Abstract:
An electronic device includes an adjustable filter with a first filter element, and a second filter element coupled to the first filter element. The second filter element includes a field effect transistor (FET) including a source terminal, a drain terminal, and a gate terminal. The source terminal and the gate terminal are coupled to a reference voltage. A control circuit is coupled to the drain terminal and is configured to apply a control voltage thereto to vary a capacitance between the source and drain terminals to adjust the adjustable filter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of electronic devices, and, more particularly, to electronic devices including adjustable capacitance elements and related methods. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wireless communication technology is evolving at an ever growing pace in order to meet the demanding performance characteristics of new mobile wireless communication devices. It is desired that these new mobile wireless communications devices are able to transmit data with a minimum amount of signal distortion. 
         [0003]    There are two common types of transmitters employed in today&#39;s wireless infrastructures. In some transmitters an information signal (e.g. audio, video, etc.) modulates a radio frequency (RF) signal. This is known as direct modulation, and these direct modulation transmitters are relatively simple. 
         [0004]    Other, more complicated transmitters are called superheterodyne transmitters. In a superheterodyne transmitter, the information signal first modulates an intermediate frequency signal. After stages for filtering and amplification, the intermediate frequency signal is converted to a RF signal by a frequency mixing stage. These superheterodyne transmitters are more complex than direct modulation transmitters, although they do provide numerous advantages. 
         [0005]    When the intermediate frequency signal is converted to the RF frequency through a mixer, a variety of undesirable frequencies in addition to the desired frequencies are generated. The undesirable frequencies are based upon both the intermediate frequency and the information signal Common undesired signals include local oscillator feed through and the IF image frequency response. Subsequent stages, including filters, are used to remove these undesirable frequencies. When a given device is capable of transmitting at multiple frequencies, advanced filtering stages can be utilized to filter out the different undesirable frequencies corresponding to which transmit frequency is currently being employed. 
         [0006]    For example, U.S. Pat. Pub. 2008/0287089 to Alles discloses an input filter for a superheterodyne receiver for image frequency suppression. The input filter includes a first filter circuit with bandpass characteristics and a center frequency. The first filter circuit has a varactor diode and a first filter inductor that are connected in parallel and form a parallel-resonant circuit, and the center frequency of the first filter circuit can be set by application of a control voltage to the varactor diode. The receiver also includes a second filter circuit with band stop characteristics that includes a varactor diode and a second filter inductor being connected in series and forming a series-resonant circuit. 
         [0007]    Similarly, U.S. Pat. No. 7,221,924 to Zheng et al. discloses a superheterodyne receiver including a notch filter. The notch filter includes a varactor. Tuning of the varactor tunes the notch of the filter. 
         [0008]    The filters in the above references may not provide the desired performance, because varactors may behave in a highly nonlinear fashion at higher frequencies, or may not be able to handle a desired amount of power, due to the overall small size of varactors. Consequently, new filter designs for electronic devices are required. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing background, it is therefore an object of the present invention to provide an electronic device with an adjustable center frequency bandpass filter that may perform in a linear fashion at high input powers at high carrier frequencies. 
         [0010]    This and other objects, features, and advantages in accordance with the present invention are provided by an electronic device that includes an adjustable filter comprising a first filter element, and a second filter element coupled to the first filter element. The second filter element comprises a field effect transistor (FET) including a source terminal, a drain terminal, and a gate terminal, the source terminal and the gate terminal being coupled to a ground reference voltage. A control circuit is coupled to the drain terminal and is configured to apply a control voltage thereto to vary a capacitance between the drain and source terminals to adjust the center frequency of the adjustable filter. 
         [0011]    This use of a FET as a variable capacitance in the bandpass filter, as opposed to conventional circuit design which would teach the use of a varactor, helps to reduce nonlinear distortion. In addition, a FET is capable of handling more power before failure than a typical varactor due to its larger structure. 
