Variable rf group delay equalizer

[57] ABSTRACT

A tunable microwave phase equalizer primarily for use in satellite communication systems is disclosed comprising in combination a circulator and a two-knob tunable microwave cavity wherein one knob adjusts the resonant frequency, and the other knob adjusts the coupling. By means of the frequency adjustment a delay curve of practically constant shape and width can be moved into any 5, 10, 20 or 40 MHz band of the 6 GHz satellite range. By means of the coupling adjustment, the bandwidth is varied and with it the time delay curvature at the frequency of delay maximum to cancel third order intermodulations generally generated by the integral of the parabolic time delay curve. Second order intermodulation is eliminated by the frequency adjustment which changes the slope of the time delay curve and moves the curve away from the center frequency.

Claims, 6 Drawing Figures

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PATENTED 00117 ΒΪ2

3,699,480

SHEET 1 OF 3

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FIG I

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FIG 2

MARTIN MUELLER

BY

Rosen &amp; steinhilper*

ATTORNEYS

PATENTEDOCT 17 !9T2

SHEET 2 OF 3

3,699,480

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FIG 4

MARTIN, MUELLER INVENTOR

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U * «.'HiXtX S

PATENTEOOCT 171972

3.699,480

SHEET 3 OF 3

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516

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I' 527

512

513.2

526

513

514

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506.1

511

515 ⁵⁵⁰³ 5Ο8_?|^

507

5505

509

510.1

513.1

506

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FIG 5A

MARTIN MUELLER INVENTOR fiflSEN &amp; SlElNHILEtR

ATTORNEYS

3,699,480

V ARI ABLE RF GROUP DELAY EQUALIZER

BACKGROUND OF THE INVENTION

Satellite communication links are playing a more and more important part in world-wide transmission of TV ⁵and multi-channel telephony signals. As in terrestrial microwave radio links, FM is used to modulate the carrier. The frequency deviation, however, is great; usually 10 to 15 dB higher than in terrestrial links. This is necessary because of the power and noise limitations in the “down” links. These same limitations require deviating the carrier closer to the band edges of the channel filters. It is known that the prime causes of distortion in an FM system are the nonlinear phase characteristics of selective circuits, and phase, eqnalization is necessary in terrestrial links, while in satellite FM systems, phase equalization is not only necessary but of great importance.

The well-known approach to phase or group delay equalization is by incorporating all-passes in the IF (70 MHz) portion of the receiver. (An ideal all-pass is defined, for the purposes of this disclosure, as a lossless network that transforms its characteristic terminating impedance into the same impedance at the input terminals and hence neither dissipates, nor reflects, any power.) These all-passes are designed so as to compensate for the nonlinearities of IF filters, as well as those of the RF filters, in the transmitter and receiver. If both ends of a radio link are designed by the same supplier (which is the rule in terrestrial links) this lumping together of the compensation means is adequate. A number of publications and articles such as, for example, “A Broadband Variable Group Equalizer,” by Hammer, R., and Wilkinson, R. G., Post Office Electrical Engineers Journal, July 1957, pp 174-175, deal with the solution to this problem, and comprise generally the use of bridged T networks of lumped constants.

In a satellite system, however, where the transmitter is in the ground station, the receiver in the satellite, and vice versa, both ends of the link are generally designed by different suppliers. In this case, equalization requirements must be specified and met for transmitter and receiver separately. Whereas in the receiver the known IF-equalization is applicable, a direct modulated transmitter (klystron or solid state voltage-controlled oscillator) does not contain 70 MHz IF stages. Hence, for the transmitter an alternate approach to the equalization problem is required. Since the phase versus frequency response, or more conveniently, its frequency derivative, the group delay response, of frequency selective circuits, is basically a double humped curve (see FIG. 1), equalization is accomplished by adding a bell-shaped positive time delay, “filling-in” the center portion of the response. This type of response is produced by an all-pass.

Circuits that are capable of bell-shaped positive time delay are known in the art. One such circuit consists of a bridge configuration consisting of 4 tuned circuits, basically arranged in the form of a Wheatstone bridge.

