Abstract:
A microwave frequency synthesizer utilizing a phase locked loop for generating a wide range of microwave frequencies in response to a control signal, in combination with a network including a plurality of distinct frequency local oscillators that are appropriately selectable for mixing with the signal output of the phase locked loop for translating it into a desired range, thereby expanding the effective frequency range of the phase locked loop. The combination provides a stable source of microwave frequency signals over a wide frequency range. The combination allows a simple, low cost circuit implementation and may be switched between output frequencies at a very fast speed.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to signal generators, and more particularly to those in the microwave frequency range that are rapidly switchable between specific frequencies for, as an example, simulating a field including numerous distinct frequency radar signals. 
     A phase locked loop is a commonly used element for a variable frequency signal generator. Frequency output can be changed among discreet frequencies within the output frequency range. The frequency range of a phase locked loop, however, is limited by the range of frequencies which its components can handle. If a signal generator with a wider frequency range is desired, a plurality of phase locked loops may be utilized, each handling a distinct portion of the expanded frequency range, but this is complicated and expensive. Also, phase locked loops of the type presently utilized are slow in changing frequency. 
     Another technique for a broad frequency band signal generator is to utilize a separate fixed frequency oscillator for each of the discrete frequencies that are desired to be generated within the band. This, of course, involves a great deal of circuit elements and thus is complicated and expensive. For example, if a microwave signal generator has a range of 0.5 GHz to 18.0 GHz by frequency steps of 10 MHz, a total of 130 oscillators at distinct frequencies would be required. Existing techniques suggest that fewer oscillators could be used by selectively combining the oscillators to provide the desired discrete frequency steps but this is also complex and cumbersome if a wide frequency spectrum is desired. 
     One application of such a signal generator is in the simulation of a radar field that contains a number of different radar signals. Such signal generators are used for training, as an example, of military or other personnel that have a job of detecting and analyzing radar signals. Rather than having separate signal generators for each simulated radar signal, it involves considerably less hardware to utilize one broad band microwave signal source that is rapidly switched between frequencies. After one microwave signal pulse at one particular frequency is emitted then there is a time delay before a second pulse at that frequency needs to be emitted in simulating the radar signal. In this interim, the same oscillator can be used to generate one or more additional microwave pulses at other distinct frequencies to simulate additional radar signals in the field. To do this, however, requires extremely fast switching between frequencies. 
     Therefore, it is a principal object of the present invention to provide a wide band microwave frequency generator that may be rapidly switched between specific frequencies within its frequency band, is stable in frequency output over time and yet is relatively simple so that the cost and amount and size of hardware can be minimized. 
     SUMMARY OF THE INVENTION 
     This and additional objects are accomplished by the present invention wherein, broadly, a phase locked loop is provided for generating microwave signals of a plurality of distinct frequencies within the broadest band width that is possible, coupled with a frequency translation circuit that takes the phase locked loop output and shifts it in frequency by mixing with a fixed frequency output of a selected one of a plurality of different frequency local oscillators. The result is a microwave signal generator having a frequency range that can be several times that of the range of the phase locked loop itself, yet with less complexity in hardware than would be required if only phase locked loops or frequency translation techniques were utilized alone. The frequency stability of a phase locked loop is realized with this combination. Further, a high reference oscillator frequency for the phase locked loop is utilized in order that its frequency may be more rapidly changed than existing phase locked loops, the reference oscillator frequency being in excess of 500 KHz. 
     In a preferred form of the phase locked loop, in order to permit it to operate over the widest possible frequency range, a frequency translation circuit is provided in its feedback loop ahead of a frequency divider so that the frequency range that needs to be handled by the divider is minimized. The particular circuit can be another phase locked loop or several local oscillators. In a least complicated circuit, a very few local oscillators are utilized for reducing an output signal frequency range of the phase locked loop to a narrower frequency range. For better loop output frequency resolution, according to another aspect of the invention, a larger number of local oscillators (or some other means such as a second phase locked loop) are provided to generate a selected frequency from a larger number of distinct frequency signals that are closer together in frequency. This frequency difference can even be made a small fraction of the frequency of the reference oscillator for the phase locked loop, thereby to combine (1) the advantage of fast frequency switching speed that a phase locked loop has when a high reference oscillator frequency is provided, and (2) the advantage of a large number of closely spaced output signal frequencies upon which the loop can be locked. 
