Abstract:
A method is provided for reducing non-linear effects in an electronic circuit including an amplifier. The method may include receiving a modulated signal at an input of the amplifier, the modulated signal comprising a baseband signal modulated by an oscillator frequency. The method may further include substantially attenuating counter-intermodulation in the modulated signal caused by harmonics of the oscillator frequency and the baseband signal by a resonant circuit. In some embodiments, the resonant circuit may include at least one inductive element and one capacitive element coupled to the at least one inductive element, the at least one inductive element and the at least one capacitive element configured to substantially attenuate counter-intermodulation in the modulated signal.

Description:
RELATED APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 13/968,995 filed Aug. 16, 2013, now issued as U.S. Pat. No. 8,942,649; which is a Continuation of U.S. patent application Ser. No. 12/719,692 filed Mar. 8, 2010, now U.S. Pat. No. 8,515,364 Issued Aug. 20, 2013, the contents of which are incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to radio-frequency communication and, more particularly, to a radio-frequency transmitter and amplifier. 
     BACKGROUND 
     Radio communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using radio-frequency transmitters. To communicate such information, radio communications systems employ radio transmitters. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. 
     A common problem in radio transmitters, radio-frequency amplifiers, and other electronic devices is non-linearity of signal gain. Non-linearity may cause amplifier gain to be dependent upon input signal amplitude and as a result may cause harmonic distortion and other undesired effects. Of particular concern is third-order non-linearity which is in many cases the dominant type of non-linearity, resulting in a phenomenon known as third-order intermodulation. In a radio transmitter, harmonics may be introduced from both a baseband signal and a local oscillator used to modulate the baseband signal. Such harmonics can impact the performance of a transmitter in at least two ways, both of which can generate signal components outside of an allowed spectral mask, and thereby, may cause spectral interference to other devices. 
     The first mechanism is the intermodulation between harmonics of the local oscillator and baseband signals, know as counter-intermodulation. The second mechanism is the harmonic distortion of an upconverted radio-frequency signal by the radio-frequency amplifier of the transmitter. Modulators may be designed to reduce counter-intermodulation. However, non-linearties in radio-frequency amplifiers may cause regeneration of the counter-intermodulation. 
     Traditional approaches to solving the above problems have disadvantages. For example, a technique known as inductive degeneration is often applied in radio-frequency circuits to improve circuit linearity. However, this technique does not address the problem of counter intermodulation. As another example, a technique known as derivative superposition may be used to cancel third-order non-linearity. However, derivative superposition is not effective in reducing counter-intermodulation regeneration. As a further example, an output of the modulator of a transmitter may be high-pass filtered to reduce counter-intermodulation, but such approach does not eliminate non-linearities of the amplifier which may regenerate the counter-intermodulation. 
     SUMMARY 
     In accordance with a particular embodiment of the present disclosure, a method is provided for reducing non-linear effects in an electronic circuit including an amplifier. The method may include receiving a modulated signal at an input of the amplifier, the modulated signal comprising a baseband signal modulated by an oscillator frequency. The method may further include substantially attenuating counter-intermodulation in the modulated signal caused by harmonics of the oscillator frequency and the baseband signal by a resonant circuit. In some embodiments, the resonant circuit may include at least one inductive element and one capacitive element coupled to the at least one inductive element, the at least one inductive element and the at least one capacitive element configured to substantially attenuate counter-intermodulation in the modulated signal. 
     Technical advantages of one or more embodiments of the present invention may include reducing counter-intermodulation caused by harmonics of a baseband signal and an oscillator. 