         [0012]    In some applications, the adjustable filter comprises an adjustable bandpass filter. Further, the first filter element comprises an inductor. In some applications, the first filter element comprises a capacitor. Additionally or alternatively, the FET comprises a source region underlying the source terminal, a drain region underlying the drain terminal, and a channel extending therebetween. 
         [0013]    Radio frequency (RF) transmitter circuitry is coupled to the adjustable filter. The RF transmitter circuitry comprises an up converter and a power amplifier cooperating with the adjustable filter. 
         [0014]    The radio frequency (RF) receiver circuitry is coupled to the adjustable filter. The RF receiver circuitry comprises a down converter and an amplifier cooperating with the adjustable filter. 
         [0015]    A method aspect is directed to a method of forming an electronic device. The method includes forming an adjustable filter by coupling a first filter element to a second filter element, the second filter element comprising a field effect transistor (FET) comprising a source terminal, a drain terminal, and a gate terminal. The method also includes coupling the source terminal and gate terminals to a reference voltage, and coupling a control circuit to the drain terminal and configuring the control circuit to apply a control voltage to the drain terminal to vary a capacitance between the source and drain terminals to adjust the adjustable bandpass filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic block diagram of an electronic device according to the present invention. 
           [0017]      FIG. 2  is a schematic block diagram of another embodiment of an electronic device according to the present invention. 
           [0018]      FIG. 3  is a circuit diagram of the bandpass filter such used in  FIG. 1 . 
           [0019]      FIG. 4  is a cross sectional view of a field effect transistor of  FIG. 3 . 
           [0020]      FIG. 5  shows the filter loss of the electronic device of  FIG. 1  when using FETs as opposed to varactors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments. 
         [0022]    Referring initially to  FIG. 1 , an electronic device  10  is now described. The electronic device  10  illustratively comprises a transmitter, although it should be understood that the electronic device may also include other components, such as a receiver. The transmitter  10  is illustratively a superheterodyne transmitter, but may be any suitable transmitter, may operate in any suitable frequency band. 
         [0023]    The baseband input signal is generated by the digital to analog converter (DAC)  12 , which, in turn, is coupled to a second mixer  14 . Also coupled to the second mixer  14  is a second local oscillator  30  so that the second mixer is driven to a fixed intermediate frequency. 
         [0024]    The output of the second mixer  14  is in turn coupled to a fixed bandpass filter  16  so as to filter out undesired tones, phase noise, and spurious responses. The mixer  14  produces both sum and difference beat frequency signals, each one containing a copy of the desired signal. The frequencies at the output include the sum and difference frequencies as well as a number of undesirable frequencies which are 3rd and higher-order intermodulation products. 
         [0025]    The fixed bandpass filter  16  selects the desired signal depending on design parameters, and is in turn coupled to a first mixer  18 . A first local oscillator  32  is coupled to the first mixer  18  such that the first mixer is driven to a desired transmit frequency. The first local oscillator  32  is variable and can operate at a plurality of frequencies, enabling the electronic device  10  to function at a variety of transmit frequencies. 
         [0026]    The first mixer  18  is coupled to a second filter  20 , which will be described in detail below. This filter  20  is adjustable in center frequency so that it can deliver satisfactory performance at a variety of transmit frequencies, and is illustratively a bandpass filter. The first mixer  18  produces both sum and difference beat frequency signals, each containing a copy of the desired signal. The frequencies at the output include the sum and difference frequencies as well as a number of undesirable frequencies, such as the 3rd and higher-order intermodulation products. The adjustable bandpass filter  20  filters out either the sum or the difference frequency, leaving the desired frequency. 
         [0027]    The adjustable bandpass filter  20  is coupled to a power amplifier  22 , which is in turn coupled to a low pass filter  24  to filter out undesirable harmonic content generated by the power amplifier  22 . The low pass filter  24  is coupled to the antenna  28  for transmission of the signal. 
