Abstract:
A frequency conversion apparatus includes a frequency generation circuit, which is based on a first semiconductor material having a first elemental composition and is coupled to generate one or more Local Oscillator (LO) signals. The apparatus further includes a conversion circuit, which is based on a second semiconductor material having a second elemental composition different from the first elemental composition. The conversion circuit is coupled to accept an input signal in a first frequency range and to convert the input signal to an output signal in a second frequency range by mixing the input signal with the one or more LO signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to Radio Frequency Integrated Circuits (RFICs), and particularly to RF Multi-Chip Modules (MCMs) that include multiple substrate types. 
     BACKGROUND OF THE INVENTION 
     Radio Frequency Integrated Circuits (RFICs) are commonly used for performing frequency up- and down-conversion in radio transmitters and receivers. For example, Mimix Broadband, Inc. (Houston, Tex.), produces a Gallium Arsenide (GaAs) Miniature Microwave Integrated Circuit (MMIC) transmitter device called XU1005-BD. Some RFICs are fabricated as Multi-Chip Modules (MCMs), in which multiple semiconductor dies are packaged in a single package. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide a frequency conversion apparatus, including: 
     a frequency generation circuit, which is based on a first semiconductor material having a first elemental composition and is coupled to generate one or more Local Oscillator (LO) signals; and 
     a conversion circuit, which is based on a second semiconductor material having a second elemental composition different from the first elemental composition, wherein the conversion circuit is coupled to accept an input signal in a first frequency range and to convert the input signal to an output signal in a second frequency range by mixing the input signal with the one or more LO signals. 
     In some embodiments, the first semiconductor material includes one of Silicon-Germanium (SiGe) and a metal-oxide semiconductor. In an embodiment, the second semiconductor material includes one of Silicon-Germanium (SiGe) and Gallium Arsenide (GaAs). 
     In another embodiment, the output signal has distortion, and the apparatus includes a digital correction circuit, which is coupled to accept the output signal and to correct the distortion to produce a corrected output signal. The digital correction circuit may include at least one circuit type selected from a group of types consisting of a carrier recovery loop, an In-phase/Quadrature (I/Q) gain imbalance compensation circuit, an I/Q phase imbalance compensation circuit and a LO leakage compensation circuit. 
     In yet another embodiment, the apparatus includes another conversion circuit, which is based on the second semiconductor material and is coupled to accept another input signal, which is in the second frequency range, and to convert the other input signal to produce another output signal, which is in the first frequency range. The apparatus may have a first operational mode in which the conversion circuit is active and the other conversion circuit is deactivated, and a second operational mode in which the conversion circuit is deactivated and the other conversion circuit is active. 
     In still another embodiment, the conversion circuit is assembled on a die of the second semiconductor material, the frequency generation circuit is assembled on a die of the first semiconductor material, and the apparatus includes a single multi-chip module package, which contains both of the dies. 
     In some embodiments, the conversion circuit includes at least one Image-Reject Mixing (IRM) module, which includes first and second mixers and is coupled to mix the input signal with the one or more LO signals using the first and second mixers. In an embodiment, the first frequency range is partitioned into multiple sub-bands, and the at least one IRM module includes multiple IRM modules assigned to the respective sub-bands, such that each IRM module is coupled to mix the input signal with one of the LO signals when a frequency of the input signal is in the respective sub-band. 
     One of the IRM modules may include a coupler, which is operative to produce first and second Quadrature components of the one of the LO signals, so as to drive the first and second mixers with the first and second Quadrature components, respectively. The coupler may include a narrowband coupler matched to a sub-band to which the one of the IRM modules is assigned. 
     In another embodiment, the first frequency range is partitioned into multiple sub-bands, the one or more LO signals includes multiple LO signals corresponding to the sub-bands, and the at least one IRM module includes a single broadband IRM module, which includes a switch that is operative to drive the first and second mixers with one of the multiple LO signals when a frequency of the input signal is in the respective sub-band. 
