SUBHARMONIC FREQUENCY CONVERTER FOR APPLICATIONS TO MILLIMETRIC RADIO WAVES

Abstract: The following description concerns a subharmonic frequency converter for applications to millimetric radio waves. The circuit structure comprises a diplexer filter realized in thin film on alumina layer set up by a pass-band section for an incoming local oscillator signal LO/2 with subharmonic frequency, and a low pass section for a signal with intermediate frequency IF. The point common to both sections is connected to a gate of a low pass filter LPS realized in microstrip on a quartz layer letting making pass the subharmonic and blocking the fundamental. The other gate of the LPS filter is connected to a head of a pair of mixer diodes in GaAs put in antiparallel on the same quartz layer; the other head of the pair is connected to ground. The diodes are at the centre of a rectangular cavity closed on one side by a short-circuit plate and on the opposite communicating with a rectangular waveguide carrying a radiofrequency signal RF.

Description: SUBHARMONIC FREQUENCY CONVERTER FOR APPLICATIONS TO MILUMETRIC RADIO WAVES Field of the Invention The present invention relates to the sector of transmitting and receiving microwave signals, and more precisely to a subharmonic frequency converter for applications to the millimetric radio waves. In the range of the millimetric radio waves concerning frequencies from 30 GHz upwards it is possible to realize very compact and miniaturized frequency converters, thanks also to the reduced dimensions of the cross sections of the rectangular waveguides interconnecting with those. This type of converters are generally realized in thin film, and the layers are put into metal packages. The frequency conversion is carried out by mixer diodes. A problem coming up regarding the propagation of the radiofrequency signal from the waveguide to the diodes, and vice versa, is that of carrying out a transition between the metallic waveguide and the circuit in microstrip connected to the diodes. Background Art A known solution for this problem consists in the introduction into the guide of an end of a small cylindric conductor which other end is welded to the microstrip. The above- mentioned conductor operates as an antenna in the guide and transfers the here present radiofrequency signal to the microstrip, or vice versa. Summary of the Invention The present invention is addressed to a subharmonic frequency converter in thin film with a simple and compact structure, in which the waveguide of the radiofrequency signal is directly facing a cavity containing the diodes, avoiding the use of additional transition means between the guide and the part in microstrip of the circuit, because these mixer diodes are carrying out the transition from guide to microstrip or the inverted transition. Those objects are solved, according to the invention, with the features of claim 1. Brief Description of the Drawings The feature of the present invention which are believed to be novel are set fort with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be understood with reference to the following description taken in conjunction with the accompanying drawings, and the several figures of which like referenced numerals identify like elements, and in which: fig. 1 shows a perspective view of the subharmonic frequency converter as the object of the present invention; fig. 2 shows a section view according to plane A-A of fig. 1 ; and fig, 3 indicated in a more detailed way the central part of fig. 1 related to the layout in thin film. Referring to fig. 1 the here shown microwave frequency converter of the subharmonic type is set up by a metallic body 1 with a configuration of a parallelepipedon in which three cavities have been realized communicating among them. A first cavity 2 has a rectangular section, the longest dimension of said section is perpendicular to an axis A-A of horizontal symmetry of the metallic body. Cavity 2 communicated with a narrower and longer rectangular cavity 3 set along the same Axis A-A. The latter communicates at its turn with a third and larger rectangular cavity 4. the cavities 2 and 3 turn out to be symmetric compared to axis A-A, which is not the case of cavity 4 which extends towards an edge of body 1. Cavity 2 passes through the entire thickness of the metallic body 1 , while the cavities 3 and 4 finish at approximately half of the thickness. Cavity 4 continues fort a short way beyond cavity 2. Two perpendicular to each other sidewalls of body 1 are perforated in correspondence to cavity 4 in order to receive two cylindric glass feedthroughs 5 and 6 called hereafter glass-beads. The latter are at their turn connected to two coaxial connectors 7 and 8 concentric to the glass-beads 5 and 6 which penetrate into respective metal supports 9 and 10 screwed on the sides of the metallic body 1. The cavities 2, 3 and 4 contain the circuit par in thin film of the converter, which will be illustrated when examining fig. 3. At connector 7 arrives a local oscillator signal LO/2 and from connector 8 comes out an intermediate frequency signal IF. Referring