# Patent

## Title
Oscillators having arbitrary frequencies and related systems and methods

## Abstract

Systems and methods for operating with oscillators configured to produce an oscillating signal having an arbitrary frequency are described. The frequency of the oscillating signal may be shifted to remove its arbitrary nature by application of multiple tuning signals or values to the oscillator. Alternatively, the arbitrary frequency may be accommodated by adjusting operation one or more components of a circuit receiving the oscillating signal.

## Background
RELATED APPLICATIONS
This application claims the benefit as a continuation under 35 U.S.C. §120 of U.S. patent application Ser. No. 12/721,472, filed Mar. 10, 2010 and entitled “Oscillators Having Arbitrary Frequencies and Related Systems and Methods”, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/289,984, filed on Dec. 23, 2009 entitled “Oscillators Having Arbitrary Frequencies and Related Systems and Methods”, both of which applications are hereby incorporated herein by reference in their entireties.


BACKGROUND
1. Field
The technology described herein relates to oscillators providing oscillating signals having arbitrary frequencies and to systems and methods for using the same.
2. Related Art
Oscillators are ubiquitous components in electronic equipment including wireless and wireline communications systems, entertainment electronics, aerospace systems, and timing systems. The oscillators traditionally are used to provide a reference signal or clock signal, such that precision of the signal frequency is important. Conventionally, crystal oscillators having quartz crystals as the resonating element have served as the oscillators of choice because they can be manufactured to provide precise signal frequencies within ±1.5 parts-per-million (ppm) of a target frequency value, frequency stabilities of ±2.5 ppm over the entire operating temperature range from −40° C. to +85° C., aging of below ±1 ppm/year (at 25° C.), typical phase noise of −138 dBc/Hz at 1 kHz, and power consumption as low as 1.5 mA.
Standard frequencies for reference signals and clock signals have developed, and oscillator manufacturing has conformed to these standard frequencies. Typical frequency values are as low as 32.768 kHz for watch crystals and real time clocks. Frequencies in the MHz range are commonly used in cell phones and GPS receivers, including 12.6 MHz, 13 MHz, 14.4 MHz, 16 MHz, 16.368 MHz, 16.9 MHz, 19.2 MHz, 19.8 MHz, 20 MHz, 23.104 MHz, 24.554 MHz, 26 MHz, 27.456 MHz, 32 MHz, 33.6 MHz, 38.4 MHz, and 52 MHz. Owing to the ability to manufacture quartz crystals to provide a precise target frequency, it is conventional for crystal oscillators to be manufactured to provide one of the several standard frequencies.
Thus, circuits and systems including crystal oscillators or receiving signals from crystal oscillators are conventionally designed to work with one of the standard frequencies corresponding to the particular crystal oscillator being used. FIG. 1A illustrates a conventional apparatus 100 including an oscillator 102 and system 106 that receives at its input port 105 an oscillator signal 104 output from an output port 103 of the oscillator 102. The system 106 is designed to work with a signal of precisely 26 MHz. Therefore, a 26 MHz oscillator is selected for the oscillator 102. If the system 106 receives a different frequency, it will not operate properly.
In some conventional devices, circuitry is designed to operate with a frequency other than that provided by the oscillator, but which can be precisely generated from a known, precise oscillator frequency conforming to one of the standard frequencies. Referring to FIG. 1B, the apparatus 150 includes the previously described oscillator 102 and a system 156, which itself includes a frequency synthesizer 158 and a sub-system 162. The sub-system 162 is designed to operate with a frequency other than the 26 MHz of oscillator signal 104 provided by the oscillator 102. The frequency synthesizer receives the oscillator signal 104 at its input port 155 and generates a synthesized signal 160, which can be referred to as an internal signal since it is generated and used internally to system 156, having the frequency required by sub-system 162. If the synthesizer 158 does not receive a precise 26 MHz signal from the oscillator, it will not generate the precise frequency required by subsystem 162, and therefore the subsystem 162 will not operate properly.
In the event that an oscillator does not provide a frequency precisely matching that required by a system, some conventional devices include circuitry to provide a tuning signal to the oscillator, referred to as automatic frequency control (AFC), as shown in FIGS. 2A and 2B. The apparatus 200 of FIG. 2A includes a 26 MHz oscillator 202 which provides the oscillator signal 104 to a system 206. Although the oscillator 202 is shown as a 26 MHz oscillator, for conventional oscillators the oscillator signal 104 can differ from 26 MHz by ±2 ppm. The system 206 determines whether the oscillator signal 104 has a frequency of precisely 26 MHz, and includes an output port 208 from which is provided an AFC tuning signal 210 to tune the oscillator if the oscillator signal 104 is not precisely 26 MHz. The AFC tuning signal 210 is received at an electronic frequency control input port (EFC_tune) 204 of the oscillator.
In FIG. 2B, the apparatus 250 includes the 26 MHz oscillator 202 and a system 256 having an input terminal 255 to receive the oscillator signal 104. The frequency synthesizer 258 generates a synthesized, or internal, signal 212 which is provided to the subsystem 262. The subsystem 262 detects whether the synthesized signal has the precise frequency required for proper operation of the sub-system and provides, via output port 264, the AFC tuning signal 210 to tune the oscillator 202 if the frequency of synthesized signal 212 does not precisely match the required frequency.
Conventional AFC tuning is limited to ±30 ppm of an initial frequency by the properties of the quartz crystals used as the resonating elements of conventional crystal oscillators, and is typically limited to ±10 ppm in practice.
SUMMARY
According to one aspect of the present invention, a method of generating an oscillating signal having a target frequency from an oscillator having a mechanical resonator is provided, the oscillator being coupled to a circuit to provide the oscillating signal to the circuit. The method comprises applying to the oscillator, from the circuit, a first tuning signal having a first value, and applying to the oscillator, from the circuit, an automatic frequency control (AFC) tuning signal having a second value different than the first value.
According to another aspect of the present invention, an apparatus is provided comprising an oscillator having a mechanical resonator, at least one input port, and at least one output port, the oscillator being configured to provide an oscillating output signal at the at least one output port. The apparatus further comprises a circuit having at least one input port coupled to the at least one output port of the oscillator to receive the oscillating output signal, and further having at least one output port coupled to the at least one input port of the oscillator. The circuit is configured to provide at its at least one output port at least one tuning signal for tuning the oscillator, the at least one tuning signal comprising an automatic frequency control (AFC) tuning signal and at least one additional tuning signal.
According to another aspect of the present invention, an apparatus is provided comprising an oscillator comprising a mechanical resonator and a memory storing at least one value indicative of a frequency of the oscillator and/or of the mechanical resonator.
According to another aspect of the present invention, a method is provided, comprising outputting, from an oscillator comprising a mechanical resonator to a circuit coupled to the oscillator, at least one value indicative of a frequency of the oscillator and/or of the mechanical resonator.
According to another aspect of the present invention, a method is provided, comprising downconverting a cellular telephone signal, the cellular telephone signal modulated with data, using an oscillating reference signal having an arbitrary frequency to generate a downconverted signal including the data. The method further comprises sampling the downconverted signal with an analog-to-digital converter (ADC) using a sampling rate selected to induce a shift of the data of the downconverted signal in a frequency domain.
According to another aspect of the present invention, a method is provided, comprising generating a digital data signal having digital data, and sampling the digital data signal with a digital-to-analog converter (DAC) using a sampling rate selected to induce a shift of the digital data in a frequency domain, the sampling resulting in an analog signal. The method further comprises upconverting the analog signal using an oscillating reference signal having an arbitrary frequency to generate an upconverted signal including data corresponding to the digital data.
According to another aspect of the present invention, a method is provided, comprising downconverting a first signal, the first signal modulated with data, using an oscillating reference signal having an arbitrary frequency to generate a downconverted signal including the data. The method further comprises sampling the downconverted signal with an analog-to-digital converter (ADC) to produce a digital signal including digital data corresponding to the data. The method further comprises shifting, using a digital signal processor (DSP), the digital data in a frequency domain.
According to another aspect of the present invention, a method is provided, comprising generating a digital data signal having digital data, and shifting the digital data in a frequency domain using a digital signal processor (DSP). The method further comprises sampling the shifted digital data with a digital-to-analog converter (DAC) to produce an analog signal, and upconverting the analog signal using an oscillating reference signal having an arbitrary frequency to generate an upconverted signal including data corresponding to the digital data. Shifting the digital data in the frequency domain comprises shifting the digital data by a frequency amount selected to account for a deviation of the arbitrary frequency from a standard oscillator frequency.
According to another aspect of the present invention, a method is provided, comprising downconverting a first signal, the first signal modulated with analog data, using an oscillating reference signal having an arbitrary frequency to generate a downconverted signal including the analog data, the arbitrary frequency differing from a standard oscillator frequency. The method further comprises sampling the downconverted signal with an analog-to-digital converter (ADC) to produce a digital signal including digital data corresponding to the analog data. The sampling is performed using a sampling rate corresponding to the standard oscillator frequency. The method further comprises applying the digital signal to a carrier tracking loop.



