Abstract:
A radio-frequency (RF) front-end circuit includes a tunable filter, a negative transconductance circuit coupled with the tunable filter to produce a tuning oscillation signal, and a counter arranged to determine a frequency of the tuning oscillation signal. The RF front-end circuit also includes a control circuit arranged to shift the frequency of the tuning oscillation signal by adjusting the tunable filter until the frequency of the tuning oscillation signal falls within an acceptable frequency range corresponding to a desired channel frequency band.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/318,844, filed on Mar. 30, 2010, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to tuning a radio-frequency (RF) front-end circuit having an embedded antenna, and in particular to a method for tuning the RF front-end circuit over a desired RF band by using an on-chip negative transconductance circuit to make an oscillator. 
     2. Description of the Prior Art 
     Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, radios and other devices that receive RF signals. For example, with respect to frequency modulation (FM) audio broadcasts, within the United States FM audio signals are broadcast in the frequency band from 76 MHz to 108 MHz. 
     In conventional systems that receive terrestrial audio broadcasts, filter circuitry is often used to filter out unwanted parts of a signal spectrum that is received through an antenna. This filter circuit, therefore, acts to tune, at least in part, the incoming signal to a desired channel or portion of the RF signal spectrum. For example, with respect to FM terrestrial audio broadcasts, this filter will help tune the receiver to the desired FM channel. 
     FM receivers typically use headphone wires as a main long antenna. A problem with this is there is no signal reception after the headphones are disconnected from the receiver. As a result, customers now demand that receivers come with embedded antennas that provide support for receiving FM signals. 
     Similarly, in some applications customers demand to have an FM transmitter circuit that can take the music from a digital library device and transmit it on FM band to be played back on the car radio while driving for example. Such FM transmitters also use embedded antennas for transmission. 
       FIG. 1  illustrates an embedded antenna  12  formed on a printed circuit board (PCB)  10 . The embedded antenna  12  can be formed in many different ways, for example as a PCB trace with no ground layer beneath. The embedded antenna  12  can also be formed as a simple wire that is wound around the housing of a device, such as a mobile phone. The embedded antenna  12  is used as an antenna for FM and other broadcast applications. The equivalent circuit model for this embedded antenna  12 , which has a length that is much less than the signal wave length of the signal being received over the embedded antenna  12 , is simply a capacitor, referred to here as C ANT . The equivalent capacitor C ANT  of the embedded antenna  12  can have a range from 1-10 pF for example. 
     The reception of the embedded antenna  12  is several tens of dB lower than that of a conventional long antenna used for FM reception. In order to boost the signal level at the antenna output, a shunt inductor can be used to resonate with the equivalent capacitance of the embedded antenna  12  to form a high resonance (high-Q) resulting in voltage gain. Since the desired bandwidth of the receive band is generally wideband, tank resonance frequency must be tuned. In the prior art, tunable on-chip capacitor arrays have been used, consisting of a number of capacitor branches connected in parallel via switches are used to shift the resonance frequency. However, a problem that remains in the prior art is how the tank&#39;s resonance frequency can be measured automatically and accurately in order to be tuned to the right value. 
     Therefore, there is a need for an improved method of tuning an embedded antenna system. 
     SUMMARY OF THE INVENTION 
     According to one embodiment, a radio-frequency (RF) front-end circuit includes a tunable filter, a negative transconductance circuit coupled with the tunable filter to produce a tuning oscillation signal, and a counter arranged to determine a frequency of the tuning oscillation signal. The RF front-end circuit also includes a control circuit arranged to shift the frequency of the tuning oscillation signal by adjusting the tunable filter until the frequency of the tuning oscillation signal falls within an acceptable frequency range corresponding to a desired channel frequency band. 
     According to another embodiment, a filter calibration system for a radio-frequency (RF) front-end circuit includes a tunable filter configured to be tuned to a desired channel by adjusting a tuning control signal, the tunable filter being tunable across a frequency spectrum including a plurality of channels. The filter calibration system also includes a negative transconductance circuit coupled with the tunable filter to produce a tuning oscillation signal in a calibration mode of operation. A control circuit is used to receive a feedback signal based on the tuning oscillation signal and accordingly shift a frequency of the tuning oscillation signal by adjusting the tuning control signal to shift until the frequency of the tuning oscillation signal falls within an acceptable frequency range corresponding to a desired channel frequency band. The negative transconductance circuit and the control circuit are integrated on a same integrated circuit of the RF front-end circuit. 
