FR2499786A1 - Wide stop band UHF filter - has quarter wave transformer which is in exact tune for given frequency set in middle of attenuation band - Google Patents

Abstract

The filter is partic. useful in an ultra high frequency system where an active component is supplied by a DC power supply. The filter (11) is a microstrip circuit formed on a dielectric ceramic substrate (12) as follows: one quarter-wave transformer (13) with characteristic impedance Z1 is exactly a quarter-wave long for a frequency FO1. It is followed by another quarter-wave transformer (14) with characteristic impedance Z2 lower than Z1 and which is exactly a quarter-wave long for another frequency FO2; this transformer (14) is followed by a transmission line (15) whose impedance varies progressively from Z3 lower than Z2, at the transformer end (16), to Z4 higher than Z3 at the other end (17). The frequencies FO1 and FO2 are in the middle part of the stop band.

Description

"HYPERFRBQUENCY FILTER WITH BROADBAND MITIGATION
AND HYPERFREQUENCY SYSTEM COMPRISING THIS FILTER "
The present invention relates to a microwave attenuation broadband filter, mainly made in the form of a microstrip circuit deposited on a dielectric ceramic substrate. It also relates to a microwave system comprising at least one DC power supply circuit.

 In a very general way, the invention finds an advantageous application in the realization of any microwave circuit comprising at least one active component requiring a DC power supply. Indeed, every time, in a microwave circuit, a transmission line co.

it exists with a DC power supply placed parallel to the transmission line, it is essential to isolate said line with respect to the power supply circuit. In order not to lose microwave power in the power supply circuit, it must have a high impedance to the microwave waves while behaving as a shortcircuit for the direct current. Hence the need to incorporate in the power supply circuit a filter significantly attenuating the microwave signal in a frequency band covering at least the band of use of the microwave circuit considered. For example, the patent application French Patent No. 2460049 describes a microwave filter comprising in particular a particular combination of filter elements made of microstrips deposited on an insulating substrate. However, most microwave filters of this type do not make it possible to obtain an attenuation band of greater width than the half-octave, which is quite disadvantageous when one wants to implement a microwave system covering a band. of use reaching or exceeding the octave.

 The object of the present invention is to remedy this disadvantage.

 In fact, according to the present invention, a microwave / broadband attenuation filter, mainly produced in the form of a microstrip circuit deposited on a dielectric ceramic substrate, is notably remarkable in that it consists of a first transformer of the quarter-wave type, of characteristic impedance ZI, and which is in exact quarter-wave for a frequency Fol situated in the median part of the attenuation band, this first transformer being followed by a second transformer , also of the quarter-wave type, of characteristic impedance Z2 lower & Z1, and which is in exact quarter-wave for a frequency Fo2 situated in the median part of the attenuation band, this second transformer being the sameAme prolonged by a line whose impedance varies progressively from a value Z3 lower than 22, measured at the end of the line situated on the side of the second transformer, to a value Z 4 upper Q Z3.

 As is demonstrated below, this filter structure makes it possible to obtain an enlarged attenuation band in which the real part of the impedance of the microwave filter according to the invention retains a sufficiently high value to behave as an open circuit. - & vis microwaves waves.

 The width of the attenuation band of the microwave filter according to the invention can be further enlarged by taking into account that, as will be seen below, the real part of the complex impedance of the filter according to the invention decreases at the frequencies extremes of the attenuation band, this phenomenon being due b the dispersion with the frequency of the imaginary part of the complex impedance of said filter. Thus, a substantial widening of the attenuation band of the microwave filter in accordance with the invention is made possible by limiting the dispersion of the imaginary part of the complex impedance of said filter. The Applicant has shown that this goal is achieved when the respective lengths li and 12 of the first and second transformers, on the one hand, and the impedances Z1, Z2, Z3, on the other hand, are chosen such that there exists in the attenuation band two frequencies F1 and F2 for which the impedance of the microwave filter takes the same complex value, so that by successively traversing all attenuation band frequencies, the figurative point of the complex impedance R + j X of the microwave filter is described in a diagram ( R, X) a gamma-shaped loop.

 Thus, by folding on themselves variations with the frequency of the imaginary part X, the invention makes it possible to maintain the real part R in a range of sufficiently high values and this in a wide frequency band.