     Additional objects, advantages and features of the present invention will become apparent from the following description of its preferred embodiments, which should be taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block schematic diagram of a microwave signal generator utilizing the various aspects of the present invention; and 
     FIG. 2 is a block schematic diagram of a variation of one sub-system of the microwave signal generator of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a very specific example of a microwave signal generator according to the present invention is described. The example chosen is for one having an output frequency range of from 0.5 GHz to 18.0 GHz. Specific frequencies within that range are selectable at 10 MHz increments, thus being its resolution. Of course, the various aspects of the present invention are not limited to such a specific generator but rather a complete system is being described in detail in order to better convey the concepts and implementation of the invention. 
     A reference oscillator 11 generates at an output 13 a very stable 10 MHz reference signal. This reference signal is utilized by a phase locked loop 15 and is also used as a phase reference by every one of the many local oscillators (L.O.) in the system of FIG. 1. The phase locked loop circuit 15 generates at an output line 17 a variable frequency in the range of 2.0-3.0 GHz in increments of 10 MHz. Cooperating with the phase locked loop 15 is a translator circuit 19 that provides by a line 21 to the phase locked loop 15 a frequency shifting signal. 
     The phase locked loop output in line 17 is made within the range of 2-3 GHz primarily because of limitations of available circuit elements for the phase locked loop 15. It is desired, however, to have the signal within a range of 16.0-18.0 GHz, the top end of the desired output range. It is preferable to start at the top end of the range and translate the frequency downward, rather than the other way around, in order to reduce spurious signals. Therefore, before further use, the output signal in line 17 is shifted to the 16-18 GHz range by a translator circuit 25 which generates in its output line 27 a selected frequency for application to a mixer 23 along with the signal in the line 17. An output 29 of the mixer 23 is thus a signal within the range of 16-18 GHz selectable at 10 MHz increments, as determined by setting of the phase locked loop 15 and the frequency translators 19 and 25. By limiting the output 17 to a 1 GHz frequency range, the quality of the signals and ease of construction of the phase locked loop 15 are maintained. The phase locked loop 15 can change frequency very rapidly because of its high frequency in the reference oscillator line 13 and the wide band width of the amplifier and filter 75. 
     The signal in the line 29 is a continuous wave signal. For radar simulation work which has been referred to previously, this signal is modulated by a modulator 31 to emit in an output line 33 a series of pulses or bursts of a microwave signal which has been shaped with a modulating amplitude envelope. These are well known techniques and not described further herein. Other applications of a microwave signal generator according to FIG. 1 may leave out the modulator 31 altogether. 
     The signal in the line 33 is applied simultaneously to three frequency translating circuits 35, 41 and 47. The translator 35 shifts the signal in line 33, that is within a 16-18 GHz range, down to a signal within a 0.5-2.0 GHz range. A line 37 will output the desired signal if within a range of 0.5-1.0 GHz. A line 39 will output the desired signal if within a range of 1.0-2.0 GHz. 
     Similarly, the translator 41 shifts the signal within the 16-18 GHz range to one within a 2-8 GHz range. A line 43 will output the desired signal if within a range of 2-4 GHz. A line 45 will carry the output signal if within a range of 4-8 GHz. 
     The third translator 47 shifts the signal within the 16-18 GHz range in line 33 to one within an 8-18 GHz range. A line 49 carries the output signal if within a range of 8.0-12.4 GHz. A line 51 outputs the signal if within a range of 12.4-18.0 GHz. 
     Thus, the desired frequency is obtained in one of the output lines 37, 39, 43, 45, 49 or 51. Separate output lines are utilized for the specific frequency ranges specified because that is the custom in microwave systems. Each of these lines carries an octave of frequencies. Of course, a single output line carrying the full frequency range of 0.5-18 GHz could be provided if desired for other applications. 
     In chosing a particular frequency at a given instant to be provided in an appropriate one of the output lines 37-51, a specific setting of a number of switches within the phase locked loop 15 and the translators 19, 25, 35, 41 and 47 need to be made. This is preferably, but not necessarily, accomplished by the use of a central controller 53 that utilizes current microprocessor technology. A digital signal output bus 55 of the controller 53 carries a signal representative of the desired output signal frequency as requested by an operator through a keyboard or other device input to the controller 53. Decoding circuits 57 and 59 respond to that digital signal on the bus 55 to emit proper signals in circuits 61, 63, 65, 67, 69, and 71 which are all connected with the various switching elements, either directly or through further decoding circuits. Of course, any number of other known techniques may be utilized for effecting a proper combination of switch settings in order to generate a desired signal frequency at any instant. An Appendix `A` to this application is provided to illustrate the various combination of switch settings that are made for some of the output frequencies. 
