Waveguide switch circuit

Abstract: An RF switch circuit includes a waveguide transmission line (19) having an input port (18a), an output port (18b), a first wall having a first cavity disposed therein and a second wall having a second cavity disposed therein with the first and second cavities being aligned along a centerline of the waveguide transmission line. An RF choke (44) provides a substantially short circuit impedance characteristic between the first cavity and the waveguide transmission line to RF signals propagating along the waveguide transmission line. An electrically conductive member (36) disposed in a first region of the second cavity has a PIN diode (38) having a first electrode disposed on a first surface of the electrically conductive member (36). An electrically conductive post (46) has a first end disposed in the first cavity and a second end disposed in the second cavity with the second end of the post electrically contacting a second electrode of the diode (38). Bias drive is applied to the upper second, end of the post (46), the electrically conductive member (36) being earthed through the body (23) of the transmission line (19). Spacer rings (40) set the diode (38) in resonant circuit. The choke (44) is also spaced (42) from the transmission line (19) to provide a quarter wavelength termination. The switch circuit is closed when the diode (38) is fully conducting, and open when the diode (38) is nonconducting.

Description: This invention relates to a radio frequency switch circuit comprising: means having a first wall region and a second opposed wall region and defining therebetween a waveguide transmission line having an input port and an output port, the first wall region having a first cavity disposed therein with an aperture to the waveguide transmission line, and the second opposed wall region having a second cavity disposed therein with an aperture to the waveguide transmission line; an electrically conductive member disposed within the second cavity; a diode disposed within the second cavity and having a first electrode disposed on a first surface of the said electrically conductive member; an electrically conductive post extending through the said apertures, the post having a first portion disposed in the first cavity and a second portion disposed in the second cavity, with the second portion electrically contacting a second electrode of the diode; and means for providing a substantially short circuit impedance characteristic at the first cavity between the said post and the said first wall region to RF signals propagating along the waveguide transmission line. As is known in the art, a radio frequency (RF) switch circuit (hereinafter switch or switch circuit) is a device used to connect and disconnect RF signal paths in an electrical circuit. When connecting RF signal paths, a switch generally provides a bi-directional RF signal path. Thus, signals fed to an input port of the switch appear at an output port of the switch and vice-versa. When the switch provides a signal path having a relatively low insertion loss characteristic between the input and the output port, the switch is generally referred to as being in the "on" position. When the switch provides a signal path having a relatively high insertion loss characteristic between the input port and the output port the switch is generally referred to as being in the "off" position. The electrical characteristics of an RF switch include isolation, insertion loss, switching speed and RF power handling capability. Depending upon the particular application it is often necessary to optimize one of these electrical characteristics by trading off the performance of the other electrical characteristics of the switch. For example, in a pulse radar systems which uses a common antenna to both transmit and receive signals, a component such as a duplexer, a circulator or the like provides isolated signal paths which couple a transmitter and a receiver to the common antenna. Components such as duplexers, circulators or the like, however, have a finite isolation characteristic. During transmit, portions of the transmit signal may leak back to the receiver due to the relatively poor isolation characteristics of the components which provide the receive path and transmit path to the common antenna. Moreover impedance mismatches between the antenna input port and the transmitter may cause high power RF signals from the transmitter to be reflected to the RF receiver. Thus, to protect the receiver from such undesired signals which are provided during the transmit mode, an RF switch circuit capable of withstanding high power RF signal levels may be disposed between the RF receiver and the duplexer for example. Furthermore, in the pulse radar system the RF switch circuit must be able to switch between its "on" and "off" states at a rate greater than the pulse repetition frequency of the