Abstract:
? An RF Voltage Controlled Oscillator (VCO) design having improved power supply noise immunity. More particularly, a VCO resonant circuit that provides a high circuit Q, immunity to noise, and is tunable over multiple distinct bands. The resonant circuit is implemented in conjunction with an integrated circuit oscillator that requires a tuned circuit to determine the frequency of operation. When the integrated circuit oscillator is used as a Local Oscillator (LO) within a wireless phone it is subjected to numerous sources of power supply noise. In a Code Division Multiple Access (CDMA) wireless phone system the power supply to portions of the RF transmit path are cycled on and off depending on the transmitted data rate. The present invention provides an oscillator with increased immunity to the noise induced on the power supply due to power supply cycling.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    I. Field of the Invention  
           [0002]    The present invention relates to electronic circuits. More particularly, the present invention relates to a novel and improved band switched Voltage Controlled Oscillator (VCO) with noise immunity.  
           [0003]    II. Description of the Related Art  
           [0004]    Wireless communication systems rely on the predictable performance of over the air Radio Frequency (RF) links. Wireless phone systems are required to simultaneously monitor and control numerous RF links.  
           [0005]    A mobile unit or wireless phone integrates numerous complex circuits. An RF transceiver is used to provide the wireless communication link with base stations. The RF transceiver is comprised of a receiver and a transmitter. The receiver receives the RF transmission from the base station via an antenna interfaced to the mobile unit. The receiver amplifies, filters, and downconverts the received signal to baseband signal. The baseband signal is then routed to a baseband processing circuit. The baseband processing circuit demodulates the signal and conditions it for broadcast through a speaker to the user.  
           [0006]    User input via keypad presses or voice input to a microphone is conditioned in the baseband processing circuit. The signal is modulated and routed to the transmitter. The transmitter takes baseband signals generated at the mobile unit and upconverts, filters, and amplifies the signal. The upconverted RF signal is transmitted to the base station through the same antenna as used for the receiver.  
           [0007]    Frequency synthesizers are used to generate the local oscillator signals required to perform the downconversion in the receiver and the upconversion in the transmitter. Frequency synthesis is used to generate the local oscillator signal because of the synthesizer&#39;s frequency stability, the spectral purity of the resultant signal, and the ability for digital control.  
           [0008]    Frequency synthesizers are classified as direct or indirect. In Direct Digital Synthesis logic circuits generate a digital representation of the desired signal and a D/A converter is used to convert the digital representation into an analog waveform. One common way of implementing DDS is to store a table of waveform phases in memory. Then the rate at which the phases are clocked out of memory is directly proportional to the frequency of the output signal. While DDS can generate an extremely accurate representation of a sine wave, the output frequency is limited by the clocking rate.  
           [0009]    Indirect synthesis utilizes a phase lock loop locked to the output of an oscillator. Indirect frequency synthesis is more popular for high frequency designs because the output of a high frequency oscillator can be divided down to a frequency within the operating range of the phase lock loop.  
           [0010]    [0010]FIG. 1 shows a block diagram of an indirect frequency synthesizer utilizing a phase lock loop. A VCO  110  capable of tuning over the desired frequency range is used to provide the LO output  112 . The output of the VCO  110  is also sent to the input of a frequency divider circuit  120 , denoted ÷N where N represents the divider ratio. The divided output is provided as a first input to a phase detector  130 . A second input to the phase detector  130  is the output of a reference oscillator  140 . The phase lock loop operates to tune the output of the VCO  110  such that the output of the frequency divider  120  is identical to the output of the reference oscillator  140 . The phase detector  130  provides an output signal corresponding to a phase error between the two input signals. The phase detector  130  output is conditioned through a Low Pass Filter (LPF) before it is provided to the frequency control input of the VCO  110 . Thus, the VCO  110  is controlled to maintain phase lock with the reference oscillator  140 . It can be readily deduced from the block diagram that incrementing or decrementing the value of the divider ratio N results in a frequency change in the LO output  112  equal to the reference oscillator  140  frequency. The frequency of the reference oscillator  140  determines the frequency step size of the LO.  
           [0011]    Frequency variations in the VCO  110  output can only be corrected by the phase lock loop if the rate of the frequency variations is less than the loop bandwidth. The phase lock loop is unable to correct for VCO frequency variations that occur at a rate higher than the loop bandwidth. The settling time of the phase lock loop will depend on the initial frequency offset and the loop bandwidth. A wider loop bandwidth results in a faster settling time. A VCO with good noise immunity will reduce frequency variations thereby reducing the settling time of the phase lock loop. Therefore, it is important to design a VCO with good noise immunity while maintaining the frequency tuning characteristics.  
