[1]United States Patent@ @Office 3PI55@914 P a t e n t e d N o v . 3 , 1 9 6 4 3,155,914 COA,XL4,L REACTFM TUNING STUB FOR TUNING A LOWER FREQUENCY SIGNAL WITHOUT AFFE4CTING A HIGHER FREQUENCY SIGNAL 5 David George Vice, John E. Mowle, and David G. Jard'me, Offawa, Ontario, Canada, assignors to Northern Electric Company Limited, Montreal, Quebec, Canada Ffled Apr. 29,1963, Ser. No. 276,650 5 Claims. (Cl. 330-4.9) Th@s invention relates to a coaxial reactive tuning stub which may be used to tune -a non-degenerate para-rnetric amplifi er. In the micronvave region, signals widely separated in frequenc y are often mixed or combined in a co@mmon circuit 15 such as may be found in a mixer or a parametric amplifier. A coaxial reactive tuning stub is often used to tune the circuit at one of the signal frequencies. The reactatice of the stub is varied by changing the electrical length @of the sttib at the desired freqaency. This is @enerally accom- 20 plishe d by varying the position of a mechanical shortcircuit placed across one end of the stub. A disadvanta,,e of such a stub is that varying the position @of the short circuit not only affects its reaelance at the desired frequency but also at all the other signal frequencies present in the 25 circuit. This disadvantage is overcome by the present invention by providing a coaxial reactive tuning sttib having an electri cal length equal to a whole number of quarter wavel engths at the higher frequency and not equal to a 30 whole number of quarter wavelengths at the lower frequenc y. The reactance of the stub is varied at the lower freque ncy by changing the stub's characteristic impedance. Becau se the electrical length of the stub is a whole number of quirter wavelengths long, the reactance o'l the stub at 35 the higher frequency remains virtually open or shoi-t circuited when the characteristic impedance is changed. The reactance of the ciretiit at the higher frequency is indep endently varied by coupling the higher frequency to the circuit through wavegaide components. The size of 40 the waveguide is so chosen that the lower frequency is below the cut-off wavelength of the -@vave,-uide and the lower frequency will, therefore, not propagate in it. Standa rd tuning elements such as screws and irises are emplo yed in the waveguide for tuning th@- circuit at the 45 higher frequency. Since the lo,,ver frequency does not propa gate in the waveguide and the higher frequency does not propagate in the sttib, no interaction between the two tuning elements at the two signal frequencies can exist. The invention is fartlier described with reference tG the ro accom panying drawings in which: FIGU RE 1 is a perspective view of a one-port parametric amplifier; and FIGU RE 2 is a vertical cross section of the amplifier taken on the line 11-11 of FIGURE 1, and illustrating 55 details of the coaxial reactive tuning stub and the waveguide tuning elements. , In a non-degenerate parametric amplifier, the following three frequencies are of primary importanr-e. The signal freque ncy f, is the input radio frequency that is to be 60 amplifi ed by the parametric amplifier. The pump is a source of power of high radio frequency, fp, generally greate r than twice the frequency of the signal, that is used to drive a varactor diode and generate increased ainplification of the si.anal frequency fed to it. The idler fre@ g5 quency fl=f,-f, is the difference between the pump frequency and the signal frequency. It is often referred to as the lower sideband. A fourth frequency - f@=f,+f, known as the upper sideband is also present but is generally suppressed in the tuned circuits of the amplifier. 10 Referring now to FIGURES I and 2, the aniplifier I @comprises a rectangular waveguide pumo arm 10, an idler 2 arm 11, a coaxial signal input line 12 having an inner conduetor 12a and an outer conductor 12b, and a coaxial simal tuning stub 13 ha-ving an in-@ier conductor 13a and an outer conductor 13b. The pump arm 10 and the idler arm 11 are constracted of rectangular waveguide components in m7hich the dominant (TE@10) mode is employed. The coaxial signal input line 12 and the coaxial signal tuning stub 13 also use the dominant (TEM) mode. The pump arm 10 has an input port 14 for connecting lo a source (not shown) of pump power of frequency f,. The pump pavier is connected f@om @the input port 14 to a varactor diode 15 located in the idler arm 11, throug.,i a two-section, iris-coupled waveguide filter 16 having three indur-tive coupling irises 17, 18 and 19, and two frequency tuning screws 20 and 21. The filter 16 has a relatively narrow pass band and is tuned to pass only the pun-ip frequency fp and reflect both the idler frequency f, and tipper sideband frequency f,,. The idler arm 11 extends from the inductive coupling iris 17 of the waveguide filter 16 to an end plate 22 and ineludes an inductive couplin-g iris 23 and frequency tuning screws 24 and 25. The iris 23 in conjunction with the end plate 22 and the tuning screw 24 forms an idler tuning circuit in the waveguide idler arm 11. The varactor diode @15 is symmetrically placed across the narrow dimension of the rectangular waveguide idler arm 11, parallel to the electric field of the guide. The si,-nal frequency f, is coupled to oiie end of the varactor diode 15 from the center conduct@or 12a of the signal input line 12 by a lo@w pass filter 26. The purpose of the filter 26 is to eliminate the pump and idler frequencies from the signal i@qput line 12. The other end of the varagtor diode 15 is connected to the inner conductor 13a of the coaxial signal tuning stub 13. The outer conductor !3b of the signal tuning stub 13 is connected at one end to the broad dii-nension of the waveguide idl6r arm 11, and at the other end to an inner conductor