Abstract:
A frequency scalable, low self-generated noise frequency source generates coherent or mostly-coherent local oscillator signals and includes a common reference, a coherent set of high frequency references and specific local oscillators which may be non-coherent for each specific output frequency. Delay lines may be included in the paths to ensure time delay alignment. The use of these elements with this modular design allows the generation of multiple coherent local oscillators via replication of the modular design elements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the generation of multiple frequencies and more particularly, the present invention relates to a frequency scalable, low self-generated noise frequency source. 
     2. Description of the Related Art 
     In a typical communications channel, the phase noise is introduced on the desired signal being received or transmitted. This phase noise degrades the overall signal to noise ratio (SNR), thereby leading to degraded signal acquisition or bit error rate performance. Typically, phase noise is introduced when the signal is multiplied by a local oscillator (LO) signal in a frequency conversion stage, the LO signal being the main source of the phase noise. Multiple frequency conversions in a communications channel and corresponding multiple LO signals may result in significant SNR degradation due to phase noise. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a frequency scalable, low self-generated noise frequency source having a modular architecture for generating coherent or mostly-coherent local oscillator signals. The frequency source in accordance with the present invention includes a common reference, a coherent common set of high frequency references and specific local oscillators which may be non-coherent for each specific output frequency. Delay lines may be included in the paths to ensure time delay alignment at the payload level. The use of these elements maximizes design reuse since just the specific IF local oscillators and delays and specific filters need custom design. 
     A frequency source in accordance with the present invention may include the following elements: 
     a modular design allowing the generation of multiple coherent local oscillators via replication of the modular design elements; 
     the use of a common low-frequency reference for all local oscillators; 
     the generation of a set of higher frequency tones as high frequency reference signals, most economically, via the transfer of an intermediate reference signal to all modules and the local generation of the set of higher frequency reference tones, the set of high frequency reference signals generally being evenly spaced in frequency; 
     the incorporation of delay elements to time-match all local oscillators; 
     the generation in each module of an intermediate frequency specific local oscillator signal which is based on the reference but is not fully coherent with respect to the high frequency reference tones; 
     the generation in each module of the final local oscillator signal by mixing the coherent high-frequency reference signal with the specific IF local oscillator signal, the mixing being either high-side or low-side mixing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and a better understanding of the present invention will become apparent from the following detailed description of example embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this invention. While the foregoing and following written and illustrated disclosure focuses on disclosing example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and that the invention is not limited thereto. The spirit and scope of the present invention are limited only by the terms of the appended claims. 
     The following represents brief descriptions of the drawings, wherein: 
     FIG. 1 is a block diagram of exemplary payload circuitry of a satellite which may include the preferred embodiments of the present invention. 
     FIG. 2 illustrates a frequency source arrangement in accordance with a first embodiment of the present invention. 
     FIG. 3 illustrates a frequency source arrangement in accordance with a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before beginning a detailed description of the subject invention, mention of the following is in order. When appropriate, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing drawing figures. Furthermore, in the detailed description to follow, example sizes/models/values/ranges may be given, although the present invention is not limited thereto. Arrangements may be shown in block diagram form in order to avoid obscuring the invention and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the present invention is to be implemented, that is, such specifics should be well within the purview of one skilled in the art. 
     Before describing details of embodiments of the present invention, a brief overview of a satellite payload will be provided. The satellite payload to be described is capable of receiving high frequency uplink beams at a plurality of receive antennas, converting the higher frequency to a lower frequency for switching and filtering of channels, converting the lower frequency signals to a higher frequency, and distributing the high power signals to one of the plurality of transmit antennas. 
     FIG. 1 illustrates electronics in a payload for a multi-beam satellite according to an example embodiment of the present invention. Other embodiments and configurations are also within the scope of the present invention. As one example, the satellite may be a communications satellite for use with broadband communications such as for the Internet. The satellite may include numerous antenna structures such as disclosed in U.S. Pat. No. 6,236,375, the subject matter of which is incorporated herein by reference. Each antenna may be an offset Cassegrain antenna having a subreflector, a main reflector and a separate feed array. FIG. 1 illustrates the electronics in the payload for one beam group of a satellite. The satellite payload may include similar electronics for each of the other beam groups. As one example, the satellite may include antenna structures for receiving and transmitting numerous beam groups, for example, eight beam groups. 