         [0028]    With reference to  FIG. 2 , another embodiment of the electronic device  10 ′ is now described. Here, the electronic device  10 ′ comprises a superheterodyne receiver, but may be any suitable receiver, may operate in any suitable frequency band. In addition, it should be understood that the electronic device  10 ′ may include both the transmitter describe above and the receiver described below. 
         [0029]    Indeed, the electronic device  10 ′ includes an antenna  28 ′ coupled to an adjustable bandpass filter  20 ′, which will be described in detail below. The adjustable bandpass filter  20 ′ is in turn coupled to an RF amplifier  13 ′, which amplifies the received signal and is selectively tuned to pass a desired range of channels. The RF amplifier  13 ′ is in turn, coupled to a mixer  15 ′. A variable local oscillator  23 ′ is coupled to the mixer  15 ′, which drives the mixer to a desired channel. Adjustment of the local oscillator  23 ′ allows for different channels to be selected as will be understood by one of skill in the art. 
         [0030]    A reason to convert to an intermediate frequency is to convert the various different frequencies of the stations to a common frequency for processing. Superheterodyne receivers such as the electronic device  10 ′ tune in different stations simply by adjusting the frequency of the local oscillator  23 ′ and processing thereafter is done at the same frequency, the intermediate frequency. Without using an intermediate frequency, the complicated filters and detectors in a radio would have to be tuned in unison each time the station was changed, which may not be desirable. 
         [0031]    Yet a further reason for using an intermediate frequency is to improve frequency selectivity. In communication circuits, a common task is to separate out or extract signals or components of a signal that are close together in frequency. With most filtering techniques the filter&#39;s absolute bandwidth increases proportionately with the frequency. So, a narrower bandwidth and more selectivity can be achieved by converting the signal to an intermediate frequency and performing the filtering at that frequency. 
         [0032]    The mixer  15 ′ produces both sum and difference beat frequencies signals, each one containing a copy of the desired signal. 
         [0033]    The output of the mixer  15 ′ is coupled to an intermediate frequency amplifier  17 ′, which is in turn coupled to a bandpass filter  19 ′ which selects the desired signal and rejects the rest. The bandpass filter  19 ′ is in turn coupled to a second mixer  21 ′, which has a second local oscillator  29 ′ coupled thereto to drive signal to a desired frequency. The second mixer  21 ′ is in turn coupled to an amplifier  25 ′, which then feeds the signal to an analog to digital converter (ADC)  27 ′. The ADC  27 ′ samples the signal for further processing in the digital domain. 
         [0034]    With reference to  FIG. 3 , the adjustable bandpass filter  20  is now described. The adjustable bandpass filter  20  comprises a plurality of inductors  40 ,  42 ,  44 ,  46 ,  48 ,  49 ,  50  and a plurality of capacitors  52 ,  54 ,  56  coupled thereto. The bandpass filter  20  further includes three FET&#39;s  64 ,  66 ,  68  coupled to the inductors  40 ,  42 ,  44 ,  46 ,  48 ,  49 ,  50  and capacitors  52 ,  54 ,  56 . 
         [0035]    The FET&#39;s  64 ,  66 ,  68  are illustratively NMOS transistors, but in other embodiments some or all thereof may instead be PMOS, or other types of insulated gate transistors. 
         [0036]    Each FET  64 ,  66 ,  68  includes a source terminal  94 , a drain terminal  98 , and a gate terminal  96 . The source terminal  94  and the gate terminal  96  are coupled to a reference or ground voltage. When biased this way, the capacitance between the source  94  and drain  98  terminals of the FETs  64 ,  66 ,  68  changes based upon a voltage applied to the non-grounded source/drain terminal. 
         [0037]    The structures of the FET  64  are now described with reference to  FIG. 4 , although it should be understood that the other FETs  66 ,  68  may have similar structures. The FET  64  comprises a source region  82 , a drain region  88 , and a doped substrate region  84  adjacent the source and drain regions. In operation, such as in a depletion mode or in an enhancement mode, a channel  92  extends between the source region  82  and drain region  88 . 