     According to yet another embodiment, a method of tuning a radio-frequency (RF) front-end circuit includes producing a tuning oscillation signal with a negative transconductance circuit coupled with a tunable filter, determining a frequency of the tuning oscillation signal, and shifting the frequency of the tuning oscillation signal by adjusting the tunable filter until the frequency of the tuning oscillation signal falls within an acceptable frequency range corresponding to a desired channel frequency band. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embedded antenna formed on a printed circuit board (PCB). 
         FIG. 2  illustrates a functional block diagram of a RF front-end circuit. 
         FIG. 3  is a flowchart summarizing the frequency tuning method performed by the RF front-end circuit illustrated in  FIG. 2 . 
         FIG. 4  illustrates a detailed block diagram of an RF transceiver front-end circuit. 
         FIG. 5  is an equivalent circuit diagram modeling parts of the RF transceiver front-end circuit and their effect on an oscillation frequency of the resulting resonance tank. 
         FIG. 6  illustrates a block diagram of an RF receiver front-end circuit. 
         FIG. 7  illustrates a block diagram of an RF transmitter front-end circuit. 
     
    
    
     DETAILED DESCRIPTION 
     A radio-frequency (RF) front-end circuit with enhanced tuning method is proposed.  FIG. 2  illustrates a functional block diagram of a RF front-end circuit  100 . The RF front-end circuit  100  contains a tunable filter  102  that is controlled by a tuning control signal  124  output by a control circuit  112 . A negative transconductance circuit  104  is connected to the tunable filter  102 . At resonance the negative transconductance of the negative transconductance circuit  104  cancels the tank loss of the other elements in the RF front-end circuit  100  in order to sustain oscillation and produce a tuning oscillation signal  122 . 
     A counter  110  measures the oscillation frequency of the tuning oscillation signal  122  to calculate a counting value. During the tuning process, the counter  110  counts the received number of pulses of the tuning oscillation signal  122  during a counting period to calculate the counting value. Meanwhile, with the aid of an on-chip precision clock, the control circuit  112  calculates an expected number of pulses of the tuning oscillation signal  122  that should be received during the counting period if the RF front-end circuit  100  is properly tuned to the correct frequency, which is a desired channel frequency band. The counter  110  then outputs the counting value to the control circuit  112  in order for the control circuit  112  to compare the counting value with the expected number of pulses. If the counting value is not within the predetermined range of the expected value, the control circuit  112  changes the value of the tuning control signal  124  to adjust the tunable filter  102 , thereby adjusting the oscillation frequency of the tuning oscillation signal  122 . Once the tuning oscillation signal  122  is within an acceptable range of the desired channel frequency band, the control circuit  112  latches the desired value of the tuning control signal  124  and then the negative transconductance circuit  104  is disabled for normal mode operation. Using a feedback loop created by the tunable filter  102 , the negative transconductance circuit  104 , the counter  110 , and the control circuit  112 , the frequency of the RF front-end circuit  100  can be tuned quickly, accurately, and automatically. 
       FIG. 3  is a flowchart summarizing the frequency tuning method performed by the RF front-end circuit  100  illustrated in  FIG. 2 . In step  150 , a desired tuning frequency is selected for the RF front-end circuit  100 . In step  152 , the tuning oscillation signal  122  is produced with the combination of the negative transconductance circuit  104  and the tunable filter  102 . Next, the frequency of the tuning oscillation signal  122  is counted by the counter  110  in step  154 . The control circuit  112  then determines in step  156  if the frequency of the tuning oscillation signal  122  is within an acceptable frequency range for the desired tuning frequency. If so, the step  160  is executed. Otherwise, step  158  is executed. In step  158 , the control circuit  112  adjusts the tunable filter  102  with the tuning control signal  124  in order to shift the frequency of the tuning oscillation signal  122 . The tuning method ends in step  160 . 