     FIG. 1 shows a first dual-polarization antenna  100 , a second dual-polarization antenna  101 , a third dual-polarization antenna  102  and a fourth dual-phase antenna  103  each to receive uplink beams from Earth in a well-known manner. Upon receipt of the uplink signals (such as broadband communication signals) at an antenna, the received signals may pass through an ortho-mode transducer (OMT)  104  to a band pass filter (BPF)  105 . The filtered signals may pass to a low noise amplifier downconverter (LNA D/C)  106  that converts the received signal from a higher frequency (such as approximately 30 GHz) to a lower frequency (such as approximately 4 or 5 GHz). 
     The lower frequency communications signals may then be amplified by an amplifier  107  and proceed to an input multiplexer (IMUX) and switching assembly  120 . The IMUX and switching assembly  120  may include an uplink connectivity switching network  121 , which may be a power dividing switching network. Signals output from the uplink connectivity switching network  121  may be input to either one of the two outbound multiplexers (IMUX)  122  or to the 4:1 inverse IMUX  123 . The IMUXes  122  output signals along forward channels O 1 , O 2 , O 3  and O 4  to a redundancy switching network  130 . The 4:1 inverse IMUX  123  outputs signal along return channel  11  to the redundancy switching network  130 . 
     The redundancy switching network  130  outputs signals to the up converters (U/C)  131 . The U/Cs  131  convert the lower frequency signals to higher frequency signals (such as approximately 20 GHz) that will be used for transmission back to the Earth. The higher frequency signals may then pass through channel linearizer/amplifiers  132  and TWTAs  133 . The TWTAs  133  are high power amplifiers that supply the transmit RF power to achieve the downlink transmission. The TWTAs  133  output high power outbound signals O- 1 , O- 2 , O- 3 , O- 4  and inbound signal I- 1  to the redundancy switching network  134 . The redundancy switching network  134  provides the signals I- 1 , O- 1 , O- 2 , O- 3  and O- 4  to an output multiplexer (OMUX) and switching assembly  140 . 
     The OMUX and switching assembly  140  may include mechanical switches  141  that couple the signals I- 1 , O- 1 , O- 2 , O- 3  and O- 4  to multiplexers (OMUX)  142 . The signals pass through the OMUXes  142  and are appropriately distributed to mechanical switches  143 . The switches  143  distribute the signals to one of the downlink OMTs  150  and the corresponding downlink antenna such as a first dual-polarization downlink antenna  151 , a second dual-polarization downlink antenna  152 , a third dual-polarization downlink antenna  153  and a fourth dual-polarization downlink antenna  154 . 
     A power converter unit  108  may also be provided to supply DC power to the LNA D/Cs  106  and the amplifiers  107 . Additionally, a frequency source unit  109 , which may correspond to the present invention, may supply a local oscillator (LO) signal to the LNA D/Cs  107  and to the U/Cs  131 . 
     FIG. 2 illustrates a frequency source arrangement in accordance with a first embodiment of the present invention. As illustrated in FIG. 2, three LO (local oscillator) channels  205 ,  206 , and  207  are shown. However, the present invention is not limited to this number of channels but rather, any combination and number of LO channels may be provided, depending upon the number of LO frequencies to be supplied. 
     A frequency reference  200  supplies a reference frequency for all of the channels. Channel  205  is a channel for generating a frequency which is an integer multiple of the reference frequency. For example, the reference frequency may be 10 MHz. The output of the frequency reference  200  is inputted to a multiplier  210  where it is multiplied in frequency by a factor N, where N is an integer. The output of the multiplier  210  is inputted to a bandpass filter  220  to eliminate spurious frequencies generated by the multiplier  210 . The output of the bandpass filter  220  is inputted to a PLL (Phase Locked Loop)  230 . While an SAW (Surface Acoustic Wave) PLL is shown in FIG. 2, it is of course understood that the present invention is not limited thereto but rather any suitable PLL may be used. 