           [0012]    A VCO is merely a tunable oscillator. A typical oscillator circuit is comprised of an amplifier and a resonant circuit commonly referred as a resonant circuit. The resulting oscillator has a frequency output where the gain is greater than unity and the phase is equal to zero. The resonant circuit sets this frequency of oscillation. The relationship is most easily seen on a Bode diagram. FIG. 2A illustrates a Bode diagram for a typical oscillator. Curve  210  is representative of the gain in decibels of the oscillator as referenced to the left vertical axis and Curve  220  is representative of the phase in degrees as referenced to the right vertical axis. As indicated by Point  230 , the oscillation occurs when the oscillator gain is approximately 14 dB and the phase is zero producing an oscillation at approximately 124 MHz.  
           [0013]    To create a VCO the resonant circuit is comprised of at least one variable component wherein the reactance of the variable component is a function of a control signal, typically a voltage level, so that the frequency of zero phase, and consequently the frequency of oscillation, is also variable. When the VCO is required to tune over a large frequency range the variable component must be capable of tuning the resonant circuit over the large frequency range. Possible circuit implementations for a variable resonant circuit capable of covering a large frequency range include a resonant circuit incorporating a highly sensitive variable component or a resonant circuit requiring an extended control voltage range. The first alternative presents some problems because the VCO gain, measured in terms of MHz/Volt, becomes very high. This results in large frequency changes for relatively small control voltage changes and makes the VCO more susceptible to noise induced on the tuning line. The second alternative also has disadvantages since the required control voltage range is very large. Large control voltages can present a problem in mobile battery powered electronics having limited available supply voltage ranges.  
           [0014]    A third alternative to designing a VCO to cover a wide tuning range can be implemented in applications where distinct frequency bands must be supported. This situation occurs commonly in the design of a dual band wireless phone. Wireless phones most commonly operate in the cellular band (Transmit band 824-849 MHz, Receive band 869-894 MHz) and the Personal Communication System (PCS) band (Transmit band 1850-1910 MHz, Receive band 1930-1990 MHz). A single phone can be designed to operate in both cellular and PCS bands. The frequency plan within the phone is typically designed to minimize the number of oscillators thereby minimizing the cost of the phone. However, even the most judicious frequency plan requires different LO frequencies when operating in one band over the other. In order to support both the cellular and PCS operating bands, components are selectively switched in the resonant circuit of the oscillator. Components are included in the resonant circuit of an oscillator and switched to the diode switches. The circuit&#39;s operating frequency limits the particular type of diode used for the switch. When the switch is in the closed position the diode must be capable of carrying varying RF currents while maintaining a minimal resistance. When the switch is in the open position the diode must be capable of isolating the RF voltages and maintaining a high resistance. A PIN diode switch is commonly used at RF frequencies for a switch although other types of diodes may be used as a switch. Additionally, the circuit is not limited to the use of a diode switch. Any switch that is capable of carrying RF currents in the closed position and is capable of RF isolation in the open position can be implemented within the circuit.  
           [0015]    When the diode switch is forward biased the switched component becomes active within the resonant circuit. When the diode switch is not forward biased, the component does not contribute electrically to the resonant circuit. Switching a component in the resonant circuit greatly extends the tuning range of the oscillator without a corresponding increase in the VCO gain.  
           [0016]    It is not sufficient that the resonant circuit tune the oscillator to the desired operating frequency. The Q of the resonant circuit is important in maintaining a specific output frequency at a given control voltage level. FIG. 2B depicts the phase response of two resonant circuits having different Q values. A lower circuit Q generates a more gentle phase response, whereas a higher circuit Q generates a sharper phase response. A higher circuit Q is desired to minimize the effects of small phase variations on output frequency. The phase response of a circuit having a relatively low circuit Q is shown in curve  240 . Curve  250  illustrates a circuit having a higher circuit Q. It can be seen for a given phase variation the change in frequency is more pronounced in the circuit having the lower circuit Q. The magnitude of f 2 , the frequency change in a low Q circuit for a given phase variation, is greater than the magnitude of f 1 , the frequency change in a high Q circuit for the same phase variation.  
           [0017]    Application specific integrated circuits are available that integrate many wireless phone functions into a single IC. Frequency synthesizer IC&#39;s are available that integrate nearly all of the required synthesizer circuits onto one chip. Typically, the user of one of these IC&#39;s only needs to provide a resonant circuit, loop filter, and reference oscillator in addition to the IC in order to produce a synthesized LO. The remaining elements of the synthesizer, the amplifier portion of the VCO, the frequency divider, and the phase detector are integrated onto one IC. The user provides the resonant circuit required generating the desired output frequency. The user also provides the low pass filter design generating the desired loop bandwidth.  
