Patent Publication Number: US-7711346-B2

Title: System and method for frequency translation using an image reject mixer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is continuation of commonly assigned patent application Ser. No. 09/552,760 entitled “SYSTEM AND METHOD FOR FREQUENCY TRANSLATION USING AN IMAGE REJECT MIXER,” filed Apr. 18, 2000 now U.S. Pat. No. 7,184,724, via the commonly assigned, continuation of said patent application (Ser. No. 11/523,785 filed Sep. 19, 2006) now U.S. Pat. No. 7,403,761 which is titled the same, the disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to the transmission of modulated signals and more particularly to the translation of a signal from one carrier frequency to another carrier frequency. 
     BACKGROUND OF THE INVENTION 
     The translation of modulated signals between different carrier frequencies is common in signal transmission. For example, upconverters are used in the television industry to translate information that is present at a particular low intermediate frequency (LIF) to a higher frequency for final transmission. Specifically, it is common in the United States to convert a signal at a LIF of 44 MHZ to a frequency division multiplex (FDM) final transmission frequency in the range of 53 MHZ to 857 MHZ. Similarly, European television transmission often utilizes up conversion of a signal at a LIF of 36.125 MHZ to a FDM final transmission frequency which may vary from a few tens of MHZ to nearly 1 GHz. 
     However, due to the relatively close packing of the FDM frequency bands associated with the various transmission channels, upconverters used as described above in the television industry are required to have a spectrally pure output so that information is transmitted in the desired FDM channel, without producing interfering signals in other FDM channels. Meeting the specifications generally required in the television industry is often very difficult due to the relatively close channelization scheme, the typically broad range of spectrum translated between, the relatively high transmission power requirements, the typically large amount of information modulated in the signal, and the like. For example, although often not providing the final transmission signal, upconverters are often deployed in the system such that their output signal must have enough output power to pass through a number of passive splitters and combiners that are disposed in the signal path. Upconverters used in these applications are not only required to translate a signal from one frequency to another within tight tolerances, but must also amplify the signal to a specified relatively high level. 
     Accordingly, the television industry presently uses upconverters that are designed using mostly discrete components, i.e., transistors, inductors, capacitors, resisters, deployed on printed circuit boards to provide discrete mixers, such as may be relatively easily relied upon to accommodate the relatively high transmission powers, provide the sharp cutoff filters needed to remove unacceptable spurious signals, and operate throughout the desired frequency spread. However, the use of such components tends to result in an upconverter product which is large, expensive, and which consumes large amounts of power. For example, a typical state of the art upconverter providing head end quality for use in television transmission applications requires a 19 inch rack mount form factor. Such a size requirement significantly limits the situations in which the upconverter may be deployed. Moreover, the use of such discrete components generally results in relatively high power requirements, further limiting deployment opportunities. 
     Additional circuit limitations associated with the use of such discrete components further encumber the design and operation of prior art devices. For example, it is often very difficult and/or expensive to achieve good matching of components when utilizing discrete components. However, matching of components in order to implement particular circuit designs is often critical, e.g., as little as 1° of mismatch in certain circuit configurations introduce spurious signals that generally cannot be tolerated in the aforementioned television transmission applications. Accordingly, particular circuit designs are often precluded from use due to the difficulty and/or expense associated with suitably matching components to implement a circuit design. 
     Accordingly, a need exists in the art for a frequency translation function to be provided in a reduced form factor to reduce size, expense, and/or power consumption. A further need exists in the art to provide such frequency translation function in an integrated circuit or circuits to achieve the aforementioned reduced size, expense, and/or power consumption as well as to facilitate component matching and, therefore, broaden the circuit designs available for use in providing frequency translation functions. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other objects, features and technical advantages are achieved by a system and method which utilizes an integrated circuit utilizing low cost filters to provide frequency translation in a low cost solution that occupies little space and consumes little power. A preferred embodiment upconverter architecture according to the present invention utilizes matching characteristics achievable in an integrated circuit design to implement image reject or single sideband mixers that relax the filter requirements of the circuit, such as the filter requirements of a high intermediate frequency (HIF), while providing frequency translation within the desired tolerances. Additionally, the preferred embodiment utilizes single sideband mixing to reduce the linearity requirements of components, such as amplifiers used in the frequency translation circuit. 
