Method and apparatus for pulse width modulation

Abstract

A ternary pulse width modulation (PWM) method and apparatus. In one embodiment, the start of the pulse sequence in the current frame is referenced to the end of the pulse sequence in a previous, reference frame, rather than to the frame boundary at the start of the current frame, thereby allowing the compensation portion of the pulse sequence to overlap into the preceding or following frame, thus achieving a higher modulation index without dropping the compensation pulses. Although in most instantiations, the reference frame will be the frame immediately preceding in time the current frame, in other instances, the reference frame may be any frame preceding the current frame that falls within the constraints of the timing facility.

Claims

1. A ternary pulse width modulation (PWM) method adapted for use with a PWM signal chain, the method comprising using the PWM signal chain to perform steps of: [1.1] receiving a first input sample during a reference frame; [1.2] developing a first compensated composite waveform as a function of at least the first input sample; [1.3] receiving a second input sample during a current frame; and [1.4] developing a second compensated composite waveform as a function of the second input sample and a selected one of the first compensated composite waveform and a boundary of the reference frame that is not a boundary of the current frame.

2. The method of claim 1 wherein the reference frame immediately precedes in time the current frame.

3. The method of claim 2 wherein a compensation portion of the second compensated composite waveform overlaps in time a frame boundary between the reference frame and the current frame.

4. The method of claim 2 wherein a compensation portion of the second compensated composite waveform precedes in time a frame boundary between the reference frame and the current frame.

5. The method of claim 2 wherein a compensation portion of the first compensated composite waveform overlaps in time a frame boundary between the reference frame and the current frame.

6. The method of claim 2 wherein a compensation portion of the first compensated composite waveform succeeds in time a frame boundary between the reference frame and the current frame.

7. In a ternary pulse width modulation (PWM) method adapted for use with a PWM signal chain, the method comprising using the PWM signal chain to perform steps of: [7.1] receiving a first input sample during a first frame; and [7.2] developing a first compensated composite waveform as a function of the first input sample; an improvement comprising using the PWM signal chain to perform additional steps of: [7.3] receiving a second input sample during a second frame; and [7.4] developing a second compensated composite waveform as a function of the second input sample and a selected one of the first compensated composite waveform and a boundary of the first frame that is not a boundary of the current frame.

8. The method of claim 7 wherein the first frame immediately precedes in time the second frame.

9. The method of claim 8 wherein a compensation portion of the second compensated composite waveform overlaps in time a frame boundary between the first frame and the second frame.

10. The method of claim 8 wherein a compensation portion of the second compensated composite waveform precedes in time a frame boundary between the first frame and the second frame.

11. The method of claim 8 wherein a compensation portion of the first compensated composite waveform overlaps in time a frame boundary between the first frame and the second frame.

12. The method of claim 8 wherein a compensation portion of the first compensated composite waveform succeeds in time a frame boundary between the first frame and the second frame.

13. A ternary PWM facility configured to perform the method of any one of the claims 1 to 12.

14. A digital signal processing system comprising the ternary PWM facility according to claim 13.

15. A non-transitory computer readable medium including executable instructions which, when executed in a processing system, causes the processing system to perform the steps of the method according to any one of claims 1 to 12.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) My invention may be more fully understood by a description of certain preferred embodiments in conjunction with the attached drawings in which:

(2) FIG. 1 illustrates, in block diagram form, a general purpose computer system adapted to practice my invention;

(3) FIG. 2 illustrates, in block diagram form, a typical integrated system adapted to practice my invention;

(4) FIG. 3 illustrates, in time line format, an example of a positive-modulated PWM signal developed by a prior art ternary PWM method, with a relatively low modulation index and compensation pulses;

(5) FIG. 4 illustrates, in time line format, an example of a positive-modulated PWM signal developed by a prior art ternary PWM method, with a maximum modulation index compatible with compensation pulses;

(6) FIG. 5 illustrates, in time line format, an example of a positive-modulated PWM signal developed by a prior art ternary PWM method, with a higher modulation index than in the example of FIG. 4 but without compensation pulses;

(7) FIG. 6 illustrates, in block diagram form, one embodiment of a pulse width modulation signal chain adapted to develop ternary PWM signals to drive a bridge-connected load;

(8) FIG. 7 illustrates, in block diagram form, one embodiment of a digital signal processor that may be employed to perform interpolation filtering and other processing to support my ternary PWM method;

(9) FIG. 8 illustrates, a pulse width modulation facility adapted to practice my ternary PWM method in a PWM signal chain such as is shown in FIG. 6;

(10) FIG. 9 illustrates, in state table format, one example control flow of the finite state machine shown in FIG. 8;

(11) FIG. 10 illustrates, in time line format, an example of a PWM signal developed by a ternary PWM method in accordance with my invention, with positive polarity and high signal amplitude;

(12) FIG. 11 illustrates, in time line format, an example of a PWM signal developed by a ternary PWM method in accordance with my invention, with negative polarity and high signal amplitude; and

(13) FIG. 12 illustrates, in time line format, an example of a PWM signal developed by a ternary PWM method in accordance with my invention, illustrating a positive to negative zero crossing.