           [0018]    Although application specific IC&#39;s simplify the implementation of the LO in a wireless phone, the wireless phone operating environment presents additional noise sources which must be considered. Cost and space limitations in a wireless phone further constrain available noise filtering solutions.  
           [0019]    The mobile phone design differs greatly depending on the particular mobile system it is supporting. Specifications outlining mobile phone design include Telecommunications Industry Association (TIA)/Electronic Industries Association (EIA) IS-95-B MOBILE STATION-BASE STATION COMPATABILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM SYSTEMS as well as TIA/EIA IS-98-B, RECOMMENDED MINIMUM PERFORMANCE STANDARDS FOR DUAL-MODE SPREAD SPECTRUM CELLULAR MOBILE STATIONS. The specification covering the operation of a CDMA system in the Personal Communication Systems (PCS) band is the American National Standards Institute (ANSI) J-STD-008 PERSONAL STATION-BASE STATION COMPATIBILITY REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL COMMUNICATIONS SYSTEMS. Similarly, the phone, or personal station, is specified in ANSI J-STD-018, RECOMMENDED MINIMUM PERFORMANCE REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL STATIONS. Additionally, the mobile phone specification defines features which, when implemented in phone hardware, tend to increase sources of noise within the phone.  
           [0020]    One beneficial feature that is utilized in CDMA phone systems such as those specified in IS-95 and J-STD-008 is multiple data rate sets. In order to take advantage of the variable nature of a wireless phone communication link, the CDMA specifications provide for data transmission at reduced rates. When a person is engaged in a telephone conversation there are numerous periods in which only one party will be speaking. During periods of reduced speech activity the telephone can reduce the data rate of the transmission resulting in a lower average transmit power level.  
           [0021]    The communication link from the wireless phone back to the base station is termed the reverse link. On the reverse link, reduction in average transmit power is accomplished by turning off the transmitter for a fraction of the time during periods when activity is low. In a CDMA reverse link the phone always transmits at the full data rate however, when the internal structure allows operation at a reduced data rate the data is repeated a number of times. As an example, when the phone is able to operate at one-half of the full data rate the information is repeated twice to bring the transmitted data rate up to the full data rate. Similarly, one-fourth rate data is repeated four times to achieve a full data rate.  
           [0022]    To conserve power on the reverse link, each 20 mS data frame is subdivided into sixteen 1.25 mS time groupings. When the phone is operating at a full data rate all sixteen of the groups within the frame are transmitted. However, when the phone is operating at a reduced data rate only a fraction of the sixteen groups is transmitted. The fraction of groups transmitted is equal to the reduction in the data rate. When the phone operates at one-half the full data rate one-half of the groups is transmitted. However, note that no data is lost since data is repeated in inverse proportion to the data rate reduction. One-half rate data is repeated twice but only half of the data is transmitted. The redundant portion of the data is not transmitted. Similarly, one-eighth rate data is repeated eight times but only one-eighth of the data is transmitted.  
           [0023]    When the phone operates at a reduced data rate, power is gated to select active circuits on the transmit path. The power to the circuits is gated off when the data is not being transmitted. The power is gated back on to the circuits prior to transmitting the desired data group. Power gating serves to conserve power within the wireless phone. This results in a much desired extended battery life.  
           [0024]    An adverse effect of power gating is the sudden load changes applied to the phone power supply. The portions of the RF transmit path that are switched on and off present the greatest loads on the power supply. Therefore, during power gating, the phone power supply is subjected to the greatest load variations that it will experience. Since no power supply is insensitive to load variations the output of the power supply will exhibit voltage ripple at the rate that power gating occurs. The actual voltage ripple on the supply voltage lines is a function of the power supply load rejection, the rate of power gating, and the change in power supply load due to power gating. The change in power supply load varies in relation to the RF communication link the phone is maintaining with the base station. The change in load current will be greater when the phone is transmitting at a higher RF power level than when the phone is transmitting at a decreased RF power level. The power gating may occur at each 1.25 mS time grouping used for each data frame on the reverse link. This results in a power supply load variation with a significant 800 Hz frequency component.  
           [0025]    What is desired is a voltage controlled oscillator design that maintains a stable output frequency with a constant control voltage applied. The VCC must be able to be switched such that it is tunable over two distinct frequency bands. Moreover, the VCO output must be insensitive to power supply noise. Specifically, when the VCO is implemented in a CDMA phone the VCO output must be insensitive to power supply noise created by power gating the RF transmit path. Another object of the invention is the design of a high Q, low cost, low component count, component switched, noise insensitive circuit for use as a resonant circuit within a VCO.  