to fig. 2 in which the same elements of fig. 1 are indicated by the same symbols, we can see the metal body 1 and the cavities "2, 3 and 4 sectioned along a plane cutting perpendicularly the body 1 along the centre line A-A. The upper part of the cavities 2, 3 and 4 are closed by a metal plate 1 1 fixed to the body 1 by screws 12. In the lower part of cavity 2 a radiofrequency signal RF in reception is indicated which enters cavity 2. For simplicity reasons the fig. 1 and 2 do not show a metallic flange fixed by screws to the lower part of the body 1. The flange supports a rectangular waveguide (also this is not shown in the fig.) which communicates with cavity 2 where the radiofrequency signal RF converges. Referring to fig. 3 in which the same elements of the fig. 1 and 2 are indicated by the same symbols, it is possible to notice that an alumina layer 13 is placed in cavity 4. A second quartz layer 14 is placed in cavity 3 where it extends for the whole length of this one, passes through the central zone of cavity 2 and ends in the part 3' which extends cavity 3 beyond cavity 2. The ground planes of the backside of the layers 13 and 14 are welded by thermal compression to the metallic body 1. Layer 13 supports a diplexer filter set up by a pass-band section PBS and by a low pass section LP. One end of the pass-band section PSB is connected to the glass- bead 5, and therefore to the coaxial connector 7 of the local oscillator signal LO/2 (shown in fig. 1); the second end of the section PSB is connected to a short microstrip 15 put along the axis A-A of fig. 1 and ending near the alumina layer 13. One end of the low pass section LP is connected to the glass-bead 6, and therefore to the coaxial connector 8 of the intermediate frequency signal IF (shown in fig. 1); the second end of the section LP is also connected to the microstrip 15 in the same point in which the second end of the pass-band section PBS is connected. The filters PBS and LP are realized in microstrips according to known techniques, the filter PSB comprises a block for the direct current set up by two short lines of which one is connected to microstrip 15. The quartz layer 15 supports a low pass filter LPS realized in microstrip according to known techniques. To both ends of the filter LPS which correspond to the same number of gates for the signals two microstrips 17 and 18 are present. A metal strap 16 interconnects the short microstrip 15 of the diplexer filter with the microstrip 17 of the filter LPS. The microstrip 18 at the other end of LPS is connected to a head of a pair of mixer diodes D1 and D2 put in antiparallel, the other head of which is connected to a short microstrip 19 which extends inside the part 3' which extends cavity 3 beyond cavity 2. The metallization of the ground plan present on the backside of layer 14 is interrupted in cavity 2 just next to zone 20 (indicated by hatching). Microstrip 19 is connected to the metal body 1 by a welding 21. Summarizing the microstrips 18 and 19 inside the cavity 2 are equivalent to a single line passing through the cavity 2 along the centre line in the sense of the shortest dimension, the above-mentioned line is interrupted in one zone in the centre of the cavity where the diodes D1 and D2 are placed in antiparallel connected between the heads of the interruption. As already said before, to the lower side of the metal body 1 a rectangular waveguide (not shown) is rigidly connected facing cavity 2 with which it communicates. The latter one is in reality a part of the above-mentioned waveguide closed at the upper end by the metal plate 1 1 which for said cavity 2 represents a short-circuit plate 11 in all respects (fig. 2). The shortest dimension of the rectangular section of cavity 2 is smaller than that of the rectangular guide which converges the signal RF (cavity in reduced height guidance) in order to avoid power losses and so losses of conversion due to the impedance missmatching between guide and cavity. Referring to the previous figures to illustrate the functioning of the converter the circuit structure of fig. 3 is that of a mixer in single-ended configuration operating in subharmonic. The local oscillator signal LO/2 with subharmonic frequency injected to connector 7 passes in the following order through: glass-bead 5, pass-band section PBS, the short microstrip 15, metal strap 16, low-pass filter LPS and the pair of diodes D1, D2. As already known due to the nonlinearity of the conductivity of the diodes and to the high level of the local oscillator signal LO/2 with subharmonic frequency, the configuration of the diodes of figure 3 is such that a fundamental with double frequency compared to the subharmonic one is generated plus a certain number of equal subharmonics. The reception radiofrequency signal RF present in the guide is directly injected into the cavity 2, which is equal to a part of the same guide for what concerns the propagation of the electromagnetic field, the signal RF injected in cavity reaches the diodes D1 and D2 where the beating with the fundamental and the following generation of the intermediate