     A preferred embodiment of the present invention utilizes an integrated circuit design providing very close matching of particular circuit components in order to provide at least one single sideband mixer. Preferably this single sideband mixer utilizes a phase splitter providing in-phase (I) and quadrature (Q) components of a local oscillator (LO) frequency used in up conversion of a signal. Accordingly, a single substrate may be utilized according to the present invention to provide a fully integrated frequency translation circuit. 
     A most preferred embodiment of the present invention utilizes multiple stages of single sideband mixers in frequency translation. For example, a preferred embodiment utilizes a first single sideband mixer, preferably utilizing a corresponding first phase splitter, to translate a baseband or intermediate frequency (IF) input signal to a first IF. The first IF signal may be filtered and/or amplified, or otherwise manipulated as desired, before passing to a second single sideband mixer, preferably utilizing a corresponding second phase splitter, to translate the first IF to a second IF. Again the output of the second single sideband mixer stage may be filtered and/or amplified, or otherwise manipulated as desired, before being passed as an output signal. 
     According to a preferred embodiment of the present invention, the above described multiple stages of mixing utilized in frequency translation provide both frequency up conversion and down conversion. For example, a most preferred embodiment utilizes the above described first mixing stage to up convert a baseband or IF signal to a high intermediate frequency (HIF) and the second mixing stage to down convert the HIF to a desired second IF which is at a lower frequency than the HIF. The use of up conversion and down conversion in frequency translation according to the present invention is preferred in order to simplify filtering of signals and, thereby, synergize the integrated configuration of the preferred embodiment upconverter. 
     It should be appreciated that because the preferred embodiment of the present invention utilizes single sideband mixing, images and other spurious signals associated with the input signal and/or first IF signal are eliminated or greatly reduced, such as on the order of −35 to −40 dB, and therefore the filtering requirements of the circuit are relaxed. Accordingly, a preferred embodiment of the present invention utilizes filter elements, such as bandpass, lowpass, and/or highpass filters, which are provided on a same single substrate as the preferred embodiment single sideband mixer. Although such integrated circuit filter elements typically do not provide as well defined cutoff points or as sharp of frequency rejection as is available with discrete filter elements, the preferred embodiment of the present invention may be relied upon to provide frequency translation to within tolerances associated with television transmission head end quality signals through the use of matched components and single sideband mixing. Of course, filter elements external to the preferred embodiment single substrate, such as discrete filter elements, may be utilized if desired. 
     Additionally, high output signal quality is achievable according to the present invention utilizing active components, such as amplifiers, having relaxed linearity requirements. Utilizing the single sideband mixers of the preferred embodiment greatly reduces the signal images and other spurious signals which often cause a circuit design to implement highly linear active components in order to avoid subsequent distortion of the signals. However, the design of a preferred embodiment of the present invention relies upon active components which are linear over a smaller range of frequencies and signal levels due to the elimination and/or reduction of spurious signals. Accordingly, implementation of such active components on the same single substrate as the preferred embodiment single sideband mixers may be easily accomplished to thereby further synergize the integrated configuration of the preferred embodiment upconverter. 
     A technical advantage of the present invention is realized in providing frequency translation functions in an integrated circuit form factor. Further technical advantages are provided in the reduced size, expense, and power consumption associated with implementation in an integrated circuit form factor. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  shows a block diagram of a typical prior art upconverter circuit; and 
         FIG. 2  shows a block diagram of a preferred embodiment frequency translation circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In understanding the present invention it is helpful to understand the current state of the art with respect to upconverter solutions. Directing attention to  FIG. 1 , a block diagram of a typical prior art upconverter solution is shown. The upconverter circuitry of  FIG. 1  is often utilized in providing up conversion of a low intermediate frequency (LIF) video signal, such as might be provided by a video signal modulated at approximately 44 MHZ in the United States or approximately 36 MHZ in Europe, to frequencies associated with particular television channels, such as may be defined in the spectrum from approximately 53 MHZ to approximately 857 MHZ. 
     In the architecture of  FIG. 1 , amplifier  111  is provided to amplify the input signal to a desired level prior to frequency translation. Although not illustrated, according to some prior art solutions amplifier  111  may be a variable gain amplifier which is operated to maintain the input to mixer  121  at a constant magnitude. 
