Method and apparatus for generating pulse width modulated waveforms

Pulse width modulation of a digital signal using a nonlinear type of pulse generator is described. The nonlinear pulse generator is characterized by its transfer characteristic which has alternating stable and unstable operating regions. The pulse width modulated signal is generated by varying the operating point between among the unstable and stable operating regions.

This application is related to commonly-owned U.S. Pat. No. 6,259,390. This application is further related to the following commonly owned and co-pending applications: U.S. application Ser. No. 09/805,845, filed Mar. 13, 2001, entitled “Circuitry with Resistive Input Impedance for Generating Pulses From Analog Waveforms” and U.S. application Ser. No. 09/839,810, filed Apr. 19, 2001, entitled “Method and Apparatus for Generating Pulses Using Dynamic Transfer Function Characteristics,” both of which are herein incorporated by reference for all purposes.

NOT APPLICABLE

NOT APPLICABLE

BACKGROUND OF THE INVENTION

This invention relates generally to signal modulation and more specifically to the generation of pulse-width-modulated signals.

In a communication system, data is typically transmitted in the form of modulated signals. Pulse Width Modulation (PWM) is an important category of modulation techniques. PWM is based on the use of a pulsed signal in which pulses of varying width represent different data being transmitted. Pulse width modulated signals also have widespread applications in many sectors of technology other than communications. For example, servo motor controllers, DC/AC converters, switching power supplies and dimming circuits are just some of the applications in which pulse width modulated signals can be used.

The basic blocks include a modulating signal source which is a sine wave generator and a free running sawtooth generator. The frequency of the sawtooth signal is several times higher than that of the sine wave and it is usually derived from a very stable frequency oscillator. The pulse width modulated signal is generated by comparing the amplitude of the sine wave and the sawtooth wave using a high gain voltage comparator.

FIG. 9is a block diagram of a typical prior art PWM generating circuit900. The basic components include a modulating signal source902which represents the signal to be transmitted by PWM, and a free running sawtooth generator904. The frequency of the sawtooth signal912produced by the sawtooth generator is several times that of the sine wave signal914. The sawtooth signal is usually derived from a very stable frequency oscillator. The pulse width modulated signal910is generated by comparing the amplitude of the sine wave and the sawtooth wave using a high gain voltage comparator906.

FIGS. 10A–10Cillustrate an analog input waveform1002applied to a conventional technique for generating PWM signals. The analog input waveform1002represents the data to be transmitted.FIG. 10Billustrates a sawtooth waveform1004generated by a free running sawtooth generator.FIG. 10Cillustrates a pulse width modulated signal1006generated by comparing the amplitude of the analog input waveform1002and the sawtooth waveform1004, according to this conventional technique. It can be seen inFIG. 10Bthat when the amplitude of the analog input waveform1002exceeds the amplitude of the sawtooth waveform1004, the comparator906outputs a high level signal shown inFIG. 10C. Otherwise, the comparator outputs a low level signal. Note that a duty cycle can be defined as the ratio of the pulse width (TW) to the pulse period (TP) shown inFIG. 10C.

This technique faces a number of difficulties in implementation. For example, the free running sawtooth generator904must be accurately controlled to operate at a frequency several times higher than that of the analog input wavefonn wave1002. Also, the average amplitude of both the input waveform1002and the sawtooth waveform1004must be carefully matched. If there is a significant mismatch, the comparator output could be diminished due to a resulting DC component.

Conventional techniques also exist for generating PWM signals where the data to be transmitted is digital data. Typically, such techniques use an oversampling clock in connection with a counter and/or appropriate combinatorial logic to generate a PWM signal. However, the accuracy of the PWM signal is directly dependent upon the oversampling rate provided by oversampling clock. The requirement for an oversampling clock of sufficiently high rate significantly increases the cost of devices implementing such techniques.

U.S. Pat. No. 5,789,992 describes a PWM method using purely digital logic circuit. The method basically generates a series of PWM component signals. These component signals will be operated with the digital word input using AND and OR logic operations to produce a PWM signal that corresponds to the digital word. It can be appreciated that this method requires an oversampling clock in order to produce a sufficiently accurate PWM signal that is useful.

U.S. Pat. No. 6,044,113 describes a circuit and method for generating digital PWM using an oversampling clock signal and Voltage-to-Frequency Conversion (VFC). The VFC converts an analog input signal to produce a digital word. This digital word, along with the oversampling clock, is provided to a counter to generate a PWM signal that is proportional to the digital word.

Clearly, it can be seen that there is room for improvement over prior art PWM techniques.