     In yet another embodiment, one of the first and second frequency ranges includes a Radio Frequency (RF) range, and the other of the first and second frequency ranges includes an Intermediate Frequency (IF) range. In some embodiments, the apparatus includes an IF circuit based on the first semiconductor material, which includes at least one Quadrature IF hybrid that is coupled to perform one of Quadrature combining and Quadrature splitting in the IF range. In a disclosed embodiment, the IF range is partitioned into multiple sub-bands, and the at least one Quadrature IF hybrid includes multiple IF hybrids corresponding to the respective sub-bands. The IF circuit may be further coupled to perform frequency conversion between the IF range and a third frequency range that is lower than the IF range. The third frequency range may include a baseband frequency range. 
     In some embodiments, the first frequency range is partitioned into at least first and second sub-bands such that the first sub-band is higher than the second sub-band, and the frequency generation circuit is coupled to set a frequency of one of the LO signals to be lower than the frequency of the input signal when the input signal is in the first sub-band, and to be higher than the frequency of the input signal when the input signal is in the second sub-band. In an embodiment, the apparatus includes a modem for producing the input signal, which is coupled to invert a spectrum of the input signal when the input signal is in one of the first and second sub-bands, and to refrain from inverting the input signal when the input signal is in the other of the first and second sub-bands. 
     In another embodiment, one of the first and second frequency ranges includes a Radio Frequency (RF) range, and the other of the first and second frequency ranges includes a baseband frequency range. In yet another embodiment, the conversion circuit is coupled to multiply a frequency of one of the LO signals before mixing the input signal with the one or more LO signals. 
     There is additionally provided, in accordance with an embodiment of the present invention, a receiver, including: 
     a frequency generation circuit, which is assembled on a first die of a first semiconductor material having a first elemental composition and is coupled to generate at least one Local Oscillator (LO) signal; 
     a Radio Frequency (RF) down-converter, which is assembled on a second die of a second semiconductor material having a second elemental composition that is different from the first elemental composition, and is coupled to accept an RF input signal and to down-convert the RF input signal to an Intermediate Frequency (IF) signal by mixing the RF signal with the at least one LO signal; and 
     an IF down-converter, which is coupled to down-convert the IF signal produced by the RF down-converter to produce a baseband output signal. 
     There is also provided, in accordance with an embodiment of the present invention, a transmitter, including: 
     a frequency generation circuit, which is assembled on a first die of a first semiconductor material having a first elemental composition and is coupled to generate at least one Local Oscillator (LO) signal; and 
     an up-converter, which is assembled on a second die of a second semiconductor material having a second elemental composition that is different from the first elemental composition, and is coupled to up-convert an input signal in an input frequency range to produce a Radio Frequency (RF) output signal by mixing the input signal with the at least one LO signal. 
     There is further provided, in accordance with an embodiment of the present invention, a method for frequency conversion, including: 
     accepting an input signal in a first frequency range; 
     generating one or more Local Oscillator (LO) signals by a frequency generation circuit, which is based on a first semiconductor material having a first elemental composition; and 
     converting the input signal to an output signal in a second frequency range by mixing the input signal with the one or more LO signals by a conversion circuit, which is based on a second semiconductor material having a second elemental composition that is different from the first elemental composition. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram that schematically illustrates a Radio Frequency (RF) receiver, in accordance with an embodiment of the present invention; 
         FIG. 2  is a block diagram that schematically illustrates an RF transmitter, in accordance with an embodiment of the present invention; and 
         FIGS. 3-6  are block diagrams that schematically illustrate Radio Frequency Multi-Chip Modules (RF MCMs), in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     Embodiments of the present invention that are described hereinbelow provide frequency conversion RFICs, in which Local Oscillator (LO) generation and frequency conversion are implemented using different RF technologies. Typically, the devices described herein comprise Multi-Chip Modules (MCMs), in which at least some of the frequency conversion circuitry is fabricated on a Gallium Arsenide (GaAs) die using GaAs technology, and LO generation circuitry is fabricated on a Silicon Germanium (SiGe) die using SiGe technology. In some embodiments, some of the lower-frequency conversion functions are implemented in SiGe, as well. 