     Mixer  121  is provided to convert the input signal frequency, to a high intermediate frequency (HIF) utilizing local oscillator (LO)  131 . Amplifier  112  is provided to amplify the HIF signal, such as to compensate for losses associated with mixer  121  or to otherwise amplify the HIF signal to a desired HIF output level. Because of the images and other spurious signals associated with the HIF signal as provided by mixer  121 , amplifier  112  is typically a highly linear component in order to avoid distortion of the amplified signals. 
     Filter  141  is a bandpass filter utilized to filter the HIF signal to remove the images and other undesired spurious signals. In order to sufficiently filter the spurious signals associated with mixing the input signal with the carrier provided by LO  131 , filter  141  must typically provide very sharp pass band cutoffs. Amplifier  113  is provided to amplify the filtered HIF signal, such as to compensate for filter loses, prior to final conversion to a desired frequency via mixer  122 . 
     Mixer  122  is provided to convert the HIF signal frequency to a desired output frequency signal utilizing LO  132 . In order to provide selection of a desired output channel of a frequency division set of channels, LO  132  may be a variable oscillator as shown. 
     Amplifier  114  is provided to amplify the selected output frequency signal to a desired output signal level. As illustrated, amplifier  114  may be a variable gain amplifier to provide selection of a desired output signal level. Because of the images and other spurious signals associated with the output signal as provided by mixer  122 , amplifier  114  is typically a highly linear component in order to avoid distortion of the amplified signals. Moreover, although illustrated in  FIG. 1  as a single circuit element, often amplification provided at the output stage of typical prior art upconverters is provided by a combination of one or more fixed gain amplifiers and a variable attenuator. 
     Filter  142  is may be a variable frequency filter utilized to suppress undesired spurious signals on either side of the desired output signal frequency. As with filter  141  discussed above, filter  142  is a bandpass filter which, in order to sufficiently filter the spurious signals associated with mixing the HIF signal with the carrier provided by LO  132 , must typically provide very sharp pass band cutoffs. 
     Alternatively, the filtering function of filter  142  may be provided by a bank of filters where the filters of the bank filter different frequency ranges. Accordingly, depending upon the desired frequency of the output signal, a particular filter or filters may be switched in the output signal path to provide the desired filtering. However, such an embodiment adds to the size and expense of the resulting upconverter solution. 
     It should be appreciated that in the prior art, the above described elements are provided by discrete components in order to accommodate the desired frequency ranges and power levels, as well as to provide an output signal of a desired quality. 
     In order to better understand limitations of the prior art circuitry of  FIG. 1 , assume that the input signal is provided at a frequency of f LIF  and that it is desired to provide an HIF signal from mixer  121  at a frequency of f LO1 +f LIF . Mixer  121  will not only provide output of an HIF signal at frequency f LO1 +f LIF , but will also provide an image signal at frequency f LO1 −f LIF . This image signal will approximately be of the same magnitude as the desired HIF signal at frequency f LO1 +f LIF . Additionally, because of the physical limitations of mixer  121  result in signal leakage, some of the carrier signal f LO1  will also be present in the output of mixer  121 . However, the carrier power level of the carrier signal f LO1  will typically be on the order of 20 dB below the desired HIF signal level in video up conversion implementations. 
     In meeting video head end signal quality requirements, the power levels of the above identified spurious signals must be at least 60 dB below (−60 dBc) the power level of the desired signal. Accordingly, filter  141  must attenuate the unwanted image (f LO1 −f LIF ) by 60 dB or more and the leaking carrier signal (f LO1 ) by 40 dB or more. If filter  141  does not provide sufficient filtering of these spurious signals they will be passed on to mixer  122  and converted, resulting in not only the original spurious signals but also additional spurious signals spawned therefrom. Accordingly, in such a situation filter  142  is not only required to provide filtering sufficient to supplement that provided by filter  141 , but must also provide filtering sufficient to redress the additional spurious signals as well as the spurious signals (i.e., f LO2 +f HIF  and f LO2 ) associated with mixer  122 . Accordingly, the filtering requirements of filter  142  may be substantial. Moreover, where the output frequency is variable, the center frequency of filter  142  must be correspondingly variable. The provision of a variable filter meeting such requirements results in added size, complexity, and cost to the upconverter solution. 
