Abstract:
A wideband frequency generator has two or more oscillators for different frequency bands, disposed on the same die within a flip chip package. Coupling between inductors of the two oscillators is reduced by placing one inductor on the die and the other inductor on the package, separating the inductors by a solder bump diameter. The loosely coupled inductors allow manipulation of the LC tank circuit of one of the oscillators to increase the bandwidth of the other oscillator, and vice versa. Preventing undesirable mode of oscillation in one of the oscillators may be achieved by loading the LC tank circuit of the other oscillator with a large capacitance, such as the entire capacitance of the coarse tuning bank of the other oscillator. Preventing the undesirable mode may also be achieved by decreasing the quality factor of the other oscillator&#39;s LC tank and thereby increasing the losses in the tank circuit.

Description:
BACKGROUND 
     1. Field 
     Apparatus and methods described in this document relate to frequency generators and methods for frequency generation. More specifically, the apparatus and methods relate to frequency generation using multiple oscillators. 
     2. Background 
     Tunable frequency generators are used in many different electronic devices. Wireless communication devices, for example, use frequency generators for upconversion of transmitted signals to intermediate and RF frequencies, and for downconversion of received signals to intermediate and baseband frequencies. Because operating frequencies vary, the generators&#39; frequencies need to be tunable. 
     Frequency coverage required for multiple communication standards and multiple bands typically necessitates wide tuning range oscillators, such as voltage controlled oscillators (VCOs) and digitally controlled oscillators (DCOs). The extent of an oscillator&#39;s tuning range is one important performance parameter. It is often desirable to increase the tuning range, for example, in order to cover multiple bands. 
     Other performance criteria of tunable oscillators include phase noise performance, power consumption, and size. The different performance criteria are sometimes competing. 
     Conventional tunable oscillators may be tuned by applying a varying biasing voltage to a variable capacitor (varactor or varicap), and by switching capacitors in the oscillator inductance-capacitance (LC) tank. For a variety of reasons, the frequency range of a single oscillator obtained through these capacitance-varying techniques is limited. For this reason, multiple tunable oscillators may need to be used within the same device. Especially in the case of portable devices, such as cellular handset and other handheld communication devices, it is often desirable to implement an oscillator on the same integrated circuit (IC or chip). 
     Inductors (the “L” in the “LC”) occupy substantial area of a small IC. It is, of course, desirable to reduce the physical size of ICs. Locating two or more LC oscillators on the same IC therefore presents certain design difficulties. Thus, it is desirable to reduce the IC area occupied by the inductors of the multiple oscillators. Furthermore, it may be desirable to reduce the coupling between or among the inductors of the different oscillators built on the same IC. 
     Given the physical proximity of the inductors located on the same IC, however, can make substantial inductor-to-inductor coupling difficult to avoid. Such coupling may result in unwanted oscillation modes of a particular oscillator, in addition to the desired oscillation mode resulting from the resonance of the LC tank of the oscillator. It may be desirable to suppress such additional oscillation modes, so that the particular oscillator will generate frequencies based on its own LC tank. 
     Therefore, there is a need in the art for tunable oscillators with extended frequency range. There is also a need in the art for reducing the size of the IC packages containing multiple tunable oscillators. There is an additional need in the art for suppressing undesirable modes of oscillation in oscillators with non-trivial coupling between their inductors. 
     SUMMARY 
     Embodiments disclosed herein may address one or more of the above stated needs by locating the inductor of the first oscillator on the die of a flip chip integrated circuit (IC), and locating the inductor of the second oscillator on the package of the IC. Varying the capacitance in the LC tank of one of the oscillators may extend the tunable range of the other oscillator within the same package, by changing the oscillation mode. Controlling the capacitance and/or the quality factor (Q) of the LC tank of one of the oscillators in a certain way may suppress undesirable oscillation modes of the other oscillator where the inductors of the LC tanks of the two oscillators are loosely coupled. 