Another such circuit consists of a bridged T configuration of two tuned circuits being coupled to each other by means of a lossless voltage divider, which divides the voltage across the bridge equally.

Still another such circuit is a reflection type all-pass consisting of one tuned circuit and a circulator. Irrespective of the number (n) of reactive elements in the networks, only two may be chosen arbitrarily; (n-2) elements are determined by the all-pass condition. Therefore, only the last type above described allpass is an “unconditional all-pass.” (Unconditional allpass is herein defined as a circuit with all-pass characteristics irrespective of the magnitude of the components provided the circulator is ideal.)

SUMMARY OF THE INVENTION

It is proposed in this invention to equalize a given time delay curve such as that shown in FIG. 1 by matching both the delay slope and curvature. This ₁₅ function is performed in the transmitter immediately after the filter and before the transmitting antenna.

An equalizer according to the invention comprises generally a circulator and a two adjustment electromagnetic tuned cavity resonator. One adjustment of ₂₀ the cavity resonator varies the resonant frequency; the other varies the degree of coupling. The input to the cavity resonator is taken from the circulator to make the system an “all-pass.” This system permits the relatively narrow bandwidths necessary in some cases, for ₂₅ example, at the center frequency of 6 GHz and with high Q values, in the order of several thousand realizable with microwave cavity resonators, to be attained by loosely coupling a resonator to the transmission line. Since Q_t (Q_£ = Q loaded) varies inversely with the 30 square of the coupling capacitance, the detuning of the resonator by varying the coupling, and therefore, the Qi, is much smaller than it would be in an equivalent series tuned circuit when C_e is varied. (C_c is herein defined as that equivalent capacitance necessary to 35 resonate the equivalent circuit at the center frequency f&gt; ·)

It is a general object of this invention to compensate . for the distortion in an FM system.

It is another object of this invention to equalize a 40 given time delay curve by matching delay slope and curvature.

Still another object of this invention is to eliminate at the origin the AM signal associated with group delay nonlinearity.

Yet another object of the invention is to compensate for the slight detuning of the resonator caused by the variation of the coupling capacitance.

Additional objects, features and advantages of this invention will become apparent in the following 50 detailed description of certain illustrative embodiments of the principles of this invention.

DESCRIPTION OF THE INVENTION

The invention is described with reference to the ac³ companying drawings, in which:

FIG. 1 is a graph of Group Delay Time versus Frequency of some circuits and combinations thereof;

FIG. 2 is an equivalent schematic illustration of the instant invention shown with lumped elements;

FIG. 3 is a vertical section through an embodiment of the invention;

FIG. 4 is a vertical section through another embodiment of this invention;

₆₅ FIG. 5A is a view (not to scale), part pictorial and part vertical section of an embodiment of the invention;

FIG. 5B is a plan view of FIG. 5A.

3,699,480

To equalize a given time delay curve, such as for example the filter time delay curve shown on FIG. 1, delay slope and curvature must be matched or filled in by the equalizer time delay curve. In the vicinity of the center frequency or frequency of delay maximum f_o on FIG. 1, the curvature of the filter time delay curve is nearly constant, and the slope is a linear function of detuning Δ/, where detuning is defined for the purposes of this invention as the frequency difference between the actual frequency of the signal and the center frequency of the filter. Tuning of the curvature of the equalizer, therefore, means variation of the time delay maximum r₀, or bandwidth of the equalizer B, since r₀is inversely related to B. On known equalizers, therefore, tuning of curvature requires ganged tuning of all capacitors and inductors in counteracting fashion. Adjustment of the slope for constant curvature is less involved in prior art devices since it only requires variation of an inductance of a parallel tuned circuit and variation of a capacitance of a series tuned circuit. However, if both slope and curvature are to be varied, the invention described below has definite advantages which will become clear as the invention is further described.

