US2586926A - High-frequency filter for radio receivers - Google Patents

Description

Feb. 26, 1952 J. M. DYER HIGH-FREQUENCY FILTER FOR RADIO'RECEIVERS 'Filed May 28. 1949 JNVENTOR. JOHD m. w/f

- enjoy, no detailed discussion thereof is deemed Patented F eb. 26, `1952 UNITED STATES PATENT OFFICE J HIGH-FREQUENCY FILTER EUR RADIO RCEIVERS John M. Dyer, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, P a'., a corpoapplicationMay 28, 194;?, ,Serial 95,945

4 claims. (121.2459520) kthus saved, the omission of the line transformer makes impossible the complete line-to-chassis isolation ordinarily afforded by the transformer. Certain of these transformer-less receivers are capable of operating from either A.C. or D.-C. lines, while others, particularly those employing conventional voltage-doubler power supply circuits, operate only on A.C. power. All receivers of this general class, however, have the common feature that the cathode heaters are connected serially across the A.C. or D.C. supply line as opposed to being connected in parallel across a low-voltage winding of a line transformer. Due to the wide acceptance which such receivers necessary here.

The utilization of the power line, or of the line-cord, as an antenna, together with coupling means for applying radio frequency signals thus picked up to the R.F. input of the receiver, is also known in the art. Such line-cord antennas are of particular applicability in the very high frequency ranges, such as, for example, those used in commercial frequency-modulation broadcasting. For a specic showing of certain practical embodiments of line-cord antennas, reference is made to the copending United States patent application No. 745,325, filed May l, 1947, now Patent No. 2,581,983, issued January 8, .1952, in the name of Milton L. Thompson and assigned to the assignee of the present invention.

Unfortunately, the combination of a transformerless radio receiver with a line-cord antenna gives rise to a phenomenon which is very detrimental to the proper reproduction of intelligencecontained in the received radio-frequency signals. This arises from the fact that some of the R.F. energy intercepted bythe line-cord and thence applied to the input stage of the conventional receiver is practically certain to nd. its` way into the heater of the tube which is cornprised in this input stage. Since the heaters of all the tubes comprised in the receiver are, ordinarily, serially connected across. the.,line,. this results inthe doubly injuriousphenomenon of R.F. energy appearing in the other stages ofA the receiver-as well as in the line-cord where it now appears as a spurious signal which is T {eturnedto'the input stage along with the desired signal. The amplitude of' this i'ed back signal can easily become so large that the amplier circuits break into oscillation, thereby completefly disrupting the operation of the receiver. This phenomenon is, of course, most serious at very high radio 'frequencies where interelectrode capacitances in the receiver tubes play an increasingly important role in coupling energy from one electrode to another. However, under some circu'mstances, even the ylower radio frequency signal's are capable of producing these deleterious effects.'

Ithas been common practice, in the construction of transformerless receivers, to connect a Vline filter condenser directly across the'power input "terminals of the set and, consequently, intermediate the antenna portion of the line-cord and the vacuum tube heaters. Unfortunately, such afilter condenser was, ordinarily, incapable of sufficiently attenuating R.F. energy which might be fed back through the heaters. This was due to thefa'ct that the heaters had relatively low impedance to R.F. signals, so thatl a condenser of 'any practical sizeA would have presented an R.F. impedance of the same order of magnitude asthat of' the heaters themselves, thus producingfo'nly slight attenuation of the fed back signals'. Any attempt at reducing the R.1T'. im-

pedance ofthe filter condenser substantially belowthat ofthe heaterswould have necessitated theiujse of an impractical'ly bulky and prohibitively costly condenser.

V,In accordance with the invention, improved filtering means are interposed intermediate the line-cord antenna and the serially-connected vacuumtube filaments of a transformerless radio receiver. This ltering means isso arranged as to preclude the feedback 'of R..F. energy from the filaments, or heaters,vto the antenna, while, at the same time, permitting readyapplication of line power to the heaters via the same antenna.

