Abstract:
A microwave synthesizer includes a drift-cancel loop having a narrow-band input, a low-frequency comb input, a wide-band input, and an output for providing an adjustable-frequency output signal. A narrow-band synthesizer is coupled to the narrow-band input, and a comb generator is coupled to the low-frequency comb input. Instead of using a wide-band synthesizer to drive the wide-band input, as conventional topologies have done, the instant invention employs a highly stable, low noise high frequency oscillator. The output of the oscillator is mixed with the output of the comb generator to produce low-noise, high frequency combs. The low-noise, high frequency combs are then used to drive the wide-band input of the drift-cancel loop. Significant reductions in phase noise can be achieved as compared with conventional designs.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to automatic test equipment for electronics (ATE) and, more particularly, to the synthesis of low-noise, high frequency waveforms for testing microwave and RF circuitry. 
     BACKGROUND OF THE INVENTION 
     Significant improvements in the accuracy of high-frequency devices used in consumer products such as cellular telephones, pagers, and wireless personal data assistants (PDAs) have created a need for more accurate testing of these devices. ATE systems generally include one or more microwave synthesizer for testing microwave devices. In one typical testing scenario, a microwave synthesizer within the tester supplies a signal directly to the DUT. The DUT provides a response, which the tester measures and tests. In another testing scenario, a tester receives a microwave signal (e.g., 900 MHz) from a device under test (DUT). The tester mixes this signal with the output of one of its microwave synthesizers to generate an intermediate frequency signal (e.g., 10 MHz). The tester then samples the intermediate frequency signal to ascertain its characteristics. If the characteristics are within predetermined limits, the test passes. Otherwise, the test fails. 
     One common testing technique is to compute a power spectrum of the intermediate frequency signal derived from the device under test. A power spectrum reveals meaningful information about the DUT as well as phase noise. To accurately test the phase noise of the device under test, it is essential that the synthesizer&#39;s phase noise be small compared with that of the DUT. If the synthesizer&#39;s phase noise is large compared with that of the DUT, the DUT&#39;s phase noise becomes lost in the synthesizer&#39;s phase noise, and it becomes impossible to tell whether the DUT meets its phase noise specification. As devices are continually improved to deliver lower and lower phase noise, microwave synthesizers must correspondingly be improved if testing is to remain accurate. 
     FIG. 1 illustrates a conventional microwave synthesizer  100 , which operates as follows. A narrow-band synthesizer  112  generates an output signal that can be varied over a relatively narrow range, e.g., a 200 MHz range between 800 MHz and 1 GHz. Simultaneously, a wide-band synthesizer  122  generates an output signal that can be varied over a relatively wide frequency range, e.g., a 2 GHz range between 4.4 GHz and 6.2 GHz. Simultaneously, a comb generator  116  produces a series of harmonically spaced tones, or “combs,” e.g., at 200 MHz tone spacing. The output of the narrow-band synthesizer  112 , the wide-band synthesizer  114 , and the comb generator  116  are respectively fed to a narrow-band input  152 , a wide-band input  154 , and a comb input  156  of a drift-cancel loop  150 . 
     Within the drift-cancel loop  150 , a power splitter  130  divides the output of the wide-band synthesizer  122  into first and second circuit paths. Amplifiers  132  and  134  boost the levels of signals along the respective paths. A first mixer  138  combines the output of the amplifier  132  with the output of the comb generator  116 , to produce a different pair of sum and difference tones for each tone produced by the comb generator  116 . By appropriately tuning the frequency of the wide-band synthesizer  122 , one of the sum or difference tones from the mixer  138  can be made to equal a target frequency, F K . For normal operation, the inputs to the drift-cancel loop  150  are always adjusted to produce a tone at the output of the mixer  138  that equals F K . 