frequency signal IF happens. In a more general way the beating happens also with the subharmonic LO/2 and the harmonics equal to the fundamental. Therefore the signal RF is transferred in frequency symmetrically around the above-mentioned frequencies a part compared to the fundamental. The low pass filter LPS blocks all components generated in the diodes D1 , D2 with the frequencies higher than the subharmonic preventing the latter ones from reaching the two sections PBS and LP of the diplexer filter. For this purpose the filter LPS carries a short circuit for said signals on the plan of the wall of cavity 2 passed through by the microstrip 18. The only signals passing completely the filter LPS are therefore the local oscillator signal LO/2 and the intermediate frequency IF. Also the striking between RF and LO/2 at the difference frequency passes , even if diminished, through the low pas filter LPS, but it will be blocked by both sections LP and PBS of the diplexer filter. The intermediate frequency signal IF generated in the diodes passes as follows through: filter LPS, strap 16, microstrip 15, low pass section LP, glass-bead 5 and comes out of connector 8. In the diplexer filter the low pass section prevents the signal IF from joining the gate of the local oscillator signal set up by the connector 7, and the low pass section LP prevents the local oscillator signal LO/2 from joining the gate of the intermediate frequency signal set up by the connector 8. According to what described before the function of the diplexer filter set up by the two sections PBS and LP, which is in fact that of isolating the gate of the signal LO/2 from the signal IF and of isolating the gate of the signal IF from the signal LO/2. As already said, the isolations RF/(LO/2) and RF/IF the gates 7 and 8 are guarantied by the filter LPS. For what concerns instead the isolations (LO/2)/RF and IF/RF inside the guide these are assured by the fact that the same acts as a high pass filter which blocks the signals IF and LO/2. Another aspect to be taken into consideration is the reduction of the losses caused by unwanted reflections of the signal RF because of the impedance missm etchings. In order to avoid this type of inconveniences the distance between the metal plate 12 which closes at the bottom cavity 2 setting up a short-circuit for the signal RF, and the reference plan of the diodes D1, D2 corresponding to the front side of the layer 14 must be such to carry an open circuit for the signal RF on said plan. Moreover the characteristic impedance ZQ of the rectangular waveguide must coincide with the characteristic impedance ZQAV °f cavity 2 including the contribution due to the impedance of the diodes; if this condition does not occur between the waveguide and cavity 2 2 rectangular wave shaft with a length equal to a quarter wave with the centre band frequency of the signal RFb and the characteristic impedance : Z τ «■ -JZ- σ ' ^CAV In the converter of the not limiting example just described before the signal RF has a centre band frequency of 55 GHz, the signal LO/2 has a frequency of 27 GHz and the signal turning out as IF has a centre band frenquency of 1 GHz. The coaxial connectors 7 and 8 for the signals LO/2 and IF are of the type K, the rectangular waveguide for the signal RF is of the type WR 19 (UG-599 U), the used diodes are of the type GAAs Hp HSCH-9201. The electric performances are summarize in the following table: The down-converter of the example operates also as an up- converter to obtain a transmission signal RF without the necessity to apply any modification to the described circuit structure, in that case the signal IF is incoming and the signal RF outgoing. The functioning at lower frequencies requires an increase of the dimensions of the rectangular wave guide in addition to those of the converter, and therefore the lowering of the frequency turns out to be less convenient. Apart from this, whenever the use of the same structure is wanted, the quartz layer 14 may be realized by using a more convenient alumina layer. Finally it is necessary to note that regarding the electric functioning of the converter the object of the cavities 4 and 3 (with the part 3') is to support the diplexer and LPS filters avoiding - once closed by the upper metal plate 1 1 - the propagation of spurious radiation in the surrounding space. Completely different is the function of cavity 2 which, as already said, is equal to a section of the waveguide enabling the diodes placed in the centre to act as frequency mixers. Furthermore: cavity 2, the short-circuit plate 11 , the diodes D1 and D2, the lines 18 and 19 without metallization below and the welding 21 set up a transfer from guide to microstrip, and vice versa. While a particular embodiment of the present invention has been shown and described, it should be understood that the present invention is not limited thereto since other embodiments may be made by those skilled in the art without departing from the scope thereof. It is thus contemplated that the present invention encompasses any and all such embodiments covered by the following claims.