transmitter. When the transmitter provides a signal pulse, the switch is in its "off" or protection state and thus the switch protects the components of the receiver from high power RF signals by disconnecting the RF signal path to the RF receiver. When the transmitter is not providing a signal pulse the switch is in its "on" or "non-protection" state and thus the switch couples RF signals from the duplexer to the receiver. A microwave switch for 5,585 Mhz is described in US-A- 3 387 665 in which two switched diodes are used. Each diode is disposed within a waveguide, the cathode being connected to a movable short circuit to ground in a tuning stub extending from an upper broad wall of the waveguide, and the anode being connected to the centre conductor of a female connector extending from a lower broad wall of the waveguide. The female connector contains a radio frequency choke and a bypass capacitor. A capacitive post or turning screw is inserted into the waveguide midway between the two diodes to be adjusted for minimum insertion loss with the switch in the on condition, i.e. passing the RF signal. The diodes are located within the waveguide midway between the narrow faces of the waveguide, and are separated by one quarter wavelength. The diodes are IN419 type and are driven by square pulses having a duration in the range 0.2 microseconds to 5 microseconds and a repetition rate in the range 100 Hz to 100 kHz. One type of switch circuit which protects receivers from high power RF signals includes a plurality of PIN diodes connected in shunt across a transmission line at points on the transmission line separated from one another by one quarter wavelength at a particular frequency of operation. Since the diode connected closest to the input port of the switch has the highest power level incident thereon, this diode should have a higher breakdown voltage characteristic than the diode connected closest to the output port of the switch. Thus, the breakdown voltage of each PIN diode should correspondingly decrease from the input port to the output port of the switch. The increase in breakdown voltage is typically achieved by increasing the thickness of the intrinsic region of the PIN diode. As is known, as the intrinsic region thickness increases the shunt resistance of the diode increases. However, as is also known, as the intrinsic region thickness is increased the capacitance of the diode decreases. This results in a concomitant decrease in the switching speed of the diode (i.e. its takes longer to switch the diode between its conducting and nonconducting states). Thus a trade off is made between the power handling capability and switching speed of the switch. When the switch is in the non-protection mode the diodes are reversed bias and provide high shunt resistance so that substantially all of the RF signal power fed to the input port of the switch propagates along the RF transmission line relatively unattenuated to the output port of the switch. However when the switch is in its protection mode the diodes are placed in their forward conducting state and provide a low impedance path between the RF transmission line and ground. RF signals fed to the input port of the switch are shunted to ground through the forward biased diodes. A portion of the RF power dissipates in the diodes due to the resistance of the diodes and thus the diodes are heated. If the diodes are unable to dissipate the heat which is generated the diodes are damaged. Thus it is desirable to channel the heat away from the PIN diodes via a heat sink. Conventional waveguide switch circuits include a plurality of diodes disposed in a transverse plane of the waveguide transmission line. By disposing the diodes in the transverse plane of the waveguide transmission line a problem is that the diodes are not provided with a heat sink. Thus, such a waveguide switch circuit is not able to handle high power RF signals. Alternatively, in another approach a diode may be disposed in a bottom wall of a waveguide transmission line having a rectangular cross-section and partially protruding into the waveguide transmission line with an electrically conductive post disposed between the diode and a top wall of the waveguide transmission line. By disposing the diode partially in the waveguide wall and partially in the waveguide transmission line a heat sink is provided to the diode. However, one problem with this technique is that such circuits are relatively difficult to tune. Nevertheless, this technique is used in waveguide switch circuits because of the low insertion loss characteristic the switch provides in its "on state. Thus, when the diode is forward biased placing the switch in its "on" state the structure is highly resonant and therefore provides the switch having a relatively low insertion loss characteristic. GB-A-2 192 311 describes a radio frequency switch circuit of the kind defined hereinbefore