     A first band-pass filter  142  filters the output of the mixer  138 . The first band-pass filter  142  has a center frequency at F K , and has a narrow bandwidth for passing only the mixing product at F K  and substantially rejecting all other frequency components. The output of the first band-pass filter  142  is passed to a second mixer  146 , which combines the output of the first band-pass filter  142  with the output of the narrow-band synthesizer  112 , thus producing another pair of sum and difference tones. These sum and difference tones are passed to a second band-pass filter  144 , which generally rejects the sum tone and transmits the difference tone to its output. 
     The transmitted tone is passed to a third mixer  140 . The third mixer  140  combines the transmitted tone with the output of the amplifier  134  to produce yet another pair of sum and difference tones. A low-pass filter  148  blocks the sum tone and transmits the difference tone to the output of the synthesizer  100 . The output may be coupled to additional stages (not shown), for selectively multiplying the frequency and adjusting the amplitude of the output signal. 
     The output frequency of the synthesizer  100  is adjustable in two ways. First, the wide-band synthesizer  122  can be adjusted to vary the overall output frequency in large increments. Second, the narrow-band synthesizer  112  can be adjusted to vary the overall output frequency in small increments. The narrow band synthesizer  122  generally operates via direct digital synthesis (DDS) to produce a nearly continuous range of output frequencies. The frequency range of the narrow-band synthesizer  112  preferably equals or exceeds the spacing of consecutive combs produced by the comb generator  116 , to allow the narrow-band synthesizer to fully tune between adjacent combs. With this arrangement, the wide-band synthesizer  122  effects gross frequency changes, whereas the narrow-band synthesizer  122  effects fine frequency changes. The combination allows the frequency of the synthesizer  100  to be adjusted over a wide range with high precision. 
     As is known, the wide-band synthesizer  122  tends to produce significant amounts of phase noise. This phase noise is greatly reduced, however, by the action of the drift-cancel loop  150 . Owing to the summing and differencing actions of the mixers  138 ,  140 , and  146 , the frequency of the wide-band synthesizer  122  is made to cancel from the output of the synthesizer  100 . Along with the frequency of the wide-band synthesizer  122 , much of its phase noise is made to cancel as well. 
     In more elaborate implementations, a delay circuit  136  is placed between the second amplifier  134  and the third mixer  140 . The delay circuit  136  causes the inputs of the third mixer  140  to convey signals that represent the output of the wide-band synthesizer  122  at corresponding instants of time. By delaying the signal conveyed along the second circuit path to match the delay incurred by the signal along the first circuit path, a great deal of phase noise is canceled by making corresponding phase perturbations common to both inputs of the mixer  140 . Because the low-pass filter  148  passes only the difference of input frequencies produced by the mixer  140 , noise that is common to both inputs of the mixer  140  is cancelled out. 
     Even with the addition of the delay circuit  136 , the synthesizer  100  still fails to reject some of the phase noise of the wide-band synthesizer  122 . Low frequency, or “close-in,” phase noise (less than 1 MHz offset) of the wide-band synthesizer largely cancels out, whereas high frequency, “far-out,” phase noise (above 1 MHz offset) generally does not. In implementations that tightly control the phase noise of the narrow-band synthesizer  112  and the comb generator  116 , the overall far-out phase noise of the microwave synthesizer  100  tends to be dominated by the unreduced, far-out phase noise of the wide-band synthesizer  122 . 
     SUMMARY OF THE INVENTION 
     With the foregoing background in mind, it is an object of the invention to reduce the far-out phase noise of signals produced by microwave synthesizers in automatic test equipment. 
     To achieve the foregoing object, as well as other objectives and advantages, a microwave synthesizer according to the invention includes a drift-cancel loop having a narrow-band input, a low-frequency comb input, a wide-band input, and an output for providing an adjustable-frequency output signal. A narrow-band synthesizer is coupled to the narrow-band input, and a comb generator is coupled to the low-frequency comb input. Instead of using a wide-band synthesizer to drive the wide-band input, as conventional topologies have done, the instant invention employs a low noise, high frequency oscillator. The output of the oscillator is mixed with the output of the comb generator to produce low-noise, high frequency combs. The low-noise, high frequency combs are then used to drive the wide-band input of the drift-cancel loop. Replacing the wide-band synthesizer with high frequency combs can significantly reduce the far-out phase noise of the synthesizer as compared with conventional designs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which— 