BRIEF DESCRIPTION OF THE DRAWINGS
Description of various aspects and embodiments of the invention will be given by reference to the following drawings. The drawings are not necessarily drawn to scale. Each identical or nearly identical component illustrated in multiple drawings is illustrated by a like numeral.
FIG. 1A illustrates a conventional configuration of an oscillator providing to a system an oscillating signal having a standard frequency.
FIG. 1B illustrates a detailed view of a conventional system for generating an internal signal from an oscillating signal of standard frequency received from an oscillator.
FIG. 2A illustrates a conventional configuration of an oscillator providing an oscillating signal to a system and the system applying an automatic frequency control (AFC) signal to the oscillator.
FIG. 2B illustrates a conventional configuration of an oscillator providing an oscillating signal to a system which generates an internal signal, and in which the system applies an AFC signal to the oscillator.
FIG. 3 illustrates in block diagram form an apparatus comprising a system coupled to an oscillator configured to generate an oscillating signal having an arbitrary frequency, according to one embodiment of the present invention.
FIG. 4 illustrates in block diagram form an apparatus comprising a system coupled to an oscillator configured to generate an oscillating signal having an arbitrary frequency and in which the system generates an internal signal from the oscillating signal, according to an alternative embodiment of the present invention.
FIG. 5 illustrates a radio frequency (RF) front-end employing an oscillator configured to generate an oscillating signal having an arbitrary frequency, according to one embodiment of the present invention.
FIG. 6 illustrates an alternative RF front-end employing an oscillator configured to generate an oscillating signal having an arbitrary frequency, and in which an analog-to-digital converter is configured to operate as a mixer, according to another embodiment of the present invention.
FIG. 7 illustrates an alternative RF front-end employing an oscillator configured to generate an oscillating signal having an arbitrary frequency, and in which a digital signal processor (DSP) is configured to induce a frequency shift of digital data, according to another embodiment of the present invention.
FIG. 8 illustrates an RF front-end including a carrier tracking loop and employing an oscillator configured to generate an oscillating signal having an arbitrary frequency, according to an embodiment of the present invention.