     It will be understood that the various embodiments of the present invention may include some, all, or none of the enumerated technical advantages. In addition, other technical advantages of the present invention may be readily apparent to one skilled in the art from the figures, description and claims included herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a block diagram of an example radio transmitter, in accordance with certain embodiments of the present disclosure; 
         FIG. 2  illustrates a block diagram of an example amplifier for use in a radio transmitter, in accordance with certain embodiments of the present disclosure; 
         FIG. 3  illustrates a block diagram of another example amplifier for use in a radio transmitter, in accordance with certain embodiments of the present disclosure; and 
         FIG. 4  illustrates a block diagram of yet another example amplifier for use in a radio transmitter, in accordance with certain embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a block diagram of an example radio transmitter  100 , in accordance with certain embodiments of the present disclosure. As shown in  FIG. 1 , radio transmitter  100  may include a digital signal processor (DSP)  102 , a baseband filter  104  coupled to DSP  102 , an oscillator  106 , a modulator  108  coupled to baseband filter  104  and oscillator  106 , an amplifier  110  coupled to modulator  108 , a power amplifier  112  coupled to amplifier  110 , and an antenna  114  coupled to power amplifier  112 . DSP  102  may include a microprocessor-like circuit configured to processes digital signals input to DSP  102  using numerous techniques including filtering, transforms, and others. For example, DSP  102  may process and/or transform signals to make such signals suitable for modulation and transmission by radio transmitter  100 . DSP  102  may output an analog signal to baseband filter  104 . 
     Baseband filter  104  may be coupled to the output of DSP  102  and may be any suitable device, system, or apparatus configured to pass signal frequencies from approximately 0 Hz to a maximum frequency, to produce a baseband signal. In radio transmitter  100 , baseband filter  104  may filter the output of an analog signal produced by DSP  102  to produce a baseband signal for modulation and transmission by radio transmitter  100 . 
     Oscillator  106  may be any suitable device, system, or apparatus configured to produce an analog waveform of a particular frequency for modulation of the baseband signal produced by baseband filter  104 . In embodiments in which radio transmitter  100  is a fixed-frequency transmitter, oscillator  106  may comprise a resonant quartz crystal or other device tuned for a desired frequency. In embodiments in which radio transmitter  100  is a variable-frequency transmitter, oscillator  106  may comprise a variable-frequency oscillator, phase-locked loop frequency synthesizer, or other device configured to produce a variable frequency. 
     Modulator  108  may be coupled to the outputs of baseband filter  104  and may be any suitable device, system, or apparatus configured to modulate a baseband signal produced by baseband filter  104  at the frequency of a waveform produced by oscillator  106  in order to produce a modulated signal. In certain embodiments, modulator  108  may comprise an IQ modulator that may produce a modulated output signal based on an inphase electrical carrier signal and a quadrature electrical carrier signal, as is known in the art. 
     Amplifier  110  may be coupled to the output of modulator  108  and may be any suitable device, system, or apparatus configured to receive an input signal (e.g., current or voltage) and amplify the input signal by a gain to produce an output signal that is a multiple of the input signal. In certain embodiments, amplifier  110  may include a non-inverting amplifier, an inverting amplifier, or any combination thereof. Example embodiments of amplifier  110  are discussed in greater detail below with respect to  FIGS. 2-4 . 
     Power amplifier  112  may be coupled to the output of amplifier  110  and may be any suitable device, system, or apparatus configured to receive an input signal (e.g., current or voltage) and amplify the input signal by a gain to produce a signal with high power at its output relative to its input signal for transmission via antenna  114 . In certain embodiments, power amplifier  112  may include a non-inverting amplifier, an inverting amplifier, or any combination thereof. 
     Antenna  114  may be coupled to the output of power amplifier  112  and may be any suitable device, system, or apparatus configured to convert electrical currents into electromagnetic waves and transmit such electromagnetic waves. 
     Although  FIG. 1  is shown as only having one amplifier  110 , some embodiments of radio transmitter  100  may include multiple amplifiers (e.g. multiple amplifiers arranged in a parallel configuration). 
       FIG. 2  illustrates a block diagram of an example amplifier  110  for use in a radio transmitter (e.g., radio transmitter  100 ), in accordance with certain embodiments of the present disclosure. As shown in  FIG. 2 , amplifier  110  may include a transistor  202 , a first impedance  204  coupled to a first terminal of transistor  202 , a resonant degeneration impedance  206  coupled to a second terminal of transistor  202 , and a load impedance  208  coupled to the first terminal of transistor  202  and first impedance  204 . 
     Transistor  202  may be any device having at least three terminals for connection to a circuit external to transistor  202 , such that a voltage or current applied to at least one of transistor  202 &#39;s terminals may control the magnitude of current flowing through at least one other terminal. Although  FIG. 2  depicts transistor  202  as a n-type field effect transistor, transistor  202  may comprise any suitable type of transistor, including without limitation a p-type field effect transistor, a bipolar junction transistor, insulated gate bipolar transistor, or any other type of transistor. The type of transistor used for transistor  202 , as well as the physical characteristics of transistor  202  (e.g., the current gain, voltage gain, transresistance, or transconductance of transistor  202 ) may be selected based on desired characteristics for amplifier  110  (e.g., desired gain) and/or radio transmitter  100  (e.g., desired transmission frequency). 