BRIEF SUMMARY OF THE INVENTION

A method and apparatus for modulating a digital data stream to produce a pulse width modulated signal includes producing a first intermediate signal based on the digital data stream. The intermediate signal is applied to a nonlinear circuit. In response, the nonlinear circuit produces a pulse width modulated signal representative of the digital data stream.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a generalized high level functional block diagram of an illustrative embodiment of a pulse width modulated (PWM) signal generating device in accordance with the present invention. As shown in the figure, a digital input103is provided to an input of a conversion block100. The input comprises a digital data stream of single bits (M=1) or M-bit data. A pulse signal101is provided to another input of the conversion block100. The pulse can be synchronized with the clocking of the M-bit digital input. In one embodiment, the pulse101can be derived from a clock signal (not shown), or it can be the clock signal itself. The conversion block outputs an intermediate signal110to serve as a control signal to operate a nonlinear pulse generator circuit106. The nonlinear circuit outputs a PWM signal108.

As shown inFIG. 1, the conversion block100contains a switch structure102. The switch structure102receives the digital input103at D1to DMand the pulse signal101. The pulse signal is switched to one of a number of outputs S1to SNbased on the M-bit data. The outputs of the switch structure feed into an amplifier module104having an input port G1to GNfor each of the switch outputs. An output of the amplifier module constitutes the output110of the conversion block.

From the description to follow, it will be appreciated that the conversion block100can be implemented by any of a variety of known, well-understood, conventional digital design methodologies. The conversion block might be appropriately implemented using a digital signal processing (DSP) architecture, any of a variety of field programmable gate array (FPGA) architectures and the like, or even analog circuits and combinations of analog and digital circuitry. The specific implementation will depend on factors not relevant to the practice of the present invention.

FIG. 2shows an illustrative example of the amplifier104. In this particular example, the amplifier comprises a summing amplifier configuration. Each input port Gnprovides a gain determined by

Gn=RfRn,
where n=1, 2, 3, . . . N. The active component200can be a conventional op-amp (e.g., LM7171). In this particular implementation, the op-amp is biased with a +15 V and −15 V DC supply. The components value are Rf=510 Ω, R1=680 Ω, R2=100 Ω, R3=20 Ω, and R4=1 Ω. Of course, other appropriate gain settings and component values can be used. An output202of the amplifier104feeds into the pulse generator circuit106.

FIGS. 3 and 3Ashow respectively a circuit30and a corresponding transfer function provided in accordance with an embodiment of the present invention. The circuit30is an example of an implementation of the pulse generator106shown inFIG. 1. The circuit can be constructed with an op-amp300and the circuit components shown. The op-amp used in this particular implementation is an LM7171. The DC voltages applied to the op-amp are VCC=4.25 V and VDD=−1.5 V. The components values are L=10 mH, RA=1 kΩ, RB=56 Ω, and RC=220 Ω. The operation of the circuit is explained below. Additional discussion is provided in commonly owned U.S. Pat. No. 6,259,390.

FIG. 3Ashows a transfer function, I=Ψ(V), of the circuit30. For the purposes of the present invention, the “transfer function” (characteristic) of a circuit refers to the relationship between any two state variables of a circuit. Electronic circuits are typically characterized by their I-V curves, relating the two state variables of current and voltage. Such curves indicate how one state variable (e.g., current) changes as the other state variable (voltage) varies. As can be seen inFIG. 3A, the transfer function for the circuit30includes a portion which lies within a region303, referred to herein as an “unstable” region. The unstable region is bounded on either side by regions301and305, each of which is herein referred to as a “stable” region.

The circuit30has an associated “operating point”314which is a location on the transfer function312. The nature of the output of the circuit depends on the location of its operating point. If the operating point is positioned along the portion of the transfer function that lies within region303, the output of the circuit will exhibit an oscillatory behavior. It is for this reason that the region303is referred to as an unstable operating region. If the operating point is positioned along the portions of the transfer function that lie within either of regions301and305, the output of the circuit will exhibit a generally time-varying but otherwise non-oscillatory behavior. It is for this reason that regions301and305are referred to as stable operating regions.

The operating point314of the circuit is a function of the signal supplied to the input of the circuit30.FIG. 3shows such a control signal332, having a first region332aand a second region332b.A line V=VSis shown inFIG. 3ato illustrate the relation of the amplitude of the control signal332to the transfer function312. The intersection of line V=VSwith the transfer function sets the operating point314of the circuit30. Thus, as the control signal amplitude varies between amplitudes VS1and VS2, it can be seen that the operating point of the circuit30moves between its stable and unstable operating regions, with corresponding changes in the behavior of the circuit output.

