US2370758A - Ultra high frequency converter circuit - Google Patents

Description

March 6, 1945. l. E. THOMPSON 4 2,370,758

ULTRA-HIGH FREQUENCY CONVERTER CIRCUIT 7 Filed April 3, 1943 (2W ZF= 304/6:

Z/NABAE arm? 95 727 370/:

pom 7 0F I l I INVENTOR laA/vo E ZZ/OMPJON ATTORN EY Patented Mar. 6, 1945 ULTRA men FlCE I Leland E. Thompson, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application April 3, 1943, Serial No. 481,687

20mins.

' My present invention relates to ultra-high frequency converter circuits, and more particularly to an ultra-high frequency triode converter circult. Y

' As is well known, there are special problems involved in the reception of radio frequency signals in the ultra-high frequency band. Oneof the most important problems which confronts the designer of an ultra-high frequency receiver is to providea high sensitivity at the converter circuit itself. In a receiver utilizing no radio frequency amplification prior to the input circuit of the converter, and covering, for example, a frequency range of 225 to 400 megacycles (me), it

is also especially important to provide a high signal-to-noise ratio at the converterinput circuit.

I have found in an ultra-highfrequency converter. circuit utilizing a triode tube that increasing the magnitude of the inductance in the oathode lead, will materially increase the sensitivity and the signal-to-noise ratio of the converter up to a certain inductance value. "Ihave found that beyond this value the sensitivity drops very rapidly. It is one of the main objects of my present invention to provide an ultra-high frequency converter with an increase in sensitivity and signalto-noise ratio of at least two times. This has been obtained in an actual receiver by maintaining the converter cathode circuit at a predetermined inductive value.

- It is another important object of this invention to provide an ultra-high frequency receiver adapted to operate to receive signals whose freq y is as high as mo. (megacycles), the receiver having its signal collecting device coupled directly to the tunableinput circuit of the converter, and the local oscillator being coupled to an inductance of predetermined magnitude in the cathode lead of the converter tube.

Still other objects of my invention are to improve generally the sensitivity and efficiency of ultra-high frequency converter circuits of the triode type, and more particularly to provide a simple andefiective method of securingan appreciable increase in signal-to-noise ratio in such a converter circuit.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims;v the invention,

Fig. 2 is a purely qualitative curve to illustrate the advantage of the invention.

In Fig. l'there is shown a triode I which is used as the converter tube of the ultra-high frequency receiver to which this invention is directed. The tube l is schematically represented,-but it will be clearly understood that it will be of a type suitable for functioning as aconverterin the ultrahigh frequency band of operation of this receiver. For example, a type 955 triode could be used. Let

-' it be assumed that the'desired frequency band is of the order of 225 to 400 inc., and that the receiver is of the superheterodyne type. The, Dreliminary resonant selector circuit consists of a resonant line circuit employing capacitytuning. The numeral 2 denotes a one-quarter wave length concentric line of any well known form. The numeral 3 designates'the capacity tuning means. It will be understood that an actual physical condenser need not beemployed, since any well known device can be used to vary the capacity of the resonant line 2. The network 2-3, then, pro-; vides a tunable selector circuit adapted to be adjusted over the given frequency range of 225 to 400 me. The cathode 4 of tube l is connected toground'through an inductance coil L. The con trol grid 5 is connected to the high potential side of'circ'uit 2-3 through a grid coupling condenser 6. The numeral 1 designates the usual grid return resistor connected from grid 5 to ground. The plate 8 is connected to the +3 terminal of the direct current energizing source through a path comprising the primary winding 9 of intermediate frequency output transformer 10 and the resistor II. I

The resistor H is bypassed to ground for-the ultra-high frequency currents by condenser l2.

The. condenser i3 is connected to ground from the plate end of coil 9. The function of condenser 13.

is to bypass ultra-high frequency currents, and

. also to act as a tuning capacitor for the primary circuit 13-9 will be tuned to the operating 1. F.

value, and the latter will'have any desired value.

e For example, an I. F. of 3B mc., could be used.

The numeral I4 designates the secondary circuit of the I. F. transformer l0, and the latter will also be tuned to the operating 1. F. value. The subsequent networks are not shown, because they are not a part of this invention. It is sufficient to point out that the usual networks of a superheterodyne receiver may be employed subsequent put coil I! of the local oscillator is shown magnetically coupled to the cathode coil L. .It will,

therefore,-be seen that the local oscillations areinjected into the cathode circuit of the converter ous frequency ranges are purely illustrative, and

outthat some benefit is obtained by the adding of inductance in the cathode circuit long before the oint of optimum operation is reached. For example, at 400 megacycles the inductance L acare in no sense restrictive as to the scope of my invention.