support 27. The face of the inner conductor support 27 which is in contact with the end of the outer conduetor of the sigiial tuning stub 13, forms a shortit)g plane 27a for the signal tuning stub 13. An adjustable sleeve 28 makes contact wiffi the inner conductor of the tuning stub 13 and the inner conductor support 27; the combination tlius forniing a vaiiable short eirruited tuning stub. The electrical length of the signal tuning stub from the inner surface of the wavegaide idler arm 11 to the shorting plane 27a of the inner conductor support 27 is made equal to 4 and is less than 4 where n is a positive integer, Xi is the wave length at the idler -'Lrequency,and N, is the wave length at the signal frequency. The eiid of the adjustable sleeve 28 is a stepped impedance transformer 29 with an electrical length of 4 The steps produce fringing discontiniiities which by desig-.q, are equal and opposite in value and therefore, cancel each other at the idler frequency fi. Because the electrical length of the tuning stub l@ at the idler frequency fi is by design an odd iiumber wave lengths long, the reflected reactance is independent of the characteristic in@ pedance of the stub 13, and it will therefore, always present a substantially open circuit to the varac@.or diode 15. However, by changing the insertion
[2]length of the adjustable sleeve 28, the characteristic impedance of the tuning stub 13 is altered, thus varying the reac'Lar.ce across the diode 15 at the signal frequency f,. This may be better understood by analysing the relalionship belween the cliaracteristic impeda-.ice, line length, and reactance for a s;mple short eircuited stub which is giveil by the forniula: 2-d XL=JZ tan where the characteristic impedance is given by the formula: Z= 138 log b a where: XL is the inductive reactance of the stub, Z is the characteristic impedance of the stub, x is the wave length of the frequency under consideration, d is the electrical length of the stub, b is t'iie inner diameter of the outer conductor and a is the outer diameter of the inner conductor or ttic sleeve. From the above, it can be seen that the reactance of the stub 13 can be altered by varying its characteristic impedance except at the open and short circiiit points where the reactance will be infinity or zero. Thus, if the adjustable sleeve 28 is fully withdrawn from the signal tuning stub 13 until the front face of the sleeve 23 is flush with the sorting plane 27a of the inner conductor support 27, the reactance presented to the varactor diode 15 at the signal frequency f, will be that of a short circuited coaxial stub with a fixed electrical length having its charactelistic impedance determined by the diameters of the outer and inner conductors of the stub 13. If the adjustable sleeve 28 is fully inserted into the stud, the reactance now presented to the diode 15 at the signal frequency f, will be that of a short circuited coaxial stub with the same electrical length but having a characteristic impedance delermined by the inner diameter of the outer conductor of the stub 13 and the diarneter of the adjustable sleeve 28. Adjustin.- the insertion length of the adjustable sleeve 28 provides acontinuously variable reactance at the si.-nal frequency fs between these two values. In both the above examples, the reactance of the stub 13 at the idler frequency fi was extremely high since by design the overah length of the short eircuited stub 13 has been made an odd number of quarter wavelengths long. When the adjustable sleeve is set to an intermediate setting, the normalized input impedance of the signal tuning stub 13 is as follows: jZ' tan Bdl+j tan Bd2 Zi. Z2 1- ZI tan Bdi tan Bd2 Z2 where: Zi. is the normalized input impedance of the signal tuning stub 13 - Z, is the characteristic impedance of the section of the stub 13 determined by the inner diameter of the outer conductor 13b and the diameter of the sleeve 28. Z2 iS the characteristic impedanr-e of the section of the stub 13 deterniined by the inner diameter of the outer conductor 13b and the outer diameter of the inner conductor 13a. d, is the electrical length of the section of the stub 13 having the characteristic impedance Zl. d2 is the electrical length of the section of the stub 13 having the characteristic impedance Z2. 3,155,914 B is equal to 27r/x where x is the wavelength of the frequency under consideration. From the above 'Lormula, it can be shown that while the reactance of the stub 13 will vary at the signal frequency f., when an intermediate setting of the sleeve 28 is employed, it remains virtually open circuited to the varactor diode 15 at the idler frequency fi. The idler arm 11 is tuned by varying the imerlion depth of the frequency tuning screws 24 and 25. The 10 signal frequency f., is below the cut-off wavelength of the wave guide and will, therefore, not propagate in the guide. Thus, varying the insertion depth of the frequency tuning screws 24 and 25 tunes the varactor diode 15 at the idler frequency fi but does not affect the tunin-. of the 15 diode 15 at the signal frequency f,, and varying the reactance of the coaxial signal tuning stub 13 tunes the varactor diode 15 at the signal frequency f, but does not affect the tunin.- oi. the diode 15 at the idler frequency fl. In the above described embodiment, the impedance of 20 the coaxial signal tuning stub presented to the varactor diode 15 at the idler frequency f, is virtually an open circuit. IL can be readily seen that if the electrical le@igth of the stub 11 is made an even number of quarter wavelengths at the idler frequency fi rather than an odd num25 ber of quarter wavelengths, the impedance presented to the varactor diode 15 at the idler frequency f, wifl then be virtually a short circuit. What I
