Method and apparatus for spectrum spreading of a pulse-density modulated waveform

Abstract

Methods and systems are provided for spreading spectral density of digital-to-analog conversion output signals. A spreading circuit may spread a digital-to-analog converter (DAC) output signal over a particular frequency spectrum, with the spreading circuit receiving the DAC output signal; generating a plurality of internal control signals; and generating based on the DAC output signal and the one or more internal control signal a corresponding spread output signal. The Internal control signals may comprise at least a first control signal, generated based on sequences meeting at least one particular criterion, a second control signal, generated based on a feedback corresponding to an intermediate output generated within the spreading circuit. The spreading circuit may generate the first control signal based on zero-sum sequences. The spreading circuit may generate a stream of pulses based on the intermediate output, and may generate the feedback signal based on the stream of pulses.

Claims

1. A system comprising: a digital-to-analog converter (DAC) circuit that generates an output signal; and a spreading circuit that spreads the DAC output signal over a particular frequency spectrum, wherein the spreading circuit: receives the DAC output signal; generates a plurality of internal control signals, wherein; the spreading circuit generates at least a first control signal based on sequences meeting at least one particular criterion; and the spreading circuit generates a second control signal based on a feedback corresponding to an intermediate output generated within the spreading circuit; and generates based on the DAC output signal and the plurality of internal control signals, a corresponding spread output signal.

2. The system of claim 1, wherein generating the second control signal comprises applying an adjustment to the intermediate output.

3. The system of claim 1, wherein the spreading circuit sets the second control signal based on applying of inversion to the intermediate output.

4. The system of claim 1, wherein the spreading circuit generates a stream of pulses based on the intermediate output; and generates the feedback signal based on the stream of pulses.

5. The system of claim 4, wherein the spreading circuit sets a number of pulses in the stream of pulses to equal to a number of pulses in the DAC output signal.

6. The system of claim 4, wherein the spreading circuit sets the second control signal based on comparing of a value corresponding to the intermediate output with a pre-defined value.

7. The system of claim 1, wherein generating the second control signal comprises applying a delay.

8. The system of claim 7, wherein applying the delay is synchronized to the generating of the first control signal.

9. The system of claim 8, wherein applying the delay and generating the first control signal are synchronized to the DAC output signal.

10. The system of claim 1, wherein the spreading circuit generates the first control signal based on zero-sum sequences.

11. The system of claim 10, wherein the zero-sum sequences comprise sequences of 1s and 1s, each zero-sum sequence having an equal number of 1s and 1s.

12. The system of claim 10, wherein the zero-sum sequences comprise four bit long sequences.

13. The system of claim 1, wherein the spreading circuit generates the first control signal based on randomly generated number.

14. The system of claim 13, wherein the spreading circuit generates the randomly generated number based on linear feedback shifting.

15. The system of claim 13, wherein generating the first control signal is synchronized to the DAC output signal and generating the randomly number is synchronized to a clock set based on a frequency of the DAC output signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosed method and apparatus, in accordance with one or more various embodiments, is described with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict examples of some embodiments of the disclosed method and apparatus. These drawings are provided to facilitate the reader's understanding of the disclosed method and apparatus. They should not be considered to limit the breadth, scope, or applicability of the claimed invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

(2) FIG. 1 is a simplified block diagram of a prior art pulse width modulation (PWM) digital to analog converter (DAC).

(3) FIG. 2 is a timing diagram of the signals associated with the prior art PWM DAC of FIG. 1.

(4) FIG. 3 is a simplified block diagram of a prior art pulse density modulation (PDM) DAC.

(5) FIG. 4 is an illustration of the relationship between the input and output of a prior art bit reversal module.

(6) FIG. 5 is a timing diagram of the signals associated with the prior art PWM DAC of FIG. 3.

(7) FIG. 6 is a simplified schematic of a spreading circuit in accordance with one embodiment of the presently disclosed method and apparatus.

(8) FIG. 7 is a table of the values applied to the input ports to the accumulator of FIG. 6 and at the output of the spreading circuit of FIG. 6.

(9) FIG. 8 is an illustration of the spectral density output of the spreading system of FIG. 6 compared to the spectral density of the PDM of FIG. 3.