     First impedance  204  may be coupled between a first terminal of transistor  202  and a signal supply voltage and may include any combination of passive circuit elements (e.g., resistors, capacitors, and inductors) selected based on desired characteristics for amplifier  110  (e.g., desired gain) and/or radio transmitter  100  (e.g., desired transmission frequency). Although first impedance  204  is shown as being coupled to the drain of transistor  202  in  FIG. 2 , the first terminal to which first impedance  204  is coupled may be determined based on the type of transistor used (e.g., first impedance may be coupled to a source terminal of a p-type field effect transistor, or an emitter or collector terminal of a bipolar junction transistor). In addition, in other embodiments first impedance  204  may be coupled between a terminal of transistor  202  and signal ground. 
     Resonant degeneration impedance  206  may be coupled between a second terminal of transistor  202  and signal ground and may include any combination of at least one inductive circuit element  210  and at least one capacitive circuit element  212  selected to have a resonant frequency such that a particular nth-order counter-modulation is not generated by amplifier  110 . For example, in certain embodiments, inductive circuit element  210  and capacitive circuit element  212  may be configured in parallel (as shown in  FIG. 2 ) and may have a resonant frequency approximately equal to three times the frequency of oscillator  106 , thereby substantially preventing generation of 3rd-order counter-intermodulation. As another example, in other embodiments, inductive circuit element  210  and capacitive circuit element  212  may be configured in parallel (as shown in  FIG. 2 ) and may have a resonant frequency approximately equal to five times the frequency of oscillator  106 , thereby substantially preventing generation of 5th-order counter-intermodulation. In some embodiments, resonant degeneration impedance  206  may have multiple resonant frequencies (e.g., both three times the frequency of oscillator  106  and five times the frequency of oscillator  106 ). Although resonant degeneration impedance  206  is shown as being coupled to the source of transistor  202  in  FIG. 2 , the second terminal to which resonant degeneration impedance  206  is coupled may be determined based on the type of transistor used (e.g., resonant degeneration impedance may be coupled to a drain terminal of a p-type field effect transistor, or an emitter or collector terminal of a bipolar junction transistor). In addition, in other embodiments resonant degeneration impedance  206  may be coupled between a terminal of transistor  202  and a signal supply voltage. 
     Load impedance  208  may be coupled to the first terminal and may include any combination of passive circuit elements (e.g., resistors, capacitors, and inductors) selected based on desired characteristics for amplifier  110  (e.g., desired gain, impedance matching) and/or radio transmitter  100  (e.g., desired transmission frequency). In certain embodiments, load impedance  208  may not be a part of amplifier  110 , but may instead serve to model an output impedance seen at the first terminal of transistor  202  (e.g., an input impedance of power amplifier  112  or other component coupled to the output of amplifier  110 ). 
     It is understood that  FIG. 2  may depict only a subset of the elements and components of amplifier  110  for the purposes of clarity and exposition. Accordingly, amplifier  110  may include elements and components other than those shown in  FIG. 2  (e.g., components for input or output impedance matching, direct current biasing components, and other components). 
     In addition, resonant degeneration impedance  206  may be used in multiple embodiments of amplifier  110 , such as the embodiments shown in  FIGS. 3 and 4 , for example.  FIG. 3  illustrates a block diagram of an embodiment of amplifier  110  (with resonant degeneration impedance  206 ) in a cascode configuration.  FIG. 4  illustrates a block diagram of a embodiment of amplifier  110  (with resonant degeneration impedance  206 ) utilizing derivative superposition. Embodiments other than those depicted in  FIGS. 2-4  may also be utilized. 
     Modifications, additions, or omissions may be made to radio transmitter  100  and/or amplifier  110  from the scope of the disclosure. The components of optical networks radio transmitter  100  and/or amplifier  110  may be integrated or separated. Moreover, the operations of optical networks radio transmitter  100  and/or amplifier  110  may be performed by more, fewer, or other components. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     Although the present invention has been described with several embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.