     In an embodiment, a flip chip includes a die with electronic circuitry of a first oscillator, electronic circuitry of a second oscillator, and a first inductor of the first oscillator. The flip chip also includes a package with a second inductor of the second oscillator. 
     In an embodiment, an integrated circuit includes electronic circuitry of a first oscillator, a first inductor configured for use in an LC tank of the first oscillator, electronic circuitry of a second oscillator, a second inductor configured for use in an LC tank of the second oscillator, and a second oscillator control module. The second inductor is loosely coupled to the first inductor. The second oscillator control module is configured to switch a second capacitance into the LC tank of the second oscillator when the second oscillator is inactive. Switching the second capacitance causes oscillation mode of the first oscillator to change from a first mode to a second mode. 
     In an embodiment, a method of generating signals includes providing a first oscillator having a first inductor configured for use in an LC tank of the first oscillator. The method also includes providing a second oscillator having a second inductor configured for use in an LC tank of the second oscillator. The method additionally includes operating a second oscillator control module configured to switch a second capacitance into the LC tank of the second oscillator when the second oscillator is inactive. Switching the second capacitance into the LC tank of the second oscillator causes oscillation mode of the first oscillator to change from a first mode to a second mode. 
     In an embodiment, an integrated circuit includes electronic circuitry of a first oscillator, a first inductor configured for use in an LC tank of the first oscillator, electronic circuitry of a second oscillator, a second inductor configured for use in an LC tank of the second oscillator, and a means for causing oscillation mode of the first oscillator to change from a first mode to a second mode when the second oscillator is inactive. The second inductor is loosely coupled to the first inductor. 
     In an embodiment, a flip chip includes a die having electronic circuitry of a first oscillator, electronic circuitry of a second oscillator, and a first inductor of the first oscillator. The flip chip also includes a second inductor of the second oscillator. The flip chip further includes a means for packaging the die and keeping the second inductor loosely coupled to the first inductor. 
     In an embodiment, an electronic device includes a first oscillator with a first LC tank. The first LC tank includes a first inductor. The electronic device also includes a second oscillator with a second LC tank. The second LC tank has a second inductor. The second inductor is magnetically loosely coupled to the first inductor. The electronic device further includes a coarse tuning circuit for coarse tuning the second oscillator by selectively switching capacitors from a capacitor bank into the second LC tank. The coarse tuning circuit is configured to switch all capacitors of the bank into the second LC tank when the first oscillator is operating and the second oscillator is not operating in order to suppress tendency of the first oscillator to oscillate in an undesirable mode. 
     In an embodiment, an electronic device includes a first oscillator with a first LC tank. The first LC tank has a first inductor. The electronic device also includes a second oscillator with a second LC tank. The second LC tank has a second inductor. The second inductor is magnetically loosely coupled to the first inductor. The electronic device further includes a first quality factor reducing circuit coupled to the first LC tank. The first quality factor reducing circuit is configured to reduce quality factor of the first LC tank when the second oscillator is operating and the first oscillator is not operating, in order to suppress tendency of the second oscillator to oscillate in an undesirable mode. 
     In an embodiment, an electronic device has a first oscillator and a second oscillator. The first oscillator has a first LC tank with a first inductor. The second oscillator has a second LC tank with a second inductor. The second inductor is magnetically loosely coupled to the first inductor. The electronic device also includes a means for reducing quality factor of the first circuit when the second oscillator is operating and the first oscillator is not operating in order to suppress tendency of the second oscillator to oscillate in an undesirable mode. 
     In an embodiment, a method is disclosed for operating a frequency generator with a first oscillator and a second oscillator, where the second oscillator has a plurality of capacitors for coarse tuning the second oscillator. The inductors of the LC tanks of the two oscillators are loosely coupled. The method includes activating the first oscillator while not activating the second oscillator, and loading LC tank of the second oscillator with the plurality of capacitors when activating the first oscillator. 
     In an embodiment, a method of operating a frequency generator having a first oscillator and a second oscillator includes activating the first oscillator while not activating the second oscillator, and loading LC tank of the second oscillator with an energy dissipating element when activating the first oscillator. 