     Several exemplary configurations of up-converting and down-converting MCMs are described below. The MCM designs described herein typically cover extremely large bandwidths, such as 6-40 GHz. In some embodiments, the entire bandwidth is covered using broadband circuits. In other embodiments, the frequency range is divided into sub-bands, which are processed separately. 
     The RFICs described herein are typically deployed in transmitters and receivers of digital microwave links, whose digital modulation schemes are often highly sensitive to noise. As such, these links are often designed to operate with frequency conversion circuits having low phase noise characteristics. On the other hand, SiGe LO generation circuits are often characterized by higher phase noise and other distortion, in comparison with GaAs circuits. In some embodiments, the receiver comprises a modem, which comprises a phase correction loop that corrects the phase noise that is generated partly by the SiGe LO generation circuits. The correction performed by the modem makes it feasible to use an RFIC having SiGe-based LO generation and GaAs-based frequency conversion integrated in a single package. 
     Unlike some known RFICs, which carry out both frequency conversion and LO generation using GaAs technology, the methods and devices described herein implement some functions using SiGe technology, whose cost is considerably lower than the cost of GaAs. Other known RFICs perform only frequency conversion, and use externally-supplied LO signals, because of the requirement for low noise performance. Unlike these known RFICs, the devices described herein perform both frequency conversion and LO generation internally. As a result, the cost and size of radio transmitters and receivers that use these devices can be reduced, and their level of integration can be increased. 
     System Description 
       FIG. 1  is a block diagram that schematically illustrates a Radio Frequency (RF) receiver  20 , in accordance with an embodiment of the present invention. Receiver  20  may be part of a microwave or millimeter-wave link, or of any other suitable communication system. Receiver  20  receives Radio Frequency (RF) signals using a receiving antenna  24 . The receiver receives signals over a wide range of frequencies, in the present example between 6-40 GHz, although any other suitable frequency range can also be used. 
     The receiver comprises an RF Multi-Chip Module (MCM)  28 , which down-converts the received RF signal to a desired Intermediate Frequency (IF). The IF signal is further down-converted to baseband by an IF-to-baseband (IF-BB) converter  32 . The baseband signal is sampled by an Analog-to-Digital Converter (ADC—not shown in the figure), and then processed by a modem  34 . The modem comprises a digital correction circuit  35 , which corrects residual phase errors, or phase noise, which may be present in the received baseband signal. In alternative embodiments, the digital correction circuit may correct other types of distortion, such as In-phase/Quadrature (I/Q) gain imbalance, I/Q phase imbalance and/or LO leakage. 
     (Elements that are normally found in transmitters and receivers and are not necessary for the understanding of the methods and devices described herein, such as analog-to-digital circuits, digital-to-analog circuits, filters and various control circuits, have been omitted from  FIG. 1 , as well as from other figures of the present patent application, for the sake of clarity.) 
     MCM  28  comprises multiple substrates, or semiconductor dies, of different elemental compositions and associated RF technologies. In the present patent application and in the claims, the terms “a circuit based on a certain RF technology,” “a circuit based on a certain semiconductor material having a certain elemental composition” and “a circuit assembled on a certain substrate type” are used interchangeably. For example, a certain circuit may be referred to as a GaAs-based circuit, a circuit based on GaAs technology or as a circuit that is fabricated on a GaAs substrate, all meaning that the circuit is implemented using GaAs technology and processes. References to SiGe-based circuits, circuits based on SiGe technology and circuits assembled on SiGe substrates are also made interchangeably. 