(10) The figures are not intended to be exhaustive or to limit the claimed invention to the precise form disclosed. It should be understood that the disclosed method and apparatus can be practiced with modification and alteration, and that the invention should be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION

(11) FIG. 6 is a simplified schematic of a spreading circuit 600 in accordance with one embodiment of the presently disclosed method and apparatus. The spreading circuit 600 receives a stream of PDM pulses 602 from a source, such as the PDM DAC shown in FIG. 300. It should be understood that the PDM pulse stream can come from any source. The PDM pulses 602 are a series of pulses representing the analog value of a digital input. It should be noted that the output filter 320 is used to smooth the pulses (integrate the output) to generate an analog signal. Therefore, the output from the PDM DAC is coupled to the spreading circuit prior to being filtered. The spreading circuit includes: a random source, such as a linear feedback shift register (LFSR); a Zero-Sum Sequence Register 606; an accumulator 604, such as a summing module; an accumulation register 608; a comparator 610; and an inversion module 612.

(12) The PDM pulse stream 602 is received and coupled to the first of four input ports to the accumulator 604. Input port 2 to the accumulator is coupled to the Zero-Sum Sequence Register 606. The Zero-Sum Sequence Register 606 provides a stream of bits that can have values of either 1 or 1, and which, when summed together, equal zero (i.e., an equal number of 1s and 1s). For example, a four value wide Zero-Sum Sequence Register 606 could have the following sets of values (1, 1, 1, 1); (1, 1, 1, 1); (1, 1, 1, 1); and (1, 1, 1, 1) stored therein. As noted above, each of the four values in each set sums to zero. Furthermore, as noted by the fact that the values can be either 1 or 1, the output from the Zero-Sum Sequence Register 606 is a signed value, such as a 2-bit signed two's complement output. However, any set of bits that can represent the values of 1 and 1 can be output by the Zero-Sum Sequence Register 606.

(13) The output port of the accumulator 604 is coupled to the Accumulator Register 608. The value output from the Accumulator 604 is a 3-bit signed value. The output port of the Accumulator Register 608 is coupled back around to input port 3 of the Accumulator 604. The Accumulator Register 608 stores the value output from the accumulator 604. In accordance with one embodiment of the presently disclosed method and apparatus, the clock signal 311 generated by the clock 317 of the PDM circuit 300 (see FIG. 3) is coupled to a clock input port of the Accumulator Register 608. The clock signal 311 synchronizes the output of the Accumulator Register 608 with the incoming PDM bit stream rate. More particularly, the clock signal 311 to the Accumulator Register 608 clocks the value output from the accumulator 604 into the Accumulator Register 608 each time a new bit is presented at the input port to the accumulator 604 by the incoming PDM bit stream 602.

(14) In one embodiment of the disclosed method and apparatus, a delay is created between the time each bit of the PDM bit stream is received at the first input of the accumulator 604 and the time the output of the accumulator 604 is clocked through to the output port of the accumulator register 608. In one such embodiment, the delay is created by having the value at input port 1 to the Accumulator 604 change on the rising edge of the clock signal 311. The input to the Accumulator Register 608 is clocked through to the output port of the Accumulator Register 608 on the falling edge of the clock signal 311. The Accumulator Register 608 holds that value until the next falling edge of the clock signal 311. Accordingly, all of the input signals at the input ports to the Accumulator 604 will be stable when the value is clocked through (e.g., during the falling edge of the clock signal 311). In accordance with one embodiment of the present invention, the clock signal 311 is output directly from the clock 317 of the PDM circuit 300. Alternatively, the clock signal coupled to clock input port of the Accumulator Register 608 is derived from and synchronized to the output of the clock 317. In one such embodiment, the clock signal may be offset in phase from the clock signal 311. In yet another embodiment, the clock signal may be filtered or otherwise processed to provide edges that are more appropriate to the spreading circuit 600.

(15) In one embodiment of the disclosed method and apparatus, the Accumulator Register 608 is capable of storing 3-bit signed values that range from 4 to 3. However, in one such embodiment, the values that are output from the accumulator 604 will only be in the range of 2 to 2, as will be seen from some examples of the operation of the spreading circuit provided below.

(16) The output port from the Accumulator Register 608 is also coupled to the input port of the comparator 610. The comparator 610 outputs a value of 1 for input values greater than zero. All other values will output a zero. Accordingly, values of 2, 1 and zero output from the Accumulator Register 608 will all cause the output port of the comparator 610 to output a zero. The output port from the comparator 610 is coupled to the output port 618 of the spreading circuit 600 and also to the input port of an inverter 612. The inverter 612 will cause a 1 to be output when a 1 is presented at its input. A zero input to the inverter 612 will result in a zero output. Since the comparator 610 only outputs either a zero or a 1, the output from the inverter 612 is constrained to the values 1 and zero. The output from the inverter 612 is coupled to input port 4 to the accumulator 604.

