Abstract:
Centering pulses within the output sample period corrects for instantaneous phase errors in multi-reference switching amplifiers. In the preferred embodiment, the pulses are ordered by the relative magnitude of each switched reference.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/494,163, filed Aug. 11, 2003, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to multi-reference switching amplifiers and, in particular, to a modulation method and apparatus for correcting instantaneous phase errors in multi-reference switching amplifiers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Multi-Reference Switching Amplifiers, of the type shown in U.S. Pat. No. 6,535,058 and PCT/US99/26691, the content of these being incorporated herein by reference, yield significantly higher instantaneous resolution than standard switching amplifiers, through the summation of multiple modulated outputs. In that they operate with pulsewidth modulated (PWM) signals, however, multi-reference amplifiers (like all Class D amplifiers) are subject to distortion unless phase correction is applied.  
         [0004]     Switching amplifiers usually rely upon modulation of pulsewidths controlling switching devices which gate power to the load. Resultant output power then approximates the integral of the gated voltage or current over the output sampling period. As these pulsewidths change with dynamic data, however, pulsewidth position within the output sample period becomes significant. Considerable distortion is induced unless the relative phase of each output pulse remains constant. Centering output pulses in the output sample period typically ensures instantaneous phase coherency. It is for this reason that many analog switching amplifiers use triangle, as opposed to sawtooth, reference waveforms.  
         [0005]     Multi-reference amplifiers, which sum and/or time-multiplex several switched references, require coherent relative phase of all switched reference pulsewidths to ensure low distortion. Additionally, pulsewidths unrelated to modulation values are typically added to each of these switched reference pulsewidths, as illustrated in my U.S. Pat. No. 6,492,868 “Dynamic Range Enhancement Technique,” the entire content of which is also incorporated herein by reference. To ensure coherent relative phase of this plurality of pulsewidths can be a difficult task. A need therefore exists for an effective method of correcting instantaneous phase errors in multi-reference switching amplifiers.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention resides in a method of centering within the output sample period all output pulses of a multi-reference switching amplifier. In the preferred embodiment, the pulses are ordered by the relative magnitude of each switched reference. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows a block diagram of a typical multi-reference amplifier using two references;  
         [0008]      FIG. 2  shows the control waveforms of the amplifier of  FIG. 1  when operated conventionally with minimum pulsewidths added to each reference, prior to the present invention; and  
         [0009]      FIG. 3  shows the control waveforms of the amplifier of  FIG. 1  when operated per the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]     Referring now to  FIG. 1 , incoming data stream  100  is applied to modulator  101 , which provides switching control signals to switching devices  107 ,  108 ,  109 ,  110 ,  111 , and  112 , the outputs of which are in turn coupled to load  113 . Note that switching devices  107  and  110  selectively connect the load  113  to the V+ reference voltage, and that switching devices  108  and  111  selectively connect the load  113  to a lower reference voltage  106  from reference source  102 . Switching devices  109  through  112  selectively connect the load  113  to ground. Details of such a representative multi-reference amplifier can be found in the applications referenced above.  
         [0011]     Referring now to  FIG. 2 , voltage traces  201 ,  202 ,  203 ,  204 ,  205 , and  206  represent control pulses from modulator  101  to switching devices  107 ,  108 ,  109 ,  110 ,  111 , and  112  respectively, all of  FIG. 1 . The time span between markers  207  and  213  represents one output sample period of the amplifier of  FIG. 1 ; a second sample period is shown between markers  213  and  217 . At time marker  207 , control voltages traces  201  and  204  indicate activation of V+ switching devices  107  and  110 , respectively.  
         [0012]     Activation time of both V+ switching devices from time marker  207  until marker  208  represents the minimum pulsewidth applied thereto, hence one switching device ( 110 , as seen in trace  204 ) is deactivated at marker  208 . Pulsewidth continuation of switching device  107 , as seen in trace  201 , until time marker  209  represents the actual coarse modulation pulsewidth. At deactivation of switching device  110  indicated in trace  204  at marker  208 , the minimum pulsewidth activation of low-voltage reference switching device  111  is seen in trace  205 . The minimum pulsewidth activation continues until marker  209 , which denotes the start of the fine resolution modulation period.  
         [0013]     At marker  210 , the deactivation of switching device  107  is seen in trace  201 , marking the end of the coarse modulation period. Concurrently at marker  210 , low-voltage reference switching device  108  is seen to be activated in trace  202  for its minimum pulsewidth, until marker  211 . Ground switching device  109  is indicated in trace  203  from marker  211  until the end of the sample period at marker  213 . Deactivation of low-voltage switching device  111  at marker  212 , seen in trace  205 , denotes the end of the fine modulation period. Ground switching device  112  activation, indicated in trace  206 , ensues from marker  212  to the end of the sample period at marker  213 .  