     In the exemplary configuration of  FIG. 1 , MCM  28  comprises an RF down-converter  36 , which is implemented on a GaAs substrate, and a receive IF circuit  40 , which is implemented on a SiGe substrate. RF down-converter  36  down-converts the input RF signal to a certain IF frequency, typically in the range 2-3 GHz. This frequency is referred to herein as high-IF. IF-BB converter  32  down-converts the high-IF signal to baseband, sometimes via another intermediate frequency, referred to as low-IF, typically in the range 80-200 MHz. Several exemplary configurations of RF down-converter  36  and IF circuit  40  are shown in  FIGS. 3 and 4  below. Although in the MCM configurations described herein IF-BB converter  32  is external to the MCM, in alternative embodiments the IF-BB converter can be implemented using SiGe technology internally to the MCM. 
     MCM  28  comprises a Local Oscillator (LO) generation unit  44 , which generates one or more LO signals for performing the different frequency conversion operations in the MCM. Unit  44  is fabricated on a SiGe substrate. Exemplary LO generation unit configurations are shown further below. 
     In some embodiments, MCM  28  is a dual-function device, which can be used for frequency down-conversion in a receiver, as well as for frequency up-conversion in a transmitter. For example, in the configuration of  FIG. 1 , MCM  28  comprises a transmit IF circuit  48 , which processes input high-IF signals, and an RF up-converter  52 , which converts the high-IF signals to RF. IF circuit  48  is implemented using SiGe technology, and RF up-converter  52  is implemented using GaAs technology. Exemplary RF up-converter and transmit IF circuit configurations are shown in  FIGS. 5 and 6  below. 
     In the present example, each MCM  28  comprises both down-conversion and up-conversion circuits, and the selection is performed by hard-wiring during production. In other words, an MCM that is intended for use as a down-converter is hard-wired during production so that its RF up-converter is deactivated, its RF down-converter is active and its LO generation unit is configured to produce the appropriate LO signal for down-conversion. An MCM that is intended to function as an up-converter is hard-wired so that its RF up-converter is active, its RF down-converter is deactivated and its LO generation unit is configured to produce the LO signal for up-conversion. 
     In alternative embodiments, the MCM may be externally-configurable to function as a down- or up-converter after production, or even after installation in the transmitter or receiver. The MCM may comprise suitable interfaces and switching circuitry to enable external configuration of its intended function. Further alternatively, the MCM may comprise a single-function device, i.e., a device that performs only down-conversion or only up-conversion. In yet another embodiment, the MCM may perform both down-conversion and up-conversion simultaneously. 
       FIG. 2  is a block diagram that schematically illustrates an RF transmitter  56 , in accordance with an embodiment of the present invention. Transmitter  56  comprises a baseband-to-IF (BB-IF) converter  60 , which converts an input baseband signal to high-IF, sometimes via an intermediate low-IF. The transmitter comprises an RF MCM, which is similar to MCM  28  of  FIG. 1  above. In the present example, however, the MCM is configured to function as an up-converter. transmit IF circuit  48  processes the input high-IF signal, and RF up-converter  52  up-converts the high-IF signal to RF. A Power Amplifier (PA)  64  amplifies the RF signal, and the signal is transmitted via a transmit antenna  68 . 
     In alternative embodiments, BB-IF converter  60  may be implemented as part of the MCM. Further alternatively, the MCM may perform direct up-conversion from baseband to RF, thus eliminating the need for BB-IF converter  60 . The different MCM configurations enable various system partitioning and installation schemes. For example, in a certain configuration, conversion between baseband and IF are performed in an Indoor Unit (IDU) and conversion between IF and RF are performed in an Outdoor Unit (ODU). The ODU and IDU are connected by a transmission line, such as a coaxial cable, which carries IF signals. Alternatively, the entire conversion between baseband and RF (either directly of via one or more intermediate frequencies) can be performed in a single unit. 