     The PLL  230  produces an output signal which bears a relationship to the signal outputted by the bandpass filter  220 . This relationship along with the value of the multiplication factor N are selected so as to produce a signal output of the PLL  230  of the desired frequency. For example, the relationship and the multiplication factor N may be selected to produce an output frequency of 1 GHz. It is to be noted that the multiplier  210 , bandpass filter  220 , and PLL  230  may be omitted if the frequency of the frequency  200  is increased. 
     The output of the PLL  230  is used to drive a delay element  250  as well as driving the delay elements in the other channels. The delay element  250  is used to add a predetermined time delay to the signal inputted thereto. 
     The output of the delay element is inputted to a comb generator  260  to produce a multitude of harmonic signals stepped-up in frequency from the signal inputted thereto. The output of the comb generator  260  is inputted to a filter  270  where a desired one of the multitude of signals generated by the comb generator  260  is selected. For example, in this case, an output signal of 12 GHz has been selected. 
     Channels  206  and  207 , which have essentially the same elements, are used to generate output signals which are not integral multiples of the reference frequency. Delay elements  251  and  252  may be similar to or identical to delay element  250  but are designed to produce predetermined time delays which may differ for each channel. 
     Similarly, comb generators  261  and  262  may be similar to or identical to comb generator  260  and filters  271  and  272  may also be similar to or identical to filter  270  but are designed to produce selected frequencies, 12 GHz, for example, in the case of filter  271  and 14 GHz, for example, in the case of filter  272 . The outputs of filters  271  and  272  are respectively inputted to mixers  285  and  286 . 
     The output of frequency reference  200  is inputted to both PLL  231  of channel  206  and PLL  232  of channel  207 . PLL  231  and PLL  232  are illustrated as being second harmonic SAW PLL&#39;s but it is to be understood that the present invention is not limited thereto. These two PLL&#39;s may be designed to produce specific respective frequencies bearing predetermined relationships to the output of frequency reference  200 . For example, PLL  231  may be designed to produce an output signal in the range of from 1000 to 1500 MHz and more specifically 1042.5 MHz, for example. Similarly, PLL  232  may also be design to produce an output signal in the range of from 1000 to 1500 MHz and more specifically 1357.5 MHz, for example. 
     The outputs of PLL  231  and PLL  232  are respectively inputted to filters  240  and  241  to eliminate spurious signals. The outputs of filters  240  and  241  are respectively inputted to mixers  285  and  286 . The outputs of filters  271  and  272  are respectively inputted to the inputs of mixers  285  and  286 . 
     The outputs of mixers  285  and  286  are respectively inputted to filters  280  and  282  to select the suitable output signals, a signal of frequency 12 GHz plus 1042.5 MHz, for example, in the case of filter  280  and a signal of frequency 14 GHz plus 1357.5 MHz in a case of filter  282 . Note that each mixer produces an output signal having a frequency equal to the sum of the frequencies of the signals inputted thereto and an output signal having a frequency equal to the difference of the frequencies of the signals inputted thereto. The respective filter connected to the output of each mixer chooses either the sum or difference output signal of the mixer. While the sum has been chosen in this example, it is to be understood that the present invention is not limited thereto and the difference signal can just as easily have been chosen. 
     FIG. 3 illustrates a frequency source arrangement in accordance with a second embodiment of the present invention. FIG. 3 differs from FIG. 2 in that lower frequency PLLs, for example, operating below 1 GHz, are used. As illustrated in FIG. 3, frequency reference  200  provides a reference frequency which is inputted to filter  310  of channel  305  and to low-frequency PLLs  315  and  316 . 
     Filter  310  filters out spurious signals and provides an output to frequency multiplier  320  where the frequency is multiplied by the multiplication factor N1, where N1 is an integer. The output of the frequency multiplier  320  is fed to another filter  330 , which may be a crystal filter, which eliminates various spurious frequency signals. The output thereof is inputted to a second frequency multiplier  331  where the frequency is multiplied by the multiplication factor N2, where N2 is an integer. The output of the second frequency multiplier  331  is inputted to a filter (which may be a SAW filter)  340  to produce an output signal which is inputted to delay elements  250 ,  251 , and  252  and to mixers  317  and  318 . As with elements  210 ,  220 , and  230  of FIG. 2, one or more of the multipliers  320  and  331  and one or more of the filters  310 ,  330 , and  340  may be eliminated if the frequency of the frequency reference  200  is increased. 
