Modulation circuit with integrated microelectro-mechanical system (MEMS) components

A modulation circuit includes a microelectronic electromechanical system (MEMS) based resonant structure having a resonant frequency, an excitation input and an output. A control module is coupled to the excitation input of the MEMS based resonant structure. The control module modifies resonant characteristics of the MEMS based resonant structure to modulate the resonant frequency of the MEMS based resonant structure to produce a modulated signal at the output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to signal generator circuits, and, more particularly, to a modulation circuit configured with integrated microelectro-mechanical system (MEMS) components.

2. Description of the Related Art

Modern electronic devices often use microprocessors or other digital circuits which require one or more clock signals for synchronization. For example, a clock signal permits the precise timing of events in the microprocessor. Typical microprocessors may be supervised or synchronized by a free-running oscillator, such as driven by a crystal, an LC-tuned circuit, or an external clock source. Over the years, clocking rates have continued to increase, and currently clock rates in personal computes may exceed 2.0 gigahertz (GHz). As clock rates increase, the circuits generating and processing the clock signals are susceptible to generating and radiating electromagnetic interference (EMI). The spectral components of the EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of the clock circuit.

In order to comply with such government limits on EMI emissions, spread spectrum clock generation (SSCG), such as that disclosed in U.S. Pat. No. 5,631,920, has been used to reduce EMI emissions. In summary, an SSCG circuit may include a clock pulse generator for generating a series of clock pulses, and a spread spectrum modulator for frequency modulating the clock pulse generator to broaden and flatten amplitudes of EMI spectral components which would otherwise be produced by the clock pulse generator. The spread spectrum modulator frequency modulates the clock pulses with specific profiles of frequency deviation versus the period of the profile. Currently, for example, the SSCG clock signal is generated on the system printed circuit board by either a discrete clock integrated circuit (IC), or by a phase lock loop (PLL) that is integrated into one or more application specific integrated circuits (ASICs).

SUMMARY OF THE INVENTION

The present invention provides a modulation circuit, such as for example, an SSCG, that is configured using integrated tunable micro electromechanical system (MEMS) components.

The invention, in one exemplary embodiment, is directed to a modulation circuit, including a microelectronic electromechanical system (MEMS) based resonant structure having a resonant frequency, an excitation input and an output. A control module is coupled to the excitation input of the MEMS based resonant structure. The control module modifies resonant characteristics of the MEMS based resonant structure to modulate the resonant frequency of the MEMS based resonant structure to produce a modulated signal at the output.

The invention, in another exemplary embodiment, is directed to a modulation circuit, including a microelectronic electromechanical system (MEMS) based resonant circuit having a resonant frequency and an output. A signal source is coupled to the MEMS based resonant circuit. The signal source applies an excitation signal to the MEMS based resonant circuit to modulate the resonant frequency of the MEMS based resonant circuit to produce a modulated signal at the output.

The invention, in another exemplary embodiment, is directed to a spread spectrum clock generation (SSCG) circuit including a MEMS based circuit having an output for supplying a spread spectrum clocking signal.

The invention, in another exemplary embodiment, is directed to a spread spectrum clock generation (SSCG) circuit. The SSCG circuit includes a resonant circuit having a resonant frequency and an output. The resonant circuit is formed using at least one microelectronic electromechanical system (MEMS) component. A device is coupled to the at least one MEMS component. The device applies an excitation signal to the at least one MEMS component to modulate the resonant frequency of the resonant circuit to produce a spread spectrum clocking signal at the output having a predetermined period and a predetermined frequency deviation profile.

An advantage of the present invention is using MEMS technology in an SSCG system provides lower system cost by reducing the number of parts required on a system printed circuit board.

Another advantage is that using MEMs technology may provide much higher clock frequencies than most current PLL circuits.

Another advantage of MEMs component usage, such as in a SSCG, is the reduction of the application specific integrated circuit (ASIC) die space compared to similar circuits using other component technologies, thereby allowing a greater number of features to be integrated into the ASIC.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and particularly toFIG. 1, there is shown an embodiment of a modulation circuit10in accordance with the present invention.