DETAILED DESCRIPTION
While, as described above, conventional quartz crystal resonators can be manufactured to provide an oscillating signal of precise frequency, doing so requires significant effort and cost. Accordingly, Applicants have appreciated that the effort and cost associated with manufacturing conventional quartz crystal resonators may be minimized or eliminated by designing systems which may accurately operate in combination with an oscillator manufactured to produce an arbitrary frequency rather than a conventionally accepted (standard) oscillator frequency. As used herein, “arbitrary frequency” refers to a frequency not substantially matching a conventional standard oscillator frequency. For example, the arbitrary frequency may differ by at least 30 parts per million (ppm) from a standard oscillator frequency in some embodiments. In some embodiments, the arbitrary frequency may differ by at least 50 ppm from a standard oscillator frequency, by at least 100 ppm, by at least 200 ppm, by at least 500 ppm, by at least 1,000 ppm, or by between approximately 1,000 ppm and 10,000 ppm (e.g., 2,000 ppm, 5,000 ppm, or any other value within this range), among other possible amounts of deviation. The term “arbitrary frequency” as used herein does not imply the frequency is not known or cannot be measured. Rather, an arbitrary frequency may be measured or otherwise have its value determined.
Furthermore, systems as described herein which may accurately operate in combination with an oscillator manufactured to produce an oscillating signal of arbitrary frequency may enable the use of mechanical resonator technologies which cannot be manufactured with the precision of conventional quartz crystal resonators, but which may offer various advantages over quartz crystal resonator technology. For example, oscillators employing MEMS resonator technology may not be easily manufactured to conform to one of the standard oscillator frequencies, but rather may be manufactured with less precision to provide an arbitrary frequency, thus making them less desirable than quartz crystal resonators for many present day applications in which a frequency precisely matching a conventional standard oscillator frequency is required. However, oscillators employing MEMS resonator technology may offer benefits compared to conventional quartz crystal resonators in terms of, for example, frequency stability, ease of manufacturing, manufacturing compatibility of the materials of the oscillator and/or mechanical resonator, cost, or other beneficial characteristics. Accordingly, it may be desirable to use oscillators employing MEMS resonator technology for some applications. One or more of the aspects of the invention described herein may enable or facilitate use of such technologies.
Accordingly, aspects of the present invention provide oscillators configured to produce oscillating signals having arbitrary frequencies and related systems and methods which may properly operate in connection with such oscillators. For purposes of the following discussion, the described systems and methods may be grouped into one of two classes, although it should be appreciated that the classes are not necessarily mutually exclusive and may overlap in one or more embodiments. The first class includes systems and methods which generate, from an oscillator configured to produce an oscillating signal of arbitrary frequency or from the oscillating signal of arbitrary frequency, an oscillating signal having a standard oscillator frequency. For example, the arbitrary frequency may differ from a standard oscillator frequency (e.g., 26 MHz) by up to ±10,000 ppm or more, and the systems and methods according to the first class discussed herein may generate from the oscillator or the oscillating signal of arbitrary frequency a signal having the standard frequency. A second class of systems and methods described herein are those which operate with a received oscillator signal of arbitrary frequency and do not shift the oscillator signal to a standard frequency, but rather adapt the configuration and/or operation of one or more components of the system to account for the arbitrary frequency.
Thus, according to one aspect of the present invention, a method of generating an oscillating signal having a target frequency (e.g., a standard oscillator frequency) from an oscillator having a mechanical resonator and configured to provide an arbitrary frequency is provided. A first tuning signal may be applied to the oscillator to shift a frequency of the oscillating signal produced by the oscillator. The method may further involve applying an automatic frequency control (AFC) tuning signal to the oscillator. The first tuning signal and the AFC tuning signal may have different values, and may form distinct signals in some embodiments. In alternative embodiments, the first tuning signal and the AFC tuning signal may form different components of a single signal. As will be described further below, the first tuning signal may influence a larger frequency shift of the oscillating signal output by the oscillator than the AFC tuning signal. Thus, the first tuning signal may be thought of as a coarse tuning signal, while the AFC tuning signal may operate as a fine tuning signal in some embodiments.
According to another aspect of the present invention, a circuit or system coupled to an oscillator and configured to receive an oscillating signal from the oscillator is configured to apply multiple tuning signals to the oscillator to control a frequency of the oscillating signal output by the oscillator. The oscillator may include a mechanical resonator of any suitable resonating technology, including MEMS technology, quartz crystal resonator technology, or any other suitable mechanical resonating technology. According to some embodiments, the circuit or system is configured to apply two tuning signals to the oscillator. One of the tuning signals may correspond to an AFC tuning signal, and the other tuning signal may be distinct from the AFC tuning signal and may represent a “frequency steering” signal, as described further below. The tuning signals may be provided separately, or in some embodiments may be provided as different components of a same signal. The value of the AFC tuning signal may influence a relatively small frequency shift of the oscillating signal output by the oscillator, whereas the additional frequency steering tuning value may influence a relatively larger frequency shift of the oscillating signal. The values of the AFC tuning signal and the additional tuning signal may be selected to shift the frequency of the oscillating signal output by the oscillator from an arbitrary frequency to a desired standard oscillator frequency.
According to some embodiments of the above-described aspects of the present invention, the values of one or both of the tuning signals may be determined at least in part based on the arbitrary frequency of the oscillating signal output by the oscillator. The value of the arbitrary frequency may be determined in various suitable manners, and may be provided to the appropriate circuitry within the oscillator and/or circuit or system operating in connection with the oscillator in any suitable manner. According to one aspect of the present invention, an oscillator including a mechanical resonator also includes memory storing a value indicative of a frequency of the oscillator and/or the mechanical resonator. The value stored in memory of the oscillator and indicative of the frequency of the oscillator and/or mechanical resonator may be provided to a circuit or system operating in connection with the oscillator, for example in addition to the oscillating output signal itself. Thus, according to one aspect of the present invention, an oscillator outputs an oscillating output signal having an arbitrary frequency as well as value indicative of the arbitrary frequency.
As mentioned, a second class of systems and methods according to the various aspects of the invention described herein are those which operate with a received oscillator signal of arbitrary frequency and do not shift the oscillator signal to a standard oscillator frequency, but rather adapt the configuration and/or operation of one or more components of the system to account for the arbitrary frequency. According to one such aspect of the present invention, a method is provided for operating on a cellular telephone signal using an oscillating reference signal having an arbitrary frequency. The cellular telephone signal may be down-converted to an intermediate frequency using the oscillating reference signal of arbitrary frequency, resulting in a down-converted signal including data corresponding to the data of the cellular telephone signal. As a result of performing the down-conversion with an oscillating reference signal of arbitrary frequency, the data of the down-converted signal may be shifted in the frequency domain relative to the intermediate frequency. The down-converted signal may then be sampled with an analog-to-digital converter (ADC). The sampling rate of the ADC may be selected to induce a shift in the frequency domain of the data of the down-converted signal to compensate for the shift of the data of the down-converted signal from the intermediate frequency.
According to another such aspect of the invention, the sampling rate of a digital-to-analog converter (DAC) in a transmit path of a device, such as a cellular telephone or other transmission device, may be selected to account for an up-conversion process performed in the transmit path using an oscillating reference signal having an arbitrary frequency. The up-conversion process may be performed by suitable mixing of an analog signal including analog data, such as a cellular telephone signal or other analog signal to be transmitted, with an oscillating reference signal, resulting in an up-converted signal at a desired carrier frequency. The analog signal itself may be generated by performing digital-to-analog conversion of a digital signal having the desired data for transmission. The transmit path may be designed to operate with an oscillating reference signal having a standard oscillator frequency, such that if the oscillating reference signal instead has an arbitrary frequency the data of the resulting up-converted signal may be shifted in the frequency domain relative to the center frequency of the desired carrier frequency. Such a shift may be accounted for by suitable selection of the sampling rate of the DAC prior to up-conversion, such that the data of the up-converted signal appears at the intermediate frequency. For example, if the arbitrary frequency of the oscillating reference signal is higher than an expected standard oscillator frequency, then the sampling rate of the DAC may be selected to be lower than if the oscillating reference signal had the expected standard oscillator frequency, and vice versa.
According to another such aspect of the present invention, a method of accounting for down-conversion of a received signal using an oscillating reference signal of arbitrary frequency may comprise using a digital signal processor (DSP) to digitally shift the data of the down-converted signal in the frequency domain. A carrier signal modulated with data may be received and down-converted using the oscillating reference signal of arbitrary frequency, thus resulting in a down-converted signal including data corresponding to the data modulated on the carrier signal. The down-converted signal may then be sampled using an ADC to produce a digital signal including digital data corresponding to the data modulated on the carrier signal. Because the down-conversion of the carrier signal is performed using an oscillating reference signal having an arbitrary frequency, the digital data of the resulting down-converted and digitized signal may be shifted in the frequency domain relative to the baseband frequency and/or intermediate frequency. Accordingly, the digitized signal output by the ADC may be provided to a DSP, which may digitally shift the data of the digitized signal in the frequency domain.
According to another such aspect of the present invention, a digital shift of data to be transmitted from a transmit path of a device, such as a cellular telephone or other transmission device, may be induced to account for an up-conversion process performed using a reference oscillating signal having an arbitrary frequency. A digital data signal having digital data to be transmitted may be generated. The digital data signal may be digital-to-analog converted using a DAC and then up-converted by mixing with a suitable oscillating reference signal. The device may be designed in expectation of the oscillating reference signal having a standard oscillator frequency. In the event the oscillating reference signal has an arbitrary frequency, the up-conversion process may result in the data to be transmitted being shifted in the frequency domain relative to the center frequency of the intended carrier frequency. To account for such a shift, a digital signal processor (DSP) may be used to shift, in the frequency domain, the digital data of the digital data signal prior to the digital-to-analog conversion. By suitable selection of the amount of frequency shift to induce in the digital data signal, the subsequent DAC conversion and up-conversion using an oscillating reference signal of arbitrary frequency may result in the data of the up-converted signal appearing at a desired frequency or frequencies (e.g., near the center frequency of the desired carrier frequency).