It is, thus, a primary object of. the Vinvention to yprovide av transformerles's' radio receiver which includes a line-cord antenna and which is so arranged as to suppress radio frequency feedback .fromthe carrier-frequency circuits to the linecord antenna of the receiver.

It is another object of the invention to provide, 1n an A.C./D.C. radio receiver, ltering means,

disposed intermediate the line-cord and the vacu- .um tube heaters Yand arranged to attenuate energy in the radio frequency and intermediate frequency ranges.

These and further objects and features of the invention will become more clearly apparent from the subsequent discussion when considered in conjunction with the accompanying drawing wherein the single figure is representative of a typical transformerless radio receiver of the A.C./D.-C. type comprising a preferred embodiment of the invention.

With reference now to the drawing, there are lustrated in detail those portions of an otherwise conventional A.C./D.-C. receiver which are intimately associated with the invention, the remaining components of such receiver having been omitted in order to avoid obscuring the essential features of the invention. These omitted components are so well known and may assume so many different specific forms that no good purpose would be served by their inclusion at this point, since their particular form is entirely unrelated to the present inventive concept.

The components illustrated comprise a metal chassis I0, diagrammatically indicated by the broken line which bounds the internal components of the receiver, power supply circuits II, a conventional R.-F. amplier I2, a line-cord I3 and line-cord antenna conductor I4. The output of R.-F. amplifier I2 may be applied to the remaining components of the radio receiver collectively represented by an appropriately designated rectangle I5. The R.F. amplifier I2 is provided with a tuned tank circuit I6 which is connected in the grid circuit of a suitable amplier tube such as pentode I'I. is usually provided with a male plug at the end at which it makes connection with an external source of power, the chassis end thereof being either permanently connected between the chassis-which constitutes the: electrical ground in this embodimentand the high voltage side of the input to the power supply circuits, or else, as in the figure, the cord is provided, at the chassis end, with a female plug I8 which connects the two wires of the line-cord, respectively, to the two prongs of a recessed male plug I9 set into the chassis. In the latter case, one of the leads from chassis plug IS may be grounded to the chassis while the other lead is connected to the power supply input. In the preferred embodiment, a switch 2n is provided intermediate the normally grounded prong of plug I8 and its ground connection, this switch, when open, disconnecting the line-cord from the power supply circuits of the receiver and thus turning off the receiver.l The line-cord antenna comprises, in general terms, the line-oord I3 and the conductor I4 which interconnects the tank circuit I6 with a point on the line-cord at which the magnitude of the R.F. energy induced therein by the received signals is relatively large. Such a point may be found at a distance, measured from plug i8 along the line-cord, of approximately oneeighth to one-quarter wavelength of the R.F. energy. ,It will be understood that, in most practical receivers, where it is desired to receive R.F. energy at several dierent frequencies, the positioning of this conductor along the line-cord will have to result from a compromise between the specific requirements for each of the frequencies which it is desired to receive. A more detailed analysis of these positioning requirements lies outside the scope of the present discussion, particularly since such an analysis is to be found in the above-referred-to copending Thompson application. One end of conductor I4 may be The line-cord I3 directly connected to the coil which constitutes one of the elements of tank circuit I6, its other end being coupled to each wire of the line-cord, for example by Way of condensers 2| and 22, respectively.

Connected to the ungrounded prong of plug I9 are the power supply circuits I I which may be of any conventional form. For example, they may comprise a number of serially connected heaters 23 which are returned to ground at the other end of the series. In addition, a conventional rectier 24 may be provided for transforming the alternating current into unidirectional current, plate potential for the various vacuum tubes of the receiver being derived between ground and one or more suitable points on the rectifier filter So far, all the elements described in the discussion of the drawing are conventional. However, in a circuit constructed in accordance with the invention, there is added a filter 2'1 disposed intermediate plug I9 and power supply circuits I I. It is in the specific construction of this filter, as well as in its cooperation with the other circuit elements that the inventive features of my novel circuit reside.

