US1929874A - Prevention of parasitic oscillations - Google Patents

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Oct. 10, 1933. B. SALZBERG PREVENTION OF PARASITIC OSCILLATIONS Filed NOV. 14, 1930 Oct. 10, 1933. SALZBERG 1,929,874.

PREVENTION OF PARASITIC OSCILLATIONS Filed Nov. 14, 1930 2 Sheets-Sheet 2 N "nun" RI 6 R INVENTOR ATIIORNEY Patented Oct. 10, 1933 PATENT OFFICE UNITED STATES to Radio Corporation of America, a

tion of Delaware corpora- Application November 14, 1930 Serial No. 495,552

17 Claims. (CL 250-27) This invention has as its principal object the provision of new and useful-methods and means for the elimination of parasitic oscillations in apparatus for undulating electrical energy, and especially for the prevention of undesired oscillations along cathode or filament leads.

In a system involving one or more electron discharge devices, it is sometimes desirable to eliminate parasitic oscillations which take place along the filament leads of the devices. The main object of the present invention is to eliminate such parasitic oscillations, and this is done by causing the filament circuit to have a high impedance for the parasitic frequency to be eliminated, while providing, at the same time, a low impedance path for the operating frequency and for the filament-heating supply.

A particularly useful type of ultra-short wave length oscillator is one wherein electron discharge devices are connected in pushpull fashion, having their cathodes connected together through conductive metallic leads whose length is comparable to the working wave length, and having their intermediate electrodes maintained at high positive potentials relative to the other electrodes of the devices. By virtue of the high undirectional potential applied to the intermediate electrode, electrons are attracted from the cathode towards the intermediate electrode and pass through the interstices thereof to approach a relatively negative anode. The electrons are then slowed down, and caused to travel in a reverse direction through the intermediate electrode due to the combined attractive force of the grid and the repelling action of the anode. In the neighborhood of the cathode, the electrons are repelled by the space charge and reattracted by the grid, thus repeating the cycle of operation described and oscillating about the intermediate electrode at ultrahigh frequencies.

As the cathodes or filaments are located within the intense, high frequency electric fields which exist about the other electrodes, high frequency voltages are induced in the cathode leads, causing waves of electrical energy to travel thereover making the filament leads part of the oscillating circuit, and thereby destroying the desired mode of operation. To remove such parasitic oscillations is the main object of the present invention and this is accomplished by making the filament leads of a length such that the parasitic waves induced in the leads become self cancelling. This is accomplished, more specifically, by making the overall length of the leads and cathodes an odd number of half wave lengths. Since the long.

cathodes are subjected to high frequency fields opposite in phase at any given instant, the waves induced in the leads will be out of phase, and therefore, become self cancelling. Of course, rather than use linear leads, concentrated leads or choke coils may be utilized to good advantage to obtain the necessary effective lead length for parasitic cancellation. Still another scheme for obtaining the requisite electrical length, particularly adapted to the longer short waves. consists in the use of tuned reiector" circuits.

Another object of this invention is to supply heating energy to the filaments by way of the leads without disturbing their prearranged radio frequency characteristics, and this is accomplished by applying the energy to voltage nodal points on the leads, the branch circuits supplying the energy to the leads being made, preferably, an odd number of quarter wavelengths As the radio frequencygrounding condensers customarily used in connection with the supply leads to high frequency apparatus may effectively alter the length of the leads, a further object of the present invention is to provide for the compensation of the electrical changes in length caused by the use of such capacitors.

In some instances, the leads connecting the cathodes of a pair of electron discharge devices together may offer relatively highimpedance to energy of the desired operating frequency. To overcome this defect is a further object of the present invention and it is accomplished by making the effective lead length a whole number of wave lengths long at the operating frequency and/or providing a path coupling the cathodes of the electron discharge devices together, which path shall offer a low impedance to energy of the desired operating frequency. Of course, in the event that the filament leads do not offer an objectionably high impedance to energy of the desired operating frequency, the path may be used to supplement the transfer of high frequency energy by the cathode heating leads proper.