(14) In the drawings, similar elements will be similarly numbered whenever possible. However, this practice is simply for convenience of reference and to avoid unnecessary proliferation of numbers, and is not intended to imply or suggest that my invention requires identity in either function or structure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

(15) In accordance with my invention, the start of the pulse sequence in the current frame is referenced, not to the leading edge of the current frame boundary but rather to an event in a previous reference frame, e.g., the trailing edge of the last pulse in the signal pulse sequence, or a boundary of the reference frame that is not a boundary of the current frame. By allowing the compensation portion of the pulse sequence to overlap into the preceding or following frame, a higher modulation index may be achieved without dropping the compensation pulses. Although in most cases, the reference frame will be the frame immediately preceding in time the current frame, it is possible to select as the reference frame any frame preceding the current frame that falls within the constraints of the timing facility.

(16) In FIG. 6, I have illustrated one embodiment of a PWM signal chain 16, comprising a digital interpolation filter 18, a noise shaping quantizer 20, an absolute value facility 22 and a PWM facility 24, adapted to develop ternary PWM signals to drive a bridge-connected load 26. In general, the same datapath hardware disclosed in my Related Applications, shown in FIG. 7 for convenience of reference, may be employed to perform interpolation filtering and other processing to support my new and improved ternary PWM methods.

(17) In FIG. 8, I have illustrated one embodiment of a pulse width modulation facility 28 adapted to practice my ternary PWM method in a PWM signal chain 16 such as is shown in FIG. 6. In this embodiment, the finite state machine (FSM), operating in accordance with the state table shown in FIG. 9, selectively loads one of the following calculated values into the timer:
A=current pulse width

(18) $B=\frac{n-A}{2}$$C=n-\frac{A}{2}-\frac{A^{}}{2}$
D=compensation pulse width
E=CD
where:

(19) n=number of clock cycles per frame; and

(20) a primed value comprises the enumerated value from a reference frame.

(21) FIG. 10 illustrates an example of a pulse train developed by my method in a positive modulation, high signal amplitude mode of operation. Note that, in the current frame, F.sub.1, the leading edge of the P signal pulse in the second compensated composite waveform 30 (both the N and P compensation pulses of which are shown in dark gray) is timed with respect to the trailing edge of the P signal pulse in the first compensated composite waveform 32 (both the N and P compensation pulses of which are also shown in dark gray) in the immediately-preceding reference frame, F.sub.0, (see, time interval C in FIG. 10) rather than with respect to the leading edge of the F.sub.1 frame boundary as in Adrian. This allows the N compensation pulse to overlap the F.sub.1 frame boundary, so that at the same modulation index as in FIG. 5, the N compensation pulses can still be developed. This increases the maximum modulation index that can be achieved before the N compensation pulses are dropped, thereby delaying the onset of an increase in distortion. As can be seen in this example, the C time interval function (see, above) represents the number of clock cycles from the trailing edge of the P signal pulse in F.sub.0 until the leading edge of the first P signal pulse in F.sub.1, but not including the P compensation pulse; this clearly illustrates how my method allows modulation to be performed as a function of at least two frames. Further, the second compensated composite waveform 30 in FIG. 10 may be developed as a function of boundary FO of the reference frame, which is not a boundary of the current frame F1. In this embodiment, since the start of compensated composite waveform 30 is timed with respect to boundary FO, it is now possible for the P compensation pulse itself to overlap boundary F1.

(22) FIG. 11 illustrates an example of a pulse train developed by my method in a negative modulation, high signal amplitude mode of operation. Note that, in this example, the P compensation pulse for F.sub.0 actually crosses the frame boundary between F.sub.0 and F.sub.1.

(23) FIG. 12 illustrates an example of a pulse train developed by my method in a positive-to-negative zero crossing mode of operation. Note that, in this example, the pulse timing calculations are functions solely of events in the current frame, F.sub.1, without reference to any previous reference frame.

(24) Although I have described my invention in the context of particular embodiments, one of ordinary skill in this art will readily realize that many modifications may be made in such embodiments to adapt either to specific implementations. Thus it is apparent that I have provided a ternary pulse width modulation method and apparatus that are both effective and efficient. Further, I submit that my method and apparatus provide performance generally superior to the best prior art techniques.