     Further problems are present in the prior art circuitry of  FIG. 1  with respect to the amplification of the output signal. Assuming that the HIF signal is at the frequency f HIF , mixer  122  will produce both a desired output at f LO2 −f HIF  and an undesired image at frequency f LO2 +f HIF , as well as a carrier leakage signal at frequency f LO2 . If f LO1  and f LO2  are chosen properly, the undesired image signal at frequency f LO2 +f HIF  will be out of the band of interest and will not be a direct interferer. However, this undesired image signal will pose problems for amplifier  114 . 
     As discussed above, both the desired signal at frequency f LO2 −f HIF  and the undesired image at frequency f LO2 +f HIF  provided by mixer  122  will be of substantially the same magnitude. Accordingly, the peak signal value in amplifier  114  will be approximately twice as large as it would be if only the desired output signal at frequency f LO2 −f HIF  were provided thereto. Therefore, in order to maintain the linearity required by the system, amplifier  114  must consume much more power than would be needed if only the actual signal of interest were present and amplified. Moreover, although the above discussion is provided with reference to amplifier  114 , it should be appreciated that similar problems are associated with amplifier  112 . For example, the desired circuit gain may be distributed over the various amplifiers, such as amplifiers  112 ,  113 , and  114 , thereby resulting in appreciably high gain requirements and the power consumption problems discussed above with respect to amplifier  114 . 
     Directing attention to  FIG. 2 , a preferred embodiment signal translator  200  of the present invention is shown which accommodates a relatively broad frequency range of output signals as well as provides relatively high output signal power levels. Specifically, the preferred embodiment signal translator  200  of  FIG. 2  is adapted to provide frequency up conversion in a range of frequencies compatible with the spectrum of television channels typically used throughout the United States and Europe at a power level sufficient for use in television signal transmission systems such as cable television head end system. Moreover, the circuitry of the preferred embodiment is configured to utilize integrated components such that a synergistic cascade of component attributes provides an output signal having a desired signal quality, such as a signal quality meeting video transmission head end requirements. 
     In the preferred embodiment circuitry of  FIG. 2 , amplifier  211  preferably acts as an input buffer and/or provides a portion of the total system gain. Alternative embodiments of the present invention may omit amplifier  211  and/or utilize other components to provide desired signal conditioning attributes, such as a filter or attenuator element. 
     The signal output from amplifier  211  is provided to phase shifter  251 . Phase shifter  251  provides splitting of signals provided thereto, adjusting their relative phase to thereby provide signal components having a predetermined phase relationship with respect to each other. Preferably, phase shifter  251  provides in-phase (I) and quadrature (Q) signal components (providing a 90° phase difference), although other phase relationships may be utilized according to the present invention. 
     Each of the signal components provided by phase shifter  251  is translated to a desired frequency through the use of an associated mixer, here mixer  221  and mixer  222 . However, as LO  231  providing the carrier frequency clock utilized by both mixer  221  and mixer  222  is also provided to a phase shifter, shown as phase shifter  252 , the carriers driving mixer  221  and mixer  222  are out of phase a predetermined amount, although operating at a same frequency. According to the preferred embodiment, the carrier signal components provided by phase shifter  252  are in-phase and quadrature carrier signal components, although other phase relationships may be utilized according to the present invention. 
     It should be appreciated that each of mixers  221  and  222  operate to provide both a desired signal and an image of the signal component provided thereto. However, the signal components provided to each of mixers  221  and  222  are out of phase a predetermined amount. Moreover, the carrier signal components provided each of mixers  221  and  222  are out of phase a predetermined amount. Accordingly, by utilizing these phase differences the signal components may be provided to mixers  221  and  222  in such a way as to result in constructive combining of desired signals and destructive combining of undesired signal images. Specifically, when the outputs of mixers  221  and  222  are summed by summer  261 , the desired signal components are in-phase and constructively combine while the undesired images are out of phase and destructively combine. For example, according to the preferred embodiment of  FIG. 2 , assuming the input signal is at a frequency of f LIF , the desired signal components f LO1 +f LIF  from each mixers  221  and  222  are in-phase and combine to enhance the amplitude of the resulting HIF signal while the undesired signal image components f LO1 −f LIF  from mixers  221  and  222  are 180° out of phase and combine to substantially null the image signals. Accordingly, as only the desired signal is output and the undesired signal images at the other sideband are suppressed, phase shifters  251  and  252 , mixers  221  and  222 , and summer  261  form a single sideband (SSB) or image reject mixer (SSB mixer  201 ). 