     The various particular circuit elements within the circuit blocks of FIG. 1 will now be described in detail. Referring initially to the phase locked loop 15, a phase comparator or mixer 73 has a voltage output in a line 74 that is applied to an amplifier and filter circuit 75. The amplified signal in the line 74 is then applied to an input of a voltage controlled oscillator 77. The oscillator 77 produces an output frequency in the line 17 that is a function of the voltage applied to its input at the line 76. A directional coupler 79 initiates a feedback loop by inserting some of the signal in the line 17 through a line 81 to a mixer 83 whose output in a line 85 is applied to a frequency division circuit 87. An output line 89 of the division circuit 87 is applied at a second input to the mixer 73. When the frequencies and/or phases of the signals in the line 13 and 89 differ, an error voltage is produced whose average value in the line 74 is proportional to that difference. Such a voltage in the line 74 is then applied in the line 76 to the oscillator 77 to change its output frequency in a manner tending to drive the error voltage in the line 74 to zero. 
     The division circuit 87 divides the frequency in the line 85 by an integer between 40 and 90. That is, the frequency in the output line 89 is reduced from that in the input line 85 by a number between 40 and 90, inclusive. The mixer 83, in conjunction with the translator 19, reduces the frequency in the line 81 to within a window of 0.4 to 0.9 GHz. The division circuit 87 operates best if limited to a 0.5 GHz band width. But since it is desired that the output in the line 17 of the phase locked loop 15 be adjustable over a band width of 1 GHz, a frequency translation is required to reduce the band width of frequencies that are applied to the division circuit 87. 
     The translator circuit 19, in this specific example, includes four local oscillators 91, 93, 95 and 97. The distinct single frequency outputs of these four oscillators are applied to a single pole, four position switch 99 that is operable through a proper control signal in the line 63 to select one of the oscillator outputs at a time for connection to the line 21. Since the frequency output selected will affect the output frequency in the line 17 from the phase locked loop 15, this selection is coordinated with the other switch positions so that the desired frequency signal is generated in an appropriate one of the output lines 37-51. Each of the local oscillators 91, 93, 95 and 97 is connected to the output 13 of the reference oscillator 11 so that the outputs of these local oscillators will remain in a fixed phase relationship with each other and the reference oscillator 11. In the particular embodiment of FIG. 1, the frequency outputs of oscillators 91 and 93 differ by 10 MHz, incidentally equal to the frequency of the reference oscillator 11, as do the local oscillators 95 and 97. The spread between the outputs of the oscillators 91 and 95 is the 0.5 GHz window of the division circuit 87, as is the difference between oscillators 93 and 97. 
     The translator 25 serves a function of expanding the frequency range of the signals in the output lines 17 of the phase locked loop 15, as well as translating it to a higher frequency range. Local oscillators 101 and 103 have their distinct single frequency outputs connected to a single pole, two position switch 105 that is controlled by a signal in a line 65. The oscillator that is connected through the switch 105 to the line 27 for mixing with the signal in the line 17 depends upon the output frequency that is called for in the settings of the other switches in the circuit. 
     The translator 35 includes a mixer 107 which receives the signal in the line 33 and the single frequency signal from a local oscillator 109. A difference between these two input frequencies is applied to a single pole, three position switch 111 which selects connection of the output of the mixer 107 to one of three band pass filters 113, 115 and 117, their bands being indicated on the drawing for the specific system being described. These filters have a purpose of surpressing spurious signals that are the result of the mixing process in the mixer 107. The outputs of these filters are applied to a single pole, three position switch 119 that has a purpose of connecting one of these filter outputs with an amplifier 121. The amplifier 121 is then switched through a switch 123 to either of the output lines 37 or 39. The switches 111, 119 and 123 are controlled through a decoding circuit 125 that operates in response to signals in the circuit 67. 
     As an example of the operation of the translator 35, consider that a 0.6 GHz output is desired in the line 37. Such a frequency will be developed at the output of the mixer 107 but then it is applied to the switch 111 to the input of the filter 113 that is designed for a frequency band which includes our desired 0.6 GHz signal. The output of the filter 113 is connected through the switch 119 to the amplifier 121 and thence through the switch 123 to the line 37. The single pole switches 119 and 123 are connected as described without being connected to any of the other lines at the same time. If a 1.2 GHz output is desired, that frequency is present at the output of the mixer 107 and it is connected through the switch 111 to the filter 115. The switch 119 is then set to be connected to the output of the filter 115, and the switch 123 is set to connect the output of the amplifier 121 to the output line 39 which has been designated to carry an output of frequencies including this desired frequency. 