FIG. 2 shows a schematic equivalent circuit in lumped-element form of the principal elements of FIGS. 3 and 4. While in practice the invention will generally be operated in frequency ranges where distributed parameter elements are utilized rather than lumped elements, the invention may be realized in lumped-parameter components for use in lower frequency ranges. In FIG. 2 the resonator, generally denoted 101, has a capacitance 4 and a variable inductance 5, in shunt with each other. The variable inductance 5 corresponds, for example, to tuning piston 305 of FIG. 3.

Coupling to the electric field of the resonator is by means of a variable capacitor 3 in series with the resonator which is mechanically joined to another variable capacitor 11 in shunt with the resonator such that an increase in the capacitance of capacitor 3 is accompanied by an identical decrease in the capacitance of capacitor 11 , and vice versa. The variable capacitor 3 of FIG. 2 corresponds to capacitive rod 303 of FIG. 3, while variable capacitor 11 corresponds to capacitive rod 3303. Circulator 2 of FIG. 2 corresponds to circulator 302 of FIG. 3. Energy is supplied to the circulator 2 through an input line 8 from input connection 6 , and is removed from the circulator through an output line 9 at output connection 7 , while energy is supplied and removed to and from the resonator through line 10 , which connects the circulator and the cavity.

Referring to FIG. 3, there is shown a tunable electromagnetic cavity resonator generally denoted 301 , and having an electrically conductive outer envelope including a tubular member 309 , having first and second end portions 307 and 308 , which comprise a single unitary cavity member enclosing a volume of space 310 . The resonator cavity in FIG. 3 although shown as cylindrical may be of any shape such as rectangular or other shapes well known in the microwave art. Disposed coaxially within the tubular portion of the envelope is an electrically conductive piston 305 defining an inner end wall capable of longitudinal noncontacting motion within the cavity to vary the volume 310 of said cavity, and externally con trolled through connecting rod 311 extending through one end portion 308. The diameter of the cavity is such that τ &lt; 3.4 X radius, where r is the wavelength in the most widely used resonant mode. The length of the cavity should be slightly greater than τ /2. A unitary member or plunger comprised of dielectric rod 319 and conductive rods 303 and 3303 , is introduced transversely through the sidewalls 309 of the cavity and extends across the cavity penetrating the volume of space 310 and emerges diametrically through the opposite side of the cavity extending through the sidewalls 309 at points diametrically opposite each other. This unitary member is comprised of a dielectric rod 319 having electrically conductive rods 303 and 3303 joined to it on either end respectively, and forming with it a unitary integral member capable of unitary transverse motion through the cavity when introduced therein through aperture 313 . The conductive member 303 passes through aperture 313 without contacting sidewall 309 , whereas the conductive member 3303 does make slideable contact with sidewall 309 in passing through it. The dielectric rod 319 included between rods 303 and 3303 is such as to be transparent to micro wave energy and may be of such material as alumina. The means for coupling energy into and out of the cavity resonator 301 comprises a three-port circulator 302 having one of its ports coupled to the input-output port 306 of the cavity resonator. The other two ports 314 and 315 of the circulator 302 serve to couple microwave energy input and output, respectively, to and from the system.

The conductive piston or plunger 305 being movable longitudinally within the tubular cavity without electrically contacting it, acts as a tuning device of the cavity resonator by permitting variation of the volume of the cavity, and hence variation of its reactance; it is used to tune the resonant frequency of the cavity resonator. The unitary member or plunger composed of the dielectric member 319 and conductive end pieces or rods 303 and 3303 being capable of transverse movement in the cavity resonator acts as a coupling device and as a Q_t compensating device, as follows: electrically conductive member 303 acts as one end plate of a coupling capacitance which changes the area of the plate exposing more or less area to the electric field inside the cavity, which results in more or less electric field coupling; rod 3303 acts in the same manner but in an opposite sense, its exposed area within the field being diminished when the exposed area within the field of the other rod 303 is increased and vice versa, thus compensating for the detuning of the loaded cavity because of the change of the coupling capacitance.