       FIG. 4  illustrates a detailed block diagram of an RF transceiver front-end circuit  300  according to one embodiment of the present invention.  FIG. 5  is an equivalent circuit diagram modeling parts of the RF transceiver front-end circuit  300  and their effect on an oscillation frequency of the resulting resonance tank. A shunt inductor  306  resonates with the sum of all capacitance connected to the RF port and the resulting resonance frequency equals the desired channel frequency band. The tunable capacitance circuit  308  is controlled to tune (shift) this resonance frequency for different desired RF channels. 
     In the embodiment illustrated by  FIG. 4 , a negative transconductance circuit  304  of  FIG. 4  corresponds to the negative transconductance circuit  104  of  FIG. 2 , the shunt inductor  306  and the tunable capacitance circuit  308  of  FIG. 4  correspond to the tunable filter  102  of  FIG. 2 , a digital counter  310  of  FIG. 4  corresponds to the counter  110  of  FIG. 2 , and a digital signal processor (DSP)  312  of  FIG. 4  corresponds to the control circuit  112  of  FIG. 2 . 
     An embedded antenna  302  is used to transmit or receive RF signals, and the embedded antenna  302  can be modeled as an equivalent capacitance C ANT  in series with an equivalent resistance R ANT . One application of the RF transceiver front-end circuit  300  is supporting reception and transmission of RF signals within the frequency modulation (FM) broadcast frequency band of 76 MHz to 108 MHz. 
     It will be appreciated that the RF transceiver front-end circuit  300  satisfies the objective of automatically tuning the embedded antenna  302  for a desired FM channel within the FM frequency band of 76 MHz to 108 MHz. The tuning flexibility offered by the RF transceiver front-end circuit  300  also allows for a wide range of embedded antenna configurations to be used, allowing the circuit to be used in a variety of different products. 
     In an embodiment, an integrated circuit  325  is used for integrating several elements of the RF transceiver front-end circuit  300 . In the description below, elements referred to as being “on-chip” are located on the integrated circuit  325 , whereas those elements referred to being “off-chip” are not located on the integrated circuit  325 . In an embodiment, all off-chip elements, along with the integrated circuit  325 , are disposed on a PCB  305  for an example, and the PCB has its own equivalent capacitance C PCB . 
     In an embodiment, the shunt inductor  306  is located off-chip, and is used to resonate with the capacitance C ANT  of the embedded antenna  302 . The shunt inductor  306  is realized as an equivalent inductance L SH . The tunable capacitance circuit  308  is a variable on-chip capacitance circuit that can be discrete or continuous depending on the application and is controlled by a tuning control signal  324  output by the DSP  312  located on-chip. The tunable capacitance circuit  308  is realized as a variable capacitor C VAR . The on-chip negative transconductance circuit  304  is used to provide a negative transconductance and oscillating with the resonance tank. The negative transconductance circuit  304  is modeled as an equivalent capacitance C −gm  in parallel with an equivalent resistance R −gm . At resonance the negative transconductance of the negative transconductance circuit  304  cancels the tank loss of the other elements in the RF transceiver front-end circuit  300  in order to sustain oscillation and produce a tuning oscillation signal  322 . 
     In an embodiment, the tunable capacitance circuit  308  comprises a capacitor array, and can be located either on-chip or off-chip. The capacitance values of the tunable capacitance circuit  308  can be either discrete or continuous, and the tunable capacitance circuit  308  is digitally or analog or mixed analog and digitally controlled with the tuning control signal  324 . 
     In an embodiment, the tunable capacitance circuit  308  is a tunable capacitance array, and both the tunable capacitance circuit  308  and the shunt inductor  306  are connected to the signal path using a shunt configuration. In yet another embodiment, as shown in  FIG. 5 , the embedded antenna  302  and the shunt inductor  306  are located off-chip, whereas the negative transconductance circuit and the tunable capacitance circuit  308  are located on-chip. 
     During calibration mode, the negative transconductance circuit  304  is enabled, and the digital counter  310  measures the oscillation frequency of the tuning oscillation signal  322  with respect to a reference clock CLKref. The reference clock CLKref is a substantially constant clock frequency that can be used as a reference for counting other signals. For instance the reference clock CLKref can be a 26 MHz clock produced by a crystal. The digital counter  310  and the reference clock CLKref can both be integrated on-chip. The digital counter  310  aids in the tuning process by counting pulses of the tuning oscillation signal  322  during a counting period indicated by the reference clock CLKref in order to calculate a counting value. 