FIG. 4shows a particular implementation of the embodiment of the invention shown inFIG. 1. The amplifier104is the example implementation shown inFIG. 2. The pulse generator106is the circuit30shown inFIG. 3.

The particular switch structure102shown inFIG. 4is a one-to-four multiplexer402, can be constructed using commercially available logic blocks, or from basic logic gates. The multiplexer has four output ports labeled as S1to S4. This multiplexer is controlled digitally by 2-bit word of binary input through ports D1and D2. This pulse101is synchronized with the clocking in of the 2-bit binary word. The combination of this 2-bit word will determine the internal connection between port I to one of the output ports (S1to S4).

The output ports S1to S4are connected to the summing amplifier104. As noted earlier, the summing amplifier provides a gain port G1to G4for each signal from the switch402. For example, the summing amplifier will amplify a signal from S1with a gain of G1, S2with a gain of G2, etc. Since only one S-port contains the pulse from the port I for any 2-bit word, a pulse with given amplitude will be generated at the output of the summing amplifier. Hence, this particular combination of multiplexer and summing amplifier operates to convert the incoming pulse101to a pulse whose amplitude depends on a two-bit digital word. Thus, a data stream can be divided into M-bit data portions which are then represented by a pulse of corresponding amplitude.FIG. 5shows a table, summarizing this mapping of a data stream into an intermediate signal202comprising signal portions represented by pulses of varying amplitude.

The discussion will now turn to two approaches for producing PWM signals in accordance with the present invention.

PWM Using Nonlinear Pulse Generator's Unstable Region

Referring toFIGS. 4 and 3A, if a pulse synchronized with the digital word (D2D1) is applied to port I, it will be amplified with a gain depending on the digital word combination. For example, if the D2D1=00, the pulse will be outputted at port S1(seeFIG. 5). The summing amplifier will amplify the pulse by a factor of −Rf/R1. The output of the summing amplifier, which is fed to the nonlinear pulse generator30as VS, will determine the position of the operating point along the unstable region303, thus controlling the operation of the generator. In this example, if R1>R2>R3>R4, then the value of these resistors can be selected such that when a pulse is applied at port I, the operating point is moved to the position P, Q, R, S inFIG. 3Awhen D2D1combinations are 00, 01, 10, 11, respectively.

FIG. 6shows some of the traces obtained from the circuit configuration inFIG. 4, using the unstable region to a generate pulse width modulated signal. Trace601is the input pulse101applied to input port I inFIG. 4. The pulse has a period TPand has a width TW(TP<TW). As noted above, the input pulse is inverted because of the fact that the summing amplifier will provide the inversion and a gain factor to the pulse. Trace603is obtained at the output of summing amplifier which will be used as an input to the nonlinear pulse generator.

Trace603shows that each combination of D2D1has been input to the switch402to produce pulses603a–603d.This is achieved by controlling the multiplexer using digital word input D2D1to direct the input pulse to the desired output port S. The summing amplifier will provide a different gain to different ports. For example, D2D1is 00 when the first pulse601aof trace601enters the multiplexer. FromFIG. 5, the input pulse is outputted at port S1, and hence this pulse is amplified by a factor of −Rf/R1. When the second pulse601bof trace601enters the multiplexer, D2D1is 01. This pulse is outputted at port S2and amplified by a factor of −Rf/R1. For the third and the fourth pulse of trace601, D2D1is 10 and 11, respectively.

The trace amplitude of the pulses in trace603are provided to the circuit ofFIG. 3as VSto determine the operating point position in the unstable region303as shown inFIG. 3A. The larger the amplitude of the pulse, the further away the operating point from A but closer to B. Trace605shows the output response, as pulses605a–605d,of the nonlinear pulse generator. As can be seen, the pulse width of the fourth pulse605dis much wider than that of the first pulse605a.This is expected because the operating point for the fourth pulse is closer to B and the operating point for the first pulse is closer to A. Trace607is a result from passing the output of the nonlinear pulse generator (trace605) to a comparator (not shown). This might be desirable if the pulses produced at605were not sufficiently square. Passing the signal through a typical comparator circuit would produce square pulses.

It is noted that the width of the pulse101that is applied to port I of the switch402(FIG. 4) can be adjusted accordingly so that when the circuit30is driven into the unstable operating region to a position P-S on the transfer curve312, the output of the circuit produces a single pulse. For example, this determination can be readily made by simply adjusting the width of pulse101and watching the resulting output (e.g., trace605) of the circuit30.

It is further noted that the rising edge or falling edge of the pulse generated is not synchronized to either the rising or the falling edge of the input pulse (601,FIG. 6). However, it is still within the input pulse period TP. If such synchronization with the input pulse is desirable, then the second method described below can be used.