         SUMMARY OF THE INVENTION  
         [0026]    The present invention is a novel and improved multiple band Voltage Controlled Oscillator (VCO) having increased noise immunity. Additionally, the invention may be viewed as a novel resonant circuit configuration that contains switched components, has high Q, and is insensitive to noise. The novel resonant circuit can be implemented with an amplifier or application specific integrated circuit to generate a VCO having the characteristics of multiple band coverage, noise insensitivity, and frequency stability.  
           [0027]    In a first embodiment all of the elements of the resonant circuit are connected in a balanced configuration with the exception of the inductor. First and second coupling capacitors comprise the positive and negative balanced connections to the resonant circuit. The outputs of the first and second coupling capacitors are interconnected using an inductor in series with a switched capacitor. A first tuning capacitor connects the output of the first coupling capacitor to a first variable capacitor. A second tuning capacitor connects the output of the second coupling capacitor to a second variable capacitor. The opposite ends of the first and second variable capacitors are connected together thereby maintaining a balanced configuration with respect to the balanced connections of the resonant circuit. A diode switch is connected in parallel with the switched capacitor such that the switched capacitor is electrically connected to the resonant circuit when the diode switch is not forward biased. The switched capacitor is not electrically connected to the resonant circuit when the diode switch is forward biased.  
           [0028]    In the first embodiment the first and second tuning capacitors are utilized as a voltage controlled variable circuit. In the first embodiment the capacitance value of the variable circuit is changed with the application of a control voltage. Any type of variable circuit whose impedance changes according to an applied voltage can be used in a resonant circuit to enable the resonant frequency to be tuned using a control voltage. The preferred embodiments described in the present invention utilize variable capacitors as the variable circuit.  
           [0029]    Operation of the switch causes the center frequency of the VCO to shift between two values, f 1  and f 2 . More particularly, actuation of the switch causes the resonant frequency of the resonant circuit to vary, thereby shifting the center frequency of the VCO between f 1  and f 2 .  
           [0030]    The first embodiment has the advantage of a maximized circuit Q. This is because only one inductor is utilized in the circuit. Inductor Q is the limitation to achieving high circuit Q. The elimination of the majority of inductors in the circuit maximizes the circuit Q. However, the circuit is not as noise insensitive as the second embodiment.  
           [0031]    In a second embodiment all of the elements of the resonant circuit are connected in a balanced configuration. First and second coupling capacitors comprise the positive and negative balanced connections to the resonant circuit, just as in the first embodiment. The outputs of the first and second coupling capacitors are connected to first and second inductors. The first and second inductors are each connected to one of the coupling capacitors and ground. A first tuning capacitor connects the output of the first coupling capacitor to a first variable capacitor. A second tuning capacitor connects the output of the second coupling capacitor to a second variable capacitor. The opposite ends of the first and second variable capacitors are connected together thereby maintaining a balanced configuration with respect to the balanced connections of the resonant circuit. The second embodiment, as presently described, is completely balanced with respect to the input of the resonant circuit. One end of the switched capacitor is connected to the output of the second coupling capacitor. The switched capacitor is connected in series to the diode switch that is then connected to the output of the first coupling capacitor. The output of the first coupling capacitor is connected to the output of the second coupling capacitor using the switched capacitor in series with the diode switch. The resonant circuit is indifferent to whether the switched capacitor is connected to the output of the first coupling capacitor with the diode switch connected to the output of the second coupling capacitor or if the positions of the switched capacitor and diode switch are transposed.  
           [0032]    Operating the switch causes the center frequency of the VCO to shift between two values, f 1  and f 2 . More particularly, actuation of the switch causes the capacitance associated with resonant circuit to vary, thereby shifting the resonant frequency of the resonant circuit and thus changing the center frequency of the VCO from f 1  to f 2    
           [0033]    The second embodiment also has greater noise immunity due to an additional pole in high pass filter. When viewed from the inputs each of the balanced inputs has effectively a high pass filter configuration. This is due to the configuration of the coupling capacitors in relation to the inductors. This high pass filter effectively acts to remove the majority of noise induced onto the resonant circuit. The noise is eliminated from affecting the variable capacitors thereby eliminating the effects of induced noise on the operation of the resonant circuit.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    The features, objects, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:  
         [0035]    [0035]FIG. 1 is a block diagram of a synthesized local oscillator;  
         [0036]    FIGS.  2 A- 2 B are amplitude and phase plots characterizing oscillator circuits;  
         [0037]    [0037]FIG. 3 is a block diagram of an integrated circuit synthesized oscillator;  
         [0038]    [0038]FIG. 4 is a block diagram showing the first embodiment of the invention incorporated with an integrated circuit oscillator; and  
         [0039]    [0039]FIG. 5 is a block diagram showing the second embodiment of the invention incorporated with an integrated circuit oscillator.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    [0040]FIG. 3 illustrates a block diagram of a typical local oscillator implementation used in a wireless phone. A synthesized oscillator IC  300  incorporates a Phase Lock Loop (PLL)  302  as well as an amplifier configured as an oscillator  304 . The synthesized oscillator IC  300  requires an external resonant circuit and loop filter  310  in order to operate. The oscillator  304  is configured as a Voltage Controlled Oscillator (VCO) if the resonant circuit can be tuned by the application of a control voltage.  
