US2303388A - Tuning impedance for high radio frequencies - Google Patents

Description

Dec. 1, 1942. G. E. PRAY TUNING IMPEDANCE FOR HIGH RADIO FREQUENCIES Filed Aug. 2, 1941 2 Sheets-Sheet l INVENTOI? ear e E H-a iagw v 1942- r G. E. PRAY r 2,303,338

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A TORNEY Patented Dec. 1 1942 UNITED STATES PATENT OFFICE TUNING IIWPEDANCE FOR HIGH RADIO FREQUENCIES (Granted under the act of Marchv3, 1883. as

amended April 30, 1928; 370 0. G. 757) 1 Claim.

This invention relates to novel impedance means for resonating at high radio frequencies.

In circuits tuned to very high radio frequencies the amount of inductive reactance required to resonate with the capacitance in the circuit is of such a low value that a conventional type of wound coil inductance must be physically very small. This introduces serious practical diiiiculties in constructing inductances of like values for the proper tracking of tuning condensers. Due to the low impedance of vacuum tubes at high frequencies it is necessary that the connections between the tubes and the tuned circuit be tapped down on the inductance in order to maintain a high resonant impedance in the circuit. Small coil inductances do not provide a sufficient choice of tapping points to obtain satisfactory operation of the circuit.

One of the objects of the present invention is to provide an inductance having a low reactance for relatively large physical dimensions thus enabling it to be constructed and duplicated with precision. Another object of the invention is to provide a means for progressively adjusting the tapping points along the inductance in small increments. A further object of the invention is to provide a group of high frequency tuned circuits which may be gang-tuned with accurate tracking. Another object is to provide an inductance of confined field thereby reducing the tendency toward stray coupling to other circuits. Still another object is to provide a system of gang-tuned high Q circuits. Further objects will become apparent from a study of the following description taken together with the accompanying drawings in which:

Fig. 1 is a diagrammatic showing of a typical preselector circuit employing gang tuned segments of parallel transmission lines as impedances;

Fig. 2 is a side elevation of a typical assembly of transmission line segments of the type indicated in Fig. 1 showing the ganged tuning arrangement;

Fig. 3 is a diagrammatic showing of a typical preselector circuit employing segments of concentric transmission lines as impedances;

Fig. 4 is a diagrammatic showing of a variation of the oscillator circuit portion of Fig. 3, together with its coupling impedance;

Fig. 5 is an elevational view in cross-section of a segment of a transmission line of the type indicated in Fig. 3 showing the use of both capacitive and inductive trimmers;

Fig. 6 is a plan View of a fragment of the transmission line of Fig. 5 showing a detailed view of the capacitive trimmer, and

Fig. '7 is an elevational view in cross-section of a modification of the transmission line segment shown in Fig. 5.

This invention makes use of segments of transmission lines as inductive reactances, one end of each line being loaded by capacitance for tuning, and the other end being short-circuited and grounded. In order for a transmission line to be inductive, it must be shorter in length than any odd number of quarter waves for the frequency to which it is to be tuned. For frequencies of the order of to 500 megacycles per second, lines may be designed to be shorter than one quarter wave length for convenience. The resulting inductive reactance may then be tuned by the loading capacitance to any frequency within the limits of the design. One application for this invention is its use in the preselector circuit of a superheterodyne receiver.

In the circuit shown in Fig. 1 segments 8, 1 and 8 of parallel transmission lines are utilized as coupling means between the antenna and the amplifier circuit 9, between that circuit and the mixer circuit l0, and between the latter circuit and the oscillator circuit II. As more clearly shown in Fig. 2, each of the transmission line segments 6, I and 8 comprises a pair of parallel conductances I2 which are short-cirouited at their lower ends by the bar I 3 and grounded as shown at M in Fig. 1. At the upper end of the segment a trimmer condenser is provided, the fixed plate l5 of which is carried by one of the conductors, the movable plate l6 being carried by the other. This condenser is manually adjustable during the alignment of the receiver. A tuning condenser I1 is also connected across the upper ends of the conductors I 2 of each segment, these condensers being gang tuned by a shaft l8 which terminates in the tuning dial l9. Adjustable taps 20 are provided for connecting the segments in their respective circuits. These taps are movable along their respective conductors.