PWM Using Nonlinear Pulse Generator's Stable Region

The first and second stable regions301and305are shown inFIG. 3A. The first stable region, as in the first method, is used to rest the operating point314of the circuit30when a pulse is not present. When a pulse is present, the operating point will be moved to the second stable region305. As can be seen inFIG. 3A, this can be achieved by properly adjusting the amplitude of the pulses produced by amplifier104; in particular adjusting the gain values G1–GN.

In the second stable region305, the further away the operating point from B, the wider the pulse generated at the output of the nonlinear pulse generator. The reason is that the trajectory time spent in the second stable region is longer when operating point is moved further away from B. This can be observed in the traces shown inFIG. 7. Thus, the circuit30is alternately operated between a first stable region (e.g.,301) and a second stable region (e.g.,305), wherein the traversal of the operating point between the two stable regions results in the circuit producing a pulse output.

Referring toFIGS. 4 and 3A, if a pulse synchronized with the digital control bit (D2D1) is applied to port I, it will be amplified with a gain depending on the control bit combination. For example, if the D2D1=00, the pulse will be outputted at port S1(seeFIG. 5). The summing amplifier will amplify the pulse by a factor of −Rf/R1. The output of the summing amplifier, which is the same as VSof the nonlinear pulse generator, will determine the position of the operating point along the second stable region305. In this example, if R1>R2>R3>R4, then the value of these resistors can be selected such that when a pulse is applied at port I, the operating point is moved to the position W, X, Y, Z when D2D1combinations are 00, 01, 10, 11, respectively.

FIG. 7shows traces obtained from circuit configuration inFIG. 4using the second stable region305to generate the pulse width modulated signal. Trace701is the input pulse applied to input port D inFIG. 4. The pulse has a period TPand has a width TW(TP<TW). The input pulse in this case is inverted because of the fact that the summing amplifier will provide the inversion and a gain factor to the pulse.

Trace703is obtained at the output of summing amplifier which will be used as an input to the nonlinear pulse generator. Note that the input pulse has been inverted and gone through different amplification factor. This is achieved by controlling the multiplexer using digital input D2D1to direct the input pulse to the desired output port S. The summing amplifier will provide a different gain to different port. For example, D2D1is 00 when the first pulse of trace701enters the multiplexer. FromFIG. 5, the input pulse is outputted at port S1, and hence this pulse is amplified by a factor of −Rf/R1. When the second pulse of trace701enters the multiplexer, D2D1is 01. This pulse is outputted at port S2and amplified by a factor of −Rf/ R2. For the third and the fourth pulse of trace701, D2D1is 10 and 11 respectively.

Trace amplitude of the pulses in trace703will determine the operating point position (W, X, Y, Z) in the second stable region305as shown inFIG. 3A. The larger the amplitude of the pulse, the further away the operating point from B. Trace705shows the output response of the nonlinear pulse generator. The pulse width of the fourth pulse in705is much wider than that of the first pulse. This is expected because the operating point for the fourth pulse is much further away from point B compared to that of the first pulse. Trace707is a result from passing the output of the nonlinear pulse generator (trace705) to a comparator (not shown). This can be performed if the particular application calls for additional shaping of the pulses705.

It is clearly shown inFIG. 7that the pulse width modulated signal generated using a second stable region of the nonlinear pulse generator is synchronized with the input signal. In this particular example, the rising edge of the pulse width modulated signal is aligned with the falling edge of the input pulse.

In this particular embodiments discussed above, the nonlinear circuit has a transfer function having an N-Shape I-V characteristic. As discussed above, the parameter that moves the operating point314is the signal amplitude. It can be appreciated from commonly owned U.S. Pat. No. 6,259,390, that alternative circuit constructions are possible. For example, a circuit with an S-Shape I-V characteristic can also be used to generate PWM signal. In this alternative embodiment, however, the waveform used at the input of the multiplexer would be changed from pulse type waveform to a triangular type waveform. Thus, the M-bit segments of the digital data stream would be mapped in a similar way to corresponding signal portions comprising triangular waveforms having varying slope, where the slope of the waveform is the parameter which controls operation of the circuit.

An advantage of the present invention is it does not need the use of sawtooth wave generator. The dependence on the external clock as the main source of input is not required either. The illustrative embodiments of the invention discussed typically require a simple multiplexer (or similar switching arrangement) and conventional op-amps which are inexpensive and readily available. Memory size, processing speed and resolution problems will not be an issue in this invention since analog ICs are used. Furthermore, in accordance with the illustrative embodiments, the pulse width of the PWM signal can easily be tuned by selecting different resistor values.