US2321376A

US2321376A - Filter for suppression of harmonics

Info

Publication number: US2321376A
Application number: US396097A
Authority: US
Inventors: James L Finch
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-05-31
Filing date: 1941-05-31
Publication date: 1943-06-08
Anticipated expiration: 1960-06-08

Description

Patented June 8, 1943 one AES

. 2,321,375 FILTER FOR SUPPRESSION or HARMONICS James L. Finch, East Rockaway, N.

Y., as'signor v to Radio Corporation of America, a corporation of Delaware Application May 31, 1941, Serial No. 396,097

(onus-44) 8 Claims.

My invention relates to high frequency filters for suppressing undesirable harmonics of a fundamental output frequency. Such harmonics have in the past caused considerable interference with other services. In many cases no effective means have been used for their suppression and in other cases they have only been partially suppressed, or very expensive equipment has been required for their suppression.

It is an object of my invention to provide a radio frequency filter for use in a transmitter wherein the presence of harmonic components in the output of such a transmitter is substantially eliminated.

It is another object of my invention to provide a simple filter structure the elements of which are inexpensive and easily adjusted.

Other objects and advantages of my invention will be made apparent in the description to follow. This description is accompanied by a drawing in which Figure 1 shows a circuit diagram of a preferred embodiment ofthe invention; and

Fig. 2 is a table giving the impedance values of certain of the elements of the filter system as shown in Fig. 1 when adjusted to optimum operating conditions.

Referring to Fig. l, I show a transformer I! connected to receive the output of the last stage of a transmitter. The load l8 may be of any desired type such as an antenna. consists of conductors 2| and 22. I8 both have neutral points which are connected to ground. Parts I, 2, 3, 4 and 5 are variable or adjustable condensers, while parts ll, l2, I3, I 4, I5 and I6 are variable or adjustable inductors. These parts are connected in the circuit as shown and their values are so chosen for any particular fundamental operating frequency as to pass the fundamental frequency with very little attenuation but to have a very appreciable attenuating effect at all harmonic frequencies. "The input side of this filter, considered as a unit, is connected to the secondary of transformer I] through conductors l9 and 20.

Referring now to Fig. 2, I show in the table thereof certain impedance values of the several reactances indicated by reference numbers in Fig. 1. These reactances are correlated with respect to a given fundamental frequency. In the Parts I? and last column of this table the attenuation in decibels is given for the fundamental and each of the harmonics up to the seventh. The table itself is based upon actual performance records of a Its feed line certain embodiment which Iconstructed and used in a-transmitter having a push-pull fundamental freqiiency output. This table also gives impedances of combinations of these parts and the attenuation which is theoretically possible if we assume that the load impedance is 300 ohms each side of neutral for all of the frequencies I under consideration and if we assume the Q'of the inductors is 500 and of the condensers is infinity. One column gives the type of currents which predominate at a particular'frequency. As indicated in this column, the currents at the fundamental frequency and at odd harmonics of the fundamental are of the type known as push: pull. This means that at a particular instant when the current in conductor I9 is flowing in one direction the current in conductor 20 is in the opposite direction. At all even harmonic frequencies the type of current is push-push. This means that at a particular instant when the current in conductor I9 is flowing in one direction it is also flowing in this same direction in conductor 20. The return path for these currents is through the connections shown dotted, which include paths through the earth.

It should be noted that the values of the parts are so chosen that at the fundamental frequency pi type low pass filter,

these circuits act as a the cut-off frequency being set for a frequency higher than the fundamental frequency but lower than the second harmonic frequency. It should further be noted that at the second harmonic frequency reactances 3, I3, 4, I4 and I5 form a tuned short circuit to ground from conductors 2! and 22. This greatly attenuates the second harmonic otherwise flow to the antenna.

harmonic frequencies reactances 3, form a tuned short circuit between conductors 2| and 22. At the fourth harmonic parts I, 2'and 5, [6 form a tuned short circuit between conductors l9 .and 20 and ground. At the fifth and seventh harmonics and at all higher odd har- At the third monics these reactances effectively form an L currents which would 13,4 and 14' tion between said ca pacitors, .a second -shunt circuit across said output inductive, capacitive; capacitive and inductive the order stated, and -an tion between the two said capacitive elements, a

elements arranged in inductive connection from ground to the junc source of input. energy for saidfilter having a grounded center-tap, and aload connected across said output terminals also havinga center tap.

2. The combination according to claim 1 wherein the reactance values assigned to the several elements of the filter are such as to produce an attenuation of the fundamental frequency by not more than .01 db. and an attenuation of the second to seventh harmonics by not less than 35 3; The combination according to claim 1 wherein the reactance values assigned to the several elements of the filter are such as to produce an grounded terminals comprising 3 elements of the filter are such as to produce ah attenuation of the fourth harmonic by not less than 68 db.

6. The combination according to claim 1 wherein the reactance values assigned to the several elements of the filter are such as to produce an attenuation of the fifth. sixth and seventh harmonies by as much as 35 db., 4'1 db. and 37 db. respectively.

7. A two-conductor filter for passing a fundamental'frequency with minimum attenuation and for attenuating harmonics of said fundamental frequency by at least 35 decibels, said filter comprising equivalent inductances connecte'd in each of the leads from the two input terminals to the two output terminals, shunting capacitors connected across the input terminals, shunting reattenuation of the second harmonic by not less than 73' db.

4. The combination according to claim 1 wherein the reactance values assigned to the several elements of the filter are such as to produce an attenuation of the third harmonic by not less than 120 db.

5. The combination according to claim 1 wherein thereactance values assigned ot the several actances symmetrically disposed across the output terminals, and means for dissipating to ground certain of said harmonics, the last said means comprising a parallel tuned circuit connected between groundand the junction between said shunting capacitors, and an inductance connected between ground and a center tap on the circuit of said shunting reactances.

8. A low pass filter suitable for push-pull transmission of a given fundamental frequency, and having high attenuation properties with respect to certain harmonics of said frequency, said filter having an inductance in each of its two series conductors, reactances symmetrically arranged in each of two shunt circuits, the first of which is connected across the input terminals and the second across the output terminals, and a parallel resonant circuit connected to ground from a mid-tap on said first shunt circuit whereby the fourth harmonic component is dissipated to ground.

JAMES L. FINCH.