     FIG. 1 is a simplified block diagram of a conventional microwave synthesizer employing a drift-cancel loop; 
     FIG. 2 is a simplified block diagram of a microwave synthesizer according to the invention; 
     FIG. 3 is a simplified block diagram of a comb generator used in connection with the synthesizer of FIG. 2; 
     FIG. 4 is a simplified block diagram of a filter bank for selecting among the low-frequency combs in the synthesizer of FIG. 2; and 
     FIG. 5 is a simplified block diagram of a filter bank for selecting among the high-frequency combs in the synthesizer of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Topology and Operation 
     FIG. 2 illustrates an embodiment of a microwave synthesizer  200  according to the invention. The microwave synthesizer  200  resembles the microwave synthesizer  100  of FIG. 1 in many respects. For example, the microwave synthesizer  200  includes a drift-cancel loop  250 , a narrow-band synthesizer  212 , a comb generator  216 , and a low-pass filter  248 , which are respectively analogous to structures  150 ,  112 ,  116 , and  148  of FIG.  1 . In addition, the drift-cancel loop  250  has a narrow-band input  252 , a wide-band input  254 , and a comb input  256 , which respectively correspond to inputs  152 ,  154 , and  156  of the drift-cancel loop  150  of FIG.  1 . 
     Despite these similarities, the microwave synthesizer  200  differs from the synthesizer  100  in significant respects. This is particularly apparent with respect to the circuitry for driving the wide-band input  254 . As described above, conventional drift-cancel loops employ a wide-band synthesizer consisting of a phase-locked loop to drive the wide-band input of the drift-cancel loop. The phase-locked loop generally includes a VCO or YIG (Yttrium-Iron-Garnet) oscillator. In the embodiment of FIG. 2, however, the wide-band input  254  of the drift-cancel loop is driven by a mixing product of the comb generator  216  and an oscillator  222 . 
     The oscillator  222  generates a low-noise, high-frequency tone at F O . A mixer  226  combines this low-noise tone with one of the combs F SC  from the comb generator  216  (via a first filter bank  218  and power splitter  220 ), to generate a pair of sum and difference tones at F O ±F SC . 
     A second filter bank  228  selects one of these tones, i.e., (F O +F SC ) or (F O −F SC ), for passage to the wide-band input  254  of the drift-cancel loop  250 . Unlike the conventional design of FIG. 1, the synthesizer  200  preferably includes a first filter bank  218  for selecting a desired comb from the comb generator  216  and for blocking all other combs. The first filter bank  218  helps to prevent unwanted spurious signals from feeding into the drift-cancel loop  250 , and thus reduces overall noise. 
     Whenever the first filter bank  218  selects a different low-frequency comb, a different sum and difference pair of frequencies is provided to the second filter bank  228 . In the preferred embodiment, the low-noise oscillator  222  produces a single tone F O  at 5.2 GHz and the comb generator  216  produces combs at 200 MHz, 400 MHz, 600 MHz, 800 MHz, and 1 GHz. Given these inputs, any of the following frequencies can be provided to the wide-band input  254  of the drift-cancel loop  250 : 
     4.2 GHz, 4.4 GHz, 4.6 GHz, 4.8 GHz, and 5.0 GHz (via frequency subtraction); 
     5.2 GHz (via direct connection that avoids the mixer  226 ); and 
     5.4 GHz, 5.6 GHz, 5.8 GHz, 6.0 GHz, and 6.2 GHz (via frequency addition). 
     By appropriately selecting low frequency combs (LFCs) and high frequency combs (HFCs), the microwave synthesizer  200  can assume a variety of different frequency ranges. By adjusting the frequency of the narrow-band synthesizer  212 , these different ranges can be made to continuously blend together. 
     If the narrow-band synthesizer  212  produces output frequencies ranging from 800 MHz to 1 GHz, the microwave synthesizer  200  can produce frequencies continuously ranging from DC to 2 GHz. For practical purposes a lower frequency limit is established at 10 MHz. Table 1, below, summarizes the manner in which the microwave synthesizer  200  selects low and high frequency combs for establishing different frequency ranges: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Selected LFC 
                 Selected HFC 
                 Output Frequency Range 
               