     In an embodiment, a method of operating a frequency generator that has a first oscillator and a second oscillator includes activating the first oscillator while not activating the second oscillator, and a step for reducing quality factor of a tank circuit of the second oscillator to reduce tendency of the first oscillator to oscillate in an undesirable mode. 
     These and other embodiments and aspects of the present invention will be better understood with reference to the following description, drawings, and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  illustrate selected elements of a flip chip structure of an integrated circuit implementing two oscillators; 
         FIGS. 1C and 1D  are perspective views of an exemplary juxtaposition of loop inductors of the flip chip structure of  FIGS. 1A and 1B ; 
         FIG. 2  is a block diagram illustrating selected components of a frequency generator with two digitally controlled oscillators; 
         FIG. 3  illustrates selected components of a frequency generator configured to suppress undesired oscillation modes; 
         FIG. 4  illustrates selected aspects of examples of impedance curves of the LC tank of one of the oscillators of the frequency generator shown in  FIG. 3 ; 
         FIG. 5  illustrates selected components of another frequency generator configured to suppress undesired oscillation modes; and 
         FIG. 6  illustrates selected aspects of examples of impedance curves of the LC tank of one of the oscillators of the frequency generator shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     In this document, the words “embodiment,” “variant,” and similar expressions are used to refer to particular apparatus, process, or article of manufacture, and not necessarily to the same apparatus, process, or article of manufacture. Thus, “one embodiment” (or a similar expression) used in one place or context may refer to a particular apparatus, process, or article of manufacture; the same or a similar expression in a different place may refer to a different apparatus, process, or article of manufacture. The expression “alternative embodiment” and similar phrases may be used to indicate one of a number of different possible embodiments. The number of possible embodiments is not necessarily limited to two or any other quantity. 
     The word “exemplary” may be used herein to mean “serving as an example, instance, or illustration.” Any embodiment or variant described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or variants. All of the embodiments and variants described in this description are exemplary embodiments and variants provided to enable persons skilled in the art to make and use the invention, and not necessarily to limit the scope of legal protection afforded the invention. 
     For purposes of convenience and clarity only, directional terms, such as top, bottom, left, right, up, down, over, above, below, beneath, rear, back, and front, may be used with respect to the accompanying drawings or chip embodiments. These and similar directional terms should not be construed to limit the scope of the invention in any manner. 
     “Loosely coupled” and similar expressions referring to magnetic coupling of coils or loops means magnetic coupling (as used in transformer theory) with a coupling coefficient (k) of less than 0.25. It should be noted that in some below-described embodiments the coupling coefficient of coils or loops described as “loosely coupled” may be less than 0.2; less than 0.15; and less than 0.10. 
     “Common mode” resonance refers to resonance (or oscillation) of an oscillator LC tank that includes a first loop inductor of a pair of loosely coupled loop inductors wherein the current in a second loop inductor of the pair of loop inductors flows so that the flux of the second loop inductor generally adds to the flux of the first loop inductor. “Differential mode” resonance refers to oscillation wherein the two fluxes tend to subtract or negate each other. For substantially concentric loop inductors described below, the currents in the two loop indictors flow in generally the same direction in the common mode oscillation; the currents flow in generally opposite directions in the differential mode oscillation. 
     A “loop” inductor need not form a closed circle, but may form a partial circle. Moreover, it need not be strictly circular, but may be part of a polygonal shape, such as a hexagon or an octagon. 
     The “VCO” and “DCO” designations may be used interchangeably within the description, each referring to a tunable oscillator, particularly where the oscillator is tunable through varying of the capacitance of the oscillator&#39;s LC tank. 