(17) In one embodiment, the Zero-Sum Sequence Register 606 selects between the four possible zero-sum sequences based on the input from a 2-bit random source, such as the LFSR shown in FIG. 6. Any 2-bit random or pseudo random source can be used to randomize the order in which the sequences are selected from the Zero-Sum Sequence Register 606. The LFSR 614 is clocked by the clock signal 311 divided by 4. A divider 616 divides the clock signal. Accordingly, the LFSR 614 runs at one fourth the frequency of the signal to be spread. Therefore, for every four input bits of the PDM stream 602, the LFSR will select a new zero-sum sequence from the Zero-Sum Sequence Register 606.

(18) FIG. 7 is a table of the values applied to the input ports to the accumulator 604, the sum at the output port of the Accumulator 604 and the values that appear at the output port 618 of the spreading circuit 600. The PDM pulse stream input 602 comprises bits having a value of either zero or 1. FIG. 7 illustrates that a PDM stream having a value of 0, 1, 1, 0, 0, 1, 0, 1 is provided to input port 1 of the accumulator 604 over the time period to through t.sub.7. A first zero-sum sequence output from the Register 606 is (1, 1, 1, 1). This bit sequence is clocked out of the Zero-Sum Sequence Register 606 one bit at a time as controlled by the clock signal 311. These bits are coupled to accumulator input port 2 at times t.sub.0, t.sub.1, t.sub.2, and t.sub.3 respectively, as illustrated in FIG. 7. A second zero-sum sequence output from the Register 606 (1, 1, 1, 1) is coupled to input port 2 at times t.sub.4, t.sub.5, t.sub.6 and t.sub.7 respectively. It should be noted that these sequences are selected randomly by the value generated by the LFSR 614.

(19) The Accumulator Register 608 is initialized to zero at time t.sub.0. Accordingly, the output from the Accumulator Register 608 will be zero until another value is presented to the input of the Accumulator Register 608 and that value is clocked through to the output port of the Accumulator Register 608. Therefore, input port 3 to the accumulator 604 is zero at t.sub.0. In addition, initializing the value of the Accumulator Register 608 to zero causes the value at time t.sub.0 at the output port of the inverter 610 to be zero and thus, input port 4 to the accumulator to be zero. Thus, the output of the spreading circuit 600 is zero at t.sub.0. The value output from the accumulator 604 is the sum of these value at time t.sub.0. Therefore, the sum of the four values at t.sub.0 is 1 at the output port of the Accumulator 604. This value will sit at the input port to the Accumulator Register 608 until clocked through to the output port at time t.sub.1.

(20) Once clocked through by the clock signal 311 at t.sub.1, the 1 value is coupled to input port 3 to the Accumulator 604. This value is also coupled to the input port to the comparator 610. Since this value is not greater than zero, the output from the comparator 610 remains zero at t.sub.1. Likewise, the output of the spreading circuit 600 remains at zero at time t.sub.1. Accordingly, the output from the inverter 612 coupled to input port 4 of the Accumulator 604 remains zero at t.sub.1. The bit coupled from the PDM stream 602 to input port 1 of the accumulator is 1 at time t.sub.1. The second bit output from the Zero-Sum Sequence Register 606 is coupled to input port 2 to the Accumulator 604. That value is a 1 at time t.sub.1. Therefore, the sum of the four input ports to the Accumulator 604 is 1 at t.sub.1. This is coupled to the Accumulator Register 608.

(21) At t.sub.2, the value at the input port of the Accumulator Register 608 is clocked through to the output port of the Accumulator Register 608. Accordingly, the value at input port 3 to the Accumulator 604 at t.sub.2 is 1. Since this value is now greater than zero, the output from the comparator 610 is 1. Therefore, at t.sub.2, the output of the spreading circuit 600 will be 1. The output from the inverter is then a 1, which is coupled to input port 4 of the accumulator at t.sub.2. The PDM stream 602 applied to port 1 of the accumulator has a value of 1 at t.sub.2 and the next value of the zero-sum sequence applied to port 2 of the accumulator at t.sub.2 is 1. Therefore, the sum at the output port of the accumulator 604 is zero at t.sub.2.

(22) At t.sub.3, the output from the Accumulator 604 is clocked through to the output port of the Accumulator Register 608. Therefore, the value at input port 3 to the Accumulator 604 is zero. Also, the output from the comparator 610 is zero. The output port of the comparator 610 is coupled to the output port 618 of the spreading circuit 600. Accordingly, a zero is output from the spreading circuit 600. This value is then also applied to the inverter 612, which then outputs a zero. The zero is coupled to input port 4 of the accumulator at t.sub.3. The value at input port 1 to the Accumulator 604 is zero at t.sub.3. The value at input port 2 to the Accumulator 604 is 1. Therefore, the sum of the four input ports at t.sub.3, is 1. This value is then output from the Accumulator 604 and coupled to the input port of the Accumulator Register 608.