               
                   
               
             
             
               
                 800 MHz 
                 4.4 GHz 
                  10 MHz-200 MHz 
               
               
                 600 MHz 
                 4.6 GHz 
                 200 MHz-400 MHz 
               
               
                 400 MHz 
                 4.8 GHz 
                 400 MHz-600 MHz 
               
               
                 200 MHz 
                 5.0 GHz 
                 600 MHz-800 MHz 
               
               
                 None 
                 5.2 GHz 
                 800 MHz-1 GHz  
               
               
                 (Bypass) 
                 (Directly) 
               
               
                 200 MHz 
                 5.4 GHz 
                   1 GHz-1.2 GHz 
               
               
                 400 MHz 
                 5.6 GHz 
                 1.2 GHz-1.4 GHz 
               
               
                 600 MHz 
                 5.8 GHz 
                 1.4 GHz-1.6 GHz 
               
               
                 800 MHz 
                 6.0 GHz 
                 1.6 GHz-1.8 GHz 
               
               
                  1 GHz 
                 6.2 GHz 
                 1.8 GHz-2 GHz   
               
               
                   
               
             
          
         
       
     
     To understand how these ranges are provided, one should note that the output frequency of the synthesizer  200  satisfies the equation— 
     
       
           F   OUT   =HFC− 5.2 GHz−NBS,    
       
     
     where HFC is the frequency of the selected high-frequency comb and NBS is the frequency of the narrow-band synthesizer  212 . 
     To provide a 5.2 GHz tone at the wide-band input  254 , a switch  224  is activated to bypass the mixer  226  and transmit the 5.2 GHz output of the oscillator  222  directly to the second filter bank  228 . The filter bank  228  passes this output directly to the wide-band input  254  (see FIG.  5 ). When the second filter bank  228  selects the 5.2 GHz tone for passage to the wide-band input  254 , the drift-cancel loop  250  activates another switch  258  to bypass the first mixer  238  and send the 5.2 GHz signal directly to the first band-pass filter  242 . Under these circumstances, no mixing is required to generate F K , because the signal at the wide-band input  254  already equals F K . 
     In the preferred embodiment, the oscillator  222  is a dielectric resonance oscillator (DRO), such as the model P2579 from General Microwave Corporation of Farmingdale, N.Y. It produces a fixed frequency of 5.2 GHz and is tunable over a narrow range to allow it to be synchronized with other system components. In the preferred embodiment, the DRO  222  is synchronized with a 100 MHz oven-controlled crystal oscillator (OCXO)  214 , such as the PTI X05051-001 from Piezo Technology, Inc., of Orlando, Fla. The OCXO  214  in turn is synchronized with the system reference  210 . Synchronization is preferably accomplished using extremely narrow-band phase-locked loops with frequency dividers in their feedback to provide closed-loop frequency multiplication. 
     By replacing the wide-band synthesizer  122  with low-noise, high frequency combs, far-out phase noise of microwave synthesizer  200  is significantly reduced. Care should be taken, however, to maintain low noise throughout the synthesizer  200 , and thus to obtain the full benefits of this low-noise design. 