     Exemplary MCM Configurations 
       FIGS. 3-6  are block diagrams that schematically illustrate several exemplary configurations of RF MCMs, in accordance with embodiments of the present invention.  FIGS. 3 and 4  show exemplary configurations of RF down-converters, receive IF circuits and associated LO generation units.  FIGS. 5  and.  6  show exemplary configurations of RF up-converters, transmit IF circuits and associated LO generation units. 
       FIG. 3  shows a down-converting RF MCM  72 . MCM  72  comprises an RF down-converter  76 , which is fabricated using GaAs technology, and a SiGe assembly  96 , which comprises a receive IF circuit  100 , an LO generation unit  104  and an interface and control unit  108 , all fabricated using SiGe technology. 
     RF Down-converter  76  accepts an input RF signal, using a single-ended RF port. The input RF signal is amplified by a chain of three Low Noise Amplifiers (LNAs)  80 A . . .  80 C. The gain of the LNA chain can be controlled by switched attenuators  84 A and  84 B. The LNA chain is broadband and covers the entire range of RF signals for which the MCM is specified, e.g., 6-40 GHz. 
     Following the broadband low-noise amplification, the down-conversion operations of MCM  72  are performed in one of two sub-bands. The MCM is typically pre-configured to operate in one of the two sub-bands, either at production or during installation. In the present example, one sub-band covers 6-18 GHz and the other sub-band covers 18-40 GHz. In the RF down-converter, a band-select switch  88  selects one of the two sub-bands. 
     The RF down-converter comprises two Image-Reject Mixers (IRMs)  92 A and  92 B, which respectively cover the 6-18 and 18-40 GHz sub-bands. Each IRM accepts an LO signal from LO generation unit  104 , and converts the RF signal to high-IF. IRM  92 A accepts a Quadrature LO signal, i.e., two LO components having a 90° phase shift with respect to one another. The two LO components are denoted LO_I_LB and LO_Q_LB in the figure. The IRM mixes the RF signal with the two LO components using two respective mixers, and produces a Quadrature IF output signal, denoted IF_I_LB and IF_Q_LB. In the present example, the output of IRM  92 A has a frequency of 1630 MHz, although any other suitable frequency can also be used. 
     IRM  92 B accepts a single LO signal (denoted LO_HB) from LO generation unit  104 , and splits it into two Quadrature components internally. The IRM comprises a suitable 0°/90° coupler implemented in GaAs technology, such as a Lange coupler, for splitting the LO signal into Quadrature components. The IRM mixes the RF signal with the two Quadrature LO components using two respective mixers, and produces a Quadrature IF output, whose components are denoted IF_I_HB and IF_Q_HB. In the present example, the output of IRM  92 B has a frequency of 3260 MHz, although any other suitable frequency can also be used. 
     The different LO signals and IF outputs of the two IRMs are routed between assembly  76  and assembly  96  over differential interfaces. The mixers in IRMs  92 A and  92 B typically comprise Single-Balanced Mixers (SBMs). In some embodiments, the IRMs comprise internal driver amplifiers, which amplify the LO signals to the appropriate power levels for driving the mixers. 
     Receive IF circuit  100  comprises two IF hybrids (Quadrature combiners)  112 , which respectively combine the Quadrature high-IF outputs of IRMs  92 A and  92 B. Switches  116  select the sub-band that is currently used. A Voltage Variable Attenuator (VVA)  120  applies variable-gain amplification to the signal. The signal is further amplified by an amplifier  124 , and provided as output. As can be seen in the figure, the entire receive IF circuit is implemented using a differential configuration. 
     LO generation unit  104  accepts an external reference clock signal, which is used as a reference for the different LO signals. A Phase Locked Loop (PLL)  128  produces a signal in the range of 10-22 GHz. A frequency doubler  132  doubles the frequency of the PLL output, to produce an LO signal in the range 20-44 GHz. A band-select switch  136  selects the sub-band that is currently in use. When operating in the upper sub-band, i.e., between 18-40 GHz, the LO signal produced by doubler  132  (LO_HB) is provided as is to IRM  92 B. When operating in the lower sub-band, i.e., between 6-18 GHz, the output of doubler  132  is divided by a factor of either two or four using an externally-controlled frequency divider  140 . The frequency divider also splits the LO signal into two Quadrature components (LO_I_LB and LO_Q_LB), and the two components are provided to IRM  92 A. 