     In using an integrated circuit to implement the preferred embodiment of the present invention, double balanced mixers having very good carrier suppression can be implemented relatively easily. Accordingly, it is possible, through proper matching of components, to achieve a sideband suppression of approximately 40 dB relative to the desired signal in an integrated circuit implementation of the above described SSB mixer  201 . Even with the rather high isolation between signal and spurious signals of 60 dB required in video head end application, the filtering requirements of the preferred embodiment circuitry of  FIG. 2  may be relaxed. For example, assuming 40 dBc of image suppression, a filter providing an additional 20 dB of filtering may be relied upon to provide the desired level of signal quality. Such a filter may be implemented at a relatively low cost and in a relatively small footprint, such as upon the same substrate as the SSB mixer. 
     Referring still to  FIG. 2 , the output of SSB mixer  201 , referred to herein as the HIF signal, is preferably provided to amplifier  212 . In the preferred embodiment amplifier  212  provides a portion of the total circuit gain and/or provides the HIF signal at a desired magnitude for filtering by filter  241 . Alternative embodiments of the present invention may omit amplifier  212  and/or utilize other components to provide desired signal conditioning attributes. 
     Filter  241  of the preferred embodiment is a bandpass filter having a center frequency corresponding to the HIF signal frequency. However, as should be appreciated from the above discussion, the particular implementation of filter  241  may provide substantially less selectivity than is typically provided in a frequency translation circuit providing similar signal qualities. This is because the preferred embodiment implementation utilizes component matching to provide a high degree of image suppression, thus negating the need for a sharp cutoff filter. Synergism is provided by the preferred embodiment configuration in that such a filter may be implemented on the same substrate as the SSB mixer of the preferred embodiment without compromising a desired level of signal quality. However, this filter may be provided externally to a common substrate utilized for a majority of the remaining circuit elements of a preferred embodiment frequency translator  200 . It should be appreciated that advantages are still realized in such an alternative embodiment utilizing an external filter as the selectivity requirements of the filter remain relaxed, and thus the complexity and/or cost, of this filter is reduced. Moreover, it should also be appreciated that, where the level of image rejection provided by SSB mixer  201  provides a desired level of signal quality, filter  241  may be omitted, if desired. 
     In the preferred embodiment of  FIG. 2 , amplifier  213  is provided at the second frequency translation stage to act as an input buffer and/or provides a portion of the total system gain. Alternative embodiments of the present invention may omit amplifier  213  and/or utilize other components to provide desired signal conditioning attributes. 
     According to the preferred embodiment, a second SSB mixer, SSB mixer  202  comprising phase shifters  253  and  254 , mixers  223  and  224 , and summer  262 , is provided to down convert the HIF signal to a desired output frequency, such as may correspond to a desired television frequency division channel. Accordingly, similar to SSB mixer  201  discussed above, the signal output from amplifier  213  is provided to phase shifter  253 . Phase shifter  253  provides splitting of signals provided thereto, adjusting their relative phase to thereby provide signal components having a predetermined phase relationship with respect to each other. Preferably, phase shifter  253  provides I and Q signal components, although other phase relationships may be utilized according to the present invention. 
     Each of the signal components provided by phase shifter  253  is translated to a desired frequency through the use of an associated mixer, here mixer  223  and mixer  224 . However, as LO  232  providing the carrier frequency clock utilized by both mixer  223  and mixer  224  is also provided to a phase shifter, shown as phase shifter  254 , the carriers driving mixer  223  and mixer  224  are out of phase a predetermined amount, although operating at a same frequency. According to the preferred embodiment, the carrier signal components provided by phase shifter  254  are in-phase and quadrature carrier signal components, although other phase relationships may be utilized according to the present invention. 
     As described in detail above with respect to SSB mixer  201 , SSB mixer  202  utilizes the phase differences the signal components to result in constructive signal and destructive signal image combining at summer  262 . Accordingly, SSB mixer  202  is used to reduce the magnitude of the undesired image at frequency f LO2 +f HIF , assuming frequency f HIF  is the frequency of the desired signal at the high intermediate frequency, while enhancing the desired signal at frequency f LO2 −f HIF . 