     The translator 41 is constructed in a manner similar to that of the translator 35 except that because it handles a broader frequency range it is somewhat more complicated. A mixer 127 receives the signal from the line 33 and mixes it with a signal output of a single pole four position switch 129 that selects a single distinct frequency output of four local oscillators 131, 133, 135 and 137. The output of the mixer 127 is supplied to a single pole, four position switch 139 that connects that output with one of four band pass filters 141, 143, 145 and 147. The filter 141 handles a full frequency range between 2 and 4 GHz that is desired for the output line 43 so it is applied directly thereto through an amplifier 151. The other three filters 143, 145 and 147 are applied to a single pole, three position switch 149, the output of which is applied through an amplifier 153 to the output line 45. Each of the switches 129, 139 and 149 are positioned in response to control signals from a decoder 155 that in turn operates in response to signals in a circuit 69. 
     In operation, the translator 41 shifts the frequency in the line 33, through the mixer 127 by one of the local oscillators 131, 133, 135 or 137, to the desired output frequency at the output of the mixer 127. That output is then passed through an appropriate one of the filters 141, 143, 145 or 147 by an appropriate positioning of the switches 139 and 149. 
     The third translator 47 is even more complicated than the other two because of yet a broader frequency range. A single pole, two position switch 157 is connected to the line 33. The switch 157 connects that line to either of two lines 159 or 161. If connected to the line 159, the signal is passed to a mixer 163 which receives a local oscillator signal from the output of a single pole, six position switch 165. Six local oscillators 167, 169, 171, 173, 175, and 177 are connected to the switch 165. The output of one of these oscillators is connected at one time through the switch 165 to the mixer 163. An output of the mixer 163 is an appropriate difference of the signal frequency in the line 159 and the selected local oscillator output. This signal is applied to a single pole, seven position switch 179 that selects an appropriate one of seven band pass filters 181, 183, 185, 187, 189, 191, and 193. A switch 194 is connected to the outputs of four of these filters, namely 181, 183, 185, and 187. The switch 194 selects one of the outputs for application through an amplifier 195 to the output line 49. A second output switch 197 receives signals from the filters 189, 191, and 193 for application through an amplifier 199 to the output line 51. A fourth position of the switch 197 receives directly the line 161. No frequency shifting is necessary when the desired output signal in the line 51 is in the 16-18 GHz range of the signal in the line 33. Therefore, for that frequency range, the switch 197 is connected to the line 161 directly. Each of the switches 157, 179, 194 and 197 are operated and controlled by control signals from a decoding circuit 201 which in turn receives signals from the circuit 71. 
     Appendix `A` attached hereto shows exemplary settings of all switches of the system of FIG. 1 for representative samples of output frequencies in the lines 37-51. 
     FIG. 2 illustrates a more complicated variation of the frequency translator 19 of FIG. 1 and has been designated as translator 19&#39; since the circuit of FIG. 2 carries out the same general function. An output signal in the line 21&#39; of FIG. 2 replaces the signal in the line 21 of FIG. 1. 
     The translator 19&#39; of FIG. 2, in a very specific embodiment being described, contains 11 local oscillators 211-221 that each have output frequencies that differ from each other by 0.001 GHz, or expressed as 1 MHz. Each of these local oscillators is referenced to the output 13 of the reference oscillator 11 (FIG. 1). The outputs of the oscillators 211-221 are grouped and applied to respective single pole, multi-throw switches 223, 225, 227 and 229. The outputs of these four switches are applied through their respective filters 231, 233, 235 and 237 to another switch 239. The switch 239 is a single pole, four position switch that presents at an output line 240 a selected one of the outputs of the filters 231-237. 
     Each of the switches 223, 225, 227, 229 and 239 is controlled from a decoding circuit 257 that receives a digital signal on the microprocessor bus 55&#39;. By proper operation of these switches, a single frequency output of one of the oscillators 211-221 is switched to the line 240. This signal is then applied to a mixer 241 where it is combined with an output of a local oscillator 243 to shift its frequency. The mixer output is then connected to a coupler 245 which splits its signal into two paths, one being applied directly through a line 249 to one position of a switch 255, and the other path being in a line 247 to another mixer 251. The mixer 251 also receives the output of a local oscillator 253 for shifting the frequency in the line 247, as appears in a line 252 that is the output of the mixer 251. The line 252 is applied to a second input of the single pole, double position switch 255 which selects between the lines 249 and 252 for connection to its output 21&#39;. The switch 255 is controlled from the decoder 257 and the local oscillators 243 and 253 are synchronized by connection to the reference oscillator output line 13. 