Furthermore, the Applicant has established that, in a particular embodiment of the invention for which the frequencies Fo1 and Fo2 are similar, the impedances Z1, Z2, Z3 satisfy the inequality Z1 <(z2) 2
Z2 Z3
It is understood that, in the following, the expression "neighbors" will have to be taken in the broad sense, that is to say including equality.

 The invention also relates to a microwave system comprising at least one DC power supply circuit, a system for which it is highly preferable for the supply circuit to have a high impedance to the microwave signal. This object is achieved by incorporating the microwave filter according to the invention into said DC power supply circuit.

 Indeed, according to the present invention, a microwave system comprising at least one DC power supply circuit is mainly remarkable in that said power supply circuit is provided with a microwave attenuation broadband filter, produced mainly in the form of a microstrip circuit deposited on a dielectric ceramic substrate, and constituted by a first quarter-wave transformer of characteristic impedance Z1, and which is in exact quarter-wave for a frequency Fol located in the middle part of the attenuation band, this first transformer being followed by a second transformer, also of the quarter-wave type, with a characteristic impedance lower & Z1, and which is in exact quarter-wave for a frequency Fo2 located in the middle part of the attenuatison band, this second transformer being itself extended by a line whose impedance varies progressively since value Z3 lower u, measured at the end of the line if killed on the side of the second transformer, up to a value Z4 greater than 3.

 The following description with reference to the accompanying drawings, given by way of non-limiting example, will make it possible to clearly understand what the invention consists of and how it can be achieved.

 FIG. 1 represents a microwave filter according to the invention.

 Figure 2 is a first type of Smith diagram relating to a microwave filter according to the invention.

 FIG. 3 is a second type of Smith diagram relating to a microwave filter according to the invention.

 FIG. 4 is a diagram of a hyperfrequency oscillator provided with a DC power supply circuit comprising a microwave filter in accordance with the invention.

 FIG. 1 represents an attenuating broadband microwave filter 11b, mainly produced in the form of a microstrip circuit deposited on a dielectric ceramic substrate 12. According to the invention, this filter 11 is constituted by a first transformer 13 of the quarter-wave type, characteristic impedance Z1, which is in exact quarter-wave for a frequency Fol situated in the median part of the attenuation band. This first transformer 13 is followed by a second transformer 14, also of the quarter-wave type, of characteristic impedance Z2 which is smaller than Z1, and which is in exact quarter-wave for a frequency Fo2 situated in the middle part of The second transformer is itself extended by a line 15 whose impedance varies progressively from a lower Z3 value Z2, measured at the end 16 of the line located on the side of the second transformer 14, an upper Z4 value Z3.

avec g1 = tg 2# 101F et 92 = tg 2n lo2F
C C
C étant la vitesse de la lumière et lol et lo2 les longueurs dans le vide des transformateurs "quart d'onde" 13 et 14 de la figure 1, les longueurs réelles li et 12 sont reliées à lol et lo2 par la permétivité électrique de la céramique diélectrique.Ainsi, en obligeant la partie réelle R A prendre des valeurs particulières pour certaines fréquences, il est possible grâce t la formule (7) de déterminer un jeu de valeurs pour les paramètres 101,102,Z1,Z2,Z3, tout en veillant cependant ce ce que la largeur réelle li du premier transformateur 13 ne soit pas trop petite, inférieure i loopm 100 m par exemple, ce qui conduirait A des difficultés technologiques de réalisation.By applying the calculation results presented in the French patent application No. 2447111, it is found that, when the end 17 of the line 16 is closed again on a load impedance equal to Z4, typically 50 ohms, the variations with the frequency F of the complex impedance R + jX of the microwave filter they are governed by the equation

with g1 = tg 2 # 101F and 92 = tg 2n lo2F
DC
C being the speed of light and lol and lo2 the lengths in the vacuum of the "quarter-wave" transformers 13 and 14 of Figure 1, the actual lengths li and 12 are connected to lol and lo2 by the electrical permeativity of the Thus, by forcing the real part RA to take particular values for certain frequencies, it is possible thanks to the formula (7) to determine a set of values for the parameters 101, 102, Z1, Z2, Z3, while at the same time ensuring that the actual width li of the first transformer 13 is not too small, less than 100m loopm for example, which would lead to technological difficulties of realization.