     Interface and control unit  108  communicates with an external controller or host, such as using a Serial Peripheral Interface (SPI). In response to commands accepted from the host, the interface and control unit controls switches  88 ,  116  and  136  to select the appropriate sub-band, controls divider  140  to select the appropriate division ratio within the lower sub-band, and sets the appropriate gains or attenuations of switched attenuators  84 A and  84 B and VVA  120 . 
       FIG. 4  shows a down-converter RF MCM  150 , in accordance with an alternative embodiment of the present invention. MCM  150  comprises an RF down-converter  154 , fabricated using GaAs technology, and a SiGe assembly  170 , which comprises a receive IF circuit  174 , an LO generation unit  178  and an interface and control unit  218 , all fabricated using SiGe technology. 
     RF down-converter  154  comprises three LNAs  158  and switched attenuators  162 , which are similar to LNAs  80 A . . .  80 C and switched attenuators  84 A and  84 B in down-converter  76  of  FIG. 3  above. In the present example, however, RF down-converter  154  comprises a single broadband IRM  166 , whose mixers cover the entire 6-40 GHz range. IRM  166  accepts separate LO signals for operating in the 6-18 and 18-40 GHz sub-bands. The LO signals (LO_I_LB, LO_Q_LB and LO_HB) are similar to the LO signals described in  FIG. 3  above. IRM  166  comprises internal band-select switches, which select the sub-band that is currently in use, so as to drive the mixers with the appropriate LO signals. IRM  166  produces a Quadrature high-IF signal denoted IF_I, IF_Q. Each of the IF signal components is provided as a differential signal. 
     Receive IF circuit  174  accepts IF signals IF_I and IF_Q, and routes them through one of two IF hybrids  186  using band-select switches  182  and  190 , depending on the sub-band that is currently in use. IF hybrids  186  are similar to hybrids  112  of  FIG. 3  above, with a separate hybrid assigned to each sub-band. The signal produced by the appropriate hybrid  186  is amplified by a VVA  194  and an amplifier  198 , which are similar to VVA  120  and amplifier  124  of  FIG. 3  above. The output of amplifier  198  is provided as the IF output of the MCM. 
     LO generation unit  178  is similar to unit  104  of  FIG. 3  above. Unit  178  comprises a PLL  202 , a frequency doubler  206 , a switch  210  and a frequency divider  214 , which operate similarly to PLL  128 , frequency doubler  132 , switch  136  and frequency divider  140  of  FIG. 3  above. Interface and control unit  218  is similar to unit  108  of  FIG. 3  above. 
       FIG. 5  shows an up-converter RF MCM  220 , in accordance with another embodiment of the present invention. MCM  220  comprises an RF up-converter  222 , which is fabricated using GaAs technology, and a SiGe assembly  240 , which comprises a transmit IF circuit  244 , an LO generation unit  246  and an interface and control unit  276 , all fabricated using SiGe technology. 
     MCM  220  accepts as input a high-IF signal. The input signal is provided to circuit  244  and is amplified by a VVA  248 . A band-select switch  252  routes the amplified high-IF signal to one of two IF hybrids (Quadrature splitters)  256 , which split the high-IF signal into In-phase and Quadrature components. One of the hybrids covers the 6-18 GHz sub-band and produces a Quadrature signal denoted IF_I_LB, IF_Q_LB. The other hybrid covers the 18-40 GHz sub-band and produces a Quadrature signal denoted IF_I_HB, IF_Q_HB. 