         [0014]     The  FIG. 2  sequence above shows assertion of coarse and fine modulated pulsewidths of V+ and low-voltage reference switching devices, respectively, on opposing sides of load  113 , of  FIG. 1 . This sequence as well shows assertion of minimum pulsewidths in both V+ switching devices and both low-voltage reference switching devices as well, which, being common-mode, effect no difference signal across load  113  of  FIG. 1 . Ground switching device activation can be seen to be the default state.  
         [0015]     At marker  213 , switching device  107  can be seen in trace  201  to be again activated for a minimum pulsewidth plus coarse modulation time, until marker  216 . This activation period from marker  213  to marker  216  can be seen to be much longer than the previous activation period from marker  207  to marker  210 , which indicates increasing coarse modulation voltage. Note, however, that the resultant time from the center of the V+ switching device activation between markers  207  and  210  to the center of the activation between markers  213  and  216  is substantially longer than the output sample period between markers  207  and  213 . This represents a lagging phase error in the coarse resolution voltage.  
         [0016]     At marker  214 , trace  205  indicates activation of low-voltage switching device  111  for a minimum pulsewidth plus fine modulation time, until deactivation at marker  215 . This activation period from marker  214  to marker  215  can be seen to be much shorter than the previous activation period from marker  208  to marker  212 , which indicates decreasing fine modulation voltage. Note, however, that the resultant time from the center of the low-voltage reference switching device activation between markers  208  and  212  to the center of the activation between markers  214  and  215  is substantially shorter than the output sample period between markers  207  and  213 . This represents a leading phase error in the fine resolution voltage.  
         [0017]     Referring now to  FIG. 3 , voltage traces  301 ,  302 ,  303 ,  304 ,  305 , and  306  represent control pulses from modulator  101  to switching devices  107 ,  108 ,  109 ,  110 ,  111 , and  112  respectively, all of  FIG. 1 . The time span between markers  307  and  316  represents one output sample period of the amplifier of  FIG. 1 ; a second sample period is shown between markers  316  and  323 .  
         [0018]     At marker  310 , V+ switching device  107  is seen to be activated in trace  301  until marker  313 , the period of which represents the sum of minimum pulsewidth and coarse modulation. Note that this activation between markers  310  and  313  is centered in the sample period indicated by markers  307  and  316 . Minimum pulsewidth activations of low-voltage reference switching device  108  bound the V+ switching device  107  activation, from marker  309  to marker  310 , and from marker  313  to marker  314 ; so are as well centered in the sample period indicated by markers  307  and  316 . Default ground switching device  109  activity is seen in trace  303  at markers  309  and  314 .  
         [0019]     At marker  308 , low-voltage reference switching device  111  is seen to be activated in trace  305  for a minimum pulsewidth plus one half the fine modulation time, until marker  311 . From marker  311  to marker  312 , a minimum pulsewidth assertion of V+ switching device  110  is seen in trace  304 . At marker  312 , low-voltage reference switching device  111  is seen to be again activated in trace  305  for a minimum pulsewidth plus one half the fine modulation time, until marker  315 . Note that not only is the V+ minimum pulsewidth between markers  311  and  312  centered in the sample period between markers  307  and  316 , but that the two minimum pulsewidths plus fine modulation assertion of switching device  111 , seen in trace  305 , is as well centered in the sample period. Default ground switching device  112  activity is seen in trace  306  at markers  308  and  315 .  
         [0020]     In the following sample period between markers  316  and  323 , V+ switching device  107  activation seen in trace  301  between markers  317  and  322  is considerably longer than the previous activation between markers  310  and  313 , indicating increasing coarse modulation voltage. Note, however, that the distance between the centers of V+ switching device  107  activations in trace  301  from marker  310  to marker  313  and from marker  317  to marker  322  is exactly the same period as the sample period between markers  316  and  323 . This indicates no coarse modulation phase error.  
         [0021]     Total low-voltage reference switching device  111  activation seen in trace  305  between markers  318  and  321  is considerably shorter than the previous total activation between markers  308  and  315 , indicating decreasing fine modulation voltage. Note, however, that the distance between the centers of low-voltage reference switching device  111  activations in trace  305  from marker  308  to marker  315  and from marker  318  to marker  321  is exactly the same period as the sample period between markers  316  and  323 . This indicates no fine modulation phase error. Note that all switching device minimum pulsewidths imposed in  FIG. 3  are as well centered in their respective sample periods.  
         [0022]     From the above figures and discussion, phase-accurate control of all control waveforms of a multi-reference switching amplifier can be seen. Expansion of the technique described herein is expected to include additional references, as well as all other permutations described in ‘Multi-Reference High Accuracy Switching Apparatus’. Although center precedence is shown to higher voltages, alternate pulse order adaptations of the present invention are as well anticipated.