       FIGS. 1A and 1B  illustrate selected elements  100  of a flip chip structure of an integrated circuit implementing two oscillators. The outputs of the two oscillators may be independently (alternatively) selectable, for example, if the oscillators are parts of a wideband frequency generator. The  FIG. 1A  is a perspective view of the structure without its package  130 , while  FIG. 1B  is a cross-sectional view that includes the package  130 . The flip chip structure includes a die  101  with electronic circuitry and bond pads  110  formed on a first surface  105  of the die  101 . Solder bumps  115  are deposited or otherwise formed on the pads  110 . Electrical connections between some or all of the electrical components of the oscillator circuitry on the die  101  are made by soldering or otherwise connecting the bumps  115  to pads  135  of the package  130 . As a person of skill in the art would understand after perusal of this document, the die  101  may contain electronic circuitry in addition to the circuitry of the oscillators described herein; electrical connections to such additional electronic circuitry may be made using electrical traces in the die  101  and/or vias connecting various layers of the die  101 , and/or otherwise. One side of the package would thus adjoin and be parallel to the first surface  105  of the die  101 , as is apparent from the cross-sectional view in  FIG. 1B . The pads  135  of the package  130  are connected to connecting elements (e.g., pins or balls)  140 , which thus connect the circuitry of the die  101  to devices external to the flip chip structure  100 . 
     It should be noted that, in the above-described embodiments, various electronic components (such as decoupling capacitors) may be formed on a packaging substrate, and be coupled to the electronic components on the die  101  through solder bumps  115 . 
     The structure  100  includes an inductor  160  in the form of a loop formed on the surface  105  of the die  101 . The inductor  160 , which may be deposited on the die  101 , is connected to other devices of the die  101  by connecting elements  165 . This inductor is part of an LC tank of a first oscillator in the structure  100 . A second inductor  150  in the form of a loop is formed on a surface  132  of the package  130  that faces the surface  105  of the die  101 . In other words, the surface  132  is the surface on which the pads  135  are formed. The inductor  150  is connected to other devices of the die  101  by connecting elements  155 , the pads  135 , the solder bumps  115 , and the pads  110 . The inductor loop  150  is concentric or substantially concentric (e.g. within manufacturing process tolerances) with the inductor loop  160 . The inductors  150  and  160  may both be of the same essential shape, such as circular, hexagonal, or octagonal, or their respective shapes may differ. Note that here the inductors  150  and  160  are stacked and the vertical distance between them (i.e., the distance between the centers of the inductors along the line normal to the planes of the surfaces  105  and  132 ) is approximately equal to the diameter of the bumps  115 , and the separation between the inductors is greater than the separation available between any two metallization layers on the die  101 . In this way, the magnetic coupling between the two inductors may be reduced, for example, to a value less than 0.25, less than 0.2, and/or less than 0.15. The two inductors  150  and  160  may thus become loosely coupled. 
       FIGS. 1C and 1D  are perspective views of an exemplary juxtaposition of the inductor loops  150  and  160  of the structure  100 . 
     It should be noted that in some embodiments, the two loops are not necessarily substantially concentric, and are not necessarily disposed on the surfaces  105  and  132  as is shown in  FIGS. 1 . 
     Loose coupling of the inductors  150  and  160  of the two oscillators may be used advantageously to increase the tuning range of each of the two oscillators built on the same chip or otherwise in proximity of each other. (Proximity here means a distance causing loose coupling of the inductors.) Let us consider a case where the first oscillator that uses the first inductor  160  has a tuning range higher than the tuning range of the second oscillator using the second inductor  150 . Note that in embodiments the frequency relationship may be reversed. We will refer to the higher frequency oscillator as an “HF oscillator” and to the lower frequency oscillator as an “LF oscillator” in the description below. Note that in embodiments the frequency tuning ranges of the two oscillators overlap; in other embodiments, they merely adjoin each other; and in still other embodiments, there is a gap between the ranges. 
     Because of the loose coupling of the HF and LF oscillators, the resonance of either oscillator will be affected by the state of the LC tank of the other oscillator. Thus, varying the capacitance of the LC tank of the HF oscillator will generally change the tuning range of the LF oscillator, and varying the capacitance of the LC tank of the LF oscillator will generally change the tuning range of the HF oscillator. 