According to a further such aspect of the present invention, a method of operating on a signal down-converted using an oscillating reference signal having an arbitrary frequency comprises utilizing a carrier tracking loop. A carrier signal modulated with data may be received and down-converted using the oscillating reference signal of arbitrary frequency, resulting in a down-converted signal. The down-converted signal may then be sampled with an ADC, producing a digital signal including digital data corresponding to the data modulated on the carrier signal. The sampling rate of the ADC may be selected as if the oscillating reference signal had a standard oscillator frequency and not an arbitrary frequency. For example, the sampling rate of the ADC may be selected as if the oscillating reference signal had a standard oscillator frequency of, for example, 26 MHz, rather than an arbitrary frequency differing from the standard operating frequency by up to approximately ±10,000 ppm. As a result, the digitized signal may include digital data not accurately reflecting the data modulated on the carrier signal. The digitized signal may be applied to a carrier tracking loop, thus effectively re-sampling the digital data of the digital signal to restore its accuracy.
The various aspects described above, as well as further aspects, will now be described in further detail below. It should be appreciated that these aspects may be used alone, all together, or in any combination of two or more, to the extent that they are not mutually exclusive. Also, while various of the aspects will be described below in the context of cellular telephone systems, it should be appreciated that the aspects are not limited in this respect, and may apply to other devices and systems which use a reference oscillator, such as navigation receivers (e.g., global positioning system (GPS) receivers), personal digital assistants (PDAs), other wireless communication devices, timing circuits, or other devices using reference oscillators.
As mentioned, according to one class of systems and methods described herein, an oscillating signal having a standard oscillator frequency is generated from an oscillator configured to produce an oscillating signal of arbitrary frequency. One non-limiting example of a system and method for doing so is to provide multiple tuning signals (which may involve, in some instances, providing multiple tuning values) to the oscillator. An example of an apparatus according to this aspect of the present invention is illustrated in FIG. 3.
As shown, the apparatus 300 includes an oscillator 302 and a system 306. According to one embodiment, the oscillator and system may be formed on separate semiconductor dies (which may facilitate separate manufacture of the two), although not all embodiments are limited in this respect. The oscillator 302 is configured to provide from an output port 303 an oscillating output signal 304. The oscillating output signal 304 is received by an input port 305 of the system 306. As shown, the oscillator 302 may be configured (e.g., by its manufacture) to produce a signal of, as a non-limiting example, 25.97425 MHz, or in other words an arbitrary frequency. The system 306 may be configured to operate with a precise standard oscillator frequency, such as, for example, 26 MHz, such that the 25.97425 MHz which oscillator 302 is configured to produce is not suitable for proper operation of the system 306. Accordingly, to work with the oscillator 302, the system 306 may be configured to provide two tuning signals to the oscillator 302 to influence the frequency of the oscillating output signal 304, and in some embodiments to control the frequency of the oscillating output signal 304 to have it match a standard oscillator frequency.
As shown, a first tuning signal 308 is provided from a first output port 310 of the system 306 to a first input port 312 of the oscillator 302. An AFC tuning signal 314 is also provided from an output port 316 of the system 306 to a second input port 318 of the oscillator 302. As will be described further below, the values of the tuning signal 308 and the AFC tuning signal 314 may be selected to tune the oscillator 302 such that the oscillating output signal 304 has a standard oscillator frequency rather than the arbitrary frequency which oscillator 302 is configured to produce, or so that the oscillating output signal has a frequency enabling the system 306 itself to generate from the oscillating output signal 304 an oscillating signal (e.g., an internal signal) having a desired standard oscillator frequency.
According to one embodiment, a combination of tuning signal 308 and AFC tuning signal 314 may induce a frequency shift of up to approximately ±10,000 ppm (e.g., up to approximately ±500 ppm, ±1,000 ppm, ±2,000 ppm, ±5,000 ppm, or any other suitable amount) of the oscillating output signal 304, or any other suitable amount. Thus, a large frequency shift of the oscillating output signal 304 may be realized by use of tuning signals 308 and 314, enabling or facilitating use of an oscillator 302 configured to produce an arbitrary frequency with the system 306. According to one embodiment, the value of tuning signal 308 induces a relatively larger frequency shift of the oscillating output signal 304 than does the AFC tuning signal 314. Thus, the tuning signal 308 may be thought of as a coarse adjustment tuning signal, and is referred to herein as a “frequency steering” signal (which is why output port 310 is labeled “FS” and input port 312 is labeled “FS_tune”), while the AFC tuning signal may be thought of as a fine adjustment tuning signal. According to one embodiment, the AFC tuning signal induces a relatively small frequency shift of the oscillating output signal 304 of, for example, less than approximately ±5 ppm, less than approximately ±10 ppm, or less than approximately ±20 ppm, as a continuous value or in increments of any suitable size. Thus, it should be appreciated that the tuning signal 308 may induce a substantially larger frequency shift, for example, up to approximately ±10,000 ppm according to some embodiments. The tuning signal 308 may induce a frequency shift of certain distinct values in increments of ±50 ppm, ±100 ppm, ±200 ppm, or any other suitable amount.
The form of tuning signals 308 and 314, and the manner and timing in which they are provided to the oscillator 302, are not limiting. According to one embodiment, the form of tuning signal 308 may depend on a type of tuning technique used to tune oscillator 302. For example, according to one embodiment the oscillator 302 may be tunable by inducing a phase shift between an output signal of the oscillator and an input signal of the oscillator, for example if the oscillator comprises or is part of a feedback loop. An example of such a device with which the aspects described herein may be applied is described in co-pending U.S. patent application Ser. No. 12/699,094, filed Feb. 8, 2010 entitled “Methods and Apparatus for Tuning Devices Having Mechanical Resonators”, which application is hereby incorporated herein by reference in its entirety. In such an embodiment, the tuning signal 308 may be any signal suitable for selecting or inducing a desired amount of phase shift. According to another embodiment, the oscillator 302 may be tunable by inducing a phase shift and an amplitude shift between an output signal of the oscillator and an input signal to the oscillator, for example again if the oscillator comprises or forms part of a feedback loop. Examples of such devices are also described in U.S. patent application Ser. No. 12/699,094. In such a non-limiting embodiment, tuning signal 308 may be any signal suitable for selecting or inducing a desired amount of phase shift and/or amplitude adjustment. Other tuning techniques for tuning the oscillator 302 are also possible, and tuning signal 308 may take any suitable form for dictating or selecting the amount of frequency shift by which to shift the frequency of the oscillating output signal provided by the oscillator.
According to one embodiment, the value of tuning signal 308 may be a digital value which may effectively program the oscillator 302, thus inducing a frequency shift of the oscillating output signal 304. For example, in one embodiment the oscillator 302 may be tunable by inducing a phase shift between an output signal and input signal of the oscillator, and the tuning signal may be a digital value indicating an amount of phase shift to induce. According to one such embodiment, the tuning signal 308 may be a digital code, which may be decoded (e.g., by suitable decoding circuitry of the oscillator) to determine an amount of phase shift to induce. It should be appreciated that digital codes may similarly be used with oscillators tuned by different tuning techniques (e.g., other than by inducing a phase shift between input and output signals of the oscillator).
The tuning signal 308 may be provided once (e.g., upon powering on of the system 306) to the oscillator 302, at periodic intervals, when a device of which apparatus 300 forms a part changes a frequency of operation (e.g., when a cell phone changes frequency channels), substantially continuously, or at any other suitable time. According to an alternative embodiment, the tuning signal 308 may be an analog tuning voltage applied at any of the above-described times or any other suitable time. According to one embodiment, the value of the frequency steering signal may be stored in local memory of the oscillator upon receipt from the system.
The AFC tuning signal 314 provided to input port 318 (labeled as “EFC_tune”) may be substantially the same as a conventional AFC tuning signal, and therefore may be either an analog tuning voltage or a digital signal, as the various aspects described herein implementing an AFC tuning signal are not limited to the form of the tuning signal unless otherwise stated. The AFC tuning signal 314 may be applied continuously to the system 302, for example as an analog tuning voltage, and may vary regularly to account for relatively small deviations of the frequency of oscillating output signal 304 from a target frequency. Other forms and timing of application are also possible for AFC tuning signal 314.
As mentioned, the manner in which the tuning signals 308 and 314 are provided to oscillator 302 is also not limiting. According to one embodiment, as shown in FIG. 3, tuning signals 308 and 314 may be provided as distinct signals to the oscillator (e.g., on separate wire leads or signal traces). According to another embodiment, tuning signals 308 and 314 may be provided as a single signal (e.g., on a single wire lead or signal trace) to the oscillator. In such an embodiment, the tuning signals 308 and 314 may represent different components or portions of a single tuning signal. Thus, it should be appreciated that the form and manner of applying tuning signals 308 and 314 to the oscillator are not limiting.
While the oscillator 302 is not limited to utilizing any particular type of mechanical resonator technology, and therefore may utilize conventional quartz crystal resonator technology, MEMS resonator technology, or any other suitable technology, it should be appreciated that the amount of frequency shift which may be induced by the combination of tuning signal 308 and AFC tuning signal 314 may be limited at least in part by the type of resonator technology employed. For example, conventional quartz crystal resonator technology may not allow for tuning up to approximately ±10,000 ppm of the initial oscillator output signal frequency. Thus, it should be appreciated that those embodiments described herein relating to tuning of an oscillator output signal frequency by up to approximately ±10,000 ppm may correspond to embodiments in which the oscillator employs mechanical resonator technology allowing for such a relatively large tuning range. According to one embodiment, MEMS resonator technology, such as that described in U.S. patent application Ser. No. 12/181,531, filed Jul. 29, 2008, entitled “Micromechanical Resonating Devices and Related Methods” and published as U.S. Patent Application Publication No. 2010-0026136-A1, and U.S. patent application Ser. No. 12/142,254, filed Jun. 19, 2008, entitled “Methods and Devices For Compensating a Signal Using Resonators” and published as U.S. Patent Application Publication No. 2009-0243747-A1, may be employed, both of which applications are hereby incorporated herein by reference in their entireties.