         [0041]    The resonant circuit is comprised of an inductor  320  in parallel with a capacitive network. The capacitive network utilizes first and second variable capacitors,  342  and  344  respectively, connected in series. The first variable capacitor  342  is connected to a first side of the inductor  320  through a first tuning capacitor  332 . The second variable capacitor  344  is connected to the second side of the inductor  320  through a second tuning capacitor  334 . The point where the first variable capacitor  342  connects to the second variable capacitor  344  is tied to signal ground.  
         [0042]    The oscillator  304  within the synthesized oscillator IC  300  operates at the frequency of the resonant circuit. A sample of the output of the oscillator  304  is routed to the PLL  302 . The PLL  302  compares the phase of the oscillator  304  output signal against a reference signal (not shown). The resultant error signal is passed through a loop filter  310  then is applied to the resonant circuit. The output of the loop filter  310  is applied to the variable capacitors,  342  and  344 , through first and second bias resistors,  352  and  354 . A first bias resistor  352  connects the output of the loop filter  310  to the terminal of the first variable capacitor  342  that is connected to the first tuning capacitor  332 . Similarly, a second bias resistor  354  is used to connect the output of the loop filter  310  to the terminal of the second variable capacitor  344  that is connected to the second tuning capacitor  334 . The first variable capacitor  342  and the second variable capacitor  344  may be implemented as varactor diodes. The output of the loop filter  310  is used to reverse bias the varactor diodes. The varactor diodes change their capacitance values based on the level of reverse bias applied. Therefore, by controlling the varactor diode reverse bias voltage the frequency of the oscillator can be controlled. The control voltage is varied to maintain phase lock within the PLL  302 .  
         [0043]    When a wireless phone LO is configured as shown in FIG. 3 the oscillator  304  can only tune over the range of the resonant circuit. If the oscillator  304  is required to tune over a large frequency span such that the phone can cover multiple frequency bands, the resonant circuit must be capable of tuning over the entire range. Tuning the resonant circuit of FIG. 3 over a wide range can be accomplished in two ways.  
         [0044]    A first method utilizes highly sensitive variable capacitors,  342  and  344 . The control voltage range is maintained at a minimum value when highly sensitive variable capacitors are used. However, the sensitivity of the variable capacitors makes the resonant circuit sensitive to noise induced on the circuit. Any source of noise induced on the variable capacitor bias causes a shift in the resonant frequency of the circuit. If the rate of induced noise is higher than the loop bandwidth the PLL  302  is unable to correct the error.  
         [0045]    A second method utilizes low sensitivity variable capacitors with an extended control voltage range. However, the extended control voltage range presents a problem for wireless phones operating off of battery power. Since the voltage provided by batteries is relatively low, the expansion of the control voltage range requires a step up of the available voltage. A voltage step up is accomplished with a DC-DC converter. A DC-DC converter does not operate at 100% efficiency. The loss in the step up voltage conversion is wasted battery power. Minimizing battery power consumption is a major priority in wireless phones. Another major priority in wireless phones is minimizing physical size. Both of these high priority design constraints discourage the use of extended control voltage ranges in a wireless phone.  
         [0046]    An alternative resonant circuit configuration integrates switched components into the resonant circuit. The components are switched out of the resonant circuit for operation in a first frequency band and the components are switched into the resonant circuit for operation in a second frequency band.  
         [0047]    The first embodiment of the present invention is illustrated in FIG. 4. The LO configuration utilizes the same synthesized oscillator IC  300  and loop filter  310  described in FIG. 3. However, the configuration of the resonant circuit in FIG. 4 differs from that shown in FIG. 3. The resonant circuit shown in FIG. 4 includes a switched capacitor  414  that can be switched into and out of the circuit according to the bias across the diode switch  420 . However, the resonant circuit shown in FIG. 4 contains additional elements not incorporated in the resonant circuit of FIG. 3.  