(23) At t.sub.4, this value is clocked through to the output port of the Accumulator Register 608. When, at t.sub.4 the output of the Accumulator Register 608 goes to 1, the comparator 610 outputs a 1 as the output of the spreading circuit 600. In turn, the inverter 612 output a 1 at t.sub.4. The sum of the signals at the input ports to the Accumulator 604 at t.sub.4 is 1, which is then applied to the input port of the Accumulator Register 608.

(24) At t.sub.5, this value is clocked through to the output port of the Accumulator Register 608. When, at t.sub.5 the output of the Accumulator Register 608 goes to 1, the comparator 610 outputs a 1 as the output of the spreading circuit 600. In turn, the inverter 612 output a 1 at t.sub.5. The sum of the signals at the input ports to the Accumulator 604 at t.sub.5 is zero, which is then applied to the input port of the Accumulator Register 608.

(25) At t.sub.6, this value is clocked through to the output port of the Accumulator Register 608. When, at t.sub.6 the output of the Accumulator Register 608 goes to zero, the comparator 610 outputs a zero as the output of the spreading circuit 600. In turn, the output of the inverter 612 is zero at t.sub.6. The sum of the input signals to the Accumulator 604 at t.sub.6 is 1, which is then applied to the input port of the Accumulator Register 608.

(26) At t.sub.7, this value is clocked through to the output port of the Accumulator Register 608. When, at t.sub.7 the output of the Accumulator Register 608 goes to 1, the comparator 610 outputs a 1 as the output of the spreading circuit 600. In turn, the inverter 612 outputs a 1 at t.sub.7. This process continues on in similar fashion.

(27) It will be noted that the signal output from the output port 618 of the spreading circuit 600 is a sequence of 0, 0, 1, 0, 1, 1, 0, 1. It should be further noted that the number of pulses output (i.e., 1s output) is equal to the number of pulses applied to input port 1 of the accumulator 604. This will be the case for any sequence of input pulses assuming a valid zero-sum sequence is selected from the Zero-Sum Sequence Register 606. This process would then repeat for each randomly selected zero sum sequence, causing the sequence of pulses output by the spreading circuit 600 to vary in response to the randomly selected zero-sum sequences. This, in turn, will disrupt the otherwise periodic nature of the PDM pulse stream. Thus, the energy output from the spreading circuit 600 will spread in the frequency domain.

(28) FIG. 8 is an illustration of the spectral density 802 output of the spreading system compared to the spectral density 804 of the PDM. This data is plotted for a value stored in the Pulse Density Register 306 of 131907 (see FIG. 3) and a value of 262144 stored in the Max Value Register 308 (see FIG. 3). The clock signal 311 (see FIG. 3) was set to a clock rate of 50 MHz. Note the very large tone and harmonic power near 25 MHz for the PDM output 804. In contrast, the power is effectively spread over the band by the spreading system in the output 802 of the spreading circuit 600. Note also that near 0 Hz, the spread system has approximately the same amount of noise, so that the low-pass filtered outputs of the PDM output 804 and the spreading circuit output 802 are very similar.

(29) Those skilled in the art will appreciate that the math can be extended to other embodiments. For example, the number of bits in the LFSR 614 can be increased to 3 to allow one of 8 eight-bit zero sum sequences to be selected from the Zero-Sum Sequence Register 606.

(30) Although the disclosed method and apparatus is described above in terms of various examples of embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described. Thus, the breadth and scope of the claimed invention should not be limited by any of the examples noted above. For example, it will be clear to those skilled in the art that values other than those disclosed above stored in the Zero-Sum Sequence Register 606 can be used as long as the sum of each sequence is zero. In addition, the function performed by the LFSR 614 can be implemented by any random number generator using any technique for generating a random (or pseudo-random) sequence. It should be understood that the more randomly the sequence, the more even the spreading. Still further, the functions of each of the elements of the spreading circuit 600 can be implemented using discrete functions or a programmable module that performs some or all of the functions, a state machine that performs some or all of the functions, or any other means for implementing the functions described above.

(31) Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term including should be read as meaning including, without limitation or the like; the term example is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; the terms a or an should be read as meaning at least one, one or more or the like; and adjectives such as conventional, traditional, normal, standard, known and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future.

(32) A group of items linked with the conjunction and should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as and/or unless expressly stated otherwise. Similarly, a group of items linked with the conjunction or should not be read as requiring mutual exclusivity among that group, but rather should also be read as and/or unless expressly stated otherwise. Furthermore, although items, elements or components of the disclosed method and apparatus may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated.

(33) The presence of broadening words and phrases such as one or more, at least, but not limited to or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term module does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations.

(34) Additionally, the various embodiments set forth herein are described in terms of exemplary block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.