In accordance with well established practice before the United States Patent Ofilce, the present invention is defined in the appended claims. However, it may best be understood both as to its mode of operation and structural organization by referring to the accompanying drawings, wherein,

Figure 1 illustrates an ultra-short wave length transmitting system subject to parasitic oscillations along leads connecting together cathodes of electron discharge devices forming part of the system,

Figure 2 is an equivalent electrical circuit given to explain the presence of parasitics on the filament leads of the apparatus shown in Figure 1,

Figure 3' illustrates the choice of cathode lead length in order to eliminate undesired oscillations thereon,

Figure 4 illustrates the use of choke coils for obtaining correct lead length the coils allowing utilization of leads physically short yet electrical- 1? 1 118,

Figure 5 illustrates another compact arrangement, particularly suitable for the longer short waves,

Figure 6 illustrates a system for feeding energy to cathode leads such that their prearranged high frequency characteristics will not be disturbed,

Figures 7 and 8 illustrate the present invention applied to other forms of oscillatory circuits, and

Figure 9 illustrates a method for providing a low impedance path between the cathodes of a pushpull transmitter, for the operating frequency, and the choice of cathode lead length necessary to eliminate the undesired oscillation thereon.

Referring to Figure 1, electron discharge devices 2, 4 are shown as having their cathodes connected together by metallic conducting leads 6. The anodes of the devices are connected together through a variable, metallic U-shaped slide 8 for tuning purposes, to which a polarizing potential, preferably of a small negative value, is imparted at a voltage nodal point 10 thereof. Output energy is taken from the anode tuning circuit 8 through transmission line 12 and radiated over a suitable antenna 14.

The intermediate electrodes of electron discharge devices 2, 4, are connected together by a circuit 16 similar to anode circuit 8, circuit 16 being supplied at its voltage nodal point 18 with a high, substantially unidirectional positive potential.

Cathode heating energy is supplied through a transformer 22 through branch line 24 to lead 6. Radio frequency by-pass condensers 26 are connected in conventional form and grounded through a conductor 28.

Modulating potentials are applied from a suitable source 30 through transformer 32 to the anodes of the ultrashort wave length oscillatory system comprising electron discharge devices 2, 4 which act during oscillation generation, in pushpull fashion.

As the filaments of the electron discharge devices 2, 4 are located in the intense electric field which exist around the other electrodes of the devices 2, 4, radio frequency voltages of a parasitic frequency may be induced upon them. Since the tubes act in pushpull fashion, the cathodes are subjected to opposite instantaneous radio frequency potentials, and maximum current tends to flow through the interelectrode capacitances the overall length of filaments, chokes and lead structure, is an odd number of half wave lengths. Under such circumstances, as already indicated, the induced waves in the filament leads become self cancelling or, in other words, by virtue of the chosen length of the lead structure, the leads will have a very high impedance for induced parasitic radio frequency voltages rendering the cathode or filament circuit neutral and allowing the original oscillation circuit to remain undisturbed.

An alternative method, particularly adapte to longer waves, is illustrated in Figure 5. Here, tuned "rejector circuits, 46, are inserted in the cathode leads and tuned to the parasitic frequency or frequencies to be eliminated. Their action corresponds to that which takes place in the previously mentioned schemes. It is to be understood that more complex networks may be substituted for the elementary type of network shown, to more clearly differentiate between parasitic and operating frequencies.

In order to avoid disturbing these prearranged radio frequency characteristics by the heating energy lines, the low frequency filament heating lines should be tapped in at a voltage nodal point on thefilament leads 6, as shown in Figure 6. As illustrated, low frequency energizing lines 38 are tapped to the mid-point 40 of line 6 which, obviously, is a voltage nodal point. If desired, or if more convenient in actual construction, lines 38 may be coupled to either voltage nodal point 42 or 44.

To further minimize the effect of the cathode energizing system on radio frequency currents, supply line 38 should be made an odd number of quarter wave lengths long, for, the sending-end impedance of such lines is very high, and the supply line will have no radio frequency effect upon leads 6. As the terminating capacitors 26 tend to affect the line length, line 38 should be made slightly shorter to compensate for the effects of condensers 26.

The present invention, it is to be clearly un-, derstood, is not restricted to the oscillatory system 120 shown in Figure 1. For example, it may be applied to any conventional oscillator system.

A possible schematic arrangement is shown by the parallel-tube oscillator system of Figure 7, anode and intermediate electrode circuits being omitted for the sake of simplicity. The parasitic voltages induced in each of the filament circuits are usually in phase, in this case; and it is merely necessary, then, to take the precaution to have the filament leads of each of the tubes equal in length, or differing in length, where that is structurally necessary, by whole wave lengths of the parasitic oscillation to be eliminated. This case also corresponds, of course, to the condition which exists when the parasitic oscillations in a pushpull oscillator, or amplifier, are in pushpush. The filament heating lines 38 should be an odd number of quarter waves in length, to present a high impedance to the parasitic oscillations.