         [0048]    The resonant circuit shown in FIG. 4 incorporates first and second coupling capacitors,  402  and  404 . These coupling capacitors will decrease the level of any noise from the Oscillator pins to the resonant circuit. The first terminal of each coupling capacitor is used to connect the resonant circuit to the respective terminal of the oscillator  304 . The second terminal of each coupling capacitor,  402  and  404 , is connected to opposite ends of the remainder of the resonant circuit. The second terminal of the first coupling capacitor  402  is connected to an inductor  410  placed in series with a switched capacitor  414 . The end of the switched capacitor  414  that is not connected to the inductor is connected to the second terminal of the second coupling capacitor  404 .  
         [0049]    A diode switch  420  is connected in parallel with the switched capacitor  414 . The anode of the diode switch  420  is connected to the second terminal of the second coupling capacitor  404  and the cathode of the diode switch  420  is connected to the junction of the inductor  410  and the switched capacitor  414 . The anode of the diode switch  420  is pulled up to the supply voltage rail using a pull up resistor  462 . At the junction of the first coupling capacitor  402  and the inductor  410  is a circuit used to control the forward bias on the diode switch  420 . This circuit is comprised of a pull down resistor  464  connected to a DC switch  466 . When the DC switch  466  is closed the pull down resistor  464  provides a DC path from the inductor  410  to ground. When the DC switch  466  is open, the pull down resistor  464  is open circuited and no current flows through it. A Band Select signal driving a control resistor  468  connected to the DC switch  466  controls the DC switch  466 . The actual configuration of the switch placed in parallel with the switched capacitor  414  is not critical. The forward biased diode switch  420  shown in FIG. 4 is illustrative only and is not a limitation on the switch configuration that can be used in the resonant circuit. Any comparable switch placed in parallel with the switched capacitor  414  is allowable.  
         [0050]    The remainder of the FIG. 4 resonant circuit is configured much like that shown in FIG. 3. A first tuning capacitor  432  connects the second terminal of the first coupling capacitor  402  to a first variable capacitor  442 . A second tuning capacitor  434  connects the second terminal of the second coupling capacitor  404  to a second variable capacitor  444 . The ends of the two variable capacitors,  442  and  444 , opposite the tuning capacitors,  432  and  434 , are connected together and tied to ground. The control voltage signal out of the loop filter  310  is applied to each of the variable capacitors,  442  and  444 , through bias resistors  452  and  454 . A first bias resistor  452  connects the control voltage signal out of the loop filter  310  to the junction of the first variable capacitor  442  and first tuning capacitor  432 . A second bias resistor  454  connects the control voltage signal out of the loop filter  310  to the junction of the second variable capacitor  444  and second tuning capacitor  434 . The bias resistors  452  and  454  apply the control voltage signal to reverse bias the variable capacitors when the variable capacitors  442  and  444  are implemented as varactor diodes. In present embodiment, the control voltage may be varied from 0-3 Volts. The control voltage signal is used to adjust the present invention for variations in components and variations due to temperature.  
         [0051]    The ability to frequency band switch the first embodiment is described as follows. When the DC switch  466  is closed the diode switch  420  is forward biased and conducts. When the diode switch  420  conducts, the switched capacitor  414  is short circuited and does not electrically contribute to the resonant circuit. The resonant frequency f 1  is then determined by the value of the inductor in parallel with the capacitive tuning circuit comprised of the tuning capacitors,  432  and  434 , in conjunction with the variable capacitors  442  and  444 . When the Band Select signal controls the DC switch  466  to an open circuit condition the diode switch  420  no longer conducts.  
         [0052]    The switched capacitor  414  is electrically connected to the resonant circuit when the diode switch  420  is not conducting. The resonant frequency f 2  of the circuit is increased when the switched capacitor  414  electrically contributes to the resonant circuit. The resonant frequency f 2  is increased because the switched capacitor  414  appears in series with the inductor  410 . The series combination results in a reactance that is the sum of each reactance. Since the reactance of an inductor is opposite the reactance of a capacitor the effect of a series combination is a reactance that is less than the larger of the two reactances. The reactance of the switched capacitor  414  is chosen to be less than the reactance of the inductor  410  such that the series combination of the two elements has the reactance of an equivalently smaller inductor. A smaller inductor in the resonant circuit increases the resonant frequency.  
         [0053]    The first embodiment of the invention shown in FIG. 4 has several advantages. One advantage is that the embodiment maximizes the circuit Q. The circuit Q is maximized because the circuit is implemented with the minimum number of inductors. Because of their physical structure, inductors have much lower component Q and larger size than capacitors. A resonant circuit that achieves band switching by the addition and removal of inductors will have a lower circuit Q than a resonant circuit that achieves band switching by the addition and removal of capacitors. Circuit Q is also maximized by the series combination of the inductor  410  and switched capacitor  414 . The circuit Q is higher with the series combination of the inductor  410  and switched capacitor  414  because a larger inductor  410  value is used. Since inductor  410  component Q is determined as X L /R L  a larger value of inductance provides a higher component Q. It is also easier to center the two operating frequencies using capacitors over inductors because capacitor values are available in finer gradations than inductor values.  