     FIG. 3 shows a detailed block diagram of the comb generator  216  of FIG.  2 . The comb generator  216  receives the ultra-low noise output of the OCXO  214 . A frequency multiplier  312  multiplies the 100 MHz signal from the OCXO to produce a 200 MHz reference. An amplifier  314  boosts the 200 MHz reference, and a band-pass filter  316  filters the boosted signal. Another amplifier  318  boosts the output of the band-pass filter  316 . The band-pass filter  316  is preferably a narrow-band crystal filter, for eliminating noise beyond 10 KHz offset. A suitable narrow-band crystal filter is available from Piezo Technology, Inc. A comb generator device  320  is coupled to the output of the band-pass filter  316 , and generates combs at 200 MHz intervals. A suitable comb generator  320  is the GG 7014039, from Microsemi Corporation of Irvine, Calif. A high-pass filter  322  is applied to the output of the comb generator  320  to help equalize the amplitudes of the different combs, and a low-pass filter  324  is applied to the output of the comb generator  320  to filter combs above 1 GHz. 
     FIG. 4 shows a detailed block diagram of the filter bank  218  of FIG.  2 . The filter bank  418  preferably includes an amplifier  410  that boosts the combs received from the comb generator  216 . The filter bank includes five band-pass filters  420 ,  422 ,  424 ,  426 , and  428 . The band-pass filters  420 ,  422 ,  424 ,  426 , and  428  have center frequencies that correspond to different combs produced by the comb generator  216 . The filter bank  218  selects a desired comb from the comb generator  216  by configuring single-pole, double-throw (SPDT) switches  412 ,  414 ,  416 , and  418 . The boosted combs are transmitted from the amplifier  410  to the band-pass filter having the center frequency that corresponds to the desired comb. For example, to select the 600 MHz comb, the SPDT switches  416  and  418  close in such a way as to connect the output of the amplifier  410  to the input of the band-pass filter  424 . The selected band-pass filter passes the desired comb, and substantially blocks all other combs. On the output side of the band-pass filters, SPDT switches  432 ,  434 ,  436 , and  438  connect the selected band-pass filter to an amplifier  440 . The amplifier  440  boosts the selected comb, and passes the selected comb to the output of the filter bank  218 . 
     FIG. 5 shows a detailed block diagram of the filter bank  228  of FIG.  2 . In contrast with the filter bank  218 , which selects from among low-frequency combs (i.e., 200 Mhz to 1 GHz in 200 MHz increments), the filter bank  228  selects from among mixing products of the selected low-frequency comb and the oscillator  222 . These mixing products are spaced apart more widely in frequency than the spacing of the high-frequency combs. For example, when mixing the 1 GHz low-frequency comb with the 5.2 GHz oscillator, the closest mixing products are 2 GHz apart. By comparison, adjacent low frequency combs are only 200 MHz apart. Therefore, different band-pass filters need not be provided for each high-frequency comb, to accomplish the requisite filtering. To this end, the filter bank  228  includes four band-pass filters,  514 ,  516 ,  518 , and  520 . Band-pass filters are selected based on the desired high-frequency comb, according to table 2 below: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Desired HFC 
                 Selected Band-Pass Filter 
               