The system 306 may include any suitable circuitry for providing the tuning signals 308 and 314, and may include any suitable circuitry for determining the values of those signals. According to one aspect of the present invention, the system 306 may determine suitable values for tuning signals 308 and 314 by comparison of the frequency of the oscillating output signal 304 to a reference frequency, such as a radio frequency signal of known frequency received by a device of which apparatus 300 forms a part (e.g., a cellular telephone). For example, according to one embodiment, the system 306 receives the oscillating output signal 304 having the initially arbitrary frequency and compares the received oscillating signal to a reference signal having a known frequency, which may correspond to a target frequency. If the system 306 determines that the oscillating output signal 304 does not have the desired target frequency, the frequency difference between that of the oscillating output signal 304 and the target frequency may be determined, and suitable values for the tuning signals 308 and 314 to adjust the arbitrary frequency of oscillating output signal 304 to the desired target frequency may be determined.
According to another embodiment, the frequency of the output signal provided by the oscillator may be directly measured, for example using a frequency analyzer or any other suitable technique. Such measurement may be made after manufacture of the oscillator or at any other suitable time. The measured frequency may be compared to a target value, from which suitable values for tuning signals 308 and 314 to adjust the arbitrary frequency of oscillating output signal 304 to the desired target frequency may be determined.
According to another embodiment, the values of one or both of tuning signals 308 and 314 may be determined based on a known value of the arbitrary frequency of oscillator 302. For example, as shown in FIG. 3, according to one embodiment the oscillator 302 provides from memory 313 a value 320 indicative of the arbitrary frequency, which may be received at an input port 315 of the system 306 (labeled as port “f_zero” since the initial arbitrary frequency of the oscillator may be labeled “f_zero”). Value 320 may be one of various values which the system 306 may use to determine appropriate values for tuning signals 308 and 314. For example, according to one embodiment, value 320 is a value of the frequency of oscillator 302 (e.g., 25.97425 MHz), for example, the arbitrary frequency. According to an alternative embodiment, value 320 is a value of an offset of the arbitrary frequency from a standard frequency. For example, value 320 may be 5,000 if the arbitrary frequency differs from a standard oscillator frequency of known value by 5,000 ppm. According to a further embodiment, value 320 may be a value of a frequency of the mechanical resonator of oscillator 302 (e.g., 25.99955 MHz), which may be indicative of the arbitrary frequency of oscillating output signal 304. According to another alternative embodiment, the value 320 may be a value of an offset of the frequency of the mechanical resonator of oscillator 302 from a standard oscillator frequency (e.g., value 320 may be 1,000 if the frequency of the mechanical resonator differs by 1,000 ppm from a standard oscillator frequency). Other values are also possible, as the value 320 is not limited to representing any specific physical quantity, but rather may represent one of various quantities which the system 306 may satisfactorily use to determine appropriate values for tuning signals 308 and/or 314.
Memory 313 storing the value 320 may be any suitable type of memory. According to one embodiment, the value 320 may be provided once upon connection of the system 306 to the oscillator 302. According to an alternative embodiment, the value 320 may be provided periodically to the system 306, for example whenever a device of which apparatus forms a part changes an operating frequency (e.g., when a cellular telephone changes frequency channels). According to a further embodiment, the value 320 may be provided upon powering on of the apparatus 300. Thus, it should be appreciated that the various aspects described herein relating to an oscillator including memory providing a value indicative of a frequency of the oscillator and/or mechanical resonator of the oscillator are not limited to the time or manner in which the value is provided.
The system 306 may utilize the value 320 in any suitable manner for determining suitable values of tuning signals 308 and 314. According to one embodiment, system 306 includes a reference table, such as a lookup table, storing values for tuning signal 308 based on the arbitrary frequency of the oscillator 302, an offset of the arbitrary frequency from a standard oscillator frequency, or any other value which may be represented by value 320, and the desired target frequency of the oscillating output signal 304 (e.g., a desired standard oscillator frequency). Thus, according to one embodiment, the system 306 receives the value 320 and refers to the reference table/lookup table stored therein to determine an appropriate value for tuning signal 308 to ensure the oscillating output signal 304 has the desired target frequency. According to one embodiment, the reference table/lookup table may provide values for both tuning signal 308 and tuning signal 314, although not all embodiments are limited in this respect. The reference table/lookup table within system 306 may be stored within memory of system 306, or in any other suitable manner. Furthermore, the reference table/lookup table may be populated or uploaded to the system 306 at any suitable time. For example, the table may be provided to the system 306 upon initial manufacturing of the system 306. Alternatively, the table may be updated or uploaded to the system 306 periodically.
While FIG. 3 illustrates a non-limiting embodiment in which a value 320 is provided from memory 313 to the system 306, it should be appreciated that alternative manners for providing the system 306 with such information are possible. For example, according to one alternative embodiment, the value 320 may be programmed into system 306 upon manufacture of the system 306, rather than being provided by the oscillator 302. For example, a manufacturer of system 306 may know the value 320 prior to oscillator 302 being connected to system 306, and therefore may provide the value 320 by programming it into memory of system 306, or by providing a lookup table as previously described based on the known value. According to one embodiment, the value 320 may be printed on a package of the oscillator 302, and the manufacturer or user of system 306 may read the value from the package and provide it to system 306 in any suitable manner. Other alternatives are also possible.
As described above in connection with FIGS. 1B and 2B, some systems receive a signal from an oscillator and synthesize an internal signal having a different desired frequency. The concepts described with respect to FIG. 3 may also apply to such a system. An example is illustrated in FIG. 4.
As shown, the apparatus 400 includes the oscillator 302 coupled to a system 406, which in one embodiment may represent a non-limiting detailed version of system 306 of FIG. 3. According to one embodiment, the oscillator 302 and system 406 may be formed on separate semiconductor dies (which may facilitate separate manufacture of the two), although not all embodiments are limited in this respect. The system 406 includes a frequency synthesizer 401 having an input coupled to the input port 305 of system 406. Furthermore, the system 406 includes a subsystem 402 which receives an output of the frequency synthesizer 401. In the non-limiting example of FIG. 4, the frequency synthesizer receives the oscillating output signal 304 of oscillator 302 and synthesizes a synthesized (internal) signal 404 which it provides to the subsystem 402. In some non-limiting embodiments, the frequency synthesizer may be an integer phase locked loop (PLL) and may be preset to a fixed divider ratio N/R, where N and R are integer numbers, so that the resulting frequency output by the frequency synthesizer is related to the received frequency (e.g., the frequency of oscillating output signal 304) by the ratio N/R. In such a scenario, because the oscillator output signal 304 may have an arbitrary frequency, at least initially, the synthesized signal 404 may also differ from a desired target frequency for the internal signal being provided to subsystem 402. It should be understood that using an integer phase locked loop is only one possible embodiment for the frequency synthesizer, and any other frequency synthesizer can be used, including fractional N PLL, direct digital synthesizer (DDS), and any other method.
In those scenarios in which the frequency synthesizer is unable to generate an internal signal having a desired target frequency, for example because the frequency synthesizer is an integer PLL and the oscillating output signal 304 has an arbitrary frequency, tuning signals 308 and 314 may be applied by system 406 to the oscillator 302 to shift the frequency of oscillating output signal 304 to a value such that frequency synthesizer 401 may then synthesize a synthesized signal 404 having a desired target frequency for subsystem 402. Thus, it should be appreciated that tuning signals 308 and 314 in this non-limiting embodiment may not be used to shift the frequency of oscillating output signal 304 itself to a standard oscillator frequency (although they may in some embodiments), but rather to a frequency from which the frequency synthesizer may generate an internal oscillating signal having the desired target frequency for sub-system 402.
A non-limiting example is now given. According to one embodiment, the frequency synthesizer 401 is a PLL having discrete frequency steps of 100 ppm. The oscillator 302 may initially output an oscillating output signal 304 having the indicated arbitrary frequency of 25.97425 MHz. Due to the step sizes of the frequency synthesizer 401, the synthesized signal 404 may have a frequency that is at best within ±50 ppm of a desired target frequency for the sub-system 402. The tuning signal 308 may, in this non-limiting example, have a value that may be selected in increments of ±10 ppm, and therefore may be selected to have a suitable value for adjusting the frequency of oscillating output signal 304 such that the synthesized signal 404 has a frequency within ±10 ppm of a desired target frequency for sub-system 402. The AFC tuning signal 314 may then assume a value suitable for shifting the frequency of the oscillating output signal 304 by a suitable amount such that frequency synthesizer 401 may synthesize a synthesized signal 404 having the target frequency. It should be appreciated that this is merely one non-limiting example, and that other manners of operation of the apparatus 400 are also possible. The values of tuning signals 308 and 314 in system 406 may be determined in any of the manners described above with respect to FIG. 3, or in any other suitable manner.
The techniques described above in connection with FIGS. 3 and 4 may be applied to various applications and contexts utilizing reference oscillators to generate an oscillating reference signal. One non-limiting example of a system in which a reference oscillator is used, and in which the concepts of FIGS. 3 and 4 may be used, is a radio frequency (RF) device, such as a cellular telephone. A non-limiting example is described with respect to FIG. 5, although it should be appreciated that other devices may also utilize the techniques described herein.
FIG. 5 illustrates an RF front-end 500, as might be used in a cellular telephone, illustrating in detail the receive path and excluding the details of the transmit path 522 for simplicity of the figure. It should be appreciated, however, that the principles of operation described with respect to the receive path may be analogously applied to the transmit path, and thus that the various aspects described herein relating to RF front-ends are not limited to receive paths only.
The RF front-end 500 has a direct-conversion receiver (DCR) architecture, also referred to as a Homodyne, Synchrodyne or zero-IF receiver. However, it should be appreciated that the aspects described herein relating to RF front-ends are not limited to the exact configuration of components illustrated in FIG. 5 or to any particular type of RF front-end unless otherwise stated. For example, the aspects described herein may also apply to heterodyne receivers or other receiver architectures.
The RF front-end 500 is configured to receive an incoming radio signal 501. According to one embodiment, the radio signal 501 is a cellular telephone signal, although other types of radio signals may be used in various embodiments, as the aspects described herein relating to RF front-ends are not limited to operation with cellular telephone signals. The radio signal 501 may include a carrier signal modulated with data (e.g., cellular telephone data) or may take any other suitable form.