In the case of independently operating electron discharge devices, such, for example, as a single device in a cascaded system, it may be desirable to suppress parasitic frequency oscillations caused by the return of parasitic frequency energy over the filament circuit.

The parasitic oscillations may be eliminated, as before, by causing the filament circuit to have a high impedance for the parasitic oscillations and at the same time presenting a low impedance to energy of the operating frequency and to the 1 1,920,074 operating supp y potentials. Most simply this may be done, as illustrated in Figure 8, by making the overall length of the filament supply lead 6 and cathode an odd number of quarter wave lengths long at the parasitic frequency.

As already indicated, in order to compensate for the effect of the terminating condensers 26, the leads 6 should be made slightly shorter than what they would otherwise be made.

Conversely in ultra-short wave length transmitting systems, the filament leads within the tubes may have a high reactance at the operating frequency, and it may become necessary to provide another return path having a lower impedance. The optimum electrical length of radio frequency cathode return conductor is one which provides the path of lowest impedance to the operating frequency. For the pushpull transmitter shown in Figure 9, this is ms, where n=1, 2, 3, etc., and is represents the operating wave length. This path of low impedance is supplied by the fiat metal strip 50 which is coupled from the filament of one tube, outside the seal, to the filament of the other tube, the overall electrical length of the path being ms. The actual filament leads may be prevented from taking part in the radio frequency return, if desired, by adjusting their length so as to provide a high impedance at the operating frequency, in the manner previously outlined.

In the case of low reactance filament leads, the leads themselves may be used for the radio frequency return by adjusting their length so as to have a low impedance at the operating frequency. For the above case, this will be so when the overall electrical length is mo.

It is to be clearly understood, that although alternating current energization of the filaments has been illustrated in the drawings, the present invention may equally well be applied to D. C. operatedfilaments or cathodes, or cathodes of the indirectly heated type. Moreover, the various features of this invention are not limited to oscillators of the type described, but may be equally well applied to oscillators of the multielectrode type, for example, the tetrode oscillator.

Having thus described my invention, what I claim is:

1. In a system for undulatory electrical energy of relatively high frequency comprising an .electron discharge device having a cathode, an anode and an intermediate electrode, the method of eliminating undesired parasitic oscillations tending to render ineffectual a desired mode of operation of the system, which includes the step of making the effective electrical length of the energfzing leads to the cathode of the electron discharge device included in the high frequency system such that the leads ofifer high impedance to the undesired parasitic oscillations.

2. In a system for undulatory electrical energy of relatively high frequency comprising a pair of electron discharge devices having cathodes, anodes and intermediate electrodes, the cathodes of the devices being connected together through conductive leads, the method of eliminating undesired parasitic oscillations tending to render ineffectual a desired mode of operation of the system, which includes the step of making the electrical length of the conductive leads connecting the cathodes together such that waves of parasitic energy induced in the leads efiectively cancel each other.

3. In a system for undulatory ultra-short wave length electrical energy comprising a pair of pushpull connected electron discharge devices whose intermediate electrodes are maintained at high positive potentials relative to the other electrodes of the devices, the cathodes of the devices being connected together through conductive metallic leads, the method of energizing the oathodes and eliminating undesired parasitic oscillations tending to render ineffectual a desired mode of operation of the system comprising the pushpull connected electron discharge devices, which includes making the effective electrical length of the conducting leads connecting the cathodes such that waves of parasitic energy induced therein substantially cancel one another, and feeding energy for energizing the cathodes to the leads at ultra short wave length voltage nodal points.

4. In a system for undulatory electrical energy of relatively high frequency, an electron discharge device having an anode, a cathode and an intermed. ate electrode, supply leads for the cathode, means for feeding electrical energy to the supply leads for energizing the cathode thereby causing it to emit electrons, the length of the cathode leads being chosen such that they offer maximum impedance to energy of parasitic frequencies and little impedance to energy of a desired frequency.

5. In a system for undulatory electrical energy of relatively high frequency, an electron discharge device having an anode, a cathode and an intermediate electrode, supply leads for furnishing heating energy to said cathode, a transformer connected to the supply leads for supplying electrical heating energy thereto, and one or more 119 condensers connected across the secondary of said transformer, the length of the supply leads being chosen such that they offer maximum impedance to energy of parasitic frequencies, the length of the leads. for that purpose, being made such that the effects of the condenser capacity across the secondary of the transformer is compensated for.