The specific antenna, or signal collecting device, is not shown, but may be a dipole'which is magnetically coupled to the tunable input circuit 2-3 as schematically shown. If desired, there 'includethe cathode lead inside of tube 1. '10

tually consisted of a wire between 2 and 3 inches long in the form of a loop. This same inductance gave some benefit at a frequency of 200 megacycles. The total inductive magnitude of L would The type of tube used would then have some effect on how much external inductance should be added.

- The inductance L would probably be of the order can be employed a co-axial cable which is adapted to feed the ultra-high frequency signal energyto the converter input circuit. The coil L may be made to have an adjustable value, if desired. It can be a fixed inductance of the proper value to obtain the desired sensitivity. The converter tunedinput circuit, which is of the resonant line type, is a high Q circuit with a. consequent high resonant impedance. The coil L may be composed of two separate inductances in series, if de-- sired; one part is used to couple to the local oscillator, and the other part is predetermined .in value so that the total cathode inductance will provide the beneficial results desired. It should be understood that while the cathode circuit iniection of the local oscillator is convenient and effective, the local oscillator, if desired, may be coupled to the grid circuit.

I have found in actual experience with the circuit of Fig. 1 that there is .a point of optimum operation for the converter circuit. That is to say, with a predetermined value of inductance L inserted in the cathode circuit, the signal-to-noise ratio will be a maximum value. For inductance values less than the critical value which has been predeterminedthe sensitivity of the converter decreases. Furthermore, for. inductance values beyond the critical, value the sensitivity drops very rapidly. In Fig. 2 there is depicted, in a purely qualitative manner, that by-assigning the critical value of the inductance L there may be secured a greatly increased signal-to-noise ratio. Indeed, for the range :of 225 to 400 megacycles an increase, in sensitivity and signal-to-noise ratio of; two times has been obtained by adding. the proper amount ofinductance Lin the cathode lead. It has been found by actual experiment that the results obtained are not due to an. increase or-decrease in oscillatorexcitation voltage impressed. on the converter. I do not wish to limit myself to any particular theoretical explanation which would explain the reason for this rise. in sensitivity, since actual experience with; the circuit has demonstrated that such a sharp rise in sensitivity issecured. What ismost im'-- portant-in the present circuit is that the-increase in gain at the converter circuit. gives anincrease in signal-to-noise ratio of an. appreciable amount (about 6 decibels) which makes itparticularly valuable inan ultra-high frequency receiver;

1 may point out that I have found it necessary that the cathode circuit of the convertenbe in:

ductive at the selected signal: frequency'to-obtain the beneficial eiiectswhich I. have observed. The? selectivity of the receiver is dependent entirelyon'.

the I. F. band width, and not on the input-circuit:

of the converter stage. Therefore, theefiects described herein'cannot be' said to be due'to'a reduction in band width at the converter input cir cult. I do not wish it to. be-understcodith'at. the

adjustment of the. cathode? circuit-inductance. L. issharply critical, In; thatregard iris-1: pointed of 0.1 microhenry.

It may well be that the results obtained with this converter circuit may be due to the inductance in the cathode lead compensating, or in some way rendering less eifective, the electron transit time of the converter tube. The input conductance formula for avacuum tube is usually given for the case where the inductance of the cathode lead is purposely kept small, This formula shows that theinput resistance is higher whenL is low. Considering the voltages appear-- ing fromgrid 5 to ground of the tube I, due to signal voltage input the vector relations of these voltages show that it is possible to adjust hot the cathcdecircuit so that it is possible to make the maximum charge appear on the grid 5 at the time of maximum signal voltage. That is, the

alternations of the charge on the grid 5 may be made to be in phase with the signal alternating current. This condition appears to eliminate the effect of electron transit time. Indeed, this effeet becomes a factor of increasing importance tion is by no means limited. to the particular organization shown and described, but that-many modifications may be made without departing from the scope of my invention, as set forth inthe appended claims.

What I claim is:

1. In an ultra-high frequency converter stage of the type comprising a converter tube of the triode type provided with atunable input circuit adjustable over a range of 200 to 400 megacycles,

- andhaving a local oscillator constructed and arranged to inject-its oscillationsinto'the converter tube cathode circuit; the improvement which includes the direct connection of an inductancebetween the converter'tube cathode and grounclfor rendering the: cathode circuit essentially inductive, and. said inductance having a predetermined value t greatly increase the sensitivity of the converter stage.

2. A method of compensating.for electron transittime'ofia .c'o'nver-ter'tube operating: in the ultra-- high frequency'rang'e andhaving its signal inputcircuit directly coupled to-asignal. collector de==v vice;: thei' method comprising 'the-stepsof inject-'- ing; into: the converter cathode circuit locallyproduced: oscillations, and: maintaining the-cath ode: circuit-essentially inductive and of a prede=- termined magnitude such as to provide the said electron transit time compensation whereby the sensitivity of the converter greatly increased.- Eb THGM-PSONr-

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