Referring to Fig. l, the received signal is conducted from the antenna along the low impedance transmission line 2! to low impedance points on the first tuned transmission line segment 6. This segment is coupled to the control grid of a radio frequency amplifier tube 22 by means of a grid lead 23 tapped at a point on the segment whose impedance corresponds to the input impedance of tube 22. The signal is amplified in this tube and coupled from its anode to the I had control grid of mixer tube 24 by means of lead 25 which is tapped to a point on the second tuned segment 1, the impedance of which corresponds to the combined impedance presented by the amplifier anode and the mixer grid. The oscillator tube 26 is shown as a triode but is not restricted to this type. In order to obtain uniformly strong oscillations throughout the frequency range the segment 8 coupled to the oscillator circuit is electrically balanced with one conductor coupled to the oscillator tube grid through lead 21 and the other conductor coupled to the oscillator anode through conductor 28. The coupling points are so adjusted as to provide proper impedance matching between the segment 8 and the tube elements. The cathode is at ground potential. Oscillator voltage is coupled from segment 8 by lead 52 through a capacitance 29 to the mixer cathode for heterodyning the received signal. This capacitance also provides a by-pass path to ground for the signal, since the impedance at the tapping point of the segment connected to the oscillator frame is very low at the signal frequency. The difference frequency between the oscillator and signal frequencies is coupled from the mixer anode to the first I. F. transformer or other load circuit. It is desirable to place the capacitance 30 between the mixer anode and ground to provide a by-pass for the signal and oscillator frequencies. This capacitance becomes a portion of the I. F. resonant circuit at the intermediate frequency, so introduces no losses.

For trimming the inductance of the oscillator circuit. an adjustable slidable bar 3|, is attached to the conductors of the transmission line segment 8 associated with that circuit as shown in Fig. 2. This bar may be moved along the conductors and acts as a short-circuiting element to effectively shorten the length of the segment and produces a corresponding reduction in inductance.

the same relative positions in the circuit as the segments 6, 1 and 8 respectively in Fig. l. The circuit of this figure is essentially the same as that shown in Fig, 1 except that the oscillator 28 operates with its anode at radio frequenc ground,

its grid at high radio frequency potential and its cathode at an intermediate point. The resonant oscillator circuit is coupled to the oscillator tube grid, to its cathode and to the mixer cathode by means of inductive coupling loops 50, i and 49 respectively, and inductive coupling loop 48 is also substituted for the tapped connection of the transmission line 2| to the amplifier segment. In both circuits it is sometimes found desirable to provide radio frequency chokes 35 in the mixer heater leads effective over the oscillator range in order to prevent the oscillator injection voltage from being by-passed to ground through the mixer cathode-heater capacitance.

As a variation of the inductively coupled oscillator unit of Fig. 3 a conductively coupled oscillator circuit is shown in Fig. 4, in which the oscillator anode operates at radio frequency ground by virture of the connection of the lead 28 to the outside of the segment 34. The grid circuit 21 is tapped directly at a high potential point 54 and the oscillator and mixer cathodes are tapped at low potential points such as 55 and 59 on the inductive center conductor. The oscillator of Fig. 4 may replace that of Fig. 3 by merely connecting the three fioating leads in place of the three corresponding leads in Fig. 3. Fig. 5 shows the structural detail of one of the concentric line transmission segments utilized in Fig. 3. A tuning capacitor 36 is connected between the outer and inner conductors, its movable plates being mounted on a shaft 3'! which also carries the movable plates of the corresponding capacitors of the other segments in the circuit. The arrangement for gang tuning is the same as that illustrated in Fig. 2. A capacitive trimmer is provided and, as more clearly shown by Fig. 6, consists of a fixed plate 38 carried by the inner conductor and a movable plate 39 carried by a screw threaded shaft 46 which is carried in a threaded portion 4i attached to a bracket 42 mounted on the outer conductor.