The incoming radio signal 501 is received by the antenna 502 and passes through a band pass filter 504, which may be a duplexer. The resulting signal is then amplified by a low noise amplifier (LNA) 506 and down-converted by mixing the incoming radio signal with a reference signal 512 using mixer 508. The reference signal 512 is illustrated in this non-limiting example as having a frequency of 900 MHz for purposes of illustration, but may have any suitable frequency. It may be generated by a frequency synthesizer 530 (illustrated as a fractional PLL in this non-limiting example) which receives the previously described oscillating output signal 304. As previously mentioned, the oscillating output signal 304 may, in some embodiments, have an arbitrary frequency, at least before any tuning signals are applied to the oscillator 302 generating the oscillating output signal. After mixing the incoming radio signal 501 and the reference signal 512 using mixer 508 the resulting down-converted signal may be low pass-filtered in filter 514 and then converted from an analog signal to a digital signal using ADC 516. The resulting digital signal may contain the data or information of the radio signal 501 (e.g., cellular telephone data). The digitized signal produced by ADC 516 may then be input to the baseband electronics 518 for further processing. The baseband electronics 518 may comprise a digital signal processor (DSP) 520, which may filter and condition the received data, e.g., the cellular telephone data in those embodiments in which the RF front-end 500 is part of a cellular telephone.
The accuracy with which the data of radio signal 501 is recovered may depend, at least in part, on whether reference signal 512 has the desired reference frequency (e.g., 900 MHz in this non-limiting example). If the reference signal 512 does not have the desired reference frequency, the data of radio signal 501 may not be accurately recovered. According to some embodiments, it may be desirable for the reference signal 512 to have a frequency substantially matching the carrier frequency of radio signal 501 plus some offset, e.g., 900 MHz may correspond to the carrier frequency of the radio signal 501 plus some offset.
In the non-limiting example of FIG. 5, the reference signal 512 is synthesized by the frequency synthesizer 530, which again is a fractional N PLL in this non-limiting example, although it should be appreciated that other types of frequency synthesizers may alternatively be employed. The baseband electronics may adjust the frequency synthesizer by applying a control signal 542 from output port 540 (labeled “div_c”) to an input port 534 (labeled as “div_ctrl”), which may adjust a setting of the fractional N PLL. This may be done to adjust the frequency of the reference signal 512 to match the carrier frequency of the radio signal 501 when the frequency channel of the apparatus 500 is changed, for example as occurs in cellular telephones when changing frequency channels. The baseband electronics may therefore store information indicating what setting of the N PLL corresponds to what frequency channel, so that the correct setting may be applied via control signal 542.
To accurately recover the data of radio signal 501, the frequency synthesizer 530 may be set to generate the reference signal 512 such that its frequency is as close to the carrier frequency of radio signal 501 as possible. However, due to the finite step size of the fractional N PLL and the fact that, in this non-limiting example, the frequency provided by oscillator 302 may be arbitrary, an offset Δf between the carrier frequency of radio signal 501 and the frequency of reference signal 512 may result. The baseband electronics may apply previously described tuning signals 308 and 314 to tune the oscillator 302 so that the frequency of oscillating output signal 304 facilitates generation by frequency synthesizer 530 of a reference signal 512 having the desired carrier frequency. The values of tuning signals 308 and 314 may be determined by the baseband electronics in any of the manners previously described with respect to FIGS. 3 and 4, or in any other suitable manner. The offset Δf may, in some embodiments, be in the range of ±50 ppm to ±100 ppm, and therefore the desired carrier frequency may not be achievable with conventional RF front-ends. However, use of the frequency steering tuning signal 308 in combination with the AFC tuning signal 314 may allow the RF front-end to achieve accurate operation by enabling the reference signal 512 to have the desired target frequency.
It should be appreciated that the tuning functionality described with respect to FIG. 5 may reduce or eliminate the occurrence of electrical interference found in conventional RF front-ends, thus beneficially improving the operation of the RF front-end. Conventional RF front-ends may experience interference when the oscillator output signal or frequency synthesizer signal, or some higher harmonics of those signals, undesirably mix with the received radio signal or higher harmonics of the received radio signal. According to the techniques described herein, for example in the non-limiting context of FIG. 5, the tuning range of the oscillator 302 may exceed the frequency step size of the frequency synthesizer by a factor of two, such that there are two or more possible settings of the frequency synthesizer for any desired target frequency of reference signal 512 based on an output signal of the oscillator having a given frequency. Accordingly, there may be two or more suitable values for the frequency steering tuning signal which may be applied to the oscillator 302 for any given desired target frequency of reference signal 512, one corresponding to each of the two or more possible frequency synthesizer settings. Thus, if a particular frequency steering value would result in undesirable electrical interference, one of the other possible frequency steering values for achieving the desired reference signal 512 may be used to reduce or eliminate the interference.
It should be appreciated from the foregoing that various aspects of the present invention are directed to systems and methods for generating a reference signal having a target frequency upon receipt of an oscillating output signal having an arbitrary frequency. As previously mentioned, various aspects of the present invention are alternatively directed to systems and methods which operate upon an oscillating signal having an arbitrary frequency, and need not necessarily generate from the arbitrary frequency an oscillating signal having a desired target frequency (e.g., a standard oscillator frequency). For example, the configuration and/or operation of one or more components of a system receiving an oscillating signal having an arbitrary frequency may be adapted to permit operation of the system with the arbitrary frequency. Accordingly, the types of adaptations which may be implemented may depend upon the components and configuration of the system receiving the oscillating signal from the oscillator. Various non-limiting examples are now described, although it should be appreciated that other implementations are possible depending on the configuration and components of the system.
According to one aspect of the present invention, a system receiving an oscillating signal having an arbitrary frequency includes an ADC configured to digitize a signal resulting from mixing a first signal with the oscillating signal of arbitrary frequency, and the sampling rate of the ADC may be selected to account for such mixing. An example is given with respect to FIG. 6, which illustrates an RF front-end 600 that is similar in many respects to previously described RF front-end 500 of FIG. 5, and which uses identical reference numbers for those components that are the same as in FIG. 5.
As shown and previously described, an incoming radio signal 501 received on antenna 502 is down-converted in mixer 508 using a reference signal. The resulting down-converted signal is supplied to a filter and is then digitized using an ADC. In the context of FIG. 5, the oscillator 302 and frequency synthesizer 530 may be controlled by signals 542, 308, and 304 to produce a reference signal 512 having a desired target frequency (illustrated as 900 MHz in the non-limiting example of FIG. 5), despite the oscillator 302 initially being configured to produce an oscillating signal of arbitrary frequency (illustrated as 25.97425 MHz in FIG. 5). The desired target frequency for reference signal 512 may match the frequency of the carrier signal of radio signal 501 including an offset corresponding to the intermediate frequency. In those instances, the data of the down-converted signal output by mixer 508 may typically appear in the frequency domain at either the baseband frequency or an intermediate frequency. In such instances, the sampling rate of the ADC 516 may be selected to suitably sample the down-converted signal such that the data on the down-converted signal is accurately captured by the digitizing process implemented by ADC 516 without inducing a frequency shift of the data during the digitizing process.
However, while it was previously described that apparatus 500 may operate by generating a reference signal 512 having a desired target frequency, such is not the case according to the present aspect described with respect to RF front-end 600 of FIG. 6. According to the present aspect, oscillator 602 may be configured to generate an oscillating output signal 304 having an arbitrary frequency (e.g., 25.97425 MHz) which is not compensated by a frequency steering signal, such that the reference signal 612 does not have the desired target frequency (e.g., a frequency corresponding substantially to the carrier frequency of radio signal 501 including an offset, corresponding to the intermediate frequency). For example, the tuning signal 308 may not be applied according to this aspect, such that the arbitrary frequency of oscillating output signal 304 results in reference signal 612 having a frequency differing from the desired target frequency. In the non-limiting example of FIG. 6, the reference signal may have a frequency of, for example, 879.956 MHz, differing from a target frequency value of 880 MHz. As a result, the data of the down-converted signal output by mixer 508 is shifted in the frequency domain relative to the intermediate frequency of the down-converted signal. The down-converted signal may then be filtered in a band-pass filter 614. Due to the frequency shift of the data of the down-converted signal relative to the intermediate frequency, operating the ADC 616 at a sampling rate as though the reference signal 612 had the desired target frequency (e.g., 880 MHz) results in the data of the digitized signal being frequency shifted relative to the intermediate frequency.
Thus, according to one aspect of the present invention, the sampling rate of the ADC 616 may be selected to compensate for the shift in the frequency domain of the data of the down-converted signal relative to the intermediate frequency. The sampling rate may be selected based on a known or detected offset of the frequency of the reference signal 612 from the carrier signal of radio signal 501 including the offset related to the intermediate frequency. For example, the frequency offset may be known by receiving a value from memory 313, and the baseband electronics may then set a suitable sampling rate of the ADC 616, as a non-limiting example. Other methods of determining a suitable sampling rate of the ADC are also possible. In this manner, the RF front-end 600 may suitably operate to accurately recover the data of radio signal 501 despite the oscillator 602 providing an oscillating signal 304 having an arbitrary frequency, and despite the frequency synthesizer 530 generating a synthesized signal having a frequency differing from the carrier frequency of radio signal 501 including an offset corresponding to the intermediate frequency. Thus, the need for an oscillator providing a precise frequency matching a standard oscillator frequency may be minimized or eliminated, which may simplify design of the system and allow for use of oscillators having various beneficial characteristics, such as ease of manufacture, low cost, or other beneficial characteristics.
According to one embodiment of the present aspect, the sampling rate of the ADC may be selected to match the resulting intermediate frequency generated by mixing of the radio signal 501 with the reference signal 612 of arbitrary frequency. A non-limiting example is now given. For purposes of this non-limiting example, the intermediate frequency which would be generated by mixing the radio signal 501 with a reference signal of 880 MHz may be 20 MHz. However, if the reference signal (e.g., reference signal 612) instead is offset from 880 MHz by 50 ppm, the resulting intermediate frequency output by mixer 508 may be 20.044 MHz, rather than 20 MHz, in those embodiments in which the reference signal is 50 ppm lower than 880 MHz. Accordingly, the sampling rate of ADC 616 may be selected to be approximately equal to the intermediate frequency, i.e., 20.044 MHz in this non-limiting example. It should be appreciated that operating the ADC 616 at such a frequency in this context is below the Nyquist criterion, such that the ADC 616 may effectively operate as a mixer, compensating for the offset from 880 MHz of the reference signal 612. Again, it should be appreciated that this is merely one non-limiting example. It should also be appreciated from the foregoing example that the amount by which the sampling rate of ADC 616 may be frequency shifted compared to what would be appropriate if the reference signal 612 had the desired target frequency may equal the absolute offset of the reference signal from the desired target frequency. It should also be appreciated that if the reference oscillator is lower than a desired target frequency, the sampling rate of the ADC 616 may be selected to be higher than if the reference oscillator had the desired target frequency, and vice versa.