     For a pair of LC oscillators with loosely coupled inductors, one of the oscillators tends to oscillate in the common mode when the resonant frequency of its LC tank is lower than the resonant frequency of the LC tank of the other oscillator. The first oscillator tends to oscillate in the differential mode when the resonant frequency of its LC tank is higher than the resonant frequency of the LC tank of the other oscillator. These statements are general in nature, and the transitions between the common and differential modes are not abrupt and do not necessarily occur precisely when the LC tank resonant frequencies cross over. But the general principle holds for oscillators with loosely coupled inductors. 
       FIG. 2  is a block diagram illustrating selected components of a frequency generator  200  made with two digitally controlled oscillators  220  and  240 , which can be operated selectively to cover a wide frequency band. The low frequency oscillator  220  includes an LF coarse tuning module  237 , an LF fine tuning and acquisition module  235 , an LF core  230 , and an inductor  225 , which may be the inductor  150  from  FIGS. 1A and 1B . The high frequency oscillator  240  includes an HF coarse tuning module  260 , an HF fine tuning and acquisition module  255 , an HF core  250 , and an inductor  245 , which may be the inductor  160  from  FIGS. 1A and 1B . The low frequency oscillator  220  may be implemented using a first negative transconductance (negative Gm) circuit, and the high frequency oscillator may be implemented using a second negative Gm circuit. The first negative Gm circuit may be part of the LF core  230 , and the second negative Gm circuit may be part of the HF core  250 . 
     When the output of a particular oscillator (either  220  or  240 ) is selected as the output of the generator  200 , the core of that oscillator operates the coarse and fine tuning modules of the oscillator to select the required frequency within the band of the oscillator. Each of the fine and coarse tuning modules may be implemented as a bank of switchable capacitors, as is known. 
     The HF core  250  and the HF coarse tuning module  260  can be configured to bring the resonant frequency of the LC tank of the HF VCO  240  below the highest resonant frequency of the LC tank of the LF VCO  220 . In this configuration, the HF VCO  240  may loose some performance characteristics that it possesses within its normal frequency band of operation. But it is the LF VCO  220  that is selected now for operation, not the HF VCO  240 . The “crossover” of the HF VCO  240  LC tank resonant frequency below that of the LC tank of the LF VCO  220  advantageously switches the mode of the LC tank of the LF VCO  220  from common to differential. Consequently, the top frequency obtainable from the LF VCO  220  now moves up, beyond that normally achievable from the LF VCO  220 . The tuning range of the LF VCO  220  is broadened beyond its normal operating band, i.e., beyond the frequency band that the LF VCO  220  could provide in the absence of the coupling between the inductors of the two oscillators. 
     Analogously, the LF core  230  and the LF tuning modules  235  and  237  can be configured to bring the resonant frequency of the LC tank of the LF VCO  220  above the lowest resonant frequency of the LC tank of the HF VCO  240 . In this configuration, the LF VCO  220  may loose some performance characteristics that it possesses within its normal frequency band of operation. But it is the HF VCO  240  that is selected now for operation, rather than the LF VCO  220 . The “crossover” of the LF VCO  220  LC tank resonant frequency above that of the LC tank of the HF VCO  240  advantageously switches the mode of the LC tank of the HF VCO  240  from differential to common. Consequently, the low frequency obtainable from the HF VCO  240  now moves lower, below that normally achievable from the HF VCO  240 . The tuning range of the HF VCO  240  is thus broadened beyond its normal operating band, i.e., beyond the frequency band that the HF VCO  240  could provide in the absence of the coupling between the inductors of the two oscillators. 