     The advantage of the circuit 19&#39; of FIG. 2 over the circuit 19 of FIG. 1 is that the resulting resolution of the available frequency steps at the output line 17 (FIG. 1) of the phase locked loop 15 are very small. In the specific example circuit of FIG. 1, these steps are 10 MHz while with the circuit of FIG. 2 utilized these steps are 1 MHz. The circuit of FIG. 2 allows the resolution of the phase locked loop 15 to be a small fraction of the frequency of the reference oscillator 11. This permits a high reference oscillator frequency to be utilized, which allows for fast operation of the phase locked loop 15 in changing frequency, but does not sacrifice the available frequency resolution in so doing. 
     Although the various aspects of the present invention have been described with respect to a specific and detailed electronic system, it will be understood that the invention is entitled to protection within the full scope of the appended claims. 
     
         __________________________________________________________________________APPENDIX `A`SWITCH SETTINGS FOR CERTAIN OUTPUT SIGNAL FREQUENCIESDesired In              ResultingOutput Output          Line 29                       Translator 35                                    Translator 41Frequency Line     ÷ N        SW 99            SW 105                 Frequency                       SW&#39;s/111 &amp; 119                               SW/123                                    SW 129                                         SW&#39;s/139 &amp;__________________________________________________________________________                                         1490.50 GHz 37  40 L.O. 91            L.O. 101                 16 GHz                       Filter 113                               Line 37                                    --   --0.51 GHz 37  40 L.O. 93            L.O. 101                 16.01 GHz                       Filter 113                               Line 37                                    --   --1.50 GHz 39  40 L.O. 91            L.O. 103                 17.0 GHz                       Filter 117                               Line 39                                    --   --1.51 GHz 39  40 L.O. 93            L.O. 103                 17.1 GHz                       Filter 117                               Line 39                                    --   --2.70 GHz 43  58 L.O. 97            L.O. 101                 16.7 GHz                       --      --   L.O. 137                                         Filter 1412.71 GHz 43  60 L.O. 95            L.O. 101                 16.71 GHz                       --      --   L.O. 137                                         Filter 1414.10 GHz 45  50 L.O. 91            L.O. 101                 16.10 GHz                       --      --   L.O. 135                                         Filter 1434.11 GHz 45  50 L.O. 93            L.O. 101                 16.11 GHz                       --      --   L.O. 135                                         Filter 14310.0 GHz 49  90 L.O. 91            L.O. 101                 16.5 GHz                       --      --   --   --10.01 GHz 49  40 L.O. 95            L.O. 101                 16.510 GHz                       --      --   --   --13.51 GHz 51  80 L.O. 93            L.O. 103                 17.41 GHz                       --      --   --   --18.0 GHz 51  88 L.O. 97            L.O. 103                 18.0 GHz                       --      --   --   --__________________________________________________________________________Desired In              ResultingOutput Output          Line 29                       Translator 47Frequency Line     ÷ N        SW 99            SW 105                 Frequency                       SW 157 SW 165 SW&#39;s 179, 194 &amp;__________________________________________________________________________                                     1970.50 GHz 37  40 L.O. 91            L.O. 101                 16 GHz                       --     --     --0.51 GHz 37  40 L.O. 93            L.O. 101                 16.01 GHz                       --     --     --1.50 GHz 39  40 L.O. 91            L.O. 103                 17.0 GHz                       --     --     --1.51 GHz 39  40 L.O. 93            L.O. 103                 17.1 GHz                       --     --     --2.70 GHz 43  58 L.O. 97            L.O. 101                 16.7 GHz                       --     --     --2.71 GHz 43  60 L.O. 95            L.O. 101                 16.71 GHz                       --     --     --4.10 GHz 45  50 L.O. 91            L.O. 101                 16.10 GHz4.11 GHz 45  50 L.O. 93            L.O. 101                 16.11 GHz10.0 GHz 49  90 L.O. 91            L.O. 101                 16.5 GHz                       Mxer 163                              L.O. 173                                     Filter 18510.01 GHz 49  40 L.O. 95            L.O. 101                 16.510 GHz                       Mxer 163                              L.O. 173                                     Filter 18513.51 GHz 51  80 L.O. 93            L.O. 103                 17.41 GHz                       Mxer 163                              L.O. 169                                     Filter 18918.0 GHz 51  88 L.O. 97            L.O. 103                 18.0 GHz                       SW 197 None   Line 51__________________________________________________________________________