         [0038]    The source region  82 , drain region  88 , and doped substrate region  84  may be doped differently in different applications. Dielectric regions  80 ,  90 ,  86  are adjacent the source region  82 , drain region  88 , and channel  92 . A gate terminal  96  is carried by the dielectric region  90 . A source terminal  94  extends between the dielectric regions  80 ,  90  to contact the source  82 . A drain terminal  98  extends between the dielectric regions  90 ,  86  to contact the drain  88 . In operation, as the voltage across the source terminal  94  and drain terminal  98  increases, the width of a depletion region changes, and thus the capacitance between the source and drain terminals increases, as will be appreciated by those of skill in the art. 
         [0039]    Referring again to  FIG. 3 , control circuits  75 ,  76 ,  77  are coupled to the source terminal of each FET  64 ,  66 ,  68 . The control circuits  75 ,  76 ,  77  each comprise a resistor  58 ,  60 ,  62  coupled to a voltage source  70 ,  72 ,  74 . The resistor  58 ,  60 ,  62  acts as a high impedance at the RF frequency, isolating the control circuit from the filter. By varying the voltage produced by the voltage sources  70 ,  72 ,  74  the capacitance between the source  94  and drain terminals  98  of the FETs  64 ,  66 ,  68  can be varied, thereby moving the poles of the adjustable bandpass filter  20  and enabling fine tuning thereof. Such an adjustable bandpass filter  20  allows the electronic device  10  to be able to transmit on different frequencies without additional bandpass filters, for example. 
         [0040]    It should be understood that the voltage sources  70 ,  72 ,  74  are merely indicative of the existence of an applied voltage. Indeed, in some applications, the voltage sources  70 ,  72 ,  74  may be connections to a controller, for example. 
         [0041]    The adjustable bandpass filter  20  herein is made particularly advantageous in comparison to prior art adjustable bandpass filters by the use of the FETs  64 ,  66 ,  68  as variable capacitance units. Prior art concerning adjustable bandpass filters generally use varactors. First, the FETs  64 ,  66 ,  68  performs more linearly than a varactor. 
         [0042]    For example, a common measure of linearity of a filter is the 3 rd  order output intercept point (OIP3). This is the output power at which the fundamental power is equal to the 3 rd  order intermodulation power. Due to the fact that the capacitance between the source  94  and drain  98  terminals of the FETs  64 ,  66 ,  68  varies more linearly with voltage changes than does the capacitance of a varactor diode, the OIP3 of the adjustable bandpass filter  20  is 20 dBm at 225 MHz and 33.4 dBm at 273 MHz, which is greater than that of a prior art adjustable bandpass filter using varactors which measures 14.3 dBm at 225 MHz and 17.3 dBm at 273 MHz. In addition, the area of the FETs  64 ,  66 ,  68  is much larger than those of varactors, allowing the FETs, and consequently the adjustable bandpass filter  20 , to handle more power before failure. Furthermore, one FET is suitable for use per each pole of the adjustable bandpass filter  20  as opposed to prior art adjustable bandpass filters that use two varactors per pole. This saves money and space by reducing the parts count. 
         [0043]    To illustrate the advantage provided by the FETs  64 ,  66 ,  68  over a traditional varactor, attention is now drawn to  FIG. 5 , which graphs the filter losses of the adjustable bandpass filter  20  with the FETs  64 ,  66 ,  68 , as well as a version of the adjustable bandpass filter with three varactors replacing the FETs  64 ,  66 ,  68 . As shown in the accompanying chart, filter losses with the FETs  64 ,  66 ,  68  are less. 
         [0044]    Those of skill in the art will appreciate that the invention includes suitable methods of making the electronic device  20  disclosed above. 
         [0045]    Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.