Modulation circuit10includes a microelectronic electromechanical system (MEMS) based resonant structure12having a resonant frequency, an excitation input14and an output16. The MEMS based resonant structure12is formed using at least one MEMS component, an in some embodiments, may be configured using only MEMS components, or may be formed using a combination of one or more MEMS components and other discrete components fabricated using other technologies.

Modulation circuit10further includes a control module18. Control module18modifies the resonant characteristics of MEMS based resonant structure12to modulate the resonant frequency of MEMS based resonant structure12to produce a modulated signal at output16. Control module18is coupled via a communications link20to excitation input14of MEMS based resonant structure12.

In the various embodiments of the present invention, communications link20may provide a mechanical coupling or an electrical coupling of control module18to MEMS based resonant structure12. Accordingly, excitation input14receives from control module18an excitation signal, wherein the excitation signal may be a mechanical excitation or an electrical excitation, depending upon the configuration of MEMS based resonant structure12and the type of coupling provided by communications link20. In its simplest form, for example, where communications link20provides a mechanical coupling, communications link20may represent a direct mechanical attachment of control module18to MEMS based resonant structure12. Where communications link20provides an electrical coupling, for example, communications link20may represent single or multiple conductors providing electrical attachment of control module18to MEMS based resonant structure12, which in turn may carry one or more analog or digital signals.

In one embodiment, for example, control module18serves as a signal source to provide an excitation signal to excitation input14of MEMS based resonant structure12, and more particularly, may be coupled to at least one MEMS component in MEMS based resonant structure12. Control module18applies the excitation signal, such as a periodic signal, to MEMS based resonant structure12to modulate the resonant frequency of MEMS based resonant structure12to produce a modulated signal at output16.

FIG. 2shows a graph illustrating an exemplary embodiment of a profile22of a modulated signal at output16that may be produced by application of the excitation signal supplied by control module18to MEMS based resonant structure12. In this example, profile22may be used for producing a spread spectrum modulated output signal at output16. The frequency modulation of the resonant frequency of MEMS based resonant structure12with the excitation signal supplied by control module18results in the modulated signal at output16having profile22, which reduces the spectral amplitude of the EMI components at each harmonic of the of the resonant frequency of MEMS based resonant structure12when compared to the spectrum of the same resonant frequency without modulation. Accordingly, in this example, modulation circuit10is a spread spectrum clock generation (SSCG) circuit and the output signal at output16is a spread spectrum clock signal suitable for use, for example, in an imaging machine (e.g., printer, all-in-one, copier, etc.) and computer applications.

The graph ofFIG. 2illustrates profile22as normalized frequency deviation versus time. In this example, the maximum deviation is illustrated as one unit, and is typically represented as a percentage of the base clock frequency. In this example, the frequency of the signal modulating the profile is 30 kHz. This maximum frequency deviation is desirably programmable with an upper limit of the maximum deviation being dependent, for example, on the application. For example, the upper limit of the maximum deviation may be in the range of about 0.1 percent to 5.0 percent, or higher, depending on the application, as would be readily understood by those skilled in the art. As would be also readily understood by those skilled in the art, a standard, non-modulated clock signal corresponding to the resonant frequency of MEMS based resonant structure12may be obtained by programming the maximum deviation to 0. In addition, those skilled in the art will recognize the shape of the profile may be varied from that of profile22, if desired. For example, by varying the amount of inflection of the profile22, a triangular or sinusoidal profile may be generated. Another example would be linear combinations of various profiles.