         [0054]    Another advantage of the first embodiment is increased noise immunity. When circuits on the transmit path are power gated on/off to conserve power during reduced rate transmission in a CDMA wireless phone system, the sudden changes in the power supply load result in fluctuations in the power supply output. The fluctuations in the power supply output affect all active components. The synthesized oscillator IC  300  may be affected by the power supply fluctuations by exhibiting a corresponding voltage ripple on all output lines. The voltage ripple will be exhibited on the terminals connecting to the resonant circuit as well as on the control voltage line to the loop filter  310 .  
         [0055]    The two coupling capacitors,  402  and  404 , help to decrease the effect of any noise induced on the resonant circuit via the oscillator pins. The coupling capacitors,  402  and  404 , provide additional reactances that serve to reduce the level of voltage ripple that ultimately reaches the variable capacitors,  442  and  444 .  
         [0056]    The band switch configuration of the resonant circuit provides increased noise immunity by minimizing the VCO gain. The VCO gain is a measure of the tuning sensitivity of the VCO, is denoted K V , and is typically measured in MHZ/V. The band switch configuration minimizes the VCO gain by limiting the capacitance change required of the variable capacitors in the resonant circuit. The capacitance range is minimized because the large scale change required to switch frequency bands is performed by the inclusion of the switched capacitor  414 . Therefore, a limited voltage control range can accurately control the oscillator frequency in both bands without increasing the value of K V . In the first embodiment, the L and C values are selected such that the KV values are about the same at the operating frequencies f 1  and f 2 .  
         [0057]    A second embodiment of the invention, shown in FIG. 5, provides a greater level of noise immunity at a slight degradation in circuit Q. The second embodiment also utilizes a band switched resonant circuit configuration. The second embodiment is very similar to the first embodiment.  
         [0058]    The second embodiment incorporates first and second coupling capacitors,  502  and  504 , as the inputs to the resonant circuit. The synthesized oscillator IC  300  has a balanced pair of connections for the external resonant circuit. A first terminal of the first coupling capacitor  502  is connected to the positive resonant circuit interface connection of the synthesized oscillator IC  300 . The second terminal of the first coupling capacitor is connected to a first inductor  512 . The first inductor  512  provides a circuit path to ground. Similarly, a first terminal of the second coupling capacitor  504  is connected to the negative resonant circuit interface connection of the synthesized oscillator IC  300 . The second terminal of the second coupling capacitor  504  connects to a second inductor  514 . The second inductor  514  provides a circuit path to ground.  
         [0059]    A switched capacitor  522  in series with a diode switch  520  connects the second terminal of the first coupling capacitor  502  to the second terminal of the second coupling capacitor  504 . The anode of the diode switch  520  is connected to the switched capacitor  522  in the series connection. FIG. 5 shows the switched capacitor  522  connected to the second terminal of the second coupling capacitor  504  and the cathode of the diode switch  520  connected to the second terminal of the first coupling capacitor  502 . However, the series connection of the switched capacitor  522  and the diode switch  520  can be reversed without affecting operation of the circuit. That is, the cathode of the diode switch  520  is connected to the second terminal of the second coupling capacitor  504  and the switched capacitor  522  is connected to the second terminal of the first coupling capacitor  502  with no change in circuit operation.  
         [0060]    The circuit required to bias the diode switch  520  includes a pull up resistor  564  connected on one end to the anode of the diode switch  520  and connected on the other end to a DC switch  566 . The DC switch  566  connects the pull up resistor  564  to the power supply rail when in the closed condition. When the DC switch  566  is closed DC current flows follows a path to ground through the pull up resistor  564 , the forward biased diode switch  520  and the first inductor  512 . When the DC switch  566  is in the open condition the pull up resistor  564  is open circuited and no current flows through the pull up resistor  564 . A Band Select signal driving a control resistor  568  connected to the control terminal of the DC switch  566  controls the operation of the DC switch  566 .  