 A first type of Smith diagram of a microwave filter according to FIG. 1 is given in FIG. 2. On the diagram, the resistors and the reactances have been normalized to 50 ohms.

 As shown in FIG. 2, the width of the attenuation band of the filter according to the invention is generally limited by the fact that at the extreme frequencies F min and F max the real part R of the impedance of the filter is so described. penalizing, this phenomenon being related to the dispersion of the imaginary part X. This is why, in order to increase the width of the attenuation band in korea, the Applicant has shown that it is possible to limit the dispersion of the imaginary part X of the impedance of the microwave filter according to the invention by choosing the respective lengths li and 12 of the two transformers 13 and 14, on the one hand, and the impedances Z1, ZZ, Z3, on the other hand, so that there exist in the attenuation band two frequencies F1 and F2 for which the impedance of the microwave filter takes the same complex value, so that by successively traversing all the frequencies of the attenuation band, the figurative point of imp R + jX of the microwave filter describes in a diagram (R, X) a loop in the form of gamma.

To produce a loop-shaped impedance diagram of the type shown in FIG. 3, it suffices to force the impedance R + jX given by the formula (1) t to pass through a certain number of particular points of the diagram. (R, X). It is then possible, by successive tests, to determine a set of values for the parameters l1, 12, Z1, Z2, Z3, for which the figurative point of the impedance
R + jX of the filter actually describes a loop.

In this work, one can be guided by the simple results to which the particular case leads or the two quarter-wave transformers "are tuned to the same frequency
Fo. We then have lol = 102 = C and, by posing
4Fo
g = tg n F
2 Fo formula (1) is written

The imaginary part X vanishes by F = Fo (g = OO), the real part then being

soit encore

avec

X also vanishes for

is still

with

When 1 - is positive, two frequencies can be defined
&alpha; - ss F1 and F2 given by

and F2 = 2 Fo - F1 for which the imaginary part is simultaneously zero, the real part R taking the same value Ro ss / &alpha; The figurative point corresponding to this impedance is therefore a double point. Finally, the condition of existence of a loop is summed up in inequality
1
> O (3)
&alpha; - ss An elementary calculation shows that when c (> 1 and p) 1, let Zi>Z2> Z3, the inequality (3) is satisfied if and only if &alpha;<ss, that is Z1 / Z2 <(Z2 / Z3) 2 (4).

When these conditions are satisfied, the impedance corresponding to the frequencies F1 and F2 is real and is
Ro ss / &alpha;, a value higher than that taken for the frequency Fo.

By defining the attenuation band of the filter as being the frequency band within which the real part of the complex impedance of the filter is greater than or equal to Ro, it follows from formula (2) that the limiting frequencies of this band are defined by
F min = Fo 2 / # Arctg gm F max = Fo #
2 Arctg gm

This band may typically have a width greater than an octave and a half.

 It should be noted that the inequality (4), although established in the case where the transformers are "quarter wave" at the same frequency, still remains substantially valid as long as the tuning frequencies Fol and Fo2 remain close .

 By way of example, the Applicant has made, on a beryllium oxide substrate, a microwave filter according to the invention for which the two transformers t3 and 14 are quarter-wave at the same frequency Fo = 15 GHz and have for length in the void lo = 5 mm. The first transformer 13 is constituted by a gold wire of 2.43 n of real length, of 50 m of diameter and about 100 ohms of characteristic impedance. This gold wire is plated on the substrate whose thickness is 0.635 March. The second transformer 14 is made by a gold microband of 2.26 mm in real length and 1.4 n wide, which represents a characteristic impedance of 38 ohms. Finally, the line 15, also made of gold microband, has an impedance Z3 of 20 ohms corresponding to a width of 3.46 mm. The impedance Z4 is taken equal to 50 ohms. Under these conditions, the figurative point of the complex impedance of this filter describes a loop intersecting at frequencies F1 = 11.87 CH: and F2 = 18.13 GHz. The impedance of the filter at the frequency Fo = 15 CH: is Ro = 138.5 ohms and 190 ohms at the frequencies F1 and F2.