FIG. 3is a block diagram of one embodiment of modulation circuit10, including MEMS based resonant structure12and control module18. In this embodiment, MEMS based resonant structure12includes a tunable MEMS inductor26and a fixed capacitor28, which combine to form a MEMS based resonant tank circuit. Fixed capacitor28may be a MEMS capacitor or a standard discrete capacitor. Control module18may include, for example, a variable current source30and control logic32. Variable current source30supplies a modulation signal to excitation input14, which is an input to tunable MEMS inductor26. Control logic32is used to select the profile, such as for example the profile22shown inFIG. 2, of a modulated signal produced at output16. In this example, profile22may be used for producing a spread spectrum modulated output signal at output16. Control logic32may include, for example, a lookup table to determine the current settings associated with the modulation signal supplied to excitation input14to control the modulation of MEMS based resonant structure12.

FIG. 4is another embodiment of modulation circuit10, including MEMS based resonant structure12and control module18. In this embodiment, MEMS based resonant structure12includes a tunable MEMS capacitor34and a fixed MEMS inductor36, which combine to form a MEMS based resonant tank circuit. Control module18may include, for example, a variable voltage source38and control logic40. Variable current source30supplies a modulation signal to excitation input14, which is an input to tunable MEMS capacitor34. Control logic40is used to select the profile, such as for example the profile22shown inFIG. 2, of a signal that may be produced at output16. In this example, profile22may be used for producing a spread spectrum modulated output signal at output16. Control logic40may include, for example, a lookup table to determine the voltage settings associated with the modulation signal supplied to excitation input14to control the modulation of MEMS based resonant structure12.

FIG. 5is another embodiment of modulation circuit10, including MEMS based resonant structure12and control module18. In this embodiment, MEMS based resonant structure12includes a digitally tunable MEMS capacitor42and a fixed MEMS inductor44, which combine to form a MEMS based resonant tank circuit. Digitally tunable MEMS capacitor42has a plurality of capacitive components which may be selectively enabled or disabled to select the amount of desired capacitance. Control module18includes control logic46, which supplies one or more digital signals to excitation input14, which in this embodiment is an input to digitally tunable MEMS capacitor42. Control logic46is used to select the profile, such as for example the profile22shown inFIG. 2, that may be produced at output16. In this example, profile22may be used for producing a spread spectrum modulated output signal at output16. Control logic46may include, for example, a processor and a lookup table to determine the pulse sequence associated with the excitation signal supplied to excitation input14to control the modulation of MEMS based resonant structure12.

InFIG. 6there is shown a modulation circuit100in accordance with another embodiment of the present invention. Modulation circuit100includes a MEMS resonant structure112having a resonant frequency, an excitation input114and an output116. Modulation circuit100further includes a control module118. Control module118is coupled via a communications link120, such as for example mechanically, to excitation input114of MEMS resonant structure112. Control module118may include, for example, a variable voltage source122and control logic124. Variable voltage source122supplies an excitation signal, such as a modulation signal, to excitation input114of MEMS resonant structure112. Control logic124is used to select the profile, such as for example the profile22shown inFIG. 2, of a signal that may be produced at output116. In this example, profile22may be used for producing a spread spectrum modulated output signal at output116. Control logic124may include, for example, a lookup table to determine the voltage settings associated with the excitation signal supplied to excitation input114to control the modulation of MEMS resonant structure112.

In the various embodiments described above, the output frequency of the output signals present at outputs16,116of each of modulation circuits10,100, respectively, is tightly controlled, and this control provides the ability to modulate the output frequency to possess a specific profile, such as an SSCG frequency profile, similar to that shown for example inFIG. 2, although other profiles may be supported. The output then achieves EMI reduction when used in an electronic system, such as an imaging system or a computer system.

The equations describing the resonant behavior of a MEMS based resonant structure is typically determined through numerical analysis, as is known in the art. Thus, for applications where table values in a lookup table are used to control the modulation of the MEMS based resonant structure, these table values may be determined through similar numerical analysis, or through experimental feedback and simulation.

While the modulation circuits10,100have been described with respect to examples of EMI reduction, those skilled in the art will recognize that the principles of the present invention may be applied to other technological endeavors, such as for example, providing modulation circuits for analog and digital communication techniques.