     In the present example, the frequency of IF_I_LB and IF_Q_LB is 2120 MHz, and the frequency of IF_I_HB and IF_Q_HB is 3750 MHz, although any other suitable frequencies can also be used. The high-IF signals are provided to RF up-converter  222  using a differential interface. 
     RF up-converter  222  comprises two IRMs  224 A and  224 B, which cover the 6-18 and 18-40 GHz sub-bands, respectively. In each IRM, the high-IF signals are mixed with appropriate LO signals using a pair of mixers, so as to convert the high-IF signals to RF. When operating in the 6-18 GHz sub-band, IRM  224 A accepts a Quadrature LO signal (LO_I_LB, LO_Q_LB) from LO generation unit  246 . When operating in the 18-40 sub-band, IRM  224 B accepts a single LO signal (LO_HB) from the LO generation unit, and splits it into Quadrature components internally. 
     A band-select switch  228  selects the RF output of the appropriate IRM. The RF signal is amplified by a broadband amplification chain, which covers the entire 6-40 GHz range. The amplification chain comprises two LNAs  232  and a switched attenuator  236 , which sets the gain of the amplification chain. The output of the amplification chain is provided as RF output of the MCM. 
     LO generation unit  246  is similar to unit  104  of  FIG. 3  above. Unit  246  comprises a PLL  260 , a frequency doubler  264 , a switch  268  and a frequency divider  272 , which operate similarly to PLL  128 , frequency doubler  132 , switch  136  and frequency divider  140  of  FIG. 3  above. Interface and control unit  276  is similar to unit  108  of  FIG. 3  above. 
       FIG. 6  shows an up-converter RF MCM  280 , in accordance with yet another embodiment of the present invention. MCM  280  comprises an RF up-converter  284 , which is fabricated using GaAs technology, and a SiGe assembly  300 , which comprises a transmit IF circuit  304 , an LO generation unit  308  and an interface and control unit  312 , all fabricated using SiGe technology. LO generation unit  308  and interface and control unit  312  are similar to the corresponding units in  FIGS. 3-5  above. 
     MCM  280  accepts as input a high-IF signal, which is amplified by a VVA  316 . A band-select switch  320  switches the signal to one of two IF hybrids  328 , similar to hybrids  256  of  FIG. 5  above. Each hybrid covers one of the two sub-bands. In the present embodiment, however, another band-select switch  332  switches the outputs of the appropriate hybrid  328  to a single Quadrature IF interface, denoted IF_I, IF_Q. This signal is provided as input to RF up-converter  284 . Thus, the high-IF signal IF_I, IF_Q provided to the RF up-converter may be located anywhere within the 6-40 GHz band. 
     RF up-converter  284  up-converts the high-IF signal using a broadband IRM  288 , whose mixers cover the entire 6-40 GHz range. IRM  288  accepts both Quadrature LO signals for the lower sub-band (LO_I_LB, LO_Q_LB), and an LO signal for the upper sub-band (LO_HB). The IRM splits LO signal LO_HB into two Quadrature components internally. The IRM comprises band-select switches, which drive the broadband mixers with the appropriate LO signals depending on the currently-used sub-band. 
     An amplification chain, which comprises two LNAs  292  and a switched attenuator  296 , amplifies the RF signal produced by IRM  288 . The output of the amplification chain is provided as RF output of the MCM. 
     In any of the SiGe assemblies described above, the SiGe assembly may comprise one or more SiGe dies. For example, the LO generation unit and interface and control unit may be assembled on the same die, either together with or separately from the transmit or receive IF circuit. Alternatively, the entire SiGe assembly may be fabricated on the same die. Further alternatively, each of the transmit IF circuit, the receive IF circuit, the LO generation unit and the interface and control unit may be fabricated on a separate die. 