         [0061]    The remainder of the resonant circuit is configured in the same way as in the first embodiment. A first tuning capacitor  532  connects the second terminal of the first coupling capacitor  502  to a first variable capacitor  542 . A second tuning capacitor  534  connects the second terminal of the second coupling capacitor  504  to a second variable capacitor  544 . The ends of the two variable capacitors,  542  and  544 , opposite the tuning capacitors,  532  and  534 , are connected together and tied to ground. The control voltage signal out of the loop filter  310  is applied to each of the variable capacitors,  542  and  544 , through bias resistors  552  and  554 . A first bias resistor  552  connects the control voltage signal out of the loop filter  310  to the junction of the first variable capacitor  542  and first tuning capacitor  532 . A second bias resistor  554  connects the control voltage signal out of the loop filter  310  to the junction of the second variable capacitor  544  and second tuning capacitor  534 . The bias resistors  552  and  554  apply the control voltage signal to reverse bias the variable capacitors when the variable capacitors  542  and  544  are implemented as varactor diodes. In present embodiment, the control voltage may be varied from 0-3 Volts. The control voltage signal is used to adjust the present invention for variations in components and variations due to temperature.  
         [0062]    The second embodiment of the invention also implements a band switched oscillator. When the diode switch  520  is forward biased the switched capacitor  522  is electrically connected to the resonant circuit. The switched capacitor  522  appears in the resonant circuit in parallel with the capacitive network comprised of the tuning capacitors,  532  and  534 , and the variable capacitors,  542  and  544 . Thus, the switched capacitor  522  increases the capacitance value in the resonant circuit. The effect is to lower the resonant frequency of the resonant circuit to f 1 . When diode is off, the circuit operates at higher resonant frequency f 2 . The resonant circuit in the second embodiment maintains a low value of K V , the VCO gain, by providing coverage in both oscillator frequency bands through the use of a band switched resonant circuit.  
         [0063]    The primary advantage of the resonant circuit of the second embodiment is in relation to noise immunity. As in the case of the first embodiment, the second embodiment provides greater noise immunity when compared to a wide band oscillator not utilizing a band switched configuration. The lower value of K V , the VCO gain, in the band switched oscillator makes the band switched oscillator less sensitive to noise induced on the control voltage line Although the loop filter  310  will remove the majority of noise induced on the control voltage line, not all of the noise will be eliminated. An equivalent voltage ripple on the control voltage line of the wide band oscillator will result in a greater frequency deviation then the same voltage ripple induced on the control voltage line of the band switched oscillator. The improvement in noise immunity for the band switched design is the ratio of the VCO gains for the two oscillator designs.  
         [0064]    The fully balanced design of the resonant circuit also produces increased immunity to noise induced at the inputs to the resonant circuit. The resonant circuit is configured as a high pass filter when viewed from either input terminal. The combination of a series coupling capacitor,  502  or  504 , in conjunction with either the first or second inductor,  512  or  514 , in shunt produces a two pole high pass filter. The high pass filter configuration is particularly helpful in eliminating noise from the synthesized oscillator IC  300 . One particular source of noise from the synthesized oscillator IC  300  is attributable to power supply voltage fluctuations corresponding to power cycling of active devices in the transmit signal path. Power cycling of active devices in the transmit signal path occurs when a CDMA wireless phone operates at a reduced data rate. When the CDMA phone is operating in a reduced data rate only one copy of a number of repeated data periods is transmitted. This not only results in power savings in the phone but also a reduction in the average RF power transmitted from the phone. The reduced average phone RF transmit power results in less interference to other phones operating in the same band. Transmit power cycling results in power supply noise with a significant  800  Hz frequency component. The power supply noise is induced onto the resonant circuit via the interface connections of the synthesized oscillator IC  300 . The high pass filter incorporated into the design of the resonant circuit eliminates the noise from the variable capacitors  542  and  544 . The result is the resonant circuit is unaffected by noise induced from the synthesized oscillator IC  300  since no other components in the resonant circuit are affected by voltage variations. The output of the oscillator  304  then exhibits better phase noise because the resonant circuit is not affected by noise.  
         [0065]    The invention provides an oscillator having a majority of desired characteristics. The high Q of the resonant circuit ensures the oscillator maintains a stable operating frequency for a given control voltage. The band switched design allows the oscillator to cover multiple frequency bands while maintaining a low value of VCO gain. This improves the phase noise of the oscillator output by desensitizing the VCO output to noise on the control voltage line. Most importantly, the resonant circuit design is relatively immune to induced noise. Noise induced onto the resonant circuit from an active oscillator circuit such as a synthesized oscillator IC is filtered in the resonant circuit before it can have an effect on the tuning elements within the resonant circuit. The filter is composed by structuring the elements of the resonant circuit into a high pass configuration. Therefore, the very elements that make up the resonant circuit simultaneously serve to filter out any noise. The result is a clean oscillator output regardless of the noisy operating environment the oscillator circuit.  
         [0066]    The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The 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 the use of the inventive faculty. 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.