This filter comprises a series inductor vZ8 and a pair of shunt condensers 29 and 30, the inductor being connected in the ungrounded side of the input line, while the two condensers shunt this line to the chassis ground, the inductor. Such a filter is evidently well suited for the principal task at hand which is that of preventing R.-F. energy feedback from the heaters 23 to the line-cord I3. This is due to the fact that the series inductor 28 adds considerably to the inherently low series impedance of the heaters to R.F. signals, thus increasing by the same token the effectiveness of the condensers associated therewith in reducing the amplitude of such signals. At the same time, this inductor may be chosen with a sufficiently low value of inductance and inherent resistance to permit substantially unaffected transmission frequency power delivered by the line-cord.

It will be understood that, while filter 21 will,'

for convenience, be discussed as one unit, it is, essentially, made up of two distinct sections. One

, of these sections comprises shunt condenser 30,

while the other consists of the L-section filter composed of condenser 29 and inductor 28. This separation into component sections is fully warranted by the different functions fulfilled by the two sections. Specifically, condenser 30 is, as in prior art circuits, provided chiefly to prevent R.F. energy from penetrating to the heaters from the line-cord. It would fulfill this function satisfactorily even in the absence of the additional L- section 29, 28, as will be evident if it is recalled that the output impedance of the linecord is high for R.F. energy, while the impedance of the condenser is low, so that R.l'. signals from the line-cord will be attenuated in the large ratio of line-cord impedance to condenser impedance even in the absence of the L-section.

'This latter does not noticeably modify this operation, as it merely serves to accentuate somewhat the already strong attenuation of condenser 30 on line-cord R.-F. energy as hereinbefore described. Note, on the other hand, that condenser 30, acting alone, is very much less effective in attenuating R.F. energy derived from the heaters. This is due to the fact that the heater impedance is not much higher, and possibly even lower, than that of shunting condenser 3|), so that the latter one on each side of to the heaters of lowwill be effective to attenuate heater R.F. energy, only by the small ratio of heater impedance to "condenser impedance. Thus, while condenser 30,

acting alone, might, by virtue of the 'inherent favorable impedancerelationship, attenuate linecord R.-F. energy by a factor of the order of 1000 to l, it will attenuate heater R.-F.energyonly by a factor of the order of 2 to l, which is clearly insuiicient to guard effectively against R..F. feedback to the line-cord antenna. Y

It is to overcome this deficiency that the additional L- section 28, 29 is chiefiy provided, as hereinbefore fully explained. Thus, the two sections have practically diametrically opposed functions, which, however, complement each other to yield the desired overall effect of complete isolation of heaters and line-cord at R.F. frequencies.

It is clear that the particular values of.capaci tance and inductance of the constituent elements of the filter will depend to some extent upon the frequency range over which the receiver is to be operative, but, in general, these values should be selected so as to provide a filter having a cut-off frequency well below the intermediate frequency range of the receiver. `Such an arrangement has the effect of preventing energy at intermediate frequencies, which might be present in the vacuum tube heaters, from being fed back to the line-cord antenna from whence they might again be introduced into the receiver with the deleterious results hereinbefore described. Since a lter of the type here illustrated is a low-pass filter, placingr its cut-off frequency below the I.-F, range will, ordinarily, also result in eliminating the possibility of R.F. feedback from the filaments to the line-cord antenna, since the I.-F. range is, in most receivers, lower than the R.F. range. Since it is conceivable that, in some receivers, this usual order may be reversed, it may be said, in general terms, that the filter should be designed to have a cut-off frequency which is below the I.F. or the R.F. range, whichever is lower.

In the preferred embodiment, it has been found desirable to ground each of filter ccndensers 29 and 30 separately to chassis I0 and to ground the normally grounded prong of plug I9 to still another point on the chassis, care being taken to keep the connections between that prong, the intermediate switch 20 and the chassis connecting point as short as practicable.