FIG. 4 differs from FIG. 3, in that the transverse unitary member in FIG. 3 comprised of the dielectric port 319 and electrically conductive end rods 303 and 3303 has been replaced by a U-shaped member 419 , having an air dielectric window or space between electrically conductive rods 403 and 4403. The embodiment of FIG. 4 is in all other material respects similar to the embodiment of FIG. 3, including a circulator 302 (not shown in FIG. 4) for interchanging microwave energy with the cavity resonator 401 through an input output port 406 , and a tuning piston 405. Other similar ports bear reference characters in the 400 series of numbers.

Referring to FIGS. 5A and B, a variable volume of the resonator cavity generally denoted 510 is enclosed

3,699,480 by a conductive envelope comprised of a base 508 and cylindrical sidewall 509. A first conductive piston 5505 is mechanically mounted in tandem to a second conductive piston 505 and inserted into the cavity 510 so that the bottom face of conductive piston 505 serves as the top wall of the cavity 510 , and is capable of longitudinal motion within the cavity. Physical contact between the sidewall 509 of the cavity and the piston 505 is prevented by a dielectric spacer 510.1. This arrangement of multiple pistons is desirable in order to prevent excessive RF energy leakage from the volume of space 510 , but not essential to the operation of the invention.

Coupling to the cavity electromagnetic field is accomplished capacitively by means of electrically rod 503 which acts as one end plate of a coupling capacitor. The rod 503 is joined to dielectric rod 519 which in turn is joined to electrically conductive rod 5503, the entire unit being capable of unitary transverse motion diametrically across the cavity. Rod 5503 also acts as a capacitor and compensates for increases or decreases of the capacitance induced by rod 503 as it protrudes more or less into the cavity 510 , and tends to prevent detuning of the cavity. Since good contact with cavity sidewall 509 is required by rod 5503 , a “tulip” type spring 519.1 is provided to assure good slidable contact. No contact to sidewall 509 is made by rod 503.

Electromagnetic energy is introduced and removed from cavity 510 through input-output port 506. A standard coaxial connector 506.1 connects the input-output port of the cavity with port number two of a circulator (not shown).

The housing of the equalizer is divided into two major parts, and mechanically joined together. One part of the housing 513 encloses and protects the mechanism which imparts longitudinal motion to pistons 505 and 5505 to vary the volume of cavity 510 ; the other part of the housing 513.2 encloses the linkage 518 which imparts transverse motion to the unitary member comprised of rods 503,5503 , and 519. The housing enclosing the mechanism which imparts longitudinal motion to the pistons 5505 and 505 is further divided into three sections 513 , 514 , and 513.1 mechanically joined to each other. The center section 514 has threads 515 on its inside circumference which engage threads 511 on the outer circumference of a cylindrical member 520 ; hence the section 514 of the housing serves both to support cylindrical member 520 and to convert rotary motion into longitudinal motion. The cylindrical member 520 is mechanically connected to pistons 5505 and 505 by connecting members (not shown), and connected to tums-counting knob 516 via shaft 527 and a cam follower arrangement comprised of cam 526, pin 525 and follower 526.1. Circular motion imparted by an operator to tums-counting knob 516 is imparted via the cam-follower arrangement to cylindrical member 520, which in turn converts the rotary motion to longitudinal motion and imparts it to pistons 5505 and 505. The turns-counting knob 516 has a digital indicator 521 located on the top surface of the knob 516. The pitch of threads 511 and 515 can be chosen in such a way in relation to the tums-counting knob 516 so that the digital indicator 521 shows approximately the resonant frequency in MHz to which the cavity is tuned.

The housing 513.2 encloses a linkage 518 which converts rotary motion imparted by an operator to tumscounting knob 517 , into transverse motion of rods 5503,519, and 503 through the cavity 510.

The foregoing description of certain embodiments of the invention is by way of example only, and not intended to limit the scope of the appended claims. No attempt has been made to illustrate all possible embodiments of the invention, but rather to illustrate its princi^υ pies and the best manner presently known to practice it. Therefore, such other forms of the invention as may occur to one skilled in this art on a reading of the foregoing specification are also within the spirit and scope of this invention.