     Additional Embodiments and Variations 
     The MCM configurations of  FIGS. 3-6  illustrate single-function MCMs, i.e., MCMs that perform only up-conversion or only down-conversion. In some embodiments, elements of these single-function MCMs can be combined in a single MCM to provide dual-function MCMs, such as MCM  28  of  FIGS. 1 and 2  above. For example, RF down-converter  76 , receive IF circuit  100  and LO generation unit  104  of  FIG. 3  above can be integrated with transmit IF circuit  244  and RF up-converter  222  of  FIG. 5  above in a single MCM. Similarly, RF down-converter  154 , receive IF circuit  174  and LO generation unit  178  of  FIG. 4  above can be integrated with transmit IF circuit  304  and RF up-converter  284  of  FIG. 6  above in a single MCM. In some embodiments, the dual-function MCM may comprise a single synthesized frequency source for both up- and down-conversion operations. Such a configuration may reduce internal noise and interference effects. 
     The SiGe elements of the dual-function MCM may be fabricated on a single SiGe die or on multiple dies. Similarly, the GaAs elements of the dual-function MCM may be fabricated on one or more GaAs dies. 
     Although the embodiments described herein mainly address MCMs that perform frequency conversion between high-IF and RF, the principles of the present invention can also be used for performing direct conversion from baseband to RF, as well. For example, a baseband signal may be converted directly to RF by a direct-conversion up-converter fabricated using GaAs technology. The direct-conversion up-converter may comprise an amplification chain and either one broadband IRM or multiple IRMs assigned to up-convert respective sub-bands. An LO generation unit, which generates the LO signals for the direct-conversion up-converter can be fabricated using SiGe technology. The direct-conversion GaAs up-converter and the SiGe LO generation unit, with possibly an interface and control unit, can be integrated in a single MCM. Moreover, such a direct-conversion up-converter can be integrated with one of the down-converter configurations described herein. 
     In some applications, it is possible to restrict the RF and IF frequencies in which the transmitter or receiver (and hence the RF MCM) operates, rather than operating over the full 6-40 GHz range. The restriction may limit the frequencies to a narrow range, or even to a single operating frequency. As noted above, in some embodiments, the GaAs IRMs split the LO signal into Quadrature components internally, using an 0°/90° coupler. In these embodiments, when the operating frequency range of the MCM is restricted to a narrow range, the coupler design may be optimized for the specific frequency band in question. As a result, the coupler may have smaller size and/or better performance, in comparison with conventional broadband couplers. 
     In any of the frequency conversion operations described herein, the frequency of the LO signal may be either higher or lower than the converted signal. In some embodiments, the LO generation unit generates LO frequencies, which are higher than the converted signal when operating in the lower sub-band, and lower than the converted signal when operating in the upper sub-band. This technique considerably reduces the range of LO frequencies, which simplifies the LO generation unit and improves its performance. Note, however, that a signal that is converted with an LO frequency higher than the signal frequency has a spectrum, which is reversed in comparison with the spectrum of a signal that is converted with an LO frequency lower than the signal frequency. In order to retain the desired signal spectrum, the transmitter or receiver may apply spectrum inversion. This operation is typically carried out by the modem of the transmitter or receiver. 
     In some embodiments, the LO generation unit produces an LO signal whose frequency is a fraction (e.g., ½ or ¼) of the desired LO frequency. The RF up- and/or down-converter multiplies the frequency of the LO signal before applying it to the mixers. For example, the LO generation unit may produce an LO signal in the range 10-20 GHz, and the RF up- and/or down-converter may selectively double the LO signal when appropriate, so as to produce LO signals in the range 10-40 GHz. 
     Although the embodiments described herein mainly address RF MCMs comprising SiGe and GaAs substrates, the methods and systems described herein can also be used to provide RF MCMs using other types of substrates and technologies that are based on other semiconductor materials. For example, the IF circuits, LO generation circuitry and/or the interface and control units may be implemented using Complementary Metal-Oxide Semiconductor (CMOS) technology. As another example, the RF up- and down-converters, and in particular the IRMs, may be implemented using SiGe technology rather than using GaAs. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.