The microwave filter according to the invention is particularly intended to equip any microwave system comprising at least one DC power supply circuit, said filter ensuring the decoupling of the supply circuit with respect to the microwave signal. Such an embodiment is given in FIG. 4 in the form of a power supply circuit 21 associated with an impedance matching circuit 22 forming part of a microwave oscillator of the type described in the French patent application. nd1 02511 filed by the Applicant together with the present application and entitled "HYPERFREQUENCY OSCILLATOR AGREED BY A GARNET,
This oscillator essentially comprises a small ball of yttrium garnet and iron placed in a magnetic field H, an active component which in the example shown in FIG. 4 is a diode. Gunn, and an impedance matching circuit 22 made on the same substrate and using the same technology as for the supply circuit 21. This circuit is intended to bias the diode
Gunn 31. In the embodiment given in FIG. 4, the terminal "+" of the DC power supply is connected to a gold strip 32 located at the end of the line 15, while the terminal " - Of said supply is connected to another micro-strip 33 also made of gold, connected to the lamina 34 and forming with the microstrip 32 a decoupling capacitor 35. The end 37 of the first transformer 13, located on the opposite side with respect to the second transformer 14, is connected to the matching circuit 22 at a point which, in the example of FIG. 4, is chosen at a first transformer 41 of said matching circuit 22.

 The invention can not be limited to the sole example of application of the microwave filter according to the invention given in FIG. 4 in the form of a microwave oscillator.

More generally, it applies to any microwave system comprising active elements, and in particular to microwave amplifiers and active mixers & transistors.

Claims (6)

1. Microwave filter (11) attenuating broadband, mainly in the form of a microstrip circuit deposited on a substrate (12) of dielectric ceramic, characterized in that it consists of a first transformer (13) of quarter-wave type, of characteristic impedance Z1, and which is in exact quarter-wave for a frequency Fol situated in the median part of the attenuation band, this first transformer (13) being followed by a second transformer (14), also of the quarter wave type, characteristic impedance Z2 lower & Z1, and which is in exact quarter-wave for a frequency Fo2 situated in the middle part of the attenuation band, this second transformer (14) being itself prolonged by a line (15) whose impedance varies gradually from a lower value z3, measured at the end (16) of the line (15) on the side of the second transformer (14), to a value Z4 greater than Z3. 2. Microwave filter according to claim 1, characterized in that the respective lengths li and 12 of the first and second transformers (13, 14), on the one hand, and the impedances Z1, Z2, Z3, on the other hand, are chosen such that there exists in the attenuation band two frequencies F1 and F2 for which the impedance of the microwave filter (11) takes the same complex value, so that by successively traversing all the frequencies of the band of attenuation, the figurative point of the complex impedance R + jX of the microwave filter (11) describes in a diagram (R, X) a loop in the form of gamma. 3. Microwave filter according to claim 2, characterized in that, the frequencies Fol and Fo2 being close, the impedances Z1, Z2, Z3 satisfy the inequality Z1 <(Z2) 2 4. A microwave system comprising at least one DC power supply circuit (21), characterized in that said supply circuit (21) is provided with a microwave filter (11) with a wide attenuation band, produced mainly in the form of a microstrip circuit deposited on a dielectric ceramic substrate (12), and constituted by a first quarter-wave type transformer (13), which has a characteristic impedance ZI, and which is in exact quarter-wave for a frequency Fol located in the middle part of the attenuation band, this first transformer (13) being followed by a second transformer (14), also of the quarter-wave type, of characteristic impedance Z2 less than Z1, and which is in exact quarter-wave for a frequency Fo2 situated in the middle part of the attenuation band, this second transformer (14) itself being prolonged by a line (screw) whose impedance varies progressively from a value Z3 less than 22, measured at the end (16) of the line (15) on the side of the second transformer (14), to a value Z4 greater than Z3. 5. Microwave system according to claim 4, characterized in that the respective lengths li and 12 of the first and second transformers (13, 14), on the one hand, and the impedances Z1, 22, Z3, on the other hand, are chosen such that there exists in the attenuation band two frequencies F1 and F2 for which the impedance of the microwave filter (11) takes the same complex value, so that successively traversing all the frequencies of the attenuation band, the figurative point of the complex impedance R + jX of the microwave filter (11) describes in a diagram (R, X) a loop in the form of gamma. 6. Microwave system according to claim 5, characterized in that, the frequencies Fol and Fo2 being close, the impedances Z1, 22, z3 satisfy the inequality Z1 <(Z2) 2.  Z2 Z3