynamic conditions,

and means for feeding the output of both diodes to said tube in opposite phase relation.

EDWARD M. SARGENT.

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