     It should be appreciated that LO  232  is illustrated as a variable oscillator, such as may be implemented in the form of a voltage controlled oscillator. Accordingly, SSB mixer  202  may be controlled to translate an input signal at f HIF  to any of a plurality of output signal frequencies, such as may correspond to particular defined channel frequencies. Additionally or alternatively, LO  232  may be controlled so as to provide a frequency hopping scheme suitable for deterring useful interception of a transmitted signal and/or for use in multiplexing a signal or plurality of signals, such as in time division bursts, on various output frequencies. 
     Amplifier  214  is provided to manipulate the amplitude of the signal provided from SSB mixer  202 . Preferably amplifier  214  is utilized to provide the final amount of circuit gain to the signal output by frequency translator  200 . Although illustrated as a single circuit element, amplifier  214  may be implemented as a combination of various gain stages. It should be appreciated that amplifier  214  is shown as a variable gain amplifier, such as may be useful in providing an output signal at a controllable magnitude. However, fixed gain amplifiers may be utilized according to the present invention. Additionally, where no gain adjustment is desired, amplifier  214  may be omitted, if desired. 
     It should be appreciated that the SSB mixers of the preferred embodiment of the present invention substantially reduce or eliminate the image signals associated with translation of the signal. Accordingly, rather than being provided two signals as in the prior art circuitry of  FIG. 1 , amplifier  214  is substantially provided only the desired signal. Accordingly, as there is only one signal provided to amplifier  214 , the linearity requirements of this amplifier are substantially relaxed. For example, when multiple signals at different frequencies are present they can intermodulate, with each other and create unwanted spurious signals so the linearity becomes very crucial. However, by reducing the amplitude of the unwanted image, output amplifier of the present invention does not have that intermodulation, thus significantly relaxing the linearity requirements and thereby making implementation of the amplifier easier. Moreover, the presence of substantially a single signal results in less power being consumed in signal amplification by amplifier  214 . Accordingly, such an amplifier may be implemented on a same substrate as all or substantially all of the circuit elements of frequency translator  200  shown in  FIG. 2 . 
     The preferred embodiment of  FIG. 2  includes filter  242  in the output signal path. According to this preferred embodiment the filter is an adjustable frequency filter that may be adjusted to a particular center frequency corresponding to adjustment of LO  232 . Accordingly, filter  242  may be utilized for additional filtering of spurious signals if desired. However, because the mixers utilized by the preferred embodiment of the present invention provide a high level of sideband signal rejection, filter  242  may be implemented with a lesser degree of selectivity than that of prior art upconverter solutions. Moreover, alternative embodiments of the present invention may omit filter  242 , if desired. 
     It should be appreciated that the preferred embodiment frequency translator  200  provides a first SSB mixer stage up converting an input signal to a HIF which is preferably at a frequency above the entire range of selectable output frequencies. Accordingly, the second SSB mixer stage provides down conversion of the HIF to a particular desired frequency of the range of selectable output frequencies. By proper selection of the carrier frequency provided by LO  231  (f LO1 ) and of the carrier frequency provided by LO  232  (f LO2 ), images associated with SSB mixer  201  and/or SSB mixer  202  may be provided out of band and, therefore, their undesired effects further mitigated according to the present invention. 
     Operation of the present invention is not limited to use in upconverting a modulated signal, such as the above described modulated video signal. For example, the present invention may be utilized to provide frequency translation of a signal provided thereto in baseband I and Q. Accordingly, an alternative embodiment of the present invention may omit phase shifter  251  in favor of utilizing a baseband I and Q signal at mixers  221  and  222  respectively. 
     It should be appreciated that the frequency translation circuitry of the present invention may be utilized in a number of systems. For example, the signal quality of the above described preferred embodiment is suitable for use in cable television head end and/or video on demand type services. 
     Moreover, because of the reduced size, complexity, cost, and/or power requirements of the preferred embodiment frequency translator utilizing substantial circuit integration of the circuit elements, i.e., an upconverter on a “chip”, embodiments of the present invention are uniquely adapted for deployment in configurations heretofore not possible. For example, frequency translators of the present invention may be deployed at neighborhood nodes of a cable television system to facilitate delivery of pay per view video or video on demand to select subscribers in the neighborhood. Additionally or alternatively, the ability to distribute frequency translation functionality throughout a communication system, such as the aforementioned cable television system, allows new and improved services to be offered. For example, highly directive communications, such as neighborhood advertisements, may be inserted on particular channels in particular areas. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.