While the present aspect has been described with respect to the receive path of the RF front-end 600, it should be appreciated that the same concept may apply equally well to the transmit path 622, although that path is not illustrated in detail in FIG. 6. For example, the transmit path may include a digital-to-analog converter (DAC) configured to receive a digital signal from DSP 520 including digital data to be transmitted. The DAC may convert the digital signal to an analog signal, which may then be up-converted by mixing with a suitable oscillating reference signal (e.g., similar to reference signal 612). If the oscillating reference signal used for the up-conversion differs from a standard oscillator frequency, the data of the resulting up-converted signal may be shifted in the frequency domain relative to the intermediate frequency. Such a shift may be undesirable, and may be accounted for in some embodiments by sampling the digital data signal from the DSP using a suitable sampling rate of the DAC to account for the frequency shift which is induced during the up-conversion process using the arbitrary frequency reference signal. According to one embodiment, the sampling rate of the DAC may be selected to approximately match the frequency shift of the intermediate frequency introduced during up-conversion, in a manner analogous to that just described for the receive path. However, it should be appreciated that according to one embodiment if the arbitrary frequency of the oscillating reference signal used in the up-conversion process is greater than the expected standard oscillator frequency, then the sampling rate of the DAC may be lowered compared to what would be appropriate if the oscillating reference signal had the standard oscillator frequency, and vice versa.
According to another aspect of the present invention, a system receiving an oscillating signal from an oscillator having an arbitrary frequency includes a digital signal processor (DSP) which may be used to digitally shift data of a signal resulting from mixing a first signal (e.g., a cellular telephone signal) with the oscillating signal of arbitrary frequency. An example is now given with reference to FIG. 7, although it should be appreciated that other systems may similarly implement the described system and techniques. The RF front-end 700 of FIG. 7 is similar in many respects to RF front-end 500, and the same reference numbers are used to illustrate identical components.
As described, the RF front-end 700 may produce a down-converted signal from mixer 508, which may be low-pass filtered by filter 514 and then digitized by ADC 516. The reference signal 712 used for the mixing process may not have a frequency substantially matching the frequency of the carrier signal of radio signal 501, such that the data of the down-converted signal provided by mixer 508 may be shifted in the frequency domain relative to the intermediate frequency, as previously described. For example, the reference signal 712 may have a frequency of 900.045 MHz according to one embodiment, being offset from a target value of 900 MHz due to oscillator 602 producing an oscillating output signal of arbitrary frequency. According to the previous aspect, the sampling rate of the ADC (e.g., ADC 616 in FIG. 6) may be shifted to account for the shift in the frequency domain of the data of the down-converted signal. However, in the present aspect illustrated by FIG. 7, the sampling rate of the ADC 516 may be selected as if the reference signal 712 had a frequency matching that of the carrier signal of radio signal 501 (e.g., a target value of 900 MHz). As a result, the digital signal provided by ADC 516 may include digital data shifted in the frequency domain relative to the intermediate frequency. According to the present aspect, the DSP 720 may receive the digital signal from ADC 516 and may re-sample the digital signal at a sampling rate suitable to effectively shift the digital data of the digital signal such that it accurately represents the data of radio signal 501. According to this aspect, the DSP may effectively operate as a frequency mixer.
While the present aspect has been described with respect to the receive path of the RF front-end 700, it should be appreciated that the same concept may apply equally well to the transmit path 722, although that path is not illustrated in detail in FIG. 7. The transmit path 722 of RF front-end 700 may be similar to the illustrated receive path of FIG. 7, although the output of the DSP may be provided to a DAC, which may then be coupled to a mixer to perform up-conversion of the signal to be transmitted. As previously described with respect to the aspects relating to altering a sampling rate of a DAC to account for use of an oscillating reference signal of arbitrary frequency, the up-conversion process performed using the oscillating signal of arbitrary frequency may result in the data of the up-converted signal being shifted in the frequency domain relative to the baseband and intermediate frequencies. In some embodiments, such a frequency shift of the data may be undesirable, and may be accounted for by digitally shifting the digital data of the digital data signal output by the DSP. The frequency shift of the digital data may be induced by the DSP itself, according to one non-limiting embodiment. The amount of the frequency shift induced by the DSP may be selected to account for an expected frequency shift from the baseband and intermediate frequencies induced during up-conversion by the use of a reference signal of arbitrary frequency, and therefore in some embodiments may be selected based on a known value of the arbitrary frequency (e.g., provided from the oscillator as previously described). According to one embodiment, the DSP may shift the digital data to a lower frequency than would otherwise be used if the arbitrary frequency of the oscillating reference signal used for up-conversion is higher than the expected standard oscillator frequency. Similarly, the DSP may shift the digital data to a higher frequency than would otherwise be used if the arbitrary frequency of the oscillating reference signal used for up-conversion is lower than the expected standard oscillator frequency. By suitable selection of the amount of frequency shift to induce in the digital data signal, the subsequent DAC conversion and up-conversion using an oscillating reference signal of arbitrary frequency may result in the data of the up-converted signal appearing at a desired frequency or frequencies.
According to a further aspect of the present invention, a system receiving an oscillating signal having an arbitrary frequency may include a carrier tracking loop adapted to account for the arbitrary frequency of the oscillating signal. Reference is made to FIG. 8 for purposes of providing a non-limiting example. The reference signal 812 has a frequency that does not substantially match the frequency of the carrier signal of radio signal 501, and in this non-limiting example has a frequency of approximately 880.044 MHz, arising from the arbitrary frequency of the oscillating signal provided by oscillator 802 (which, it should be noted, is not configured to receive either a frequency steering signal or an AFC tuning signal). As a result, the down-converted signal provided by mixer 808 has data that is shifted in the frequency domain relative to the intermediate frequency. The resulting down-converted signal is filtered by band-pass filter 810. The sampling rate of the ADC 814 in this non-limiting aspect is selected as though the reference signal 812 has a frequency matching the frequency of the carrier signal of radio signal 501 including the offset equivalent to the intermediate frequency, such that the digital signal provided by ADC 814 includes digital data shifted in the frequency domain.
According to this aspect, a carrier tracking loop 815 is used at the intermediate frequency to lock to the carrier of the radio signal 501 at the intermediate frequency. For this purpose the digital intermediate frequency data is down-converted by a mixer 816 using a frequency from a numerically controlled oscillator (NCO) 822. The down-converted signal undergoes an integrate and dump 824 operation and is passed on to the receiver processor 826. The receiver processor includes a PLL discriminator and loop filter and controls the NCO 822. In this manner, the digital data of the signal output by ADC 814 is effectively down-converted to the baseband such that it accurately reflects the data of radio signal 501 independent of the frequency of the reference oscillator 802. As a result the reference oscillator 802 does not require a AFC to obtain lock to the carrier frequency of the received radio signal 501.
As mentioned, it should be appreciated that the foregoing aspects described with respect to systems and methods for operating upon an oscillating signal having an arbitrary frequency are merely non-limiting examples. Other systems and manners of adapting the configuration and/or operation of the components of the system may be implemented.
Furthermore, while the foregoing aspects have been described in the context of cellular telephones, it should be appreciated that they are not limited to such applications. For example, one or more of the aspects may apply to other types of communications systems (e.g., other wireless communications systems, WiFi systems, etc.), as well as to other systems which make use of an oscillating reference signal provided by an oscillator including a mechanical resonator, such as navigation receivers (e.g., GPS receivers), FM receivers, storage systems (e.g., Fibre storage systems, including those using a reference oscillator to generate or operate on an optical signal, etc.), video systems, wireless infrastructure (e.g., WiMax), networking systems (e.g., SPI-4, PCI Express, etc.) or other devices. Thus, it should be appreciated that the foregoing discussion is provided for purposes of illustration, and is not limiting.
Furthermore, it should be appreciated that the various aspects described herein may be used with oscillators and systems designed to provide and operate with any frequency of interest, and that the reference made to 26 MHz in describing various aspects is not limiting, but rather is used for purposes of illustration. For example, the aspects described herein may be used to generate oscillating signals having standard oscillator frequencies of 12 MHz, 12.6 MHz, 13 MHz, 14.4 MHz, 16 MHz, 16.368 MHz, 16.9 MHz, 19.2 MHz, 19.8 MHz, 20 MHz, 23.104 MHz, 24 MHz, 24.554 MHz, 26 MHz, 27 MHz, 27.456 MHz, 32 MHz, 33.6 MHz, 38.4 MHz, 52 MHz, 669.3266 MHz, any other standard oscillator frequency, or any other frequency or frequencies of interest.
It should be appreciated from the foregoing that various benefits may be attained by application of one or more of the described aspects. For example, manufacturing constraints of conventional quartz crystal resonators may be relaxed since such resonators need not provide a precise frequency matching a standard oscillator frequency to operate according to the various aspects described herein. Furthermore, resonator technologies other than conventional quartz crystal resonators may be used even if they are not easily manufactured to provide a precise output frequency matching a standard oscillator frequency. Thus, for example, MEMS resonators having output signal frequencies which may be manufactured to a precision of ±10,000 ppm may be used. Such resonators may provide beneficial characteristics in terms of signal noise (e.g., phase noise among others), reduced spurious modes, improved noise floor, jitter, ruggedness, cost, lower power consumption, and lighter weight, as well as other characteristics.
It should be appreciated that the various aspects of the invention described herein are not limited to use with oscillators employing any particular resonator technology. For example, the various aspects described herein may apply to oscillators using quartz crystal resonators, bulk acoustic wave (BAW) resonators, surface acoustic wave (SAW) resonators, plate acoustic wave (PAW) resonators, (thin) film plate acoustic resonators (FPAR), film bulk acoustic resonators (FBAR), solid mounted resonators (SMR), contour mode resonators (CMR), thin-film piezoelectric on silicon (TPoS), microelectromechanical systems (MEMS) technology, or any other type of resonator technology that uses mechanical vibrations in a solid to excite a resonance frequency and use this as a frequency reference in the oscillator. It should be appreciated that as used herein the term “mechanical resonator” encompasses at least quartz crystal resonators, BAW, SAW, PAW, SMR, FPAR, FBAR, CMR, thin-film piezoelectric on silicon (TPoS) resonator technology, and MEMS resonators. According to some embodiments, the oscillator may include a mechanical resonator comprising or formed of one or more of the following materials: Quartz, Langasite, Silicon, Silicon oxide, Aluminum Nitride, Lithium Tantalate, Lithium Niobate, Zinc oxide, Gallium Arsenide, Cadmium Sulfide, Germanium. Other resonator technologies may also be used.
Having thus described several aspects of at least one embodiment of the technology, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology. Accordingly, the foregoing description and drawings provide non-limiting examples only.