     In a particular simulated design example, the low end frequency of a tunable LF VCO is about 2.8 GHz. The high end frequency of the LF VCO is about 3.94 GHz when a tunable HF VCO with a loosely coupled inductor is configured so that the resonant frequency of its LC tank is above the cross over frequency, so that the LF VCO operates in the common mode. When the HF VCO is configured so that the resonant frequency of its LC tank is below the crossover frequency (i.e., below the frequency of the LF VCO), the high end of the LF VCO frequency band shifts up to about 4.13 GHz. In the same example, the high end frequency of the HF VCO tuning range is at about 5.5 GHz. The low end of the HF VCO tuning range is at about 3.82 GHz when the LF VCO is configured so that the resonant frequency of the LC tank of the LF VCO is below the cross over frequency. The low end of the HF VCO tuning range shifts down to about 3.7 GHz when the LF VCO is reconfigured so the resonant frequency of its LC tank moves above the crossover frequency, i.e., the resonant frequency of the LC tank of the LF VCO moves above the frequency of the HF VCO. 
     In practice, the HF VCO may include a large capacitor C Hext  that, when switched into a parallel combination with other components of the HF VCO (the inductor and any other capacitors that may at the same time appear in the LC tank of the HF VCO, such as the stray capacitances and the switchable capacitors of the HF tuning modules  255  and  260 ), may bring the resonant frequency of the LC tank of the HF VCO below the crossover frequency at the top or somewhat below the top of the normal tuning range of the LF VCO. Analogously, the LF VCO may include a capacitor C Lext  that, when switched out of the parallel combination with the inductor of the LF VCO (leaving stray capacitances that may at the same time appear in the LC tank of the LF VCO and possibly some other capacitances such is those of one of the tuning modules), may bring the resonant frequency of the LC tank of the LF VCO above the crossover frequency at the bottom or somewhat above the bottom of the normal tuning range of the LF VCO. It should be noted that the “normal” range of the LF VCO is typically with the C Lext  capacitor in the circuit, and the normal range of the HF VCO is typically without the C Hext  capacitor in the circuit. The capacitor C Hext  may be a separate capacitor, or it may be realized as a combination of capacitors within the HF Coarse Tuning Module  260 , and possibly in the Fine Tuning Module  255 . The capacitor C Lext  may be a separate capacitor, or it may be realized as one or a combination of capacitors within the LF Fine Tuning Module  235 . After perusing this document, a person skilled in the art should be able to come up with similar ways to bring the resonant frequency of the LC tank of the HF VCO below the top of the tuning range of the LF VCO, and to bring the resonant frequency of the LC tank of the LF VCO above the bottom of the tuning range of the HF VCO. For example, switchable inductors may be used. 
     As noted above, there is a need to prevent unwanted oscillation modes in oscillators of a frequency generator that includes two (or possibly more) oscillators in order to cover a wide tuning range. This need is typically most acute when the inductors of the oscillators are magnetically coupled with a relatively high coupling coefficient, but the need may arise in other cases, including in the oscillators described above. We describe two approaches for reducing the tendency of one of two loosely coupled oscillators to oscillate in an undesirable mode. One approach is to load the LC tank of the other oscillator of the generator with a large capacitance. The second approach is to reduce the quality factor (Q) of the LC tank of the other oscillator. 
       FIG. 3  illustrates selected components of a frequency generator  300  configured in accordance with the first approach. Most of the components of the generator  300  may be similar to the analogous components of the generator  200  described above. Note that the inductors  325  and  345  may or may not be disposed as shown in  FIGS. 1A and 1B . For example, the inductors  325  and  345  may be both disposed on the same die. 
     When the generator  300  enables the HF VCO  340 , it loads the unused LC tank of the LF VCO  320  with a large capacitance, for example, with the entire coarse tuning bank of the module  337 . Fewer than all coarse tuning capacitors may also be used for loading the unused LC tank of the LF VCO  320 . An additional capacitor may also be used to load the unused LC tank. The frequency generator  300  also includes a multiplexer  370 , which is configured to selectively load the LC tank of the LF VCO  320  with the entire or partial coarse tuning bank  337 . The generator is configured so that the MUX  370  loads the LC tank of the LF VCO  320  in response to a signal received on its input  372 , which may be the HF VCO select signal. When the generator  300  selects the LF VCO, the MUX  370  transmits the code corresponding to the desired coarse tuning of the LF VCO  320  to the LF Coarse Tuning Module  337 . 