An inductive trimming means is also provided for use in the segment 34 which is coupled to the oscillator. This means is shown in Fig. 5 and consists of a pair of electrically conductive plates supported on vertical rods 44 extending upward from the bottom of the segment. These plates in the magnetic field may be adjusted by rotating the rods M in which they are supported. For this purpose the rods extend through the ase of the conductor in order that their ends may be engaged by a wrench or other adjusting tool. The rods 44 may be rotated to position the trimmers at any angle between parallelism with the magnetic field and perpendicularity thereto. A rotation of of the rod carries the trimmer through its maximum range, the greatest inductive effect being secured when the trimmer is perpendicular to the inductive field. It may sometimes be desirable to provide inductance trimming in the radio frequency and mixer circuits, where extreme accuracy of alignment and tracking is required. However, it has been found that adjustment of the tapping points will normally provide sufilcient inductive trimming for these circuits.

Fig. '7 illustrates another form of concentric transmission line segment similar to that shown in Fig. 5, but having large openings such as that indicated at 45 for the purpose of allowing access to its interior. In all other respects this form of impedance is the same as that shown in Fig. 5, although the trimmers have been omitted.

In a wound inductance, with one end of the coi1 grounded, the potential on adjacent turns are in phase, and the mutual inductive efi'ect increases the net inductance of the coil. This results in the electrical length of the coil being much greater than its physical length. The high distributed capacity present in the coil tends further to increase its electrical length. The physical dimensions of a coil being so much less than its electrical dimensions it becomes very difiicult and impractical to accurately design and duplicate coils to obtain an inductance of low value.

However, in a transmission line such as those of Fig. 1 the potentials at corresponding points along the two parallel conductors are of equal magnitude in opposite phase, tending to cancel any mutual effect and confine the inductive field to the space immediately adjacent the transmission line. In such a transmission line the distributed capacity is very low. If this line had no capacity loading at the end, its physical length would be nearly equal to its electrical length. By shortening the line to the proper inductive value it may be tuned over a wide frequency range by a small variable capacitance. Accurate design and duplication of such inductances is very simple and very practical.

In a concentric transmission line such as those used in the circuit of Fig. 3 and further illustrated in Figs. 5 and '7 the outer conductor is at a nearly uniform potential throughout its length, such small variations as may occur being of opposite phase from those along the center conductor. Thus the same reasoning as given above for parallel transmission lines applies here also. The inductive field in this case is confined entirely within the transmission line. The space between conductors should be small compared to one-quarter wave length for both types of inductance. In designing a tuned segment of parallel transmission line as illustrated in Figs. 1 and 2, its inductance may be determined from its dimensions or vice versa.

X Z tan 21% where X =inductive reactance= 2DfL ohms l=physical length (usually in centimeters) A=wavelength of operation in same units as l Z =surge impedance in ohms 276 log g b=spacing, center to center, of parallel conductors a=radius of conductor.

The quantities b and a where found herein may be in any like units since only their ratio is employed. However, for convenience they are usually expressed in the same terms as Z.

In designing a concentric line segment, as shown in Figs, 3, 5 and '7, its inductance may also be determined from the above equation for XL, where where b=inner radius of outer conductor a=outer radius of inner conductor Another method for solving the short concentric line is the toroid equation where L=inductance in microhenries l=physica1 length in inches.

In aligning the circuits, the normal procedure is to adjust the capacitance trimmers near the high frequency end of the tuning range, and the inductive trimmers near the low end, repeating the process until satisfactory tracking is obtained.

It should be understood that the practice of the invention is not limited to the embodiments illustrated and described but is circumscribed only by the scope and limitations of the appended claim.

The invention described herein may be manufactured and/or used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

I claim:

A preselector circuit comprising an amplifier circuit, a mixer circuit and an oscillator circuit, a series of inductive transmission line segments coupling said circuits together and coupling said amplifying circuit to an antenna, means conductively coupling and grounding the conductors of each of said segments at one end thereof, a capacitor coupling said conductors at the other end of each segments, a capacitive trimming means associated with each of said segments, movable tap means on the conductors of each of said segments, whereby the points of connection of the leads of said circuits may be varied for the impedance trimming of said segments, and an inductive trimming means associated with the one of said segments coupling said oscillator circuit with said mixer circuit.

GEORGE E. PRAY.

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