6. In a system for undulatory electrical energy of relatively high frequency wherein it is desired to eliminate parasitic frequencies, a pair of elec- 120 tron discharge devices each having an anode, a cathode, and an intermediate electrode, metallic conductive leads for connecting the cathodes together, the length of the leads being an odd multiple of half the wave length of the energy of parasitic frequencies induced therein which it is desired to eliminate.

7. In a system for undulatory ultra-short wave length electrical energy wherein it is desired to eliminate parasitic frequencies, a pair of electron discharge devices each having an anode, a cathode and an intermediate electrode, conductive metallic leads for connecting the cathodes together, the length of the leads being chosen such that waves of ultra-short wave length energy induced 136 therein become self cancelling, and, an energy supply line for supplying energy to a voltage nodal point on said connecting leads, the energy supply line being substantially an odd number of quarter wave lengths long at, the parasitic frequencies which it is desired to eliminate.

8. In a system for undulatory electrical energy, an electron discharge device having an anode, a cathode, and leads for supplying heating energy to said cathode, and, tuned circuits serially connected in said leads offering high impedance only to energy'of predetermined frequencies.

9. In a system for undulatory electrical energy, a pair of electron discharge devices each having an anode and a cathode, metallic conductive 150 leads for connecting the cathodes together, means for supplying heating energy to said leads, and tuned circuits serially connected in said leads between said devices tuned to offer high impedance only to energy of undesired frequencies.

10. In a system for undulatory ultrashort wave length electrical energy, whereinit is desired to eliminate parasitic frequencies, a pair of electron discharge devices each having an anode a cathode and a grid electrode, ahd conductive metallic leads for connecting the cathodes together, said leads and cathodes being effectively a whole number of wave lengths long at the operating frequency whereby said cathodes and leads ofler a minimum impedance to energy of the operating frequency.

11. In a system for undulatory ultrashort wave length electrical energy wherein it is desired to eliminate parasitic frequencies, a pair of electron discharge devices each having an anode a cathode and a grid electrode, conductive metallic leads for connecting the cathodes together, a path, independent of said leads, comprising a flat metallic strip extending between said devices for capacitively coupling the cathodes thereof together, said path offering a low impedance to energy of the desired operating frequency.

12. In a system for undulatory ultrashort wave length electrical energy, a pair of electron discharge devices each having an anode, a cathode and a grid electrode, conductive metallic leads connecting the cathodes together, said leads offering a relatively high impedance to energy of the operating frequency, and a separate conductive metallic path extending between said devices, capacitively coupling the cathodes together and offering a low impedance to energy of the operating frequency.

13. A system for undulatory electrical energy of relaitvely high frequency having, in combination, a pair of electron discharge devices in pushpull relationship, each havingan anode, a cathode, and an intermediate electrode, and electrical conductive elements for connecting the cathodes together, the efiective overall length of said conductive elements and cathodes being an odd number of half wave lengths at the frequency of the parasitic energy which it is desired to eliminate from the system.

14. A system for undulatory electrical energy of relatively high frequency having, in combination, a pair of electron discharge devices in pushpull relationship, each having an anode, a cathode, and an intermediate electrode, and metallic conductive leads for connecting the cathodes together, the overall length of said leads and cathodes being equal to where n is any whole number and x and wave length of the parasitic energy induced in the cathodes which it is desired to eliminate from said system.

15. A system for undulatory electrical energy of relatively high frequency comprising an electron discharge device having a cathode, an anode, and an intermediate electrode, a source of heating energy'for said cathode, leads extending from, said source to said cathode, the effective overall electrical length of said leads and cathode being an odd multiple of one quarter of the wave length at the frequency of the parasitic energy which it is desired to eliminate from said cathode.

16. A system for undulatory electrical energy of relatively high frequency having, in combination, a pair of electron discharge devices in pushpull relationship, each having an anode, a cathode, and an intermediate electrode, and electrical conductive elements for connecting the cathodes together, the efiective overall length of said conductive elements and cathodes being an odd number of half wave lengths at the frequency of the parasitic energy which it is desired to eliminate from the system, said elements including tuned circuits which are tuned to the parasitic frequency to be eliminated.

17. In a system for undulatory ultrashort wave length electrical energy, a pair of electron discharge devices each having an anode, a cathode, and an intermediate electrode, conductive metallic leads for connecting the cathodes together, the length of the leads being an odd multiple of one half the wave length of the energy induced therein which it is desired to eliminate, and an energy supply line for supplying energy to a voltage nodal point on said connecting leads, the energy supply line being substantially an odd number of quarter wave lengths long at .the frequency of the energy which it is desired to eliminate.

BERNARD SALZBERG.

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