               
                   
                   
               
             
             
               
                   
                 4.4 GHz 
                 4.2 GHz-4.6 GHz (514) 
               
               
                   
                 4.6 GHz 
                 4.2 GHz-4.6 GHz (514) 
               
               
                   
                 4.8 GHz 
                 4.8 GHz-5.0 GHz (516) 
               
               
                   
                 5.0 GHz 
                 4.8 GHz-5.0 GHz (516) 
               
               
                   
                 5.2 GHz 
                 NONE 
               
               
                   
                 (Directly) 
               
               
                   
                 5.4 GHz 
                 5.4 GHz-5.6 GHz (518) 
               
               
                   
                 5.6 GHz 
                 5.4 GHz-5.6 GHz (518) 
               
               
                   
                 5.8 GHz 
                 5.8 GHz-6.2 GHz (520) 
               
               
                   
                 6.0 GHz 
                 5.8 GHz-6.2 GHz (520) 
               
               
                   
                 6.2 GHz 
                 5.8 GHz-6.2 GHz (520) 
               
               
                   
                   
               
             
          
         
       
     
     Advantages 
     By driving the wide-band input of a drift-cancel loop with a mixing product of the low-frequency combs and a stable oscillator, the resulting microwave synthesizer can produce exceedingly low phase noise. The synthesizer maintains low phase noise, even at high frequency offsets from the carrier, where drift-cancel loops are no longer useful at reducing phase noise. 
     Preliminary measurements of a prototype microwave synthesizer  200  show that overall phase noise is dominated not by the signal applied to the synthesizer&#39;s wide-band input, as in conventional designs, but by the signal at the narrow-band input. Driving the narrow-band input with a DDS having −155 dBc/Hz phase noise at 10 MHz offset, experiments have revealed an overall phase noise of only −153 dBc/Hz for the entire synthesizer. By comparison, designs that employ conventional voltage-controlled or YIG oscillators for driving the wide-band input produce approximately −140 to −143 dBc/Hz of phase noise, at least 10 dBc/Hz more phase noise than that of the instant design. 
     The microwave synthesizer according to the invention also has faster settling time than conventional synthesizers. YIG oscillators have response times on the order of tens of milliseconds. Voltage-controlled or YIG oscillators configured within phase-locked loops have stability requirements that tend to be satisfied at the expense of speed. By contrast, high-frequency combs can be switched in less than ten microseconds, three orders of magnitude faster than the settling time of YIG oscillators. The microwave synthesizer according to the invention is therefore able to change frequency at high speed. This enables the synthesizer to keep pace with devices that employ frequency hopping, such as those designed to the meet the Bluetooth specification. Bluetooth devices change their operating frequency at a maximum rate of once every 625 microseconds. The microwave synthesizer according to the invention can therefore test these devices as they are hopping in frequency, and it can do so with exceedingly low phase noise. 
     More generally, reduced test time for a device directly translates to reduced manufacturing cost. Another advantage of the invention is that, by reducing switching time, the synthesizer according to the invention allows customers to produce devices at lower cost. 
     Implementation 
     The microwave synthesizer  200  preferably takes the form of an instrument that plugs into a backplane of test system. The test system includes a host computer that communicates over the backplane and is capable of running test programs. The test programs include commands for controlling microwave synthesizer  200 , for example, programming its frequency, programming its amplitude, performing calibration, and reading back status. The output of the synthesizer connects to a device under test directly, via suitable cabling and connectors, or through a high frequency switching matrix. 
     To operate in this environment, the microwave synthesizer  200  preferably includes a digital control circuit (not shown). The digital control circuit receives high level commands from a test program, and translates these commands into electronic signals for controlling the activities of the synthesizer  200 . The digital control circuit also monitors activities within the synthesizer  200  and reports back to the test program. 
     The microwave synthesizer  200  preferably includes conventional output circuitry (not shown). This includes frequency multipliers for selectably providing different ranges of output frequencies under control of the digital control circuit. It also includes circuitry for adjusting the amplitudes of waveforms that the synthesizer  200  produces. 
     Alternatives 
     Having described one embodiment, numerous alternative embodiments or variations can be made. As described above, the oscillator  222  is a fixed-frequency dielectric resonance oscillator (DRO). Other types of oscillators can be used, however. For example, a variable-frequency oscillator can be used, provided that it is able to maintain low phase-noise over its operative frequency range. The preferred embodiment described above includes an oven-controlled crystal oscillator (OCXO)  214 , for providing an exceedingly quiet frequency reference. Depending upon phase noise requirements, the OCXO  214  can be replaced with other types of oscillators. 
     Although the filter bank  218  is a preferred portion of the microwave synthesizer  200 , it is not strictly required and could be omitted. Omission of the filter bank  218 , however, places an additional burden on the filter bank  228  and on the band-pass filter  242  to rejected unwanted combs produced by the comb generator  216 . Therefore, omitting the filter bank is expected to require more expensive components elsewhere in system, or to result in greater spurious signals. 
     As described above, the same comb generator  216  is used to produce both low frequency combs and high frequency combs. Alternatively, different comb generators could be used to produce the different sets of combs. For example, the output of a second comb generator could be mixed with the output of the oscillator to produce high-frequency combs. 
     Although the preferred embodiment is described above with reference to specific frequencies and ranges, nothing in the design of the microwave synthesizer  200  precludes other frequencies or frequency ranges from being used. For example, the combs need not be spaced by 200 MHz. Nor must the oscillator  222  operate at 5.2 GHz. 
     The embodiment of the synthesizer  200  described above takes the form of an instrument that plugs into a tester. The synthesizer  200  is not limited to this implementation, however. It could be provided as a bench-top instrument, for example, one that is stand-alone or programmable via an IEEE-488 bus. It could also be implemented as a modular instrument suitable for installing in a standard backplane, such as a VXI or PXI backplane. 
     Each of these alternatives and variations, as well as others, has been contemplated by the inventors and is intended to fall within the scope of the instant invention. It should be understood, therefore, that the foregoing description is by way of example, and the invention should be limited only by the spirit and scope of the appended claims.