Claims: CLAIMS 1. Microwave frequency converter comprising a diplexer filter in microstrip having one input gate (7) for a local oscillator signal (LO/2) with subharmonic frequency and an input or output gate (8) for an intermediate frequency signal (IF), said filter being set up by a pass-band section (PBS), letting get through said local oscillator signal (LO/2) towards two frequency mixer diodes (D1 , D2) connected in antiparallel, and by a low pass section (LP) letting go through towards said gate of the intermediate frequency signal (8) the homonym ous signal (IF) generated by said diodes (D1 , D2) for the beating between a fundamental with frequency double with respect to said subharmonic, generated by said diodes, and a radiofrequency reception signal (RF), or allowing the transfer towards said diodes an intermediate frequency signal (IF) entering the homonymous part (8) obtaining for the beating with said fundamental a transmission radiofrequency signal (RF), characterized in that it comprises moreover: a dielectric plate (14) which supports a second low pass filter (LPS) realized in microstrip connected between a point common to both said sections (PBS, LP) of the diplexer filter and said diodes (D1 , D2), said second low pass filter (LPS) letting go through said local oscillator signal (LO/2) towards the diodes and blocking the frequencies higher than said subharmonic, isolating in this way said gates of the local oscillator signals (LO/2) and with intermediate frequency (IF) from the radiofrequency signal (RF); a metal body (1) in which at least 2 cavities (4+3, 2) have been created which are communicating among them, and of which a first one (4+3) contains said diplexer filters (PBS, LP) and a second low pass filter (LPS), and a second (2) with a rectangular section goes through all the thickness of said metal body (1) and includes said diodes (D1 , D2); a waveguide with rectangular section which transports inside or outside said second cavity (2) the reception or transmission radiofrequency signal (RF), the second cavity (2) being equivalent to a part of said waveguide for what concerns the propagation of the electromagnetic field; a metal plate (11) which closes completely said first cavity (4+3) and which closes on one side said second cavity (2) setting up for the latter one a short-circuit plate; a metallic line (18, 19) put on said dielectric plate (14) and without metallization below which goes through said second cavity (2) along the centre line in the sense of the shortest dimension, the ends of said line (18, 19) being connected to said metal body (1) and to an end of said second low pass filter (LPS), respectively, the above- mentioned line being interrupted in a sector in the centre of said second cavity (2) where said diodes (D1 , D2) in antiparallel are connected to the heads of the interruption. 2. Microwave frequency converter according to claim 1 , characterized in that the distance between said metallic short circuit plate (11) and the side of said dielectric plate (14) on which said metallic line (18, 19) is put, is such to carry on said plan a circuit open for said radiofrequency signal (RF). 3. Microwave frequency converter according to claim 1 , characterized in that said first cavity (3) continues for a short distance (3') beyond said second cavity (2) placing one end of said dielectric plate (14) and of said metal line (18, 19) connected to the metal body (1); 4. Microwave frequency converter according to claim 1 , characterized in that said metal line (18, 19) outside of said second cavity (2) is a microstrip of said second low pass filter (LPS). 5. Microwave frequency converter according to some of the previous claims, characterized in that the shortest dimension of the rectangular section of said second cavity (2) is smaller than the corresponding dimension of said rectangular waveguide. 6. Microwave frequency converter according to the claims 1 or 5, characterised in that between said waveguide and said second cavity (2) a waveguide shaft .is inserted with rectangular section and length equal to a quarter wave with reference to the centre band frequency of said radiofrequency signal (RF) and with the characteristic impedance Z_. which satisfies the following expression: Z r — y^o ' &CAV where Z Q is the characteristic impedance of said waveguide and Z CA v is the characteristic impedance of said second cavity (2) including the contribution due to the impedance of said diodes (D1 , D2). 7. Microwave frequency converter according to claim 1 , characterized in that said dielectric plate (14) is made of quartz. 8. Microwave frequency converter according to claim 7, characterized in that it works in the range of the millimetric radio waves.

Inventors: CATTANEO FULVIO, PILONI MARCO, CATTANEO, FULVIO, PILONI, MARCO
Applicants: CATTANEO FULVIO, ITALTEL SPA, PILONI MARCO, CATTANEO, FULVIO, ITALTEL S.P.A., PILONI, MARCO
Application year: 1997
Main classification: H03D9
IPC codes: H01P1, H03D9