## Claims

1. A method comprising:
downconverting a cellular telephone signal, the cellular telephone signal modulated with data, using an oscillating reference signal having an arbitrary frequency to generate a downconverted signal including the data; and
sampling the downconverted signal with an analog-to-digital converter (ADC) using a sampling rate selected to induce a shift of the data of the downconverted signal in a frequency domain,
wherein the arbitrary frequency differs from a standard oscillator frequency by between approximately 5,000 parts per million (ppm) and approximately 10,000 ppm.

2. The method of claim 1, further comprising receiving, from an oscillator having a mechanical resonator, the oscillating reference signal having an arbitrary frequency.

3. The method of claim 1, further comprising generating the oscillating reference signal from an oscillator output signal, wherein generating the oscillating reference signal comprises receiving, from an oscillator having a mechanical resonator, the oscillator output signal at a frequency synthesizer and synthesizing the oscillating reference signal, wherein the oscillating reference signal differs in frequency from a frequency of the oscillator output signal.

4. The method of claim 3, wherein the frequency synthesizer is a phase-locked loop (PLL).

5. The method of claim 1, further comprising receiving a value of the arbitrary frequency separate from the oscillating reference signal and selecting the sampling rate of the ADC in response to receiving the value.

6. A method comprising:
downconverting a first signal, the first signal modulated with data, using an oscillating reference signal having an arbitrary frequency to generate a downconverted signal including the data;
sampling the downconverted signal with an analog-to-digital converter (ADC) to produce a digital signal including digital data corresponding to the data; and
shifting, using a digital signal processor (DSP), the digital data in a frequency domain,
wherein the arbitrary frequency differs from a standard oscillator frequency by between approximately 500 parts per million (ppm) and 2,000 ppm, and wherein sampling the downconverted signal with an ADC comprises using a sampling rate corresponding to the standard oscillator frequency.

7. The method of claim 6, further comprising receiving the first signal modulated with data.

8. The method of claim 6, further comprising receiving, from an oscillator having a mechanical resonator, the oscillating reference signal having an arbitrary frequency.

9. The method of claim 6, further comprising generating the oscillating reference signal from an oscillator output signal.

10. The method of claim 9, wherein generating the oscillating reference signal comprises receiving, from an oscillator having a mechanical resonator, the oscillator output signal at a frequency synthesizer and synthesizing the oscillating reference signal, wherein the oscillating reference signal differs in frequency from a frequency of the oscillator output signal.

11. The method of claim 10, wherein the frequency synthesizer is a phase-locked loop (PLL).

12. The method of claim 6, wherein the first signal is a cellular telephone signal.