     In channel  305 , elements  250 ,  260 , and  270  correspond to the same elements in FIG.  2  and produce the same output signal as in FIG. 2, in this case, 12 GHz, for example. 
     In channel  306 , the low-frequency PLL  315  produces a signal bearing a relationship to the signal input thereto, for example, a signal having a frequency of 200 MHz. This signal, when mixed in mixer  317  with the signal output of the SAW filter  340 , results in a signal outputted by the mixer  317  whose frequency is the sum of the frequencies of the two input signals, namely, 1.2 GHz as well as a signal whose frequency is the difference of the frequencies of the two input signals. 
     The signal outputted by the mixer  317  is filtered by the filter  391  to remove one of the two output frequency components, for example, the difference frequency component is removed, and the output of the filter  391  is inputted to another mixer  285 . Delay element  251  and comb generator  261  correspond to the same elements in FIG.  2  and produce the same output signal as in FIG.  2 . The output of the comb generator  261  is inputted to a filter  371  which extracts the appropriate harmonic generated by the comb generator  261 , a signal having a frequency of 13 GHz, for example, which is also inputted to the mixer  285 . 
     The mixer  285  produces an output signal having a frequency which is equal to the sum of the frequencies of the two input signals, namely a signal having a frequency of 14.2 GHz, as well as an output signal having a frequency which is equal the difference of the frequencies of the two input signals. The output of the mixer  285  is then filtered in filter  380  to remove one of the two output frequency components, for example, the difference frequency component is removed. 
     Channel  307  operates in the same fashion as channel  306 . Elements  252  and  262  operate in the same fashion as the corresponding elements in FIG.  2  and filter  372  is used to select the appropriate harmonic frequency output of the comb generator  262 , for example, a signal having a frequency of 15 GHz. Note that the filter  372  is labeled a tunable filter. However, the present invention is not limited thereto and the filter  372  may be a fixed filter. 
     The low-frequency PLL  316  may be designed to produce a signal having a frequency of 800 MHz, for example, which is inputted to the mixer  318  to produce a signal having a frequency equal to the sum of the frequencies of the two input signals, namely, a signal having a frequency output of 1.8 GHz, for example as well as a signal having a frequency equal to the difference of the frequencies of the two input signals. After being filtered by the filter  392  to remove one of the two output signal components, for example, the difference signal is removed, the resultant signal is inputted to the mixer  286  along with the signal outputted from the filter  372  to produce a signal output from the mixer  286  having a frequency equal to the sum of the frequencies of the two input signals, namely, a signal having a frequency output of 16.8 GHz as well as a signal having a frequency equal to the difference of the frequencies of the two input signals. 
     Note that it is possible to mix and match the channels of FIGS. 2 and 3 to fabricate a frequency source having a plurality of frequency outputs. For example, channel  205  and channel  206  of FIG. 2 may be combined with channels  306  and  307  of FIG. 3 to fabricate a frequency source. 
     As illustrated in the drawing figures, the RF reference signals may be produced using comb generators to generate multiple tones. Each comb generator may be driven from a common reference source to generate the coherent portions of the local oscillator signals as illustrated in FIG.  2 . The comb generators generate harmonics so that the desired harmonic may be selected and filtered for each local oscillator frequency. Selecting the highest feasible frequency from each comb generator results in the greatest coherency between output local oscillator frequencies. The use of common comb generators to generate the coherent portions of the local oscillator signals is combined with low-frequency sources, such as phase-locked-loops that are easily reconfigurable to generate specific frequencies. Furthermore, easily reconfigurable filters or scalable filter designs may also be used. This modular and easily scalable design reduces the cost, schedule, and risk of developing frequency sources. The number of local oscillator frequencies may be easily increased. For example, to add an additional local oscillator output frequency, an additional module is added, the module consisting of a comb generator, a delay line, a low-frequency source and a mixer or mixers. To adapt the unit for a different system application, the number of modules can easily be changed, as is the output frequency of each of the low-frequency sources and the harmonic frequency derived from each comb generator. 
     This concludes the description of the example embodiments. Although the present invention has been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings, and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.