       FIG. 4  illustrates selected aspects of examples of impedance curves of the LC tank of the HF VCO  340 . The first curve  410  is a typical impedance curve where the LC tank of the LF VCO is not loaded with the entire coarse tuning bank. Note the relatively pronounced secondary impedance peak around 4000 MHz. The second curve  420  is the impedance curve of the same HF VCO  340 , but with the LF VCO LC tank loaded with the entire coarse tuning bank. Note that the secondary peak has shifted lower in the frequency spectrum, and also became less pronounced. The frequency shift away from the primary peak makes the unwanted mode oscillation less likely, as does the suppression of the magnitude of the secondary peak. 
     The other approach is to reduce the Q of the unused LC tank. This approach may be used in both low frequency and high frequency oscillators.  FIG. 5  illustrates selected components of a generator  500  where this approach is implemented in the LF VCO  520 . Most of the components of the generator  500  may be similar to the analogous components of the generator  300  described above, but with the Q loading circuit  570  replacing the MUX  370 . When the HF VCO  540  is selected (HF VCO select is active), the Q loading circuit  570  essentially closes a “bad” switch and inserts a dissipating element across the LC tank of the LF VCO  520 . When the LF VCO  520  is operating, the switch is open and the LF VCO operates normally. 
       FIG. 6  illustrates selected aspects of examples of impedance curves of the LC tank of the HF VCO  540 . The first curve  610  is the same as the curve  410 , with the relatively pronounced secondary peak around 4000 MHz. The second curve  620  is the impedance curve of the same HF VCO  540 , but with the Q of the LF VCO LC tank reduced by the circuit  570 . Here as well the secondary peak has shifted lower in the frequency spectrum and became somewhat less pronounced, making the unwanted mode oscillation less likely. 
     It should be noted that the oscillators of the above-described embodiments may include more than a single coil in its LC tank. For example, an oscillator may include two, three, or a higher number of coils. 
     The apparatus and methods described in this document can be used in various electronic devices, including, for example, access terminals operating within a cellular radio network transporting voice and/or data packets between multiple access terminals of the network, or between the access terminals and devices connected to additional networks outside the access network. In particular, the apparatus and methods may be used in the local oscillator frequency source of an access terminal. 
     Although steps and decisions of various methods may be described serially in this disclosure, some of these steps and decisions may be performed by separate elements in conjunction or in parallel, asynchronously or synchronously, in a pipelined manner, or otherwise. There is no particular requirement that the steps and decisions be performed in the same order in which this description lists them, except where explicitly so indicated, otherwise made clear from the context, or inherently required. It should be noted, however, that in selected variants the steps and decisions are performed in the particular sequences described and/or shown in the accompanying Figures. Furthermore, not every illustrated step and decision may be required in every embodiment or variant, while some steps and decisions that have not been specifically illustrated may be desirable in some embodiments/variants. 
     Those of skill in the art would understand that some embodiments described herein require a flip-chip kind of package, but that in other embodiments the use of flip-chip packages is optional. Thus, the embodiments that prevent (or reduce the possibility of) unwanted oscillations through LC resonant frequency and/or quality factor manipulations may but need not be implemented in flip-chip packages. 
     Those of skill in the art would understand that an inductor of a first LC combination of a first oscillator may be placed on or in the package, while an inductor of a second LC combination of a second oscillator may be placed on a surface of the die facing the package, on another surface of the die, or on an intermediate layer of the die between the two surfaces of a multilayer die. In some embodiments, each of the inductors may be selectively placed on either surface or on any of the intermediate layers of a multilayer die. 
     Those of skill in the art would also understand that information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To show clearly this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps may have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware, software, or combination of hardware and software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The various illustrative logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g. a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The steps of a method or algorithm that may have been described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in an access terminal. Alternatively, the processor and the storage medium may reside as discrete components in an access terminal. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make and use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.