The design criteria hereinbefore presented are universally applicable to apparatus constructed in accordance with the invention, and their specific determination is well within the province of anyone skilled in the art. By way of example, however, the following illustrative values are given for the constituent elements of filter 2l when incorporated into an A.-C./D.C. receiver specifically constructed for the reception of F.M. broadcasts in the range of 88 to 108 megacycles:

Inductor 28 microhenries 0.2 Condenser 29 micr'omicrofarads 100 Condenser 3E) do 100 It will be understood, of course, that should practical exigencias render a single filter section of the type illustrated insufficient to give the required attenuation, it may be desirable to provide several such sections, rather than to modify the components of the individual section. These and still other practical modifications of the device may be effected by those skilled in the art within the scope of my inventive concept. I therefore desire fthatccncept -tobelimited only -by` the vappended claims. e

-I claim:

l. A. power-line operatedradioreceiver comprising: a-relatively largefelectrically conductive element forming .a V:radio-frequency ground, a pair of power input terminals, a line-cord connectedfat th'egrcjceiver end thereof to said terminals and adapted to be connected, at its other end, to an external source of electrical power, means providing a path having low impedance at radio frequencies between both said terminals and said element, a radio-frequency input stage, coupling means providing a radio-frequency connection between said line-cord and the radio-frequency input circuit of said radio-frequency input stage whereby to supply to said stage radio-frequency signals intercepted by said line-cord acting as an antenna, means connecting the heaters of said vacuum tubes in series, and means including a filter having at least one series inductance element for connecting said series-connected heaters to said terminals, said filter introducing high radio-frequency attenuation between said heaters and said line-cord, thereby to reduce radio-frequency feedback from said heaters to said linecoid.

2. In a power-line operated radio receiver having an electrically-conductive chassis and a plurality of heater-type vacuum tubes, the combination comprising a pair of power input terminals, a line-cord connected at the receiver-end thereof to said terminals and adapted to be connected at its other end to an external source of electrical power, means providing a path having low impedance at radio` frequencies between both said terminals and said chassis, a radio-frequency input stage, coupling means providing a radiofrequency connection between 'said line-cord and the radio-frequency input circuit of said radiofrequency input stage whereby to supply to said stage radio-frequency signals intercepted by said line-cord acting as an antenna, means connecting the heaters of said vacuum tubes in series, and means including a lter having at least one series inductance element for connecting said series-connected heaters to said terminals, said filter introducing high radio-frequency attenuation between said heaters and said line-cord, thereby to reduce radio-frequency feedback from said heaters to said line-cord.

3. The combination claimed in claim 2, characterized in that said lter comprises at least a single 1r-section wherein the series element consists of an inductor and the shunt elements are capacitors.

4. An A.-C./D.C. radio receiver comprising a power input circuit, a radio-frequency signal input circuit, a line-cord arranged to interconnect said power input circuit and an external electrical power source, said line-cord being adapted to intercept radio frequency signals and to develop voltages corresponding to said signals, coupling means interconnecting said line-cord and said signal input circuit, said coupling means being arranged to introduce said developed voltages in said signal input circuit, internal feedback paths whereby signals introduced into said signal input circuit are fed back to said power circuit, and filtering means connected in circuit relation intermediate said line-cord and said power input circuit, said filtering means being characterized by presenting high series and low shunt impedance to said fed back signals, while simultaneously presenting high shunt and low series impedance to energy derived from Vsaid power source, said filtering means comprising a series inductor and a pair of shunt condensers, each of said condensers being connected to a corresponding terminal of said inductor and each of said condensers being separately grounded.

JOHN M. DYER.

` REFERENCES CITED The following references are of record in the le of this patent:

FOREIGN PATENTS Country Date Great Britain Jan. 1, 1931 Number

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