     During the tuning process, the digital counter  310  counts the received number of pulses of the tuning oscillation signal  322  during the counting period to calculate the counting value. Meanwhile, the DSP  312  calculates an expected number of pulses of the tuning oscillation signal  322  that should be received during the counting period if the RF transceiver front-end circuit  300  is properly tuned to the correct frequency. The digital counter  310  then outputs the counting value to the DSP  312  in order for the DSP  312  to compare the counting value with the expected number of pulses. If the counting value received from the digital counter  310  is close enough, or within a predetermined range, of the expected value calculated by the DSP  312 , then the RF transceiver front-end circuit  300  is considered to be properly tuned. If the counting value is not within the predetermined range of the expected value, the DSP  312  changes the value of the tuning control signal  324  to adjust the variable capacitance C VAR  of the tunable capacitance circuit  308 , thereby adjusting the oscillation frequency of the tuning oscillation signal  322 . Once the tuning oscillation signal  322  is within an acceptable range, the DSP  312  latches the desired value of the tuning control signal  324  and then the negative transconductance circuit  304  is disabled for normal mode operation. Thus, using the above tuning method, the negative transconductance circuit  304  produces the tuning oscillation signal  322  that is used to adjust or tune the frequency of the RF transceiver front-end circuit  300 . The digital counter  310  counts the oscillation frequency of the tuning oscillation signal  322  and provides this counting value to the DSP  312  as feedback. Using the feedback loop, the frequency of the RF transceiver front-end circuit  300  can be tuned quickly and automatically. 
     Please continue to refer to  FIG. 4 . The RF transceiver front-end circuit  300  has the functions of both transmitting RF signals and receiving RF signals. For receiving RF signals, a low noise amplifier (LNA)  314  located on-chip is used for amplifying received RF signals that are received through the embedded antenna  302  to produce amplified received RF signals. A receiving mixer  316  located on-chip is used for frequency down converting the amplified received RF signals for further processing. The input impedance of the LNA  314  can be modeled as an equivalent capacitance C LNA  joined in parallel with an equivalent resistance R LNA . 
     For transmitting RF signals, a power amplifier  318  located on-chip is used for amplifying RF signals to be transmitted to produce amplified output transmission RF signals for transmission through the embedded antenna  302 . The power amplifier  318  can be realized as an equivalent current source I PA  joined in parallel with both an equivalent capacitance C PA  and an equivalent resistance R PA . 
       FIG. 6  illustrates a block diagram of an RF receiver front-end circuit  400 . Differing from the RF transceiver front-end circuit  300  shown in  FIG. 4 , the RF receiver front-end circuit  400  only receives RF signals and does not contain a transmitter function. Therefore, the power amplifier  318  used for transmitting RF signals are not included in the RF receiver front-end circuit  400 . For all other elements in the RF receiver front-end circuit  400 , their functions are the same as described above with respect to the RF transceiver front-end circuit  300 . 
       FIG. 7  illustrates a block diagram of an RF transmitter front-end circuit  500 . Differing from the RF transceiver front-end circuit  300  shown in  FIG. 4 , the RF transmitter front-end circuit  500  only transmits RF signals and does not contain a receiver function. Therefore, the LNA  314  and the receiving mixer  316  used for receiving RF signals are not included in the RF transmitter front-end circuit  500 . For all other elements in the RF transmitter front-end circuit  500 , their functions are the same as described above with respect to the RF transceiver front-end circuit  300 . 
     The RF transceiver front-end circuit  300 , the RF receiver front-end circuit  400 , and the RF transmitter front-end circuit  500  are well suited for receiving or transmitting FM radio signals. The embedded antenna  302  can have a length of less than λ/4, and even much less than λ/10, where the wave length λ is related to the desired tuning frequency of the RF front-end circuit used to transmit or receive signals. 
     In addition to the simplicity of the proposed tuning method, another main advantage of using the proposed solution is that the tuning algorithm is very similar to that used for the tuning of an on-chip voltage controlled oscillator (VCO) used in a synthesizer of local oscillator (LO) generation used for a receiver or a transmitter. Therefore, the same digital hardware can be re-used for both the VCO and embedded antenna tuning. As a result, no extra digital hardware is needed. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.