at the beginning in which the waveguide transmission line is provided by a rectangular section waveguide, and the diode is a PIN diode. The electrically conductive member is a conductive boss that can be screwed in or out of the bottom wall of the waveguide to alter the optimum point of tuning. The conductive post extends through the top wall to engage a flat choke through which the post can also be screwed up and down. The choke is electrically separated from the outer surface of the wall of the waveguide by an insulating layer, and the choke and the insulating layer form a low impedance for RF alternating current at the top end of the bore or cavity through which the post extends in the top wall of the waveguide. Two blocks of dielectric material, such as PTFE, are placed at the same point along the waveguide as the switch circuit, with one block on each side wall. These blocks increase the frequency bandwidth of the switch circuit. Other microwave circuits also use resonant structures disposed along the waveguide transmission line. For example, as is known in the art, a negative resistance device, such as an IMPATT diode for example, is often used as an oscillator or an amplifier to convert DC power to RF power. IMPATT diode oscillator circuits generally include a waveguide transmission line having a rectangular cross-section and having an electrically conductive member transversely disposed across one end of the waveguide to provide a short circuit impedance characteristic across the end of the waveguide transmission line. Thus the waveguide transmission line having a short circuit at one end provides a resonant waveguide cavity. A plurality of IMPATT diodes which feed RF signals into the cavity may be disposed in the side walls of the waveguide cavity and spaced along the sidewalls of the cavity at half wavelength intervals. Each of the IMPATT diodes are provided having matching circuits to match the impedance of the IMPATT diodes to the impedance of the waveguide cavity over a pre-determined range of frequencies. In one embodiment of an IMPATT diode oscillator circuit described in U. S. Patent No. 4,583,058 and assigned to the assignee of the present invention each one of the diode oscillators includes a center conductor having a first end connected to a first end of an IMPATT diode. A second end of the IMPATT diode is connected to a heat sink. A ring shaped member is disposed about the center conductor and spaces the IMPATT diode from the resonant waveguide cavity. A second end of the centre conductor has a tapered sleeve section to provide a stabilizing load used to terminate the diode oscillator in a characteristic impedance. Several of such IMPATT diode oscillator circuits may be disposed around a power combiner circuit which combines the power of such IMPATT diode oscillator circuits to provide high power RF signals over a relatively broad range of operating frequencies. According to the present invention, a radio frequency switch circuit of the kind defined hereinbefore at the beginning is characterised in that the short circuit impedance means comprises an RF choke means providing along the said first portion of the electrically conductive post a first region having a high impedance characteristic and a second region having a low impedance characteristic, and means so disposing the RF choke means as to position the said second region to provide the short circuit impedance characteristic at the said aperture of the first cavity. In a preferred embodiment of the present invention, the switch circuit includes a waveguide transmission line having an input port, an output port, a first wall having a first cavity disposed therein and a second opposing wall having a second cavity disposed therein with the first and second wall cavities being aligned along a centerline of the waveguide transmission line. The switch further includes means for providing a substantially short circuit impedance characteristic between the first cavity and the waveguide transmission line to RF signals propagating along the waveguide transmission line. An electrically conductive member is disposed in a first region of the second cavity and a diode is disposed on a first surface of said electrically conductive member with a first electrode of said diode contacting said electrically conductive member. An electrically conductive post having a first end disposed in the first cavity and having a second end disposed in the second cavity electrically contacts a second electrode of the diode. With this particular arrangement, a waveguide switch circuit is provided. In particular the switch is capable of switching high power levels while also having a fast switching speed and a relatively low insertion loss characteristic. In conventional waveguide switch circuits excessive heating damages the diode. In the preferred embodiment of the invention the diode is disposed in the second wall of the waveguide and thus the second waveguide wall provides a large thermal mass to act as a heat sink for the diode. Thus the diodes can dissipate more heat provided from absorption of the high power RF signals fed thereto. Therefore compared to prior techniques, smaller diodes having faster switching speeds may be used to provide the RF switch circuit with an overall increased switching speed. One or more electrically conductive disks (choke spacers) may be disposed about an RF choke to space the RF choke a predetermined distance from an aperture disposed between the first cavity and the waveguide transmission line. The electrically conductive post and the diode form a resonant circuit in the waveguide. Electrically conductive disks (diode spacers) may be disposed between the conductive member and the waveguide transmission line to space the diode a predetermined distance from the waveguide transmission line. By spacing the diode with the diode spacers and the RF choke with the choke spacers the resonant circuit may be "tuned" to provide the switch with a relatively low insertion loss characteristic in its "on" state. A plurality of said resonant circuit structures may be disposed in the waveguide transmission line and spaced at quarter wavelength intervals along a longitudinal axis of the waveguide transmission line to provide the switch with a high isolation characteristic in its "off" state. Brief Description of the Drawings The foregoing features of this invention as well as the invention itself may be more fully understood from the following description of the drawings in which FIG. 1 is a block diagram of an RF receiving and transmitting system; FIG. 2 is an exploded isometric diagrammatical view of an RF switch circuit of the type shown in FIG. 1; FIG. 3 is a cross-sectional view of a waveguide housing of the type used in the RF switch circuit of FIG. 2 without diode resonators taken along line 3-3 of FIG. 2; FIG. 4 is a cut away cross-sectional view of the assembled resonator section disposed in the waveguide housing of FIG. 3 taken along line 4-4 of FIG. 2; and FIG. 4A is an enlarged view of an RF choke in a region bounded by the line 4A-4A of FIG. 4 used in the switch circuit of FIGs. 2 and 4. Description of the Preferred Embodiments Referring now to FIG. 1, an RF system 10 such as a radar is shown to include an antenna 12 coupled to a duplexer circuit 14. The duplexer 14 includes a pair of isolated signal paths (not referenced) with a first signal path (not referenced) of the duplexer 14 disposed between the antenna 12 and the transmitter 16 and a second signal path (not referenced) of the duplexer 14 disposed between the antenna 12 and an RF switch circuit (hereinafter switch) 18 having an input port 18a and an output port 18b. The switch 18, as will be further described in conjunction with FIG. 2, couples RF signals from the duplexer 14 to a receiver 20. The switch 18 is here provided having three diodes 38, 38', and 38'' each of said diodes having an anode coupled to an RF propagation network 19 and a cathode coupled to ground. A driver circuit 22 provides a DC voltage or a DC current at each of the output terminals 22a, 22b and 22c. Each of the output terminals 22a-22c is coupled to a corresponding one of the diodes 38-38'' of the switch 18. The driver circuit 22 provides either a forward bias current or a reverse bias voltage at terminals 22a, 22b and 22c to respectively switch the PIN diodes 38-38'' between their conducting and non-conducting states. During a transmit mode of operation the RF system 10 the transmitter 16 provides a transmit signal to the duplexer 14 and the driver circuit 22 provides a forward bias current to the PIN diodes 38-38'' placing the diodes 38, 38', 38'' in their conducting state. In the forward biased state, the diodes 38-38" provide a short circuit impedance in the waveguide transmission line. Thus, substantially all of the RF signal energy provided to the input port 18a is reflected back toward the input port 18a. However, due to the resistance of the diodes 38-38" a portion of the RF energy is coupled to and dissipated in the diodes 38-38" and a highly attenuated RF signal emerges at the output port 18b. Thus the switch 18 protects the RF receiver 20 from RF transmit signals provided from the duplexer 14 in the transmit mode of the radar system 10. When the transmitter 16 is not providing a transmit signal, the driver circuit 22 provides a reverse bias voltage to the diodes 38-38'' and places each of the diodes 38-38'' in its non-conducting state. In this instance the switch 18 is placed in its non-protection state and couples signals from the duplexer 14 to the RF receiver 20 with a relatively small amount of attenuation. Thus the driver circuit 22 switches the switch 18 between its protection and non-protection states. Referring now to FIGs. 2 and 3, the switch circuit 18 of FIG. 1 is shown to include a housing 23 having an aperture of rectangular cross-section disposed therethrough to provide a rectangular waveguide transmission line 19 having a first port 18a corresponding to the input port of the switch circuit 18 and a second port 18b (FIG. 3) corresponding to the output port of the switch circuit 18. Here the waveguide transmission line 19 is provided as a so-called half-height waveguide transmission line. It should be noted however that waveguide transmission line 19 may alternatively be provided as a full-height rectangular waveguide transmission line, a quarter height rectangular waveguide transmission line or even a circular waveguide transmission line. A plurality of cavities having circular cross-section 26, 26' and 26'' are disposed in a first or here top wall of the housing 23 aligned with and spaced along a centerline of the waveguide transmission line 19. Likewise, a corresponding plurality of cavities 28, 28' and 28'' (FIG. 3) are disposed in a second or here bottom wall of housing 23 aligned with and spaced along the centerline of the waveguide transmission line. As shown in FIG. 3, the cavities 26-26'' and 28-28'' are disposed in the top and bottom walls respectively of the housing 23. Taking the cavity 26 as representative of the cavities 26-26", the cavity 26 is provided having a first threaded region 27, a second region 27a having no threads, a shoulder having a surface 27b and an aperture 26b having unthreaded sidewalls 27c and exposing the cavity 26 to the waveguide transmission line 19. Likewise, taking lower cavity 28 as representative of the cavities 28-28'', the cavity 28 is provided having a first threaded region 29, a second region 29a having no threads, a shoulder having a surface 29b and an aperture 28b have unthreaded sidewalls 29c and exposing the cavity 28 to the waveguide transmission line 19. Referring back to FIG. 2, the switch 18 includes three so-called resonant sections 30, 30' and 30'' spaced at odd multiples of one-quarter wavelength (λ/4) along the waveguide transmission line 19. Here said resonant sections 30-30'' are spaced at three-quarter wavelength (3λ/4) intervals along the center of the top and bottom walls of the waveguide transmission line 19. One-quarter wavelength spacing intervals can also be used and in some applications would be preferred due to the concomitant reduction that single quarter-wavelength spacing would provide in the length of the waveguide transmission line 19. Taking resonant section 30 as representative of resonant sections 30', 30'', the resonant section 30 is shown to include a diode assembly 32 and an RF choke assembly 34. The diode assembly 32 includes a diode holder 36 which here is provided as a threaded conductive member having a circular cross-section. The diode holder 36 is preferably provided from a material having relatively high electrical and a thermal conductivity for reasons to be discussed in conjunction with FIG. 4. Here diode holder 36 is provided as a single member. Alternatively, the diode 38 may be disposed on a separate threaded member 36' shown (in Fig. 4) in phantom which screws into the diode holder 36. The diode assembly 32 also includes a diode 38 having an anode 38a and a cathode (not numbered). The diode 38 is disposed on a first surface of the electrically conductive member 36 with the cathode of the diode 38 in contact with the first surface of the electrically conductive member 36. The diode assembly 32 is then disposed in the cavity 28 (FIG. 3). Here the diode assembly 32 screws into the threads 29 of the cavity 28. Prior to the insertion of the diode assembly 32 into the cavity 28, a so-called diode spacer 40 having a thickness t 1 , which will be discussed further in conjunction with FIG. 4, may optionally be disposed in the cavity 28 (FIG. 3). Suffice it here to say the diode spacer 40 contacts the surface 29b (FIG. 3) of the shoulder and thus spaces the diode 38 a predetermined distance from the aperture 28b (FIG. 3). The RF choke assembly 34 includes an optional choke spacer 42, here provided as an electrically conductive annular disk having a thickness t2, and an RF choke 44 having a spool shape and having an axial bore therein and provided from an electrically conductive material. The RF choke 44 will be further described in conjunction with FIG. 4. The RF choke assembly 34 further includes an electrically conductive post 46 having a first region 46a of a first diameter, a second region 46b of a second diameter and a cylindrically shaped sleeve 46c disposed between the first and second regions and having a third diameter. The assembly further includes a spring 48 and a dielectric member 50 having a cylindrical shape and having a stepped bore therein. To assemble the RF choke assembly 34, the optional choke spacer 42 is disposed in the cavity 26 with a first surface of the spacer 42 in contact with the surface 27b of the shoulder (FIG. 3). The RF choke 44 is then disposed in the cavity 26 with a bottom base surface thereof resting on a second surface of the choke spacer 42. The post 46 is disposed in the cavity 26 with the region 46b disposed through the bore of the RF choke 44 and through the center of the choke spacer 42. The spring 48 is disposed over the first region 46a of the post 46 and rests on a first surface of the sleeve 46c and the dielectric member 50 is disposed over the first and second regions 46a, 46c of the post 46 and rests on a top base surface of the RF choke 44. The spring 48 is thus compressed and exerts a force on the sleeve 46c of the post. A second end of the post 46 thus exerts a force on the diode 38. Here, the dielectric member 50 is threaded and thus screws into the threads of the cavity 26 to secure the RF choke assembly 34 in the cavity 26. It should be noted that resonant sections 30-30'' are here disposed along the centerline of the longitudinal axis of the waveguide transmission line 19. Alternatively, the resonant sections may be disposed adjacent to the longitudinal centerline of the waveguide transmission line 19. In this instance two of such resonant sections may be adjacently disposed about the centerline of the longitudinal axis of the waveguide transmission line 19, so that each of the resonant sections 30 to 30'' is replaced in this alternative by two resonant sections disposed one each side of the centerline. The electric field of a dominant mode RF signal propagating along the waveguide transmission line 19 has sinusoidal shape with a voltage maximum along the center of the longitudinal axis of the waveguide. By adjacently disposing the resonant sections about the center line of the longitudinal axis of the waveguide transmission line 19, less voltage is incident upon each diode. Thus the diode absorbs less RF energy and dissipates less heat allowing even smaller, faster switching diodes to be used in the switch circuit 18. Referring now to FIG. 4, in which like elements of the switch 18 of FIGs. 2 and 3 are provided with the same designations, the resonant section 30 is here shown assembled having a first end of the post 46 contacting the anode of the diode 38. The diode 38 is disposed on the electrically conductive member 36 using any technique well known to those of skill in the art which allows mounting of such diodes with good thermal contact. Here two diode spacers 40 are disposed in the cavity 28 to space the diode 38 a predetermined distance from the aperture 28b (FIG. 3) which exposes the waveguide transmission line 19. Similarly, here the choke spacer 42 spaces the RF choke 44 a predetermined distance from the aperture 26b (FIG. 3) exposing the waveguide transmission line 19. As will be further discussed below, any number of choke spacers 42 or diode spacers 40 may be disposed in the cavities 26, 28. Furthermore, such choke spacers and diode spacers need not be provided having equal thickness. Referring momentarily to FIG. 4A, an enlarged view of a portion of the RF choke assembly is shown. It should here be noted that the RF choke 44 is provided having a first region 44a having a physical pathlength L 1 a second region 44b having a physical pathlength L 2 , and a third region 44c having a physical pathlength L 3 . A dielectric material 43 is disposed between the conductive post 46 and the third region 44c of the RF choke 44 to prevent the post 46 from contacting the RF choke 44 and causing a short circuit between the post 46 and the choke 44. The dielectric material 43 should have a relatively low dielectric constant typically of about 2.54 to keep the Q of the RF choke assembly 34 high and therefore keep the loss low. The dielectric material 43 may be provided for example as rexolite or any other low loss plastic dielectric material. The RF choke 44 and the shoulder 46c of the electrically conductive post 46 are dielectrically spaced, (i.e. they are not in electrical contact) here said dielectric being air. The dielectric member 50 limits lateral movement of the shoulder 46c and thus prevents the post electrically contacting said RF choke. Similarly, a radar absorbing material (RAM) 45 is disposed on a peripheral surface of the axial bore in a top rim region 44a of the RF choke 44. The RAM 45 may be provided for example as the type manufactured by Emmerson Cummings having part number MFSOOF-117. Alternatively, any RF absorber material having similar electrical characteristics may be used. The radar absorbing material 45 prevents the peripheral surface of the axial bore from electrically contacting that portion of the post 35 disposed therethrough. Further, the RAM 45 terminates any RF signals leaking past the RF choke 44. The outer peripheral surface of the second region 44b of the spool shaped RF choke 44 provides the inside diameter of a section of coaxial transmission line 47a having a high impedance characteristic and having a physical pathlength L 2 which corresponds to an electrical path length of one quarter wavelength at a desired frequency of operation. An inner peripheral surface of the choke spacer 42 provides the outside diameter of the high impedance coaxial line section 47a. The outer peripheral surface of the third region 44c of the RF choke 44 provides the inside diameter of a section of low impedance coaxial transmission line 47b having a physical pathlength L 3 . A corresponding portion of the cavity wall provides the outside diameter of the low impedance coaxial line section 47b. A dielectric material, which may be comprised of polyamide for example, is disposed on the outer peripheral surface of the RF choke 44 in the region 44c having the pathlength L 3 . Such a dielectric coating prevents the surface of region 44c of the RF choke 44 from coming into electrical contact with the wall of the cavity 26 and also dielectrically "loads" the coaxial line section 47b to make the physical pathlength L 3 correspond to an electrical pathlength of one quarter wavelength. Thus, both the high impedance and the low impedance sections of coaxial transmission line 47a, 47b have an electrical pathlength corresponding to one quarter wavelength at a predetermined frequency of operation. The RF choke provides a short circuit impedance characteristic at the aperture 26b (FIG. 3) and thus prevents RF signal energy from propagating into the cavity 26. The choke spacer 42 is used to adjust the distance of the RF choke 44 from the aperture 26b (FIG. 3) to thus provide the short circuit impedance characteristic precisely at the aperture 26b.. Referring again to FIG. 4, the driver circuit 22 (FIG. 1) is coupled to the post 46 in the region 46a to provide bias voltages and currents to the PIN diode 38. Here, driver circuit 22 (FIG. 1) provides the diode 38 with bias signals separately from the diodes 38', 38'. In operation when a reverse bias voltage is provided to the anode of the diode 38, the diode 38 is placed in its non-conductive state and the diode 38 provides a high impedance path between the post 46 and the waveguide wall (not numbered) provided by the housing 23. The post 46 in conjunction with the capacitance of the reversed biased diode provides a resonant circuit. That is, the length of the post section 46b in combination with the impedance characteristic of the diode 38 is selected to be resonant at the desired frequency of operation. Thus the signal propagates along the waveguide transmission line 19 relatively unattenuated. However when a forward bias current is provided to the anode of the diode 38, the diode 38 is placed in its conductive state and provides a low impedance path between the post 46 and the waveguide wall (not numbered) provided by the housing 23. In this instance the post provides a highly inductive impedance to the waveguide transmission line 19. The post 46 and the diode 38 are disposed along the center of the waveguide transmission line 19 where the dominant mode electric field is concentrated. Thus signals propagating along the waveguide transmission line are shunted to ground via the conductive post 46 and diode 38 and therefore the signals are substantially reflected with only a small portion of the RF energy propagating from the input port 18a to the output port 18b. The resonant section 30 is tuned by physically adjusting the distance of the diode 38 and the RF choke 44 from the apertures 26b, 28b. The choke spacers 42 are used to adjust the distance between the RF choke 44 and the aperture 26b. Similarly, the diode spacers 40 are used to adjust the distance between the diode 38 and the aperture 28b. Each resonator section 30-30'' may be provided having a different number of choke spacers 42 and diode spacers 40. Thus each resonant section 30-30'' may be tuned separately to provide the switch 18 having the overall desired insertion loss and isolation characteristics. If all of the diodes 38-38'' are reversed biased and thus placed in their non-conducting state, the switch 18 provides a minimum insertion loss characteristic to signals fed thereto. If all of the diodes 38-38'' are forward biased and thus placed in their conducting states the switch 18 provides a maximum insertion loss characteristic to signals fed thereto. Alternatively, since each diode 38-38'' may be biased independently of the others a first one of the diodes, 38'' for example, may be reversed biased and the second and third diodes 38, 38' may be forward biased to provide an attenuation step. That is, the switch will not be completely turned off and will allow at least a portion of an RF signal fed to the input port 18a to propagate to the output port 18b. Thus the switch 18 may provide either a high insertion loss state, an intermediate insertion loss state or a low insertion loss state. Such a feature may be desirable in a radar system for example since the intermediate insertion loss state of the switch 18 may be used to allow an RF receiver to continue to receive RF signals having a power level which would otherwise damage components in the receiver 20 (FIG. 1). Thus such a feature should effectively extend the dynamic range of the receiver 20. Furthermore the diode 38'' is here provided as a so-called high power PIN diode and the diodes 38, 38' are provided as so-called attenuator diodes. By providing the diode 38'' as a high power PIN diode the switch 18 is provided having a high RF power handling characteristic. By providing the diodes 38, 38' as attenuator diodes the switch 18 is provided having a fast switching speed in the intermediate attenuation state. Moreover, since the diodes 38-38'' are disposed on the conductive members 36-36'' which provide an effective heat sink, the diodes 38-38'' are not as thermally limited as in the conventional approach. That is, here each of the diodes 38-38'' is disposed on a larger heat sink than in the conventional approach. Thus each of the diodes may be provided having a smaller intrinsic region for a given input power level compared to the diodes used in prior techniques. PIN diodes having a small intrinsic region are generally able to switch between their conducting and nonconducting states more quickly than diodes having a large intrinsic region. Thus, the switch 19 is provided having a high RF power handling capability and a fast switching speed. Having described preferred embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating their concepts may be used. This same technique for example may be used to provide a low loss passive limiter circuit. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the appended claims.

Claims: A radio frequency switch circuit comprising: means (23) having a first wall region and a second opposed wall region and defining therebetween a waveguide transmission line (19) having an input port (18a) and an output port (18b), the first wall region having a first cavity (26) disposed therein with an aperture (26b) to the waveguide transmission line (19), and the second opposed wall region having a second cavity (28) disposed therein with an aperture (28b) to the waveguide transmission line (19) ; an electrically conductive member (36) disposed within the second cavity (28); a diode (38) disposed within the second cavity (28) and having a first electrode disposed on a first surface of the said electrically conductive member (36); an electrically conductive post (46) extending through the said apertures (26b,28b), the post (46) having a first portion disposed in the first cavity (26) and a second portion disposed in the second cavity (28), with the second portion electrically contacting a second electrode (38a) of the diode (38); and means (44) for providing a substantially short circuit impedance characteristic at the first cavity (26) between the said post (46) and the said first wall region to RF signals propagating along the waveguide transmission line (19); characterised in that the short circuit impedance means (44) comprises an RF choke means (44) providing along the said first portion of the electrically conductive post (46) a first region (44b) having a high impedance characteristic and a second region (44c) having a low impedance characteristic, and means (42) so disposing the RF choke means (44) as to position the said second region (44c) to provide the short circuit impedance characteristic at the said aperture (26b) of the first cavity (26). A circuit according to claim 1, characterised by means (40) for providing a matched impedance characteristic between the diode (38) and the waveguide transmission line (19). A circuit according to claim 2, characterised in that means for providing a matched impedance comprises at least one electrically conductive ring shaped member (40) disposed between the said electrically conductive member (36) and the waveguide transmission line (19). A circuit according to claim 3, characterised in that the means for disposing the RF choke means (44) comprises at least one electrically conductive ring shaped member (42) disposed about the RF choke means (44). A circuit according to claim 4, characterised by means (48) for pressing the electrically conductive post (46) against the second electrode (38a) of the diode (38). A circuit according to claim 5, characterised in that the first electrode of the diode (38) is a cathode and a second electrode (38a) of the diode (38) is an anode. A circuit according to claim 6, characterised in that the waveguide transmission line (19) is provided as a half-height rectangular waveguide transmission line. A circuit according to any preceding claim, characterised in that the diode (38) is a PIN diode.

Inventors: BACON DONALD F, RICHARDS HOWARD M, BACON, DONALD F., RICHARDS, HOWARD M